Based on the data of E, SI, and HA variables (Table 12), PC1 had a high positive correlation with all studied traits, except PH and SD traits, while it was related to wheat GY, WUE, and PUE under both conditions in the present study. These variables of the wheat GY and its components contributed to PC1. PC2 identified all studied traits possessing positive loading factors and contributes to the variables except TI, SN, SY, and WUE traits, while the most variables studied had the highest positive loadings on PC3 and other PCs under experimental factors. Based on the studied traits (Table 13), the Al-Qasr and Abo Kwela sites in both years with 60 kg HA ha−1 influenced PC1, while PC2 was affected by the El-Neguilla site and 60 kg HA ha−1 in both seasons under normal conditions. During normal irrigation conditions, PC1 included 30 and 60 kg HA ha−1 applications in the Al-Qasr and Abo Kwela sites in both seasons, while PC2 consisted of 60 kg HA ha−1 application at the El-Neguilla site in both seasons.

Based on all measured data, PC1 and PC2 mainly distributed and distinguished the experimental factors and studied traits in different groups. Therefore, the first two PCs were employed to draw a biplot (Figure 4). The data of variables studied displayed a positive correlation between most studied traits, but they differed in their degree and consistency in quantity. The biplot diagram depicted the contribution of E, SI, and HA in creating variability of all traits measured.

In PC1 (Figure 4), GY and other investigated traits, excluding PH, SNS, and SD, were highly and positively associated with the Al-Qasr and Abo Kwela sites in both seasons, with 30 and 60 kg HA ha−1 under normal irrigation conditions, which was located in the first and fourth quarters. Regarding PC2, PH, SD, and SNS traits had a positive correlation with the El-Neguilla site in both seasons with 0 kg HA ha−1 under drought-stress conditions, which occupied the second and third quadrants. The 60 kg HA ha−1 treatment at the Al-Qasr site in the 2020/21 season was located near GY and its components traits, as well as WUE and PUE parameters under the normal and drought-stress conditions. Regarding the relationships between all studied traits by PCA, WUE and PUE are strongly correlated with GY and its component traits under normal and drought-stress conditions. The PCA scree plot for E, SI, and HA on GY and other traits evaluated showed that the PC1 and PC2 eigenvalues correspond to the whole percentage of the variance in the dataset (Figure 5).

**Table 13.** Results of principal components (PCs) for the studied bread wheat traits as affected by the environment (E) and humic acid (HA) treatments under supplemental irrigation (SI) mode (i.e., normal and drought) conditions.

HA0, HA30, and HA60 indicate the addition of 0, 30, and 60 kg ha−1 humic acid, respectively.

**Figure 4.** Biplot diagram of PC1 and PC2 shows similarities and dissimilarities in relationships between the studied traits for the studied factors under normal and drought conditions. HA0, HA30, and HA60 indicate the addition of 0, 30, and 60 kg ha−1 humic acid, respectively. PH: Plant height, TI: Tillering index, SL: Spike length, SNS: Spikelet number per spike, SD: Spikelet density, NSm2: number of spike per m2, SY: Straw yield, BY: Biological yield, GY: Grain yield, T-GW: Thousandgrain weight, WUE: Water use efficiency, and PUE: Precipitation use efficiency. **Figure 4.** Biplot diagram of PC1 and PC2 shows similarities and dissimilarities in relationships between the studied traits for the studied factors under normal and drought conditions. HA0 , HA30, and HA60 indicate the addition of 0, 30, and 60 kg ha−1 humic acid, respectively. PH: Plant height, TI: Tillering index, SL: Spike length, SNS: Spikelet number per spike, SD: Spikelet density, NSm2 : number of spike per m2 , SY: Straw yield, BY: Biological yield, GY: Grain yield, T-GW: Thousand-grain weight, WUE: Water use efficiency, and PUE: Precipitation use efficiency.

**Figure 5.** Scree plot of PCA between respective eigenvalues % and component number.

**Figure 4.** Biplot diagram of PC1 and PC2 shows similarities and dissimilarities in relationships between the studied traits for the studied factors under normal and drought conditions. HA0, HA30, and HA60 indicate the addition of 0, 30, and 60 kg ha−1 humic acid, respectively. PH: Plant height, TI: Tillering index, SL: Spike length, SNS: Spikelet number per spike, SD: Spikelet density, NSm2: number of spike per m2, SY: Straw yield, BY: Biological yield, GY: Grain yield, T-GW: Thousand-

grain weight, WUE: Water use efficiency, and PUE: Precipitation use efficiency.

**Figure 5. Figure 5.** Scree plot of PCA between respective Scree plot of PCA between respective eigenvalues % and component number. eigenvalues % and component number.

#### **4. Discussion**

The current study evaluated GY and other quantitative traits of the cultivar Sakha 94 fertilized with HA in different environments (three sites over two years) under normal and drought-stress conditions. Statistically, GY and most traits were significantly affected by E, SI, and HA, as well as first- and second-order interactions. These results indicated the existence of variability between our experimental factors for drought tolerance; thus, improvement can be achieved for wheat GY in Egypt. Some previous studies reported conclusions similar to our results; for example, [43–46] mentioned that HA and years had highly significant effects on all production components in wheat. Paˇcuta et al. [46] confirmed significant differences in the first- and second-order interactions for GY in wheat. The differences between years were oftentimes weather-related [43]. Thus, we can assume that weather conditions, HA, and SI were the causes of significant differences for all studied traits of bread wheat. Cultivar-specific differences can play an important role in helping wheat breeders to develop more climate-change-resistant wheat [47].

Based on C.V. % values, the environmental influence was low (<10%) for all studied traits, so this trial would be considered to have high precision. The SN, SY, and PUE traits showed that the C.V. % values (10 > C.V. % > 7%) were greater than that of the other traits measured. Thus, the environmental influence was high for these traits in the normal and drought-stress conditions when compared to the other traits. This would suggest the existence of substantial differences among experimental factors for the studied traits in their drought response. The magnitude of C.V. % indicated that the wheat plants fertilized with HA had exploitable variability during the selection of GY and other traits under the various environments. These findings were consistent with [48] and different from [49–51] in wheat. The values of C.V. % confirmed the existence of high diversity and it is a useful resource in providing the fundamentals for future breeding under stress conditions [51,52].

Mean values indicated that the interactions among experimental factors revealed that there was different behavior for studied traits in normal and drought-stress conditions. Thus, it is possible to use these data in the future to increase wheat GY in Egypt. Generally, the drought-stress conditions during critical stages of growth reduced all studied traits compared to normal conditions, with decreased values ranging from 1% for SD to 46% for GY. Our results are also in agreement with [47,48,50,53–55]. The decrease in GY and

its related traits under drought-stress conditions is a popular phenomenon and can be controlled by many complex morphological, physiological, and molecular factors during plant growth stages [56]. The largest impact of drought on the grain yield of wheat may be partially due to the accumulative effects that it exerts on grain yield-related characteristics [57], pre-anthesis, post-anthesis, anthesis, and booting stages [58], and the grain-filling duration [48].

Our study revealed that GY and most studied traits were significantly higher at the Al-Qasr site in both seasons than those at the Abo Kwela and El-Neguilla sites under normal and drought-stress conditions, regardless of HA rates. Wheat productivity and other traits increased at the Al-Qasr site due to the high seasonal rainfall rates during the studied seasons, and the extent of the increase was 10% and 20% in the 2019/2020 season, as well as 50% and 45% in the 2020/2021 season, compared to the El-Neguilla and Abo Kwela sites, respectively. Applying 60 kg HA ha−1 led to a significant increase in wheat GY and its traits compared to 0 and 30 kg HA ha−1 under the two conditions, regardless of the other factors studied. We also found that all studied traits of normal conditions were higher than that of drought-stress conditions, regardless of the other two factors studied. In the study by [45], the growing season affected the GY and T-GW of durum wheat differently; also, the behavior of the genotypes changed in relation to growing years. Wheat production varies greatly from year to year. Lower GY may be due to rainfall variability in the wheat-growing season [46]. As reported in [59], wheat plants respond to drought stress through changes in various metabolic and physiological processes. The significant increase in GY and other traits due to HA application compared to the control treatment was also reported previously by [27,46,60,61].

Regarding the first-order interactions, the highest GY and most traits were found in the interaction of E × SI (Al-Qasr in both seasons × normal conditions), the interaction of E × HA (Al-Qasr in both seasons × 60 kg HA ha−1 ), and the interaction of SI × HA (normal conditions × 60 kg HA ha−1 ). As for the second-order interaction, the highest GY and most studied traits were found for the interaction of Al-Qasr × normal conditions × 60 kg HA ha−1 in both seasons under the two conditions. These results may be due to enabling the plants to adapt to drought conditions. The GY and other studied traits have been observed to increase via the combination of factors in an experiment that evaluated and recorded the highest values of every single factor, as already reported by [45,46].

STI is used for the identification of high-tolerance genotypes based on the ratio of means under normal and drought-stress conditions [41]. Compared with all experimental factors in our study, the wheat plants fertilized with 60 kg HA ha−1 at the Al-Qasr site in both seasons recorded the highest STI for GY and most studied traits under normal and drought-stress conditions. The application of 60 kg HA ha−1 differed from other HA rates by showing higher performance under drought conditions, hence having higher STI values. Thus, wheat plants under the 60 kg HA ha−1 application had the lowest susceptibility to drought stress. STI was most useful to identify genotypes differing in their response to drought in wheat [50] and barley [62].

The reciprocal correlations among most studied traits were positive and insignificant or significant (*p* ≤ 0.05 or 0.01) under normal and drought-stress conditions. Generally, the GY was positively and significantly correlated with the most studied traits under both conditions. Positive correlations for studied traits indicated that selection for the increased value of one trait will result in an increase in the value of the other [63], where the contrasting GY change is a consequence of the changes in yield components [64]. Statistically, a significant correlation was noted for GY and other traits under drought-stress conditions by [49,65]. Significant correlations between most of the traits were found under rainfed and water-stress conditions in different years, which also explained why an increase in these traits would further enhance GY under both conditions [50].

Principal component analysis (PC) has been used to estimate the similarities and dissimilarities in the relationships between the studied traits across environments, supplemental irrigation, and HA variables. Similarly, Koua et al. [51] reported that the first

two PCs explain the total variance under drought-stress conditions better than in rainfed conditions. In agreement with [66], PC1 and PC2 explain more than 90% of the total variance of all variables studied in both conditions. Meanwhile, both PCs explained lower values in our results than those in [44,49,50,67,68]. PC1 explained approximately <63% of the measured data total variability in the original variables under normal and drought-stress conditions in our study, similar to other studies [66–68]. It is evident that PC1 and PC2 can be interpreted as a response related to WUE and PUE, as well as GY and its components traits, which possess positive and negative contributions to the experimental factors. PC1 is considered very important to increase wheat GY under drought-stress conditions. Likewise, PC1 characterized GY and other agronomic traits under drought stress in winter wheat in both seasons [50], while PC2 seems to represent humic substances [67]. The biplot showed the degree of correlation amongst most studied wheat traits under E, SI, and HA variables. In other studies, the statistical analysis of PCA exhibited a strong correlation among the studied traits of wheat in both seasons under drought stress [49,50]. PC1 obtained higher loading values for all traits measured, except PH, SNS, and SD, and it also included wheat plants under the 60 kg HA ha−1 application at the Al-Qasr site in both seasons under normal irrigation conditions. The results of the scree plot were harmonic with [69] who reported that there is a break in the plot that separates the meaningful components from the trivial components. Thus, most researchers would agree that PC1 and PC2 are likely meaningful.

The biplot analysis of the relationship between the variables studied revealed that wheat plants under 60 kg HA ha−1 treatment at the Al-Qasr site in both seasons gave the highest wheat GY under normal and drought-stress conditions. In line with this study, Hegab et al. [70] have already stated that wheat GY and its components increased with an increasing the application of HA rates. It is worth noting that HA application in wheat promoted plant growth, yields (grain, straw, and biological), nutrient uptake in the soil, and resistance to biotic and abiotic stress. Furthermore, previous authors [59,71–73] mentioned that HA increased the levels of 40 compounds that are associated with the stress response. HA molecules promote the osmotic adjustment ability, increase leaf water retention, as well the photosynthetic and antioxidant metabolism of plants under drought stress [74,75]. The integration of HA application and a water deficit makes it possible to assess the precision and efficiency of the system in researching the effect of HA on drought tolerance [45,76]. Moreover, the traits may respond differently across genotypes, showing different types of drought tolerance [77]. Generally, our results showed that there is a divergence between environments (sites and seasons) and HA rates under normal irrigation and droughtstress conditions, and thus, these diversities can be used to improve wheat GY under drought-stress conditions.

#### **5. Conclusions**

Significant divergences between different environments (sites and seasons) and HA rates under normal irrigation and drought-stress conditions, as well as their interactions for wheat GY and most traits evaluated, were observed via a three-way ANOVA. Drought stress markedly decreased wheat GY and its components compared to normal conditions under the studied factors. The Al-Qasr site had the highest positive impact on GY and most studied traits in both seasons. The application of HA at a rate of 60 kg ha−1 markedly increased all studied traits compared with 0 and 30 kg ha−1 . The highest level of GY and most of its traits was recorded when fertilizing wheat plants with 60 kg HA ha−1 under normal and drought-stress conditions at the Al-Qasr site. The results of STI, Pearson's correlation coefficients, and PCA in our study could be useful and used as a suitable method for studying drought tolerance mechanisms and wheat GY improvement. Finally, the application of the 60 kg HA ha−1 dose is recommended to obtain the maximum wheat productivity under drought-stress conditions in Egypt.

**Author Contributions:** Conceptualization: E.F.E.-H., M.M.A.E.-E., M.A.E.-H.A. and K.A.E.-T.; investigation, methodology, and data curation: M.A.E.-H.A. and M.M.A.E.-E.; preparing original draft: E.F.E.-H., M.M.A.E.-E., K.A.E.-T. and A.S.; review and final editing: A.S., E.F.E.-H., M.M.A.E.-E., K.A.E.-T.; M.T.E.-S. and T.A.A.E.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project was funded by the Abu Dhabi Research Award (AARE2019) for Research Excellence-Department of Education and Knowledge (ADEK; Grant #: 21S105) to Khaled A. El-Tarabily.

**Data Availability Statement:** The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

**Acknowledgments:** Khaled A. El-Tarabily would also like to thank the library at Murdoch University, Australia, for the valuable online resources and comprehensive databases. All authors give thanks and gratitude to the PRIMA project entitled Development and Optimization of Halophyte-based Farming systems in salt affected Mediterranean soils, for the availability of the requirements for conducting research in the study areas, and especially thank Hasan Mohamed El-Shaer, head of the project.

**Conflicts of Interest:** The authors declare that they have no competing interests.

### **References**

## *Article* **Soil-Mulching Influence on Spearmint Oil Yield, Ecophysiological Activities and Essential-Oil Content in Rainfed Environment of Southern Italy**

**Sebastiano Delfine 1,\* , Violeta B. Velikova 2 and Franco Mastrodonato 3**

**Abstract:** The application of soil mulching is widely used to improve crop productivity within semiarid regions of Mediterranean environments. A field study was conducted during two consecutive cycles of spearmint (*Mentha spicata* L.) within the rainfed region of Southern Italy to evaluate the effects of straw mulch cultivation practices on crop yields and ecophysiological activities. Four treatments were evaluated: (1) rainfed with straw mulch (RM), (2) rainfed without straw mulch (R), (3) well-watered control plants (W) and (4) well-watered with straw mulch (WM). The rainfed mulch treatment (RM) significantly improved oil yields and ecophysiological activity of the spearmint in comparison with rainfed (R). The rainfed mulch treatment (RM) showed lower inhibition of photosynthesis and smaller diffusive limitations than control treatment, while in rainfed plants (R) photosynthetic activity and diffusive limitations strongly decreased at the end of crop cycle. The average essential-oil content was significantly lower under the W, WM and RM treatments, in comparison to the R treatment, during the full-bloom stages (40 DAT). Instead, at the end of crop cycle, the mulching practice (RM and WM) insignificantly changed the essential-oil content compared with non-mulched well-watered treatment (W), while in rainfed plants (R) the essential-oil content strongly decreased. In addition, rainfed conditions affected the percentage of the three major monoterpenes and decreased the formation of carvone from limonene. Therefore, the current study concluded that straw mulch is an effective management practice to improve growing conditions by decreasing groundwater consumption and to increase oil yields in spearmint within this Mediterranean rainfed region.

**Keywords:** mulching; spearmint; yield; ecophysiology; essential oil

#### **1. Introduction**

Spearmint (*Mentha spicata* L.), is a creeping rhizomatous, glabrous and herbaceous perennial plant, with a pungent smell. The commercial interest in spearmint essential oil, which is among the 10 most commercialized in the world, has increased the demand in mint cultivation. Spearmint indeed contributes greatly to the economy due to its extensive use in food, perfumery, toothpaste, the pharmaceutical industry and the production of essential oil (EO).

The EO of spearmint has antimicrobial [1], insecticidal [2], larvicidal [3], antioxidant [4] and mutagenic activity due to the abundance of terpenes [5].

The crop needs sufficient water availability during summer months to ensure good growth and high yields. In spearmint, sustained deficit irrigation, reducing water availability, clearly decreased mint hay yields and affected oil yield and quality [3,6,7].

**Citation:** Delfine, S.; Velikova, V.B.; Mastrodonato, F. Soil-Mulching Influence on Spearmint Oil Yield, Ecophysiological Activities and Essential-Oil Content in Rainfed Environment of Southern Italy. *Agronomy* **2022**, *12*, 1521. https://doi.org/10.3390/ agronomy12071521

Academic Editors: Aliasghar Montazar and Robert J. Lascano

Received: 13 May 2022 Accepted: 24 June 2022 Published: 24 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The positive effect of water availability on herb yields of various mint species and aromatic grasses has been reported [8–10]. Meanwhile, in medicinal plants, a moderate water-stress condition, while reducing the accumulation of dry matter, increases the yield of essential oil and modifies its composition in respect to the well-watered plants [8,11–13].

Severe water deficit limits crop growth and yield with negative economic consequences [14]. In the last few years, rainfall has declined in some parts of the Mediterranean region due to a climatic change [15] causing substantial shortage in the available water resources for agricultural production. Furthermore, the uneven distribution of rainfall has become the main limitation to sustainable crop yields in drought-prone areas [16]. Thus, the improved utilization of limited seasonal rainfall resources through reduction in soil water loss and increased water-utilization efficiency is needed.

A number of proven mulching techniques exist to entrap and conserve rainwater, such as plastic film mulch, straw mulch and ridge-furrow mulch [17]. The use of plastic film mulch, however, leaves a large amount of plastic in the field, which has obvious negative effects on the environment and soil structure [18].

Instead, the application of straw in agriculture will improve the agricultural ecological environment, because the need for open-field straw burning will be reduced or eliminated. Straw mulch has also been shown to reduce the needed quantities of chemical fertilizers [19] as it increases soil organic matter and improves soil structure [20].

Organic mulching plays a vital role in sustainably conserving soil moisture under low water availability in a manner that enhances crop yields and water-use efficiency [16,21]. Furthermore, conserving moisture through mulching and reduction in the frequency of water supplies may subsequently reduce the losses of N through leaching and minimize NH3 volatilization by restricting soil temperature rise.

In the present study, we investigated the effect of straw mulch on rainfed spearmint (*Mentha spicata* L.) in environmental conditions typical of the Mediterranean region. In particular, we focused on plant growth, photosynthesis and oil yield. We hypothesized that mulch is useful and necessary to support plant growth and activity, and to avoid oil-yield reduction in spearmint.

#### **2. Materials and Methods**

#### *2.1. Experimental Site*

The study was conducted during 2020 and 2021 growing season at an experimental field site in Baranello (Molise Region, Italy, latitude 41◦310 N, longitude 14◦330 E, altitude 630 m a.s.l.). The area has an average annual rainfall of 845 mm and mean annual temperature of 12.7 ◦C. Molise Region is positioned on the eastern side of the Apennines watershed, and has a typical Mediterranean climate of southern Italy.

Before fertilizer applications, soil samples (0–30 cm) were taken from each block and analyzed according to standard procedures [8]. The soil was characterized by a claysand texture (45% clay, 10% silt and 45% sand) and the organic-matter content was 1.5%. The soil profile was overall uniform, containing medium amount of total N (nitrogen, 0.11%), medium amount of available P (phosphorous, 19.5 µg g−1 ) and medium quantity of exchangeable K (potassium, 133 µg g−1 ). Soil had very low active CaCO3, and pH was average neutral; salinity was low. Bean (*Phaseolus vulgaris* L.) was the previous crop within both years.

### *2.2. Plant Material and Experimental Design*

Ten-month old spearmint seedlings were transplanted during mid-April in both years, followed by a light irrigation after planting in rows 50 cm apart (plant density of 10 plants m−2 ). The field was surrounded by a buffer strip to allow for uniform growing conditions.

The experiment was arranged in a randomized block design with five replications (5 m2 each plot). After tillage was completed, the land was smoothed and firmed using a roller harrow.

The recommended doses of N (100 kg ha−1 ), P2O5 (70 kg ha−1 ) and K2O (100 kg ha−1 ), were applied before planting [22].

The following treatments were designed: plants cultivated under natural rainfed conditions without mulch (hereafter, R plants), plants cultivated under natural rainfed conditions with straw mulch (hereafter, RM plants), plants grown under well-watered conditions, considered as control (hereafter, W plants) and plants grown under wellwatered conditions with straw mulch (hereafter, WM plants). Within the straw mulch treatments, the wheat straw covered all the soil surface at the rates of 1 kg m−2 (5 cm straw length). The wheat straw was applied annually. Plants cultivated under well-watered conditions served as a control.

In the control plots, the crop evapotranspiration (ET) was fully restored by drip irrigation. After plant establishment (at 10 May, in both years), irrigation was withheld in R and RW plots [0 days after treatment (DAT)]. Briefly, potential evapotranspiration was calculated from micrometeorological data using the Penman–Monteith formula [23]. Weeds were manually controlled.

#### *2.3. Meteorological Measurements and Yield Biomass*

Main meteorological parameters were measured by means of a standard weather station, located 20 m from the experimental field. The daily mean temperature ranged between 9.1–23.5 ◦C, while the corresponding minimum and maximum temperatures were 7.6 and 34.7 ◦C, respectively. Only 10.3 mm and 11.5 mm rainfall were received during the entire spearmint growth season in the years 2020 and 2021, respectively.

The crop was harvested manually on 0, 20, 40, 60 and 80 DAT in both years. The herbal-yield biomass was weighed, after which the plant samples were heated at 105 ◦C for one hour and then dried at 75 ◦C to constant weight.

#### *2.4. Photosynthetic Gas Exchange*

Gas-exchange activities, as net photosynthesis (Pn, µmol m−2 s −1 ) and stomatal conductance (gs, mol m−2 s −1 ) were measured using a portable photosynthesis system, at ambient CO2 concentration and at time of maximum solar radiation (e.g., 12:00 to 2:00 pm) as described by Delfine [24]. Measurements were recorded on five fully expanded leaves selected randomly on different plants. For photosynthesis measurements, the leaf temperature was maintained at 30 ◦C as in the environmental conditions. Mesophyll conductance (gm, mol m−2 s −1 ) of CO2 was determined as described by Delfine [8]. Briefly, the electron-transport rate calculated by gas exchange (Jc) and that measured by chlorophyll fluorescence (Jf) were compared at an irradiance of 800 µmol m−2 s −1 for accurate measurements of intercellular CO2 concentration.

#### *2.5. Relative Water Content*

The relative water content (RWC) of leaves was measured according to Di Stasio [25]. The third fully expanded leaves from the top were measured. The fresh leaves were weighed (FW), then submerged in distilled water for 12 h to obtain turgid weight (TW). Finally, the leaves were kept at 105 ◦C for 24 h until a constant weight was reached (DW). The formula RWC (%) = (FW − DW)/(TW − DW) × 100 was used to calculate the relative leaf water content.

#### *2.6. Soil Water Content*

The water content to a depth of 1.0 m was measured every 20 days from the beginning of treatment (DAT) to observe the dynamics in soil water content of the 0–0.3 and 0.3–1.0 m layers. The soil was sampled at 0.1 m intervals by manual coring between adjacent plants in the same row, and the gravimetric water content of the soil samples was calculated on the basis of oven-dried weights (dried at 105 ◦C).

#### *2.7. Essential-Oil Analysis*

Essential oil was extracted by hydrodistillation of fresh samples through Clevenger's apparatus. The steam distillation was performed for 60 min. The separated spearmint EO collected was dried over anhydrous sodium sulfate and stored in dark-brown vials at 4 ◦C until used. Three extractions were performed for each treatment [26]. Samples from every treatment and replicate were collected and analyzed separately for oil composition.

The leaf and stem dry biomass, RWC, photosynthesis, stomatal and mesophyll conductance, essential-oil content and essential-oil yield were recorded with interval of 20 days from 0 DAT.

#### *2.8. GC and GC–MS Analysis*

Analytical gas chromatography was carried out on a Perkin–Elmer Sigma 115 gas chromatograph fitted with a HP-5 MS capillary column (30 m × 0.25 mm), 0.25 µm film thickness. Helium was the carrier gas (1 mL/min). Column temperature was initially kept at 40 ◦C for 5 min, then gradually increased to 250 ◦C at 2 ◦C/min rate, held for 15 min and finally raised to 270 ◦C at 10 ◦C/min. Diluted samples (1/100 *v*/*v*, in n-pentane) of 1 µL were injected at 250 ◦C, manually and in the splitless mode. Flame-ionization detection (FID) was performed at 280 ◦C. Analysis was also run by using a fused silica HP Innowax polyethylenglycol capillary column (50 m × 0.20 mm), 0.20 µm film thickness and operating as described above. GC–MS analysis was performed on an Agilent 6850 Ser. II apparatus, fitted with a fused silica DB-5 capillary column (30 m × 0.25 mm), 0.33 µm film thickness, coupled to an Agilent Mass Selective Detector MSD 5973; ionization voltage 70 eV; electron-multiplier energy 2000 V. Gas chromatographic conditions were as given; transfer line temperature, 295 ◦C.

#### *2.9. Identification of Compounds*

Most constituents were identified by gas chromatography by comparison of their retention indices (Ki) with those of the literature [27,28] or with those of authentic compounds available in our laboratories. The retention indices were determined in relation to a homologous series of n-alkanes (C8–C24) under the same operating conditions. Further identification was made by comparison of their mass spectra on both columns with those stored in NIST 02, Wiley 275 libraries and our home-made library or with mass spectra from literature [27,29]. Component relative percentages were calculated based on GC peak areas without using correction factors. The primary oil components that were compared were limonene, carvone, 1,8-cineole and myrcene.

#### *2.10. Data Statistics*

All data were presented as means of the five replicates, while the data for essential oil content and yield and primary oil components are presented as means of the three replicates. SPSS 18.0 was used to conduct a one-way analysis of variance, and significant differences between treatments were compared by the least-significant-difference test at *p* < 0.05.

#### **3. Results**

As the measured parameters did not show significant differences between growing seasons, data averaged over 2 years, 2020 and 2021, were presented.

#### *3.1. Plant Phenology and Soil Water Contents*

No differences in phenology were observed between the spearmint plants exposed to the different treatments up to 40 DAT. Specifically, at the beginning of the study (0 DAT) the plants, of all treatments, were in the pre-flowering stage. After 20 DAT, the plants were in early bloom, and at 40 DAT in full bloom, in all four overall treatments. At 60 DAT the W, WM and RM plants were at the end of flowering and at 80 DAT in flower withering;

while the R plants were already withering from 60 DAT until the end of the cropping cycle (80 DAT). cropping cycle (80 DAT). The soil water contents (SWC) (Figure 1) showed that the rainfed mulching treat-

As the measured parameters did not show significant differences between growing

No differences in phenology were observed between the spearmint plants exposed to the different treatments up to 40 DAT. Specifically, at the beginning of the study (0 DAT) the plants, of all treatments, were in the pre-flowering stage. After 20 DAT, the plants were in early bloom, and at 40 DAT in full bloom, in all four overall treatments. At 60 DAT the W, WM and RM plants were at the end of flowering and at 80 DAT in flower withering; while the R plants were already withering from 60 DAT until the end of the

The soil water contents (SWC) (Figure 1) showed that the rainfed mulching treatments increased the SWC within the 0–0.3 m soil profile compared with the rainfed treatment during the growing seasons of spearmint plants. ments increased the SWC within the 0–0.3 m soil profile compared with the rainfed treatment during the growing seasons of spearmint plants.

*Agronomy* **2022**, *12*, 1521 5 of 15

*3.1. Plant Phenology and Soil Water Contents*

seasons, data averaged over 2 years, 2020 and 2021, were presented.

**3. Results**

**Figure 1.** Changes in soil water content (%) in the 0–0.3 m (**a**) and 0.3–1.0 m (**b**) soil layers of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments. **Figure 1.** Changes in soil water content (%) in the 0–0.3 m (**a**) and 0.3–1.0 m (**b**) soil layers of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments.

At the end of the spearmint growth stage (flower-withering stage, 80 DAT), the SWC within the 0–0.3 m soil profile under the rainfed treatments was lower in comparison to the RM and control treatment. The SWC reductions indicated that under the mulching treatments, more water was stored within the 0–0.3 m soil profile compared with the rainfed treatment. Over the studied cropping cycle (0–80 DAT), there were no significant differences in SWC between well-watered and well-watered mulching treatments. The amplitude of the variation was much larger in the 0–0.3 m than the 0.3–1.0 m layer.

#### *3.2. Biomass Yields 3.2. Biomass Yields*  The dry herbal biomass yields of spearmint were significantly affected by the straw

The dry herbal biomass yields of spearmint were significantly affected by the straw mulch treatment during the growth season (Figure 2). mulch treatment during the growth season (Figure 2).

plitude of the variation was much larger in the 0–0.3 m than the 0.3–1.0 m layer.

At the end of the spearmint growth stage (flower-withering stage, 80 DAT), the SWC within the 0–0.3 m soil profile under the rainfed treatments was lower in comparison to the RM and control treatment. The SWC reductions indicated that under the mulching treatments, more water was stored within the 0–0.3 m soil profile compared with the rainfed treatment. Over the studied cropping cycle (0–80 DAT), there were no significant differences in SWC between well-watered and well-watered mulching treatments. The am-

*Agronomy* **2022**, *12*, 1521 6 of 15

**Figure 2.** Changes in dry herbal biomass (g m−2) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments. **Figure 2.** Changes in dry herbal biomass (g m−2 ) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments.

In W and WM plants, the dry herbal biomass content had increased over time, showing the maximum yield in full bloom (DAT 40) and no significant difference between the two treatments. The herbal yield of RM-treated plants tended to increase but significantly less than in WM plants, and the dry herbal biomass was about 20% less at the end of the experiments (DAT 80). Instead, the development of R plants stopped at the beginning of flowering, showing a reduction of about 38% at the end of the experiments compared to the control (W). In W and WM plants, the dry herbal biomass content had increased over time, showing the maximum yield in full bloom (DAT 40) and no significant difference between the two treatments. The herbal yield of RM-treated plants tended to increase but significantly less than in WM plants, and the dry herbal biomass was about 20% less at the end of the experiments (DAT 80). Instead, the development of R plants stopped at the beginning of flowering, showing a reduction of about 38% at the end of the experiments compared to the control (W).

#### *3.3. Photosynthesis, Stomatal and Mesophyll Conductances*

*3.3. Photosynthesis, Stomatal and Mesophyll Conductances*  The trends in photosynthesis (Pn), stomatal (gs) and mesophyll (gm) conductances among the different treatments are illustrated in Figures 3–5, respectively. The mean photosynthesis was 60% and 26% lower in R and RM plants compared to the corresponding controls (W and WM), respectively, at DAT 80. There were no significant differences in photosynthesis between the W and WM control plants over the studied cropping cycle The trends in photosynthesis (Pn), stomatal (gs) and mesophyll (gm) conductances among the different treatments are illustrated in Figures 3–5, respectively. The mean photosynthesis was 60% and 26% lower in R and RM plants compared to the corresponding controls (W and WM), respectively, at DAT 80. There were no significant differences in photosynthesis between the W and WM control plants over the studied cropping cycle (Figure 3).

The same trend was observed for stomatal conductance (Figure 4).

(Figure 3). At the 20 DAT, mesophyll conductance was significantly lower only in the leaves of R plants with respect to control W (Figure 5).

On the contrary, the mesophyll conductance of the RM treatments decreased and was significantly lower (*p* < 0.05), with respect to control WM, only at the 80 DAT. Over the studied treatment cycle, the mesophyll conductance of control W was similar to those of plants grown under well-watered conditions with straw mulch (WM).

*Agronomy* **2022**, *12*, 1521 7 of 15

**Figure 3.** Changes in photosynthesis (µmol m−2 s−1) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments. **Figure 3.** Changes in photosynthesis (µmol m−2 s −1 ) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments. plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments. The same trend was observed for stomatal conductance (Figure 4).

**Figure 4.** Changes in stomatal conductance (mol m−2 s−1) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and **DAT Figure 4.** Changes in stomatal conductance (mol m−2 s−1) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and **Figure 4.** Changes in stomatal conductance (mol m−2 s −1 ) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments.

overall treatments.

R plants with respect to control W (Figure 5).

80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in

At the 20 DAT, mesophyll conductance was significantly lower only in the leaves of

**Figure 5.** Changes in mesophyll conductance (mol m−2 s−1) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicate the early bloom and arrow at 40 DAT indicate the full bloom, in overall treatments. **Figure 5.** Changes in mesophyll conductance (mol m−2 s −1 ) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicate the early bloom and arrow at 40 DAT indicate the full bloom, in overall treatments.

#### On the contrary, the mesophyll conductance of the RM treatments decreased and was *3.4. Relative Water Content*

ure 6).

significantly lower (*p* < 0.05), with respect to control WM, only at the 80 DAT. Over the studied treatment cycle, the mesophyll conductance of control W was similar to those of plants grown under well-watered conditions with straw mulch (WM). The results of the RWC indicated that RM plants generally had preserved tissue water content compared to R plants over the growing seasons (Figure 6). *Agronomy* **2022**, *12*, 1521 9 of 15

**Figure 6.** Changes in relative water content (%) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments. At the end of the spearmint growth stage (flower-withering stage, 80 DAT), the av-**Figure 6.** Changes in relative water content (%) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments.

erage RWC was significantly (*p* < 0.05) lower in R and RM plants, by about 40% and 20%, respectively, in comparison to the control W and WM plants. No significant differences in

The mulching treatments increased the EO content (+16%) in the RM compared to

**Figure 7.** Changes in essential-oil content (g 100 g−1 d.w.) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40,

*3.5. Essential-Oil Content and Essential-Oil Yield*

WM plants during the full bloom (40 DAT) (Figure 7).

At the end of the spearmint growth stage (flower-withering stage, 80 DAT), the average RWC was significantly (*p* < 0.05) lower in R and RM plants, by about 40% and 20%, respectively, in comparison to the control W and WM plants. No significant differences in RWC between the W and WM control plants over the cropping cycle were observed (Figure 6). erage RWC was significantly (*p* < 0.05) lower in R and RM plants, by about 40% and 20%, respectively, in comparison to the control W and WM plants. No significant differences in RWC between the W and WM control plants over the cropping cycle were observed (Figure 6).

**Figure 6.** Changes in relative water content (%) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 5). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall

At the end of the spearmint growth stage (flower-withering stage, 80 DAT), the av-

#### *3.5. Essential-Oil Content and Essential-Oil Yield 3.5. Essential-Oil Content and Essential-Oil Yield*

treatments.

The mulching treatments increased the EO content (+16%) in the RM compared to WM plants during the full bloom (40 DAT) (Figure 7). The mulching treatments increased the EO content (+16%) in the RM compared to WM plants during the full bloom (40 DAT) (Figure 7).

*Agronomy* **2022**, *12*, 1521 9 of 15

**Figure 7.** Changes in essential-oil content (g 100 g−1 d.w.) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, **Figure 7.** Changes in essential-oil content (g 100 g−1 d.w.) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 3). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments.

There were no differences in EO content (EOC) between the W and WM plants over the cropping cycles. During the full-bloom stages (40 DAT), the EO content of the W (−26%), WM (−26%) and RM (−14%) plants were significantly lower in comparison to R plants. While EO content was strongly reduced in R plants (about-77%) compared to W plants at the last harvest (80 DAT). RM loses a negligible quantity of oils compared to WM. At 80 DAT, EO content in W and WM plants dropped by about 5% compared to full bloom (40 DAT). EOC of RM dropped by 3%, while in R plants EOC decreased by 51%, almost disappearing in leaf tissues.

The mean essential-oil yields (Figure 8) show significant decreases with increasing water deficit across the harvest time.

At 40 DAT, the essential-oil yield was significantly higher in W, WM and RM than in R, whereas the data did not differ significantly among W, WM and RM. The decrease in rainfed treatment at the end of harvest time (80 DAT) was much greater than rainfed mulching treatment compared to the respective control treatments (W and WM conditions). At the same time (80 DAT), the variation of essential-oil yields did not differ significantly between W and WM plants.

60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 3). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom,

There were no differences in EO content (EOC) between the W and WM plants over the cropping cycles. During the full-bloom stages (40 DAT), the EO content of the W (−26%), WM (−26%) and RM (−14%) plants were significantly lower in comparison to R plants. While EO content was strongly reduced in R plants (about-77%) compared to W plants at the last harvest (80 DAT). RM loses a negligible quantity of oils compared to WM. At 80 DAT, EO content in W and WM plants dropped by about 5% compared to full bloom (40 DAT). EOC of RM dropped by 3%, while in R plants EOC decreased by 51%,

The mean essential-oil yields (Figure 8) show significant decreases with increasing

**Figure 8.** Changes in essential-oil yield (kg ha-1) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 3). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments. **Figure 8.** Changes in essential-oil yield (kg ha−1 ) of spearmint exposed to the following treatments: R—rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 0, 20, 40, 60 and 80 days after the onset of the treatments (DAT). Vertical bars represent ± SE of the mean (*n* = 3). Bars with different letters indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Triangle at 20 DAT indicates the early bloom, and arrow at 40 DAT indicates the full bloom, in overall treatments.

#### At 40 DAT, the essential-oil yield was significantly higher in W, WM and RM than in *3.6. Major Oil Components*

in overall treatments.

almost disappearing in leaf tissues.

water deficit across the harvest time.

R, whereas the data did not differ significantly among W, WM and RM. The decrease in rainfed treatment at the end of harvest time (80 DAT) was much greater than rainfed mulching treatment compared to the respective control treatments (W and WM condi-The major oil components that were compared were myrcene, limonene, 1,8-cineole and carvone, which comprise over 75% of the significant monoterpenes (Table 1).

tions). At the same time (80 DAT), the variation of essential-oil yields did not differ significantly between W and WM plants. *3.6. Major Oil Components*  **Table 1.** The primary oil components (%) of spearmint exposed to the following treatments: R rainfed without straw mulch, RM—rainfed with straw mulch, W—well-watered control plants and WM—well-watered with straw mulch. Measurements were performed 40 and 80 days after the onset of the treatments (DAT). 40 DAT indicates the full bloom, in overall treatments.

Different letters in columns indicate significant differences among treatments in the given DAT at *p* ≤ 0.05. Data related to *Mean DAT* 40 and *Mean DAT* 80 are means for all treatments and different letters in columns indicate significant differences between 40 and 80 DAT at *p* ≤ 0.05.

The percentage of myrcene did not differ significantly among treatments at 40 DAT. The percentages of limonene and 1,8-cineole, at 40 DAT, were significantly higher in R than

in W, WM and RM, which did not differ significantly from each other, whereas at 40 DAT, the percentage of carvone was significantly lower in R than in W, WM and RM, which did not differ significantly from each other.

Even after 80 DAT, the percentage of myrcene did not differ significantly among the treatments. The increase in the percentage of limonene and 1,8-cineole in rainfed treatment, at the end of harvest time (80 DAT), was much greater than rainfed mulching treatment compared to the respective control treatments (W and WM conditions). At the same time (80 DAT), the variation of percentage of limonene and 1,8-cineole did not differ significantly between W and WM. At 80 DAT, the decrease in the percentage of carvone in rainfed treatment was much greater than rainfed mulching treatment compared to the respective control treatments (W and WM). The variation of the percentage of carvone did not differ significantly between W and WM. From 40 to 80 DAT, the result showed a significant increase in the myrcene, 1,8-cineole and carvone, and a significant decrease in the limonene percentage over the ranges of applied treatments (Table 1).

#### **4. Discussion**

During the whole time of the experimentation, the scant precipitations occur in the form of low-intensity precipitation events. Thus, the precipitation quantities delivered in this manner cannot satisfy the normal growth of crops. Therefore, in these droughtprone environments, the soil-surface mulch practices may reduce the evaporation losses from the soil surface [16,21]. Thus, the improvement of the soil-moisture conditions of dry-land agriculture is of particular importance [30,31]. Our study indicated that rainfed mulching treatments increased the SWC within the 0–0.3 m soil profile compared with the rainfed conditions in the growing season of spearmint (Figure 1). The SWC for the rainfed treatment was even lower when compared with the mulching treatment from the full bloom (40 DAT) to the end of the cropping cycle (80 DAT). Moreover, already after 20 DAT, the SWC within the 0–0.3 m soil profile under the rainfed treatment was generally lower than under well-watered treatment.

In our study, the mulch treatments of rainfed plants significantly increased the dry herbal biomass yields during the crop-growing stages (Figure 2) in comparison to the rainfed treatment. Similar results have been reported on a menthol mint plants studied under different moisture regimes and nitrogen fertilization in the presence of sugarcane trash mulched [10]. Instead, there were no significant differences in the dry herbal biomass yields between W and WM plants. It should be noted that as expected, well-irrigated plants (W and WM) had higher dry herbal biomass yields than rainfed ones (R and RM). The rainfed mulching treatment conserves soil water and reduces soil evaporation, thus demonstrating that it is suitable in a rainfed climate for the utilization of water [32]. The dry herbal biomass accumulation was closely associated with photosynthetic gas-exchange parameters and leaf RWC during different crop-growth stages. There were no significant differences between W and WM plants during the entire spearmint growing season, which may be attributed to the similar tissue water content (Figures 3–6).

The mulching rainfed treatment likely enhanced the accumulation of biomass by improving the photosynthesis to accelerate crop growth compared to the rainfed treatment. The photosynthesis limitations highlighted in R and RM plants compared to W and WM plants were mostly due to diffusive resistances, including the mesophyll component [8]. Indeed, until 40 DAT, stomatal conductance in our study significantly decreased in R and RM plants in comparison to the well-watered controls (W and WM). To conclude that the reduction in photosynthesis observed until full bloom (40 DAT) is only attributable to stomatal closure, one should observe a consequent additional resistance to CO2 diffusion toward the chloroplasts that is generally caused by the mesophyll components [24]. This additional resistance may increase under stress conditions [8]. However, mesophyll resistance is likely to be controlled by mesophyll anatomical structure and particularly by the chloroplast surfaces exposed to gas exchanges [33,34]. Mesophyll resistance was similar in controls (W and WM) and rainfed mulching treatment at 40 DAT (Figure 5). Thus, it

did not contribute to increasing resistance to CO2 diffusion in RM leaves, indicating that mesophyll resistances to CO2 diffusion did not directly affect photosynthesis [8]. In this condition, in which it is only the stomatal conductance that limits photosynthesis, it did not cause permanent damage to the carbon fixation apparatus, as has been observed in other studies [8,24], and therefore, it is possible to consider the stress condition as moderate. Moreover, contrary to what was found at 40 DAT, we observed that at 60 and mostly at 80 DAT, the CO2 mesophyll conductance of R leaves was significantly lower than that of the rainfed mulching treatment. Thus, mesophyll resistance only contributed to the total resistance to CO2 diffusion from 60 DAT [24]. During the harvesting period, when comparing R and RM plants it became clear that the total diffusive limitations were fewer in RM compared to R plants. Thus, the greater water availability increased the stomatal conductance, ensuring a greater input of carbon dioxide, which in turn increased photosynthetic activity in RM compared to R plants. These differences significantly accelerated the growth of the crop canopy and resulted in the significantly increased dry herbal biomass values that are shown in Figure 1 [35].

In our study, the rainfed treatments (without and with mulch) significantly increased the essential oil content, during the full bloom (40 DAT) (Figure 7). The rainfed treatment significantly increased the essential-oil content and storage in green tissues in comparison to the W, WM and RM treatments, before and during the early growth stage (20 DAT) [8]. During the withering stages, there were no significant differences in the essential-oil content between the well-watered control plants (W and WM) and RM plants, while the essentialoil content of R plants strongly decreased. Our study indicated that mulching rainfed treatment (RM) significantly increased the essential-oil content compared to the control treatment (WM) in the full-bloom phase and kept it constant until the end of the growing season during withering, extending the oil-extraction period (Figure 7).

Essential oil yield increased substantially with the increasing of bloom for all treatments (Figure 8). At 40 DAT, compared with rainfed treatment, mulching rainfed spearmint (RM) clearly yielded the same amount of essential oil as the W and WM controls. Moreover, after 80 DAT, the reduction in essential-oil yield in the mulching rainfed treatment (RM, −15%), although significant, was much less evident than in the rainfed treatment (R, −78%) compared to the respective controls (WM and W).

This indicates a possibility, at the full-bloom stage (40 DAT), for decreased grower costs through mulching rainfed treatment by cutting costs for water, harvesting, transportation and distilling spearmint hay; and for increasing profits if the oil yield and quality remain fairly constant. This encouraging result for mulching rainfed treatment (RM) is attributable to the presence of higher soil water contents compared to rainfed treatment (R), which helped sustain the crop at these rainfed conditions in mulching treatment.

The percentage of four major monoterpenes (limonene, carvone, 1,8-cineole and myrcene) showed a low sensitivity to rainfed conditions if at moderate stress levels (Table 1), or 40 DAT and in the presence of mulch (RM). Over time, from 40 to 80 DAT, the percentage of carvone significantly increased, whereas that of limonene decreased over the ranges of applied treatments (Table 1). Instead, at both 40 and 80 DAT, the result shows a significant decrease in the carvone and a significant increase in limonene percentage with the increase in water-stress conditions due to reduced water availability (R). This indicates that rainfed plants may show different metabolic maturity as limonene is a precursor to carvone [3,6].

#### **5. Conclusions**

The mulching treatment had significant and positive effects on rainfed spearmint yields. Spearmint yields were positively related to physiological activity. The beneficial effects of mulching on essential-oil content and yield primarily occurred during the early stages together with lower inhibition of photosynthesis and smaller diffusive limitations. This benefit was primarily reflected in the fact that mulching, in rainfed plants, regulated tissue water content, promoted the growth of spearmint plants in later growth stages and increased the essential-oil yields compared to rainfed treatment. Thus, the rainfed

mulching treatment may have helped decrease the consumption of groundwater by providing improved utilization of limited water availability and may have helped resolve the water-scarcity problem that limits sustainable agriculture production in rainfed environments. The findings of our investigation support the hypothesis that mulch is important for a more efficient and sustainable agricultural management.

**Author Contributions:** Conceptualization, S.D., V.B.V. and F.M.; methodology, S.D.; validation, S.D. and V.B.V.; investigation, S.D.; writing—original draft preparation, S.D.; writing—review and editing, S.D. and V.B.V.; supervision, F.M.; project administration, S.D.; funding acquisition, S.D. and F.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Dipartimento di Agricoltura, Ambiante ed Alimenti, Università degli Studi del Molise, and Società Agricola Officinali.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

#### **References**

## *Article* **Ridge-Furrow Mulching Enhances Capture and Utilization of Rainfall for Improved Maize Production under Rain-Fed Conditions**

**Muhammad Mansoor Javaid 1,\* , Hussah I. M. AlGwaiz 2,\*, Hasnain Waheed 1 , Muhammad Ashraf 3 , Athar Mahmood 4 , Feng-Min Li 5 , Kotb A. Attia 6,7 , Muhammad Ather Nadeem 1 , Muneera D. F. AlKahtani 2 , Sajid Fiaz 8 , Muhammad Nadeem 9 and Hafiz Bashir Ahmad 10**

**Abstract:** The capture and utilization of rainwater by crops under various mulching conditions have great importance in agriculture production systems, especially in dry-prone regions. Understanding the effect of mulching on rainwater use efficiency growth and yield of a crop is very important. For this purpose, field experiments were conducted in 2017 and 2018 to evaluate the potential of ridge-furrow mulching on maize growth and development under rain-fed conditions. The field study compared four treatments, i.e., ridge-furrow without mulch (WM), black plastic mulch (BM), transparent plastic mulch (TM) and grass mulch (GM). The BM treatment consistently increased the soil moisture and temperature, resulting in earlier emergence, as well as increased plant height and plant biomass, compared to the WM treatment. Compared to WM, the two-years mean yield of maize with BM, TM and GM were recorded to be increased by 33.6%, 28.1% and 10.8%, respectively. The BM produced a maximal crop growth rate at 90 days after sowing (DAS) as specified by a greater leaf area index. Transpiration rate and leaf stomatal conductance were significantly higher with BM and TM than with WM, however, the BM treatment showed the highest net photosynthetic rate in both years. Net income for the BM treatment was the highest (USD 1226 ha−1 ) of all the treatments and USD 335 ha−1 greater than WM. As growth, yield and net income of maize were improved with BM, therefore this treatment was found to be the most effective for maize production in rain-fed conditions. This system is evaluated at a small scale, hence to maximize its effectiveness on a large scale, a simulation design needs to be developed.

**Keywords:** benefit-cost ratio; physiological traits; rainwater harvesting system; rain-fed conditions and ridge-furrow mulching

**Citation:** Javaid, M.M.; AlGwaiz, H.I.M.; Waheed, H.; Ashraf, M.; Mahmood, A.; Li, F.-M.; Attia, K.A.; Nadeem, M.A.; AlKahtani, M.D.F.; Fiaz, S.; et al. Ridge-Furrow Mulching Enhances Capture and Utilization of Rainfall for Improved Maize Production under Rain-Fed Conditions. *Agronomy* **2022**, *12*, 1187. https://doi.org/10.3390/ agronomy12051187

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Academic Editor: Aliasghar Montazar

Received: 10 April 2022 Accepted: 10 May 2022 Published: 14 May 2022

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#### **1. Introduction**

Water is a crucial part of agricultural production [1,2]. Scarcity of irrigation water along with irregular and insufficient rainfall has significantly affected the agricultural productivity [3–5]. Inadequate water possessions are the key constraint for crop production [4,6,7]. The decline in future rainfall may cause a reduction in the yield of various crops and enhance the risk to food supply [8,9]. There is a dire need to develop strategies to capture and utilize the rainwater for optimum crop production in rain-fed, arid and semiarid areas [10].

Ridge-furrow covered with plastic mulch on flat land is one of the most viable approaches to maximize the rainfall usage [11,12]. It can enhance the availability of moisture in the crop root zone by reducing the surface runoff, reducing the unproductive evaporation and prolonging the water availability in dry conditions which enhanced the water use efficiency and yield of crops [13]. It also promotes rainfall penetration in deep soil and reduces the evaporation losses [14–16]. Each millimeter of rainwater collected, stored and conserved in the root zone has increased the yield of a crop up to 10 kg ha−1 [17]. It is documented that rainfall in maize crop ranging from 230–440 mm, ridge-furrow farming system increased the soil water contents in 0–100 topsoil by 5–12% when compared to that with conventional flat farming system [18]. In the northwest of China, ridge-furrow plastic film mulching and straw mulching increased the maize yield by 20% and 10%, respectively, when compared to that with conventional planting [18]. The main purpose of mulching is to improve the rainwater harvest over a small land area along with a reduction in water loss through soil evaporation [19]. Many researchers have suggested that there is a significant potential to increase agricultural productivity with a considerable increase in the rainwater use efficiency, particularly in rain-fed areas where crops depend only on rainfall [17,20,21]. Although the ridge-furrow mulching technique is considered very beneficial for achieving enhanced yield in rain-fed agriculture [22–24], there is still a need to understand the capture and utilization of rainwater under different mulches for clarifying physiological progression, growth and development of the crop. In general, crops are planted on the ridge in a ridge-furrow system that received little advantage from rain as rainwater makes a channel into the furrow areas, especially in the case of a light shower. However, in this study, the planting geometry was quite different than that in conventional methods of sowing, used in most areas of the world. In this study, we used furrows as planting zone and ridges served as water harvesting areas. Therefore, it is important to determine how different mulching practices affect crop growth, yield and resource utilization for maximizing the water management and enhancing the yield of maize. The main objective of this experiment was to investigate the role of the ridge-furrow mulching on the physiological, growth and yield of maize by collecting the rainwater and re-allocating it to the furrow where the crop was planted. This study could be helpful for water management approaches in maize production under rain-fed conditions.

#### **2. Materials and Methods**

#### *2.1. Site Description*

Experiments were conducted during the summer season of 2017 and 2018 at the Agronomic Research Area, College of Agriculture, University of Sargodha, Punjab Province, Pakistan (Latitude 31.41◦ N, Longitude 74.17◦ E and Altitude 194.4 m). The soil at the experimental site was clay loam with a pH of 7.7, organic matter 0.83%, 0.52% N, 10 mg/kg available P and 112 mg/kg available K. During the entire experimental phase, the daily maximum or minimum and rainfall distributions are shown in Figure 1. The total rainfall during the maize growing period was 443.76 mm in 2017 and 946.62 mm in 2018.

**Figure 1.** The daily maximum or minimum temperatures and rainfall during the maize growing seasons of the experimental site in 2017 (**a**) and 2018 (**b**). **Figure 1.** The daily maximum or minimum temperatures and rainfall during the maize growing seasons of the experimental site in 2017 (**a**) and 2018 (**b**).

#### *2.2. Experimental Design and Field Management 2.2. Experimental Design and Field Management*

Alternate ridge and furrow were set up on the flat land. There was a narrow ridge with a width of 30 cm and a height of 15 cm, whereas the wide ridge had a width of 60 cm and a height of 10 cm (Figure 2). The ridges served as rainwater harvesting zone and furrows served as planting zone. The treatments were comprised of ridge-furrow without mulching (WM), ridge-furrow covered with black plastic mulch (BM), transparent plastic mulch (TM) and grass mulch (GM). The experiment was carried out in a Randomized Complete Block Design with four replications. Each plot size was 5 m × 4.5 m. The thickness of the film was 0.008 mm. The edge of the plastic film was buried in the soil. Grass mulch was spread evenly on the ridges and furrows at a rate of 9000 kg ha−1. The crop was sown in each furrow at the base of the ridge at 25 cm apart with a planting density of 88,888 ha−**1** (Figure 2). A fertilizer having 150 kg N ha−1, 125 kg P2O5 ha−1 and 125 kg K2O ha−1 was applied uniformly over the furrows and plowed into the soil layer. The maize seed was drilled with a hand push maize seeder with a dibbler wheel into the plastic mulch for all treatments except the grass cover mulch where mulching was done after sowing of the maize seed with the maize seeder. Alternate ridge and furrow were set up on the flat land. There was a narrow ridge with a width of 30 cm and a height of 15 cm, whereas the wide ridge had a width of 60 cm and a height of 10 cm (Figure 2). The ridges served as rainwater harvesting zone and furrows served as planting zone. The treatments were comprised of ridge-furrow without mulching (WM), ridge-furrow covered with black plastic mulch (BM), transparent plastic mulch (TM) and grass mulch (GM). The experiment was carried out in a Randomized Complete Block Design with four replications. Each plot size was 5 m × 4.5 m. The thickness of the film was 0.008 mm. The edge of the plastic film was buried in the soil. Grass mulch was spread evenly on the ridges and furrows at a rate of 9000 kg ha−1 . The crop was sown in each furrow at the base of the ridge at 25 cm apart with a planting density of 88,888 ha−1 (Figure 2). A fertilizer having 150 kg N ha−1 , 125 kg P2O5 ha−1 and 125 kg K2O ha−1 was applied uniformly over the furrows and plowed into the soil layer. The maize seed was drilled with a hand push maize seeder with a dibbler wheel into the plastic mulch for all treatments except the grass cover mulch where mulching was done after sowing of the maize seed with the maize seeder.

Maize (cv. DK-6525) was sown on 2nd and 6th July in 2017 and 2018, respectively, using a seed rate of 25 kg ha−1 with a drill. Thinning of the crop stand was performed at the 3 to 4 leaf stage to sustain one plant per hill. All other farming practices such as the use of natural and standard insecticides were done for all the treatments. If heavy rainfall occurs, the excess rainwater will be drained out through a drain at the end of the furrows. This means that at the end of each furrow, there is a small ridge about 10–15 cm high. When the rainfall is low, all the rainwater can be trapped in the furrow. If the rainfall is heavy, the excess water will drain out automatically and will not cause serious flooding problems. To prevent flooding, the mulch film can be punctured at the point of emergence, leaving a hole for emergence. This will ensure that there is no flooding risk. Maize (cv. DK-6525) was sown on 2nd and 6th July in 2017 and 2018, respectively, using a seed rate of 25 kg ha−1 with a drill. Thinning of the crop stand was performed at the 3 to 4 leaf stage to sustain one plant per hill. All other farming practices such as the use of natural and standard insecticides were done for all the treatments. If heavy rainfall occurs, the excess rainwater will be drained out through a drain at the end of the furrows. This means that at the end of each furrow, there is a small ridge about 10–15 cm high. When the rainfall is low, all the rainwater can be trapped in the furrow. If the rainfall is heavy, the excess water will drain out automatically and will not cause serious flooding problems. To prevent flooding, the mulch film can be punctured at the point of emergence, leaving a hole for emergence. This will ensure that there is no flooding risk.

#### *2.3. Data Collection and Measurements 2.3. Data Collection and Measurements*

• *Growth and yield traits:* The number of plants grown in each plot was calculated from a selected area until a steady and constant number of plants was obtained. The mean days taken to emergence were determined from the time of sowing. Plant height and 1000-grain weight were measured from five indiscriminately selected plants from each plot at maturity and then the average was recorded. The leaf area from five consecutive plants randomly selected from a line in each plot was measured at 30, 60, 90 and 120 days after sowing [25] with the help of a leaf area meter (CI-202, CID, Bio-Science, Camas, WA, USA). The following formula was used to calculate the leaf area index (*LAI*): • *Growth and yield traits:* The number of plants grown in each plot was calculated from a selected area until a steady and constant number of plants was obtained. The mean days taken to emergence were determined from the time of sowing. Plant height and 1000-grain weight were measured from five indiscriminately selected plants from each plot at maturity and then the average was recorded. The leaf area from five consecutive plants randomly selected from a line in each plot was measured at 30, 60, 90 and 120 days after sowing [25] with the help of a leaf area meter (CI-202, CID, Bio-Science, Camas, WA, USA). The following formula was used to calculate the leaf area index (*LAI*):

$$LAI = \frac{Leaf\ area}{\text{Land area}}\tag{1}$$

To estimate the crop growth rate (*CGR*), five plants per plot were taken. Each plant was mocked, mixed well and then sun-dried. The samples were then subjected to an oven at 70 ◦C ± 5 ◦C to record dry weight. The dry weight of each plant was measured and converted into dry matter per unit of land area (m2 ). The *CGR* was calculated at 30, 60, 90 and 120 days after sowing according to the formula presented by Beadle [26]:

$$\text{CGR} = \frac{W2 - W1}{T2 - T1} \tag{2}$$

where *W*2 = Plant dry weight m−2 at 2nd harvest, *W*1= Plant dry weight m−2 at 1st harvest, *T*2 = Time consistent to 2nd harvest, *T*1 = Time consistent to 1st harvest.

To obtain grain yield, the maize plants were harvested at maturity from every plot, dried in the sun and threshed manually. The biological yield was measured by collecting the total plant biomass from every plot, dried in the sun for several days and then transformed to t ha−1 .

The harvest index was determined using the following formula:

$$\text{Harvest Index} \left( \% \right) = \frac{\text{Grain yield}}{\text{Biological yield}} \times 100 \tag{3}$$

#### *2.4. Statistical Analysis*

The statistical analysis was performed by the software Statistix 8.1 computer software (Statistix 8.1, Analytical Software, Tallahassee, FL, USA) to work out an analysis of variance (ANOVA) of each data set. The treatments' means were compared using Tukey's (HSD) test at a 5% probability level [27]. The graphical demonstration of the data was done using the Sigma plot 11.0 (Systat Software, Inc., San Jose, CA, USA).

#### **3. Results**

#### *3.1. Plant Growth and Yield Attributes*

Data showed that in BM and TM, maize emergence was two days earlier than that with WM in both years. Maize plant height was significantly influenced by various mulching treatments over the two years with the higher plant height observed in 2017 as compared with 2018 (Table 1). In 2017, higher plant height (186.5 cm) was observed with BM and lower plant height (169.2 cm) was measured with WM. In 2018, BM produced taller plants (188.2 cm) than did all other treatments. The lowest plant height (169.2 cm) of the maize crop was recorded in plots where not mulch material was used. The 1000-grain weight

of maize showed a substantial response to all mulch treatments. The BM exhibited a maximum 1000-grain weight (285.5 g) which was followed by TM in 2018. However, a minimum 1000-grain weight was recoded with WM during both growing seasons. All mulching treatments had a significant impact on grain yield as compared to WM in both years. The mulch treatments significantly influenced the grain yield of maize. The maize grain yield from every treatment was categorized as follows: BM > TM > GM > WM. Compared with WM, the mean grain yield of maize with BM, TM and GM was found to be improved significantly by 1.24 t ha−1 (33.6%), 1.04 t ha−1 (28.1%) and 0.40 t ha−1 (10.8%), respectively (Table 2). As compared to WM, the ridge and furrow mulching improved the biological yield significantly. The mean biological yield with BM, TM and GM was markedly enhanced by 1.74, 1.42 and 0.81 t ha−1 , respectively. All tested treatments showed a variable harvest index (%) of maize, ranging from 42.1% to 48% in both growing seasons. The harvest index was higher (48%) in 2017 with BM and the difference of BM and TM vs. WM was significant during both years.

**Table 1.** Influence of different ridge-furrow mulching on days taken to emergence, plant height (cm) and 1000-grain weight of maize during 2017 and 2018.

Values within each column sharing the same letter did not differ significantly at a 5% level of probability. WM = Ridge-furrow without mulch, BM = black plastic mulch, TM = transparent plastic mulch, GM = grass mulch.

**Table 2.** Influence of different ridge-furrow mulches on grain yield (t ha−1 ), biological yield (t ha−1 ) and harvest index (%) of maize during 2017 and 2018.

Values within each column sharing the same letter did not differ significantly at a 5% level of probability. WM = Ridge-furrow without mulch, BM = black plastic mulch, TM = transparent plastic mulch, GM = grass mulch.

In contrast to the WM treatment, all mulching treatments improved the leaf area index in both years (Figure 3). The leaf area index was significantly higher with BM and TM when compared with WM at each critical stage. The GM produced a slightly higher leaf area index than did the WM during both years at each developmental stage. Moreover, the major differences were perceived between the WM and GM treatments in 2018, mainly at 60 and 120 days after sowing (DAS). The variation in leaf area index was connected with a different crop growth rate under various mulch treatments (Figure 4). Of all the treatments, the ridge-furrow covered with BM or TM produced a higher crop growth rate in 2017 and 2018. At 30, 60, 90 and 120 DAS, the BM produced 27.6, 33.3, 49.3 and 33.2% higher crop growth rate, respectively, than WM in 2017 (Figure 4). The increase was 21.5% at 30 DAS, 30.8% at 60 DAS, 37.9% at 90 DAS and 26.4% at 120 DAS in 2018. However, TM produced a slightly higher crop growth rate over BM at 90 days after sowing in 2018 (Figure 4).

**Figure 3.** Leaf area index (*LAI*) of maize under ridge-furrow without mulching (WM), with black plastic mulch (BM), transparent plastic mulch (TM) and grass mulch (GM) in 2017 (**a**) and 2018 (**b**)*.* **Figure 3.** Leaf area index (*LAI*) of maize under ridge-furrow without mulching (WM), with black plastic mulch (BM), transparent plastic mulch (TM) and grass mulch (GM) in 2017 (**a**) and 2018 (**b**).

2018 (**b**).

**Figure 4.** Crop growth rate (*CGR*) of maize under ridge-furrow without mulching (WM), with black plastic mulch (BM), transparent plastic mulch (TM) and grass mulch (GM) in 2017 (**a**) and **Figure 4.** Crop growth rate (*CGR*) of maize under ridge-furrow without mulching (WM), with black plastic mulch (BM), transparent plastic mulch (TM) and grass mulch (GM) in 2017 (**a**) and 2018 (**b**).

Soil water storage (SWS) indicated a slight difference among the four treatments (Figure 5). Compared to the WM treatment, all mulching treatments augmented the SWS significantly during both growing seasons. The quantity of water storage from 1st (30 Soil water storage (SWS) indicated a slight difference among the four treatments (Figure 5). Compared to the WM treatment, all mulching treatments augmented the SWS significantly during both growing seasons. The quantity of water storage from 1st (30 DAS) to 2nd (60 DAS) sampling was recorded to be improved with BM, TM and GM treatments in 2017 and 2018, as well as being the most vital water restoring time for crop growth. While, from the 2nd (60 DAS) to the 4th (120 DAS) sampling, differences between the treatments

DAS) to 2nd (60 DAS) sampling was recorded to be improved with BM, TM and GM treatments in 2017 and 2018, as well as being the most vital water restoring time for crop

between the treatments in the value of SWS were very high in 2017, however, in 2018, the differences among the treatments were quite less. At the first sampling, higher soil water was stored in the WM treatment due to more rain in July in both growing seasons. The heavy rain caused the runoff in mulch treatments and WM facilitated the water penetration, which resulted in more soil water contents. The *p*-value showed that the effect of mulching was significant for leaf area index, crop growth rate and soil water

storage at 30, 60, 90 and 120 DAS at a 5% level of probability (Table 3).

in the value of SWS were very high in 2017, however, in 2018, the differences among the treatments were quite less. At the first sampling, higher soil water was stored in the WM treatment due to more rain in July in both growing seasons. The heavy rain caused the runoff in mulch treatments and WM facilitated the water penetration, which resulted in more soil water contents. The *p*-value showed that the effect of mulching was significant for leaf area index, crop growth rate and soil water storage at 30, 60, 90 and 120 DAS at a 5% level of probability (Table 3). *Agronomy* **2022**, *12*, x FOR PEER REVIEW 9 of 16

**Figure 5.** Average soil water storage (SWS, 0–100 cm) under ridge-furrow without mulching (WM), with black plastic mulch (BM), transparent plastic mulch (TM) and grass mulch (GM) in 2017 (**a**) and 2018 (**b**)*.*  **Figure 5.** Average soil water storage (SWS, 0–100 cm) under ridge-furrow without mulching (WM), with black plastic mulch (BM), transparent plastic mulch (TM) and grass mulch (GM) in 2017 (**a**) and 2018 (**b**).

**Parameter Mean Sum of Square F-Value** *p***-Value (0.05)**

Leaf area index at 30 DAS 0.17 0.17 8.67 5.67 0.003 0.012 Leaf area index at 60 DAS 0.77 0.75 23.2 24.9 <0.001 <0.001 Leaf area index at 90 DAS 1.00 0.84 37.1 19.3 <0.001 <0.001 Leaf area index at 120 DAS 0.65 1.31 21.7 39.2 <0.001 <0.001 Crop growth rate (gm−2 day−1) at 30 DAS 2.92 2.91 44.9 54.7 <0.001 <0.001 Crop growth rate (gm−2 day−1) at 60 DAS 10.54 12.82 148.00 46.8 <0.001 <0.001 Crop growth rate (gm−2 day−1) at 90 DAS 62.34 52.68 156.00 152.00 <0.001 <0.001 Crop growth rate (gm−2 day−1) at 120 DAS 12.30 7.42 46.70 47.10 <0.001 <0.001 Soil water storage at 30 DAS 599.72 2222.56 3.03 11.6 0.071 0.007 Soil water storage at 60 DAS 575.83 3043.23 2.51 32.2 0.108 <0.001 Soil water storage at 90 DAS 3749.06 650.72 32.6 7.97 <0.001 0.003 Soil water storage at 120 DAS 3479.50 2562.75 12.4 40.5 0.006 <0.001 Net photosynthesis rate (µmol m−2 s−1) 11.21 17.69 13.1 23.7 <0.001 <0.001 Transpiration rate (mmol m−2 s−1) 2.19 3.77 22.4 27.2 <0.001 <0.001

**Table 3.** Summary of the analysis variance for leaf area index, crop growth rate, soil water storage,

**2017 2018 2017 2018 2017 2018**

mulching materials.

**Table 3.** Summary of the analysis variance for leaf area index, crop growth rate, soil water storage, net photosynthesis rate, transpiration rate and stomatal conductance of maize under different mulching materials.

#### *3.2. Physiological Attributes* Leaf stomatal conductance (mmol m−2 s−1) 23631.9 3.77 18.2 27.2 <0.001 <0.001

In both years of study, the net photosynthetic rate of maize was significantly affected by different ridge-furrow mulches (Figure 6). During both years, the net photosynthetic rate was consistently higher with BM, TM and GM as compared to that with WM. All mulching treatments markedly improved the transpiration rate of maize. The transpiration rate was observed higher in 2017 than that of 2018. The highest transpiration rate was recorded with BM and statistically at par with that of TM during both growing seasons. Compared to WM, GM showed a slightly higher transpiration rate (Figure 7). Figure 8 shows that compared to WM, other treatments, i.e., BM, TM and GM exhibited higher leaf stomatal conductance. Among all mulching treatments, BM produced slightly higher values of leaf stomatal conductance during both years. However, the lowest leaf stomatal conductance was recorded in plots with the ridge-furrow left uncovered (WM). *3.2. Physiological Attributes*  In both years of study, the net photosynthetic rate of maize was significantly affected by different ridge-furrow mulches (Figure 6). During both years, the net photosynthetic rate was consistently higher with BM, TM and GM as compared to that with WM. All mulching treatments markedly improved the transpiration rate of maize. The transpiration rate was observed higher in 2017 than that of 2018. The highest transpiration rate was recorded with BM and statistically at par with that of TM during both growing seasons. Compared to WM, GM showed a slightly higher transpiration rate (Figure 7). Figure 8 shows that compared to WM, other treatments, i.e., BM, TM and GM exhibited higher leaf stomatal conductance. Among all mulching treatments, BM produced slightly higher values of leaf stomatal conductance during both years. However, the lowest leaf stomatal conductance was recorded in plots with the ridge-furrow left uncovered (WM).

**Figure 6.** Influence of different ridge and furrow mulching treatments on net photosynthetic rate, of maize in 2017 and 2018. WM = Ridge-furrow without mulch, BM= Ridge-furrow covered with black plastic mulch, TM = Ridge-furrow covered with transparent plastic mulch, GM = Ridge-furrow covered with grass mulch. **Figure 6.** Influence of different ridge and furrow mulching treatments on net photosynthetic rate, of maize in 2017 and 2018. WM = Ridge-furrow without mulch, BM = Ridge-furrow covered with black plastic mulch, TM = Ridge-furrow covered with transparent plastic mulch, GM = Ridge-furrow covered with grass mulch.

**Figure 7.** Influence of different ridge and furrow mulching treatments on the transpiration rate of maize in 2017 and 2018. WM = Ridge-furrow without mulch, BM = Ridge-furrow covered with black plastic mulch, TM = Ridge-furrow covered with transparent plastic mulch, GM = Ridge-furrow covered with grass mulch*.*  **Figure 7.** Influence of different ridge and furrow mulching treatments on the transpiration rate of maize in 2017 and 2018. WM = Ridge-furrow without mulch, BM = Ridge-furrow covered with black plastic mulch, TM = Ridge-furrow covered with transparent plastic mulch, GM = Ridge-furrow covered with grass mulch. *Agronomy* **2022**, *12*, x FOR PEER REVIEW 12 of 16

Different mulching materials and treatments without mulch have a variable cost-benefit ratio (Table 4). The two-year average benefit–cost ratio was ranked as follows: BM ˃ TM ˃ GM ˃ WM. The cost with different treatments followed the order:

was improved by 16%, compared to WM. The net income difference for BM vs. WM was

**Particulars WM BM TM GM**  Fertilizer cost 258 258 258 258 Seed cost 97 97 97 97 Mulching material cost - 109 109 58 Common cost a 238 238 238 238 Total cost 593 702 702 651 Grain yield (t ha−1) 3.69 4.93 4.73 4.09 Grain yield revenue (USD) 1069 1429 1371 1185 Fodder yield (t ha−1) 8.60 10.34 10.02 9.41

(USD) 415 499 484 454

**Table 4.** Average economic benefit cost ratio (USD ha−1) of maize production in 2017 and 2018.

**Figure 8.** Influence of different ridge and furrow mulching treatments on leaf stomatal conductance of maize in 2017 and 2018. WM = Ridge-furrow without mulch, BM= Ridge-furrow covered with black plastic mulch, TM = Ridge-furrow covered with transparent plastic mulch, GM = Ridge-furrow covered with grass mulch*.*  **Figure 8.** Influence of different ridge and furrow mulching treatments on leaf stomatal conductance of maize in 2017 and 2018. WM = Ridge-furrow without mulch, BM = Ridge-furrow covered with black plastic mulch, TM = Ridge-furrow covered with transparent plastic mulch, GM = Ridge-furrow covered with grass mulch.

Fodder yield revenue

USD 335 ha−1 (Table 3).

*3.3. Economic Analysis*

#### *3.3. Economic Analysis*

Different mulching materials and treatments without mulch have a variable costbenefit ratio (Table 4). The two-year average benefit–cost ratio was ranked as follows: BM > TM > GM > WM. The cost with different treatments followed the order: BM = TM > GM > WM. Net incomes of BM, TM and GM were higher than that of WM. Net income was highest (USD 1226 ha−1 ) for the BM treatment and the benefit–cost ratio was improved by 16%, compared to WM. The net income difference for BM vs. WM was USD 335 ha−1 (Table 3).

**Table 4.** Average economic benefit cost ratio (USD ha−1 ) of maize production in 2017 and 2018.

WM = Ridge-furrow without mulching, BM = Ridge-furrow covered with black plastic mulch, TM = Ridge-furrow covered with transparent plastic mulch, GM = Ridge-furrow covered with grass mulch, a Including labor cost was USD 4.34 per person per day, land preparation, pesticides and harvesting charges; cost of plastic film was USD 2.31 kg−1 , grass mulch USD 0.0063 kg−1 ; maize seed price was USD 3.86 kg−1 . Cost was calculated in USD and paid in PKR.

#### **4. Discussion**

Our study showed that seedling emergence with TM was two days earlier than that with WM. It is believed that solar radiation passed through the transparent film mulch and reached the soil surface but this film prevents the entry of long-wave radiation, hence improving the soil thermal conditions. The early emergence under TM might be due to the thermal difference in the soil. Researcher also pointed out that straw mulch caused cooling effects and plastic film caused warming effects at the early growth stage of the crops. The reduction in the emergence days under mulch treatments might be due to the availability of more moisture compared to that under WM treatments. Mulch treatments reduced the days to emergence.

Consequently, the ridge-furrow mulching treatments produced higher plant height and biological yield than that with WM. The ridge-furrows covered with plastic film raised the soil moisture, which could increase the growth of maize during the early phase [14,28]. There were little differences between BM and TM in terms of improving maize growth and biological yield which may have been due to the availability of moisture at critical growth stages.

The productivity of maize appears to correlate closely with the successive distribution of detected resources [29], in particular, the uptake and absorption of sunlight, which were linked with the leaf area index [30,31]. The mulching treatments significantly improved stored soil water by retaining adequate levels of heat and water for plant growth at the seedling phase. Therefore, the increase in leaf area might have led to an increase in the effective intercepted radiation that was later available for the fast-growing maize crop. However, significant differences were observed between the treatments in terms of crop growth rate with respect to these properties. The BM and TM treatments exhibited a greater crop growth rate, which might have been due to the higher leaf area index, and improved radiation detention [32], which might have led to more biomass production compared

to the WM treatment. The possible reason for this is that covering the soil with mulches preserves the soil water by inhibiting the evaporation and enhancing the water. Further, it is reported that when plants were small, soil water was lost mainly through soil evaporation. As the maize crop grew vigorously, the water loss shifted from soil evaporation to plant transpiration, so better water conditions under mulch conditions improved the growth of the maize crop. Compared to the GM and WM treatments, the BM and TM symbolize a more operative exercise to respond to the scarcity of available water. Our findings showed that the ridge-furrow mulching enhanced the soil moisture and augmented the yield of maize with BM and TM at 33.6 and 28.1%, respectively. The improvement in the maize yield resulted from the more efficient single plants developed under BM and TM and could have been due to the exploitation of deep soil water effectively [33,34]. In our study, the ridge-furrow covered with mulches markedly enhanced the water storage of soil. The increased soil moisture contents under mulch conditions could improve the soil nitrogen availability. High N accumulation promotes maize growth by improving nitrogen use efficiency. The soil water storage was higher with BM, TM and GM than that with WM, which might have been due to the reason that mulches inhibit the evaporation from the soil surface and increase rainfall infiltration. According to Li et al. [31] and Wang et al. [35], plastic-covered ridge-furrow mulching can improve the soil moisture status by collecting water more efficiently from light rainfall, endorsing rainfall infiltration and reducing the evaporation. Ren et al. [18] also described that various intensities of rainfall (440, 340 and 230 mm) improved the mean water contents of soil by 4.5, 5.2 and 2.3%, respectively, with the plastic-covered ridge-furrow mulching than with the flat cultivation without ridges and plastic mulches. In addition, more harvested rainwater infiltrated deeper in the covered ridge system when compared to that with flat cultivation, hence increasing the availability of water to plant for a longer period of time as well as being less subject to evaporation [18]. However, for effectiveness on a large scale, this system needs simulation design with different rainfall amounts.

The physiological attributes such as net photosynthetic rate, transpiration rate and leaf stomatal conductance were found to be improved with BM, TM and GM treatments compared to that of WM, because photosynthetic capacity is highly linked with water availability, with maize crop at the early growth stage showing an increase in photosynthesis rate under the plastic film or grass mulch compared to that under non-mulch treatment [36–38]. In our experiment, the treatments of mulches resulted in increased retention of soil moisture as compared to that with WM treatment during the partial rainy season. These results might have been due to the loss of soil moisture from the WM treatment due to the strong evaporation from the soil surface by direct sun radiation and dry air during the early phases of maize growth. The BM, TM and GM treatments produced higher net photosynthetic, transpiration rate and leaf stomatal conductance than the WM treatment, because mulches directly inhibit water evaporation from the surface of soil [30] and provide more water to plants, increasing the gas exchange activity of plants by using solar radiation and enhanced canopy transpiration [39]. The plastic and grass mulches could enhance the soil moisture of dry arable land [14,15,35,40].

#### **5. Conclusions**

The ridge-furrow covered with plastic mulches accelerated the photosynthetic capacity, growth and development by capture and utilization of rainwater. The BM significantly increased the net photosynthesis and transpiration rate during both growing seasons over the other treatments. Consequently, as compared to all other treatments, BM was found to be the most effective approach to bring an increase in the farmer's income. Therefore, BM treatment must be encouraged and applied as an effective cultivation method in rain-fed areas.

### **6. Implication**

Crop productivity in arid and semiarid regions of Pakistan is dependent on rainfall. Frequent drought is an important factor that limits crop production. Seasonal rainfall and distribution determine the success of crop in dry areas. This study indicates that ridge-furrow plastic film mulch has some advantages over conventional systems under the same volume of rainfall. First, the ridge-furrow configuration leads to capture and utilizing the small amount of rainfall, thus providing better conditions for maize growth and development. Secondly, the full cover of ridge-furrow with plastic film reduced the evaporation losses. This system is evaluated at a small scale, hence to maximize its effectiveness on a large scale in Pakistan, a simulation design needs to be developed. The cost of ridge-furrow plastic film is higher than the conventional system as the additional cost of plastic and labor is added every year. However, this cost can be compensated with higher productivity under plastic mulch, but plastic fate is a disadvantage in terms of recycling and environmental protection.

**Author Contributions:** Writing—original draft preparation and edition, M.M.J., H.W. and M.A.; project administration, F.-M.L. and M.A.; conceptualization, methodology and investigation, M.M.J., F.-M.L., A.M., H.I.M.A. and S.F.; writing—review and editing K.A.A., M.A.N., M.N., H.B.A. and M.D.F.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** Princess Nourah bint Abdulrahman University Researchers Supporting Project Number (PNURSP2022R20), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

**Institutional Review Board Statement:** Not applicable as studies not involving humans or animals.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Princess Nourah bint Abdulrahman University Researchers Supporting Project Number (PNURSP2022R20), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Article* **Interactive Effects of Nitrogen Application and Irrigation on Water Use, Growth and Tuber Yield of Potato under Subsurface Drip Irrigation**

**Amanpreet Kaur 1 , Kanwar Barjinder Singh 1 , Rajeev Kumar Gupta 1,\* , Abed Alataway 2 , Ahmed Z. Dewidar 2,3 and Mohamed A. Mattar 2,3,4,5,\***

**Abstract:** Potatoes are a high-value crop with a shallow root system and high fertilizer requirements. The primary emphasis in potato production is minimizing nitrogen-leaching losses from the shallow root zone through fertigation. Therefore, a field experiment was conducted for two consecutive years, 2018–2019 2019–2020 to assess the effect of nitrogen and irrigation amount and frequency on tuber yield, water balance components and water productivity of potatoes under surface and subsurface drip irrigation. The experiment was laid out in a split-plot design with three nitrogen levels (187.5 kg N ha−1 (N1 ), 150 kg N ha−1 (N2 ) and 112.5 kg N ha−1 (N3 )) in main plots and six irrigation levels in the subsurface (drip lines were laid at 20 cm depth) and one surface drip in subplots. Irrigation scheduling was based on 100% of cumulative pan evaporation at an alternate (I1 ) and two-day interval (I2 ), 80% of cumulative pan evaporation at an alternate (I3 ) and two-day interval (I4 ), 60% of cumulative pan evaporation at an alternate (I5 ) and two-day interval (I6 ) and 80% of cumulative pan evaporation at alternate days with surface drip (I7 ). Our results showed that potato transpiration was higher in N1 and N2 compared to N3 , while soil evaporation was higher in N3 over N1 and N2 . Irrigation regimes I5 and I6 had lower transpiration than I1 , I2 , I3 and I7 , while I7 had more soil evaporation than I1 , I2 and I3 . Leaf area index (LAI), dry matter accumulation (DMA), root mass density (RMD) and tuber yield in N1 and N2 were at par but significantly higher than N3 . The LAI and DMA were statistically at par in I1 , I2 and I3 but significantly higher than recommended irrigation (I7 ). Tuber yield was statistically at par in I1 , I2 , I3 and I7 but I3 and I7 saved 20% irrigation water compared to I1 and I2 . On the other hand, real water productivity (WPET) under N1 and N2 were comparable in I3 and I4 but significantly higher than recommended practice (I7 ) as pooled evapotranspiration (ET) and soil evaporation (E) in I7 were 19.5 and 20.6 mm higher, respectively, than in I3 . Among interactive treatment combinations, N1 I1 , N1 I2 , N1 I3 , N1 I7 , N2 I1 , N2 I2 and N2 I3 recorded the highest tuber yields without any significant differences among them. Treatment N2 I3 saved 20% nitrogen and irrigation water compared to all other combinations. Water productivity in N1 and N2 was comparable in I3 and I4 but significantly higher than recommended practice (I7 ).

**Keywords:** potato fertigation; leaf area index; root mass density; real water productivity; field water balance

#### **1. Introduction**

Potato (*Solanum tuberosum* L.) is the fourth-most-crucial vegetable crop, with a global production of 370 million tons [1]. India is the second-largest potato producer, with an

**Citation:** Kaur, A.; Singh, K.B.; Gupta, R.K.; Alataway, A.; Dewidar, A.Z.; Mattar, M.A. Interactive Effects of Nitrogen Application and Irrigation on Water Use, Growth and Tuber Yield of Potato under Subsurface Drip Irrigation. *Agronomy* **2023**, *13*, 11. https://doi.org/ 10.3390/agronomy13010011

Academic Editors: Aliasghar Montazar and Junliang Fan

Received: 30 October 2022 Revised: 4 December 2022 Accepted: 16 December 2022 Published: 21 December 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

area of 2.17 million and a production of around 53.58 million metric tons [1]. Among the potato-growing zones of India, Punjab falls in the north-western plain, which accounts for about 5% of the total area under potato cultivation in India [2]. The main-season potato crop is popular among stakeholders in spring maize–rice–potato rotation. Spring maize and rice are high-water-requiring crops, and potato is sensitive to water stress which responds to frequent irrigation [3].

Moreover, potato is grown on raised beds and irrigated through furrows, accounting for significant water losses. Hence, the region's cultivation of water-guzzler rice and spring maize crops is becoming challenging as Punjab is already facing a water crisis [4]. The mean ground table has experienced a fall of 0.4 m year− in 80% of irrigated areas in central Punjab [5] since 1990. Thus this scenario of water scarcity in the region has necessitated the adoption of improved water management technologies such as drip irrigation to improve the water productivity of cropping systems. Surface drip irrigation in potato is recommended by researchers worldwide [6–8], but studies on the feasibility of subsurface drip irrigation systems in potato are limited. Thus, this experiment was planned to study the growth and yield of subsurface-drip-irrigated potato sown after spring maize and rice on the same drip lines. Potato is a shallow-rooted crop and is highly affected by excessive water and stressed water conditions [9]. Although furrow irrigation is the most common irrigation method in the Indo-Gangetic plains, drip irrigation has proved to be a good option in managing the volumetric inefficiencies in irrigation regimes resulting from too frequent or too much irrigation [10]. Agronomic inadequacies occur when plants are stressed due to insufficient water being frequently applied or excessive water application resulting in water logging or increasing the incidence of diseases [11].

Potato production is highly dependent on nitrogen (N), phosphorous (P) and potassium (K) applications [12]. Nitrogen is a dominant nutrient determining potato's growth, development, productivity and quality, and its optimal application improves yields [13], while excessive N may contaminate groundwater [14]. Therefore, in potato, water and nutrient management through a drip irrigation system can be the best option for maximization of potato productivity. Studies [15,16] have shown that the maximum productivity of potato can be obtained while keeping the soil moist and ensuring the N available during periods of high demand, which can be managed well with drip irrigation and fertigation. This system is most economical in giving the highest water productivity and saving about 40–50% of irrigation water compared to the furrow method [17]. It gives uniform water distribution with a significant reduction in water losses by percolation and runoff. This system is also beneficial on frosty nights as it reduces frost damage in plants in main-season crops. We hypothesize that subsurface drip irrigation is also used globally for potato production [18–20], in which drip lines are buried below the soil surface, providing water and nutrients directly to the root zone. Therefore, it has the potential to save water by reducing evaporation losses compared to other methods of irrigation, particularly in sandy loam soil [21]. Therefore, this experiment was planned to study the interactive effects of nitrogen, irrigation amount and irrigation frequency on potato growth, evapotranspiration and yield under subsurface drip irrigation.

#### **2. Materials and Methods**

#### *2.1. Climate Conditions*

A field experiment was conducted at Research Farm, Soil and Water Engineering, Punjab Agricultural University, Ludhiana, India, during 2018–2019 and 2019–2020. The layout of the experiment is given in Figure S1. Ludhiana is situated at 30◦560 N and 75◦520 E at 247 m above sea level. The area has a sub-tropical to semiarid climate with a hot and dry summer (April to June), hot and humid monsoon (July to September), mild winter (October to November) and cold winter (December to February). The mean minimum and maximum temperatures show considerable fluctuation during summer and winter. Maximum air temperature above 38 ◦C is expected during summer and frequent frosty spells in December–January. The average annual rainfall at Ludhiana is 755 mm, 75%

of which is received in the summer monsoon (July to September), complemented by a few low- to medium-intensity showers in winter. During the 2018–2019 potato season, the highest mean maximum air temperature was observed during October and lowest during January, while the mean minimum air temperature was lowest during December and highest during October. The maximum relative humidity was 71% observed in January. 2018–2019 (Table 1). The potato crop during 2018–2019 received 68.6 mm of rainfall. During 2019–2020, the weekly mean maximum air temperature in October was 30.6 ◦C while the minimum temperature was observed during January. The mean maximum relative humidity was 78.4% observed in January. The total rainfall was 123 mm.

Tmax is maximum air temperature, Tmin is minimum air temperature, Epan is open pan evaporation, RF and RH are rainfall and relative humidity, respectively, and ET0 is potential evapotranspiration.

#### *2.2. Experimental Site Description*

Soil samples were collected randomly from four depths (0–15, 15–30, 30–60 and 60–90 cm) and three different field sites before the experiment started with the help of a screw auger. A composite sample was made and brought to the soil and water testing laboratory in the Department of Soil and Water Engineering to analyze the initial status of physical and chemical properties. The analysis was performed according to standard laboratory procedures. The experimental field soil was sandy loam in texture with pH 8.1, electrical conductivity 0.23 dS m−1 , organic carbon 0.48%, nitrogen 170.2 kg ha−1 , phosphorous 37.4 kg ha−1 and potassium 345 kg ha−1 in 0–15 cm soil depth. The bulk density of the soil was 1.52 g cm−3 . The physical properties of the soil are given in Table 2.

**Table 2.** Physical properties of experimental soil.

In order to prepare good tilth before planting, the disc harrow was run twice, followed by two ploughings with a tractor-drawn cultivator. Drip lines were installed at a 20 cm depth below the soil surface with a tractor-drawn machine (as shown in the layout). The distance between the two drip lines was kept at 60 cm per row-to-row crop spacing. Ridges were made manually 60 cm apart in the east–west direction (Figure S1). The experiment was laid out in a split-plot design with three nitrogen levels of 187.5 kg N ha−1 (N1), 150 kg N ha−1 (N2) and 112.5 kg N ha−1 (N3) in the main plots. Seven irrigation levels (six subsurface drip lines and one surface drip) were kept in the subplots. The irrigations were 100% of Epan at alternate day (I1) and two-day intervals (I2), 80% of Epan at alternate (I3) and two-day intervals (I4), 60% of Epan at alternate day (I5) and two-day intervals (I6) in the subsurface and 80% of Epan at an alternate day in surface drip (I7). The potato

variety *Kufri Pukhraj* was planted on 18 October, during 2018–2019 and on 15 October in 2019–2020 at 20 cm plant-to-plant distance and 7–8 cm soil depth from the top of the ridge in rows as per the layout. All the plants emerged between 14 and 18 days. All the crop management practices were followed as per the practice package for cultivating vegetable crops. Fertilizer N was applied as per treatments. The recommended doses of phosphorus (62.5 kg P2O5 kg ha−1 ) and potassium (K2O 62.5 kg ha−1 ) were applied along with N in 16 splits at weekly intervals. A water meter was installed on 63 mm PVC pipe to measure the irrigation water delivered to each plot. Tensiometers were installed at 10, 20, 30, 60 and 90 cm depths in I1, I3, I5 and I7 to monitor soil matric potential (SMP) daily at 8 am during 2018–2019. However, during 2019–2020 SMP was monitored only during three major rain events. Soil water content was determined 24 h, 48 h and 72 h after each irrigation from 0–10, 10–20, 20–30, 30–40, 40–60 and 60–100 cm profile depths with FDR (Frequency domain reflectometry). Actual crop evapotranspiration (ETa) was estimated from the soil water balance equation.

$$\mathbf{I} + \mathbf{P} = \mathbf{E} \mathbf{T}\_\mathbf{a} + \mathbf{D} + \mathbf{R} \pm \Delta \mathbf{S} \mathbf{W} \tag{1}$$

where I is irrigation water applied (mm), P is precipitation (mm), R is surface runoff (mm), D is deep drainage (mm) and ∆SW is the change in soil profile moisture storage (mm). Runoff was absent as sufficient dikes were maintained. Deep drainage was considered zero when soil profile moisture storage was less than the field capacity. When soil moisture storage exceeded the field capacity storage after irrigation or rainfall, deep drainage was calculated as the difference between the field capacity storage and soil moisture storage plus irrigation/rainfall [22]. Evapotranspiration (ETa) was partitioned into soil evaporation (E) and transpiration (Tp) [23]:

$$\mathbf{E} = \mathbf{E} \mathbf{T}\_{\mathbf{a}} \times \mathbf{e}^{-\alpha \text{ LAI}} \tag{2}$$

$$\mathbf{T\_{P}} = \mathbf{E} \mathbf{T\_{a}} - \mathbf{E\_{s}} \tag{3}$$

where α is the extinction coefficient of radiation set as 0.56 [24], and LAI is the leaf area index (measured with the Sunscan Plant Canopy Analyzer of Delta T Devices) at different crop growth stages. The following equation already calibrated for Potato LAI [23] was used to partition LAI during crop growth and senescence:

$$\text{LAI}\_{\text{(t)}} = \text{LAI}\_{\text{max}} \frac{1}{1 + \mathbf{e}^{\mathbf{a}\_1}(\mathbf{t} - \mathbf{t}\_{\text{inf}})} - \mathbf{e}^{\mathbf{a}\_2}(\mathbf{t} - \mathbf{t}\_{\text{end}}) \tag{4}$$

where LAImax is the maximum LAI value observed, a1 and a2 are shape factors controlling the growth and senescence rates, tinf is the time at which the maximum growth rate is reached (inflexion point) and tend is the time of complete senescence. The terms LAImax, tinf, and tend were measured and a1 and a2 were fitted [23]. Whole plants and roots were uprooted, sun-dried and then dried in an oven at 60 ◦C until they reached a constant weight for measuring dry matter accumulation at 60, 75 DAP and at harvest, which was expressed in Mg ha−1 . The leaf area index was recorded periodically (30, 60, 90 and 120 DAS) using Sunscan Plant Canopy Analyzer based on the principle of Beer–Lamber's law. After calibration, the leaf area index was recorded from 12:00 to 2:00 p.m. on a sunny day from two locations in each plot and then averaged for the final reading of the leaf area index. Root-mass density was determined by collecting root samples with the help of tubes having an internal diameter of 5 cm from 0–15, 15–30, 30–45 and 45–60 cm depths from three places, under the plant, 7 cm from the base of the plant towards row and 7 cm from the base of the plant towards furrow. These samples were mixed, passed through a 32 mm mesh and washed in a running water channel by providing gentle pulsing with the hand to make the roots free from soil. Then roots were transferred to Petri dishes to remove inert material. Then roots were dried in an oven and root mass density was calculated by dividing the mass of dry roots by the volume of the tube. N uptake in plant samples was determined at 90 days. The digestion process was performed for nitrogen determination in plant samples.

During digestion, complex structures were broken into simple structures, releasing nitrogen as ammonium radicals. For digestion, 0.5 g of plant sample was taken in a digestion tube and mixed with 6 mL of concentrated sulphuric acid. Then, one teaspoon of mix from a mixture of CuSO4 (100 g), K2SO4 (10 g) and selenium oxide (1 g) was added to the digestion tube. The solution was retained at room temperature for 24 h. Then, the digestion tube was heated up to 410 ◦C for 2 h. After completion of digestion, the sample turned to light green or colourless. After digestion, distillation with 4% boric acid and titration with 0.1 N H2SO4 was performed to determine the plant nitrogen as per the KEL PLUS manual. Nitrogen uptake was determined by multiplying the N content with biomass and tuber yield. Real water productivity (kg m−3 ) was calculated as the ratio of grain yield and evapotranspiration as described by [5,25]:

$$\text{WP}\_{\text{ET}} = \text{GY} / \text{ET}\_{\text{a}} \times 10 \tag{5}$$

where WPET is real water productivity (kg m−3 ), ETa is crop evapotranspiration (mm) and GY is grain yield (Mg ha−1 ).

#### *2.3. Statistical Analysis*

Analysis of variance was performed using the Proc GLM procedure of SAS version 9.4 (SAS Institute, Inc., Cary, NC, USA). Significant mean differences were tested using 54 Fisher's protected least significant difference (LSD) tests at α = 0.05. For pooled analysis, the year was the main factor to increase the precision. In addition, the regression procedure was used to test the nature of the relationship between different parameters.

#### **3. Results and Discussion**

#### *3.1. Soil Matric Potential and Soil Moisture Distribution at Different Depths*

Soil matric potential (SMP) values for different treatments throughout cropping season 2018–2019 are shown in Figure 1. At 10 cm depth, SMP in I1 and I3 varied between −38 and −48 kPa, except for two significant rain events (Figure 1 at the same depth, SMP for I5 was even less and varied between −48 and −55 kPa. However, under surface drip (I7), SMP at the surface layer was high and varied between 18 and 24 kPa. At 20 and 30 cm depths, the SMP was higher at 60 and 90 cm depths in I1, I3 and I5 due to placement of the dripper at these depths. Between these treatments, the highest SMP was observed in I1, followed by I3 and I5. However, in I7, higher SMP was observed at 10 cm and 20 cm depths compared to 30, 60 and 90 cm depths. In all these treatments, more changes in soil matric potential occurred in 10, 20 and 30 cm depths. However, fewer fluctuations were observed at 60 and 90 cm, even during the major rain events, indicating no drainage in our experiments. Mean seasonal volumetric soil moisture (MSVSM) during the growing season (except major events) is presented in Table 3. During 2018–2019, in 0–10 cm depth after 24 h of irrigation, MSVSM in I1, I2, I3, I4, I5, I6 and I7 was 11.1, 12.1, 10.9, 10.7, 9.6, 9.3 and 18.7%, respectively. It dropped to 9.2, 10.1, 8.8, 5.6, 9.1, 8.8, 8.5, and 15.8%, respectively, after 48 h of irrigation which indicated that, throughout the season, the surface soil layer remained relatively wet in surface drip and dry in subsurface drip irrigation. After 24 h in the 10–20 cm soil layer, the seasonal volumetric soil moisture was higher in I1 and I2 by 2.1 and 2.0%, respectively, over I7, while in I5 and I6, it was lower than I7 by 1.8 and 2.8%, respectively. Compared to I7, I3 had 0.5% more, but I4 contained less moisture content in this layer. After 24 h in the 20–30 cm soil layer, MSVSM was higher in I1, I2, I3, I4, I5 and I6 by 8.0, 6.2, 4.4, 3.7, 0.7, and 0.6%, respectively, over I7. A similar trend was observed after 48 h of irrigation. In 30–40 cm depth after 24 h of irrigation, MSVSM was higher in I1, I2, I3, I4, I5 and I6 than I7 by 6.0, 5.8, 4.3, 3.8, 1.7 and 0.9%, respectively. A similar trend was observed after 48 h of irrigation. In 40–60 cm soil depth after 24 h of irrigation MSVSM was higher in I1, I2, I3, I4, I5 and I6 than in I7 by 4.7, 4.0, 3.6, 3.2, 2.3 and 2.4%, respectively. Similarly, during 2019–2020 (Table 4), MSVSM in I1, I2, I3, I4, I5, I6 and I7 was 12.0, 11.8, 11.1, 10.8, 10, 9.9 and 20.2%, which dropped to 9.2, 10.1, 8.8, 9.1, 8.8, 8.5 and 15.8%, respectively, after 48 h and further dropped to 8.9, 8.1, and 8.3% after 72 h in treatments I2, I4 and I6,