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**Chapter 9**

**Provisional chapter**

**Root Growth, Morphological and Physiological**

**Root Growth, Morphological and Physiological**

**Vegetable Crops Grown in the Tropics Under Different**

Root growth and morphology are important for maximizing water uptake and mineral absorption. Similar to the plants grown in the soil, in a soilless culture such as an aeroponic system, the amount of water and nutrient available to a plant is determined by the root surface area and volume of nutrient solution with which its roots are in contact. Furthermore, plant roots can alter their nutrient acquisition capacity by adjusting their morphological and/or physiological characteristics to meet changes in shoot nutrient demand in response to environmental stress. Subtropical and temperate vegetables have successfully been grown aeroponically in the tropics by simply cooling the root zone (RZ) while their aerial portions are subjected to fluctuating atmospheric temperatures. This paper focused on RZ temperature (RZT) on root and shoot growth, and root morphology of subtropical and temperate vegetable crops grown in the tropics. The impacts of RZT on water relations as well as nitrate (NO3−) uptake and assimilation

**Keywords:** nitrate uptake and assimilation, root morphology, root-zone temperature,

Plants are photoautotrophic and manufacture their own food through the process of photosynthesis. However, they must acquire water and minerals from the environment for photo-

and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Crops Grown in the Tropics Under Different Root-Zone**

**Characteristics of Subtropical and Temperate Vegetable**

**Characteristics of Subtropical and Temperate**

**Root-Zone Temperature**

**Temperature**

http://dx.doi.org/10.5772/64824

**Abstract**

water relations

**1. Introduction**

Additional information is available at the end of the chapter

of these vegetable crops were also discussed.

Additional information is available at the end of the chapter

Jie He

Jie He

## **Root Growth, Morphological and Physiological Characteristics of Subtropical and Temperate Vegetable Crops Grown in the Tropics Under Different Root-Zone Temperature Root Growth, Morphological and Physiological Characteristics of Subtropical and Temperate Vegetable Crops Grown in the Tropics Under Different Root-Zone Temperature**

Jie He Jie He

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/64824

#### **Abstract**

Root growth and morphology are important for maximizing water uptake and mineral absorption. Similar to the plants grown in the soil, in a soilless culture such as an aeroponic system, the amount of water and nutrient available to a plant is determined by the root surface area and volume of nutrient solution with which its roots are in contact. Furthermore, plant roots can alter their nutrient acquisition capacity by adjusting their morphological and/or physiological characteristics to meet changes in shoot nutrient demand in response to environmental stress. Subtropical and temperate vegetables have successfully been grown aeroponically in the tropics by simply cooling the root zone (RZ) while their aerial portions are subjected to fluctuating atmospheric temperatures. This paper focused on RZ temperature (RZT) on root and shoot growth, and root morphology of subtropical and temperate vegetable crops grown in the tropics. The impacts of RZT on water relations as well as nitrate (NO3−) uptake and assimilation of these vegetable crops were also discussed.

**Keywords:** nitrate uptake and assimilation, root morphology, root-zone temperature, water relations

## **1. Introduction**

Plants are photoautotrophic and manufacture their own food through the process of photosynthesis. However, they must acquire water and minerals from the environment for photo-

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

synthesis to occur. While CO2 comes from the atmosphere for photosynthesis, plants obtained water and minerals from the soil or other growth media via their roots. Plants are sessile and adapt their morphological structures to the encountered environmental conditions. The morphology of the root system such as root length, number of root tip, root diameter, root surface area, and root volume varies greatly depending on the plant species, soil composition, and water and mineral nutrients availability [1, 2] and root-zone temperature (RZT) [2, 3]. Root plasticity provides the sessile plants to adjust their structure to environmental conditions as they change [1]. Changes in root morphology are important for maximizing water uptake [1, 3, 4] and mineral absorption to alleviate stress of nutrient deficiency [4–7]. Furthermore, plant roots alter their nutrient acquisition capacity not only by adjusting the morphological characteristics but also the physiological performance such as nitrate (NO3−) uptake and assimilation to meet changes in shoot nutrient demand in response to environmental stress [2, 4].

elongation in roots [16]. It was found that maximum seminal root elongation and first-order lateral root initiation and elongation of Sorghum (*Sorghum bicolo*), occurred at 25°C but they

Root Growth, Morphological and Physiological Characteristics of Subtropical and Temperate Vegetable...

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133

We have found that subtropical and temperate vegetables grown in the tropical greenhouse under cool RZTs (C-RZTs) established much bigger root systems compared to those subjected to hot ambient RZT (A-RZT) [2]. For instance, the total root length of capsicum plants (*Capsicum annuum* Indra F1-hybrid) at C-RZT increased about 400 cm in 2 weeks during the experimental period (**Figure 1A**). When plants were transferred from C-RZT to A-RZT (C → A-RZT), increases in their total root length seemed to stop compared to C-RZT plants [19]. Capsicum plants grown at A-RZT did not show any increases and had the lowest total root length during the entire experimental period. However, after transferring capsicum plants from A-RZT to C-

**Figure 1.** Total root length (A), total number of root tips (B), total surface area (C), and average root diameter (D) of capsicum (*Capsicum annuum* Indra F1-hybrid) during different RZT treatments: C-RZT (•), A-RZT (▴), C → A-RZT (❍), and A → C-RZT (Δ). RZT treatments were started 3 weeks after transplanting. Means with different letters are statisti-

Differences in total number of root tip (**Figure 1B**) and total surface area (**Figure 1C**) among the four different RZT treatments were similar to those of total root length (**Figure 1A**). Compared to plants grown at A-RZT, the average root diameter of C-RZT plants was much

cally different (*P* < 0.05; *n* = 6) as determined by Tukey's multiple comparison test. Redrawn from Tan [19].

RZT (A → C-RZT), linear increase in total root length was observed.

were severely inhibited at 40°C [17].

Root morphology and physiology are closely associated with the growth of the aerial parts and shoot productivity [2, 3, 8]. For instance, plant growth and productivity are often limited by high temperature and thus restrict the growth of subtropical and temperate crops in the tropics. However, since 1997, our research team has successfully grown certain subtropical and temperate crops in Singapore by simply cooling their roots while their aerial portions are subjected to hot fluctuating atmospheric conditions [9, 10]. Working on subtropical and temperate vegetable crops grown in the tropics, in this paper, the author first focused on effects of RZT on root morphology. The impacts of RZT on water uptake and water relations as well as NO3− uptake and assimilation were also discussed.

## **2. Effects of root-zone temperature on root morphology**

Root systems have higher ratios of surface area to volume that effectively explore a larger volume of soil [11]. Similar to the plants grown in the soil, in aeroponic systems, root surface area determines the amount of water and mineral uptake [2, 12, 13]. As root systems are responsible for acquisition of water and mineral nutrients, it is not surprising that root morphology is highly influenced by rhizosphere environments such as RZT [2].

When the plant is exposed to high temperatures, root development is adversely affected [14]. However, manipulating RZT alone has a great influence on root development, perhaps even greater than shoot temperature manipulation [15–18]. Because of the changes of RZT, there is great variation of root morphology, especially root length which is a more sensitive indicator of RZT impact compared to root biomass. For example, grown underoptimum RZT of 30°C, root length of rape (*Brassica napus* cv. "Emeralk") was fivefold longer compared to those grown under superoptimum RZT of 35°C. However, there was only twofold difference in root biomass, which was found between the plants grown under two different RZTs. Higher root length/weight biomass ratio was due to the smaller diameter of the successive orders of lateral roots [15]. It was reported that in both heat tolerant and sensitive clones of potato (*Solanum tuberosum* L), smaller number and shorter length of lateral roots were observed at 30°C RZT than 20°C RZT resulting from a reduction of cell division rate and followed by cessation of cell elongation in roots [16]. It was found that maximum seminal root elongation and first-order lateral root initiation and elongation of Sorghum (*Sorghum bicolo*), occurred at 25°C but they were severely inhibited at 40°C [17].

synthesis to occur. While CO2 comes from the atmosphere for photosynthesis, plants obtained water and minerals from the soil or other growth media via their roots. Plants are sessile and adapt their morphological structures to the encountered environmental conditions. The morphology of the root system such as root length, number of root tip, root diameter, root surface area, and root volume varies greatly depending on the plant species, soil composition, and water and mineral nutrients availability [1, 2] and root-zone temperature (RZT) [2, 3]. Root plasticity provides the sessile plants to adjust their structure to environmental conditions as they change [1]. Changes in root morphology are important for maximizing water uptake [1, 3, 4] and mineral absorption to alleviate stress of nutrient deficiency [4–7]. Furthermore, plant roots alter their nutrient acquisition capacity not only by adjusting the morphological characteristics but also the physiological performance such as nitrate (NO3−) uptake and assimilation to meet changes in shoot nutrient demand in response to environmental stress [2,

Root morphology and physiology are closely associated with the growth of the aerial parts and shoot productivity [2, 3, 8]. For instance, plant growth and productivity are often limited by high temperature and thus restrict the growth of subtropical and temperate crops in the tropics. However, since 1997, our research team has successfully grown certain subtropical and temperate crops in Singapore by simply cooling their roots while their aerial portions are subjected to hot fluctuating atmospheric conditions [9, 10]. Working on subtropical and temperate vegetable crops grown in the tropics, in this paper, the author first focused on effects of RZT on root morphology. The impacts of RZT on water uptake and water relations as well

Root systems have higher ratios of surface area to volume that effectively explore a larger volume of soil [11]. Similar to the plants grown in the soil, in aeroponic systems, root surface area determines the amount of water and mineral uptake [2, 12, 13]. As root systems are responsible for acquisition of water and mineral nutrients, it is not surprising that root mor-

When the plant is exposed to high temperatures, root development is adversely affected [14]. However, manipulating RZT alone has a great influence on root development, perhaps even greater than shoot temperature manipulation [15–18]. Because of the changes of RZT, there is great variation of root morphology, especially root length which is a more sensitive indicator of RZT impact compared to root biomass. For example, grown underoptimum RZT of 30°C, root length of rape (*Brassica napus* cv. "Emeralk") was fivefold longer compared to those grown under superoptimum RZT of 35°C. However, there was only twofold difference in root biomass, which was found between the plants grown under two different RZTs. Higher root length/weight biomass ratio was due to the smaller diameter of the successive orders of lateral roots [15]. It was reported that in both heat tolerant and sensitive clones of potato (*Solanum tuberosum* L), smaller number and shorter length of lateral roots were observed at 30°C RZT than 20°C RZT resulting from a reduction of cell division rate and followed by cessation of cell

as NO3− uptake and assimilation were also discussed.

**2. Effects of root-zone temperature on root morphology**

phology is highly influenced by rhizosphere environments such as RZT [2].

4].

132 Plant Growth

We have found that subtropical and temperate vegetables grown in the tropical greenhouse under cool RZTs (C-RZTs) established much bigger root systems compared to those subjected to hot ambient RZT (A-RZT) [2]. For instance, the total root length of capsicum plants (*Capsicum annuum* Indra F1-hybrid) at C-RZT increased about 400 cm in 2 weeks during the experimental period (**Figure 1A**). When plants were transferred from C-RZT to A-RZT (C → A-RZT), increases in their total root length seemed to stop compared to C-RZT plants [19]. Capsicum plants grown at A-RZT did not show any increases and had the lowest total root length during the entire experimental period. However, after transferring capsicum plants from A-RZT to C-RZT (A → C-RZT), linear increase in total root length was observed.

**Figure 1.** Total root length (A), total number of root tips (B), total surface area (C), and average root diameter (D) of capsicum (*Capsicum annuum* Indra F1-hybrid) during different RZT treatments: C-RZT (•), A-RZT (▴), C → A-RZT (❍), and A → C-RZT (Δ). RZT treatments were started 3 weeks after transplanting. Means with different letters are statistically different (*P* < 0.05; *n* = 6) as determined by Tukey's multiple comparison test. Redrawn from Tan [19].

Differences in total number of root tip (**Figure 1B**) and total surface area (**Figure 1C**) among the four different RZT treatments were similar to those of total root length (**Figure 1A**). Compared to plants grown at A-RZT, the average root diameter of C-RZT plants was much smaller (**Figure 1D**). It is surprising to note that there were no increases in root diameter after transferring plants from C-RZT to A-RZT (C → A-RZT) as capsicum plants were similar to those of C-RZT plants throughout the experiment. However, A → C-RZT transfer resulted in the development of new and finer roots in A → C-RZT plants. Thus, the average root diameter of A → C-RZT capsicum was significantly smaller than A-RZT by day 6 after RZT transfer (**Figure 1D**). Similar to capsicum plants, lettuce grown at C-RZT showed linear increase in total root length over the 2-week period (**Figure 2A**). Unlike the capsicum plants (**Figure 1A**), increase in total root length was also observed in C → A-RZT lettuce plant and the increase rate was similar to C-RZT lettuce plants from day 0 to day 8 after RZT transfer. However, C → A-RZT lettuce plants maintained their total root length of about 600 cm from day 10 to day 14 after RZT transfer. There was no significant increase in total root length of A-RZT lettuce over the 2-week experimental period. Again, unlike the capsicum plants (**Figure 1A**), A → C-RZT lettuce exhibited increase in total root length only after 8 days of RZT transfer (**Figure 2A**). The total root tip number of C-RZT lettuce was consistently higher than lettuce grown at other RZTs (**Figure 2B**). Unlike the capsicum plants (**Figure 1B**), there was an increase in total root tip number for C → A-RZT lettuce although the net increase decreased significantly after C → A-RZT transfer (**Figure 2B**). The total root tip number of A-RZT lettuce did not change significantly throughout the whole experiment. When lettuce was transferred from A → C-RZT, there was rapid increase in the total root tip number. The root tip number of A → C-RZT lettuce became significantly higher than A-RZT lettuce by day 6 after RZT transfer. At day 14 after RZT transfer, total root tip number of A → C-RZT lettuce was 2.8 times higher than A-RZT plants. Responses of total surface area of lettuce plants to different RZTs (**Figure 2C**) were similar to those of total root length (**Figure 2B**). Similar to those of capsicum plants (**Figure 1D**), lettuce grown at A-RZT had the highest average root diameter while C-RZT plants had the least (**Figure 2D**). The average root diameter of C → A-RZT lettuce remained similar to C-RZT plants during the first 10 days of RZT transition. However, unlike capsicum plants (**Figure 1D**), there was slight root thickening in C → A-RZT lettuce roots after 2 weeks of RZT transfer. By day 14 after RZT transfer, the average root diameter of C → A-RZT lettuce was much thicker than C-RZT plants. Similar to those of capsicum plants, A → C-

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Root morphological analysis of both subtropical and temperate vegetables revealed that high RZT inhibited root elongation, branching, and hair formation but increased root diameter. These were also observed in other plant species [16, 17]. However, effects of RZT transfer on morphology were different between capsicum and lettuce plants (**Figures 1** and **2**). These finding suggest that effects of RZT on root morphology is species-dependent. It was also reported that root length and diameter appeared to be inversely related to a study using *Secale cereale* seedlings [20]. Root thickening, or an increase in diameter, was controlled through signals emanating from shoot apices and root tips [21]. Root thickening may also be accompanied by associated changes in microfibril angles within expanding cell walls [21]. The chemical signals involved in root thickening may be ethylene [22]. The role of ethylene in inhibition of root elongation and root thickening was further confirmed by our team [13]. Our recent study with a recombinant inbred line (RIL) of lettuce and its parental lines (*Lactusa serriola* × *Lactuca sativa* "Salinas") that were grown in a tropical greenhouse under 24°C-RZT and hot A-RZT showed that higher RZ ethylene concentrations accumulated in A-RZT plants compared to that of 24°C-RZT plants. Lowest RZ ethylene concentration corresponded with highest shoot fresh weight [23]. Our results indicated that the presence of an ethylene inhibitor promoted root elongation at high RZT of 38°C. Without ethylene inhibitor, root elongation at high RZT was significantly inhibited. Our previous 14C feeding experiments implied that both capsicum and lettuce grown under C-RZT indeed had higher assimilation rates of 14C and their younger developing leaves exhibited greater sink strength [24]. Further studies concluded that plant growth was the result of interaction between source leaves and carbon partitioning among competitive sinks [25–27]. It was interesting to note that A-RZT lettuce had higher fresh weight and dry weight root/shoot ratio than that of C-RZT lettuce [19, 23]. This suggested that more photoassimilate may be distributed to the lettuce roots under hot A-RZT conditions. The RZT transfer experiments confirmed that A-RZT induced greater levels of 14C delivered to the lettuce root system. However, it was surprising that the high 14C translocated to the roots of A-RZT lettuce which was not accompanied by a greater root development [19, 24]. This may be attributed to the higher respiration rates in the roots which may require energy for the active uptake of water and nutrients in a poorly developed root system [26, 28]. The high rate of

RZT lettuce, developed new and finer root (**Figure 2D**).

**Figure 2.** Total root length (A), total number of root tips (B), total surface area (C), and average root diameter (D) of lettuce (*Lactuca sativa* L. "Panama") during different RZT treatments: C-RZT (•), A-RZT (▴), C → A-RZT (❍), and A → C-RZT (Δ). RZT treatments were started 3 weeks after transplanting. Means with different letters are statistically different (*P* < 0.05; *n* = 6) as determined by Tukey's multiple comparison test. Redrawn from Tan [19].

after C → A-RZT transfer (**Figure 2B**). The total root tip number of A-RZT lettuce did not change significantly throughout the whole experiment. When lettuce was transferred from A → C-RZT, there was rapid increase in the total root tip number. The root tip number of A → C-RZT lettuce became significantly higher than A-RZT lettuce by day 6 after RZT transfer. At day 14 after RZT transfer, total root tip number of A → C-RZT lettuce was 2.8 times higher than A-RZT plants. Responses of total surface area of lettuce plants to different RZTs (**Figure 2C**) were similar to those of total root length (**Figure 2B**). Similar to those of capsicum plants (**Figure 1D**), lettuce grown at A-RZT had the highest average root diameter while C-RZT plants had the least (**Figure 2D**). The average root diameter of C → A-RZT lettuce remained similar to C-RZT plants during the first 10 days of RZT transition. However, unlike capsicum plants (**Figure 1D**), there was slight root thickening in C → A-RZT lettuce roots after 2 weeks of RZT transfer. By day 14 after RZT transfer, the average root diameter of C → A-RZT lettuce was much thicker than C-RZT plants. Similar to those of capsicum plants, A → C-RZT lettuce, developed new and finer root (**Figure 2D**).

smaller (**Figure 1D**). It is surprising to note that there were no increases in root diameter after transferring plants from C-RZT to A-RZT (C → A-RZT) as capsicum plants were similar to those of C-RZT plants throughout the experiment. However, A → C-RZT transfer resulted in the development of new and finer roots in A → C-RZT plants. Thus, the average root diameter of A → C-RZT capsicum was significantly smaller than A-RZT by day 6 after RZT transfer (**Figure 1D**). Similar to capsicum plants, lettuce grown at C-RZT showed linear increase in total root length over the 2-week period (**Figure 2A**). Unlike the capsicum plants (**Figure 1A**), increase in total root length was also observed in C → A-RZT lettuce plant and the increase rate was similar to C-RZT lettuce plants from day 0 to day 8 after RZT transfer. However, C → A-RZT lettuce plants maintained their total root length of about 600 cm from day 10 to day 14 after RZT transfer. There was no significant increase in total root length of A-RZT lettuce over the 2-week experimental period. Again, unlike the capsicum plants (**Figure 1A**), A → C-RZT lettuce exhibited increase in total root length only after 8 days of RZT transfer (**Figure 2A**). The total root tip number of C-RZT lettuce was consistently higher than lettuce grown at other RZTs (**Figure 2B**). Unlike the capsicum plants (**Figure 1B**), there was an increase in total root tip number for C → A-RZT lettuce although the net increase decreased significantly

134 Plant Growth

**Figure 2.** Total root length (A), total number of root tips (B), total surface area (C), and average root diameter (D) of lettuce (*Lactuca sativa* L. "Panama") during different RZT treatments: C-RZT (•), A-RZT (▴), C → A-RZT (❍), and A → C-RZT (Δ). RZT treatments were started 3 weeks after transplanting. Means with different letters are statistically differ-

ent (*P* < 0.05; *n* = 6) as determined by Tukey's multiple comparison test. Redrawn from Tan [19].

Root morphological analysis of both subtropical and temperate vegetables revealed that high RZT inhibited root elongation, branching, and hair formation but increased root diameter. These were also observed in other plant species [16, 17]. However, effects of RZT transfer on morphology were different between capsicum and lettuce plants (**Figures 1** and **2**). These finding suggest that effects of RZT on root morphology is species-dependent. It was also reported that root length and diameter appeared to be inversely related to a study using *Secale cereale* seedlings [20]. Root thickening, or an increase in diameter, was controlled through signals emanating from shoot apices and root tips [21]. Root thickening may also be accompanied by associated changes in microfibril angles within expanding cell walls [21]. The chemical signals involved in root thickening may be ethylene [22]. The role of ethylene in inhibition of root elongation and root thickening was further confirmed by our team [13]. Our recent study with a recombinant inbred line (RIL) of lettuce and its parental lines (*Lactusa serriola* × *Lactuca sativa* "Salinas") that were grown in a tropical greenhouse under 24°C-RZT and hot A-RZT showed that higher RZ ethylene concentrations accumulated in A-RZT plants compared to that of 24°C-RZT plants. Lowest RZ ethylene concentration corresponded with highest shoot fresh weight [23]. Our results indicated that the presence of an ethylene inhibitor promoted root elongation at high RZT of 38°C. Without ethylene inhibitor, root elongation at high RZT was significantly inhibited. Our previous 14C feeding experiments implied that both capsicum and lettuce grown under C-RZT indeed had higher assimilation rates of 14C and their younger developing leaves exhibited greater sink strength [24]. Further studies concluded that plant growth was the result of interaction between source leaves and carbon partitioning among competitive sinks [25–27]. It was interesting to note that A-RZT lettuce had higher fresh weight and dry weight root/shoot ratio than that of C-RZT lettuce [19, 23]. This suggested that more photoassimilate may be distributed to the lettuce roots under hot A-RZT conditions. The RZT transfer experiments confirmed that A-RZT induced greater levels of 14C delivered to the lettuce root system. However, it was surprising that the high 14C translocated to the roots of A-RZT lettuce which was not accompanied by a greater root development [19, 24]. This may be attributed to the higher respiration rates in the roots which may require energy for the active uptake of water and nutrients in a poorly developed root system [26, 28]. The high rate of respiration may have taken place at the expense of root development. It is highly likely that there exists a negative feedback mechanism among root respiration, water uptake, nutrient absorption, root morphology, and high RZT. Our results showed that capsicum grown at C-RZT had lower root/shoot ratios than A-RZT plants [19]. C-RZT resulted in longer total root length, greater total root tip number, larger root surface area, and smaller root diameter in both lettuce and capsicum plants. However, the trend of the capsicum root/shoot ratios was different from that of lettuce because the capsicum grown at C-RZT had higher root/shoot ratio than A-RZT capsicum [19]. C → A-RZT transfer also caused a decline in root/shoot ratio while A → C-RZT resulted in an increase in root/shoot ratios of capsicum. These suggested more photoassimilate partitioning to roots of C-RZT capsicum and less so for A-RZT capsicum. This was further supported by the 14C feeding experiments which showed that more 14C was found in the roots of C-RZT capsicum compared to the roots of A-RZT capsicum. The transfer from C → A-RZT also resulted in decreased 14C found in C → A-RZT capsicum roots. The reverse was demonstrated in A → C-RZT capsicum which had higher root 14C than A-RZT plants. Therefore, lettuce grown at A-RZT distributed more 14C to their roots while capsicum grown at C-RZT distributed more 14C to the roots [24].

**Vegetable species 25°C-RZT A-RZT**

Predawn ψLeaf (Mpa) (*n* = 6) \* −0.51 ± 0.070 −2.95 ± 0.18 Midday ψLeaf (Mpa) (*n* = 6) \* −1.45 ± 0.092 −4.87 ± 0.24

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Predawn ψLeaf (Mpa) (*n* = 6) \* −0.21 ± 0.03 −1.07 ± 0.06 Midday ψLeaf (Mpa) (*n* = 6) \* −0.85 ± 0.07 −2.03 ± 0.12

**Vegetable species Cool-RZT\* A-RZT**

Predawn RWC (%) (*n* = 6)# 93.2 ± 0.22 80.3 ± 0.28 Midday RWC (%) (*n* = 6)# 81.3 ± 0.31 51.7 ± 0.29

Predawn RWC (%) (*n* = 6)# 97.6 ± 0.23 85.4± 0.16 Midday RWC (%) (*n* = 6)# 85.5 ± 0.27 64.9 ± 0.12

Predawn RWC (%) (*n* = 6)# 95.1 ± 0.17 88.8 ± 0.35 Midday RWC (%) (*n* = 6)# 89.2 ± 0.29 60.1 ± 0.46

**Table 2.** Leaf RWC of subtropical capsicum (*Capsicum annuum* Indra F1-hybrid) [19] Chinese broccoli (*Brassica alboglabra* Bailey) [19], and temperate vegetable crops of lettuce [39] grown at two different RZTs while their shoots were maintained at fluctuating ambient temperatures under 100% prevailing solar radiation. All the measurements

Based on the results shown in **Tables 1** and **2**, it is obviously that subtropical and temperate vegetables grown in the tropical greenhouse had experienced permanent water deficit (reduced predawn ψshoot or/and leaf RWC) and midday (lower midday ψshoot or/and leaf RWC) when they were grown at A-RZT. In another experiment with capsicum, leaf *gs*, root hydraulic conductivity, and shoot ψshoot declined after transferring plants from 20°C-RZT to A-RZT [42]. It was explained that supraoptimal RZTs caused a reduction in root hydraulic conductivity and might lower ψshoot, which in turn could cause stomatal closure [42]. However, water deficit

\* Cool-RZT, 25°C-RZT for capsicum and Chinese broccoli; 20°C-RZT for lettuce.

were done after 30 days of transplanting. Values shown are means ± standard deviation.

in 20 or 25°C-RZT plants was alleviated due to the larger root system [19].

Significant interaction between the two RZTs at *P* < 0.01.

**Table 1.** ψLeaf of subtropical vegetable crops of capsicum [19] and Chinese broccoli (*Brassica alboglabra* Bailey) [38] grown at two different RZTs while their shoots were maintained at fluctuating ambient temperatures under 100% prevailing solar radiation. All the measurements were done after 30 days of transplanting. Values shown are means ±

Capsicum

Chinese broccoli

standard deviation.

Capsicum

Lettuce

#

Chinese broccoli

\* Significant interaction between the two RZTs at *P* < 0.01.

## **3. Effects of root-zone temperature on water relations**

Changes of root morphology are important for maximizing water and dissolve mineral uptake [1, 11, 29–31]. Plant roots can alter not only their morphological but also their physiological characteristics to meet changes in shoot water and nutrient demand in response to environmental stress [29]. Manipulation of RZT resulted in changes of root morphology, water, and mineral uptake and transport [12, 32–35]. For plants grown in soil, high RZT causes not only poor root growth and development [36] but also results in spatial water and mineral nutrient availability in soil [37] and reduces uptake efficiency per unit root length [36, 37]. In our aeroponic system, plant roots are continuously sprayed with nutrient mist, and there is no spatial variation in water and mineral nutrient availability. However, water deficits and mineral deficiency occurred at high A-RZT due to poor root system development and lower rate of water uptake [9, 10, 12].

We have previously reported that supra-optimal A-RZT caused shoot water deficit by altering the balance between water uptake by the root system and water loss from the shoot [38, 39]. Water deficits resulting in stomata closure measured by stomatal conductance are due to the direct effect of reduced shoot water potential (ψshoot) and leaf relative water content (RWC) [38– 42]. **Table 1** summaries ψshoot of subtropical vegetable crops of capsicum (*C. annuum* Indra F1 hybrid) and Chinese broccoli (*Brassica alboglabra* Bailey) [38] grown at 25°C-RZT and A-RZT while their shoots were maintained at fluctuating ambient temperatures. **Table 1** shows predawn and midday ψshoot were higher in 25°C-RZT than A-RZT plants in both vegetable species. Leaf RWC were further determined in both subtropical (capsicum and Chinese broccoli) and temperate vegetables (lettuce). Similar to the results of ψshoot, RWC was significantly lower in all plants grown at A-RZT that at C-RZT measured predawn and midday (**Table 2**).

**Table 1.** ψLeaf of subtropical vegetable crops of capsicum [19] and Chinese broccoli (*Brassica alboglabra* Bailey) [38] grown at two different RZTs while their shoots were maintained at fluctuating ambient temperatures under 100% prevailing solar radiation. All the measurements were done after 30 days of transplanting. Values shown are means ± standard deviation.

\* Cool-RZT, 25°C-RZT for capsicum and Chinese broccoli; 20°C-RZT for lettuce. # Significant interaction between the two RZTs at *P* < 0.01.

respiration may have taken place at the expense of root development. It is highly likely that there exists a negative feedback mechanism among root respiration, water uptake, nutrient absorption, root morphology, and high RZT. Our results showed that capsicum grown at C-RZT had lower root/shoot ratios than A-RZT plants [19]. C-RZT resulted in longer total root length, greater total root tip number, larger root surface area, and smaller root diameter in both lettuce and capsicum plants. However, the trend of the capsicum root/shoot ratios was different from that of lettuce because the capsicum grown at C-RZT had higher root/shoot ratio than A-RZT capsicum [19]. C → A-RZT transfer also caused a decline in root/shoot ratio while A → C-RZT resulted in an increase in root/shoot ratios of capsicum. These suggested more photoassimilate partitioning to roots of C-RZT capsicum and less so for A-RZT capsicum. This was further supported by the 14C feeding experiments which showed that more 14C was found in the roots of C-RZT capsicum compared to the roots of A-RZT capsicum. The transfer from C → A-RZT also resulted in decreased 14C found in C → A-RZT capsicum roots. The reverse was demonstrated in A → C-RZT capsicum which had higher root 14C than A-RZT plants. Therefore, lettuce grown at A-RZT distributed more 14C to their roots while capsicum grown at C-

Changes of root morphology are important for maximizing water and dissolve mineral uptake [1, 11, 29–31]. Plant roots can alter not only their morphological but also their physiological characteristics to meet changes in shoot water and nutrient demand in response to environmental stress [29]. Manipulation of RZT resulted in changes of root morphology, water, and mineral uptake and transport [12, 32–35]. For plants grown in soil, high RZT causes not only poor root growth and development [36] but also results in spatial water and mineral nutrient availability in soil [37] and reduces uptake efficiency per unit root length [36, 37]. In our aeroponic system, plant roots are continuously sprayed with nutrient mist, and there is no spatial variation in water and mineral nutrient availability. However, water deficits and mineral deficiency occurred at high A-RZT due to poor root system development and lower

We have previously reported that supra-optimal A-RZT caused shoot water deficit by altering the balance between water uptake by the root system and water loss from the shoot [38, 39]. Water deficits resulting in stomata closure measured by stomatal conductance are due to the direct effect of reduced shoot water potential (ψshoot) and leaf relative water content (RWC) [38– 42]. **Table 1** summaries ψshoot of subtropical vegetable crops of capsicum (*C. annuum* Indra F1 hybrid) and Chinese broccoli (*Brassica alboglabra* Bailey) [38] grown at 25°C-RZT and A-RZT while their shoots were maintained at fluctuating ambient temperatures. **Table 1** shows predawn and midday ψshoot were higher in 25°C-RZT than A-RZT plants in both vegetable species. Leaf RWC were further determined in both subtropical (capsicum and Chinese broccoli) and temperate vegetables (lettuce). Similar to the results of ψshoot, RWC was significantly lower in all plants grown at A-RZT that at C-RZT measured predawn and midday

RZT distributed more 14C to the roots [24].

136 Plant Growth

rate of water uptake [9, 10, 12].

(**Table 2**).

**3. Effects of root-zone temperature on water relations**

**Table 2.** Leaf RWC of subtropical capsicum (*Capsicum annuum* Indra F1-hybrid) [19] Chinese broccoli (*Brassica alboglabra* Bailey) [19], and temperate vegetable crops of lettuce [39] grown at two different RZTs while their shoots were maintained at fluctuating ambient temperatures under 100% prevailing solar radiation. All the measurements were done after 30 days of transplanting. Values shown are means ± standard deviation.

Based on the results shown in **Tables 1** and **2**, it is obviously that subtropical and temperate vegetables grown in the tropical greenhouse had experienced permanent water deficit (reduced predawn ψshoot or/and leaf RWC) and midday (lower midday ψshoot or/and leaf RWC) when they were grown at A-RZT. In another experiment with capsicum, leaf *gs*, root hydraulic conductivity, and shoot ψshoot declined after transferring plants from 20°C-RZT to A-RZT [42]. It was explained that supraoptimal RZTs caused a reduction in root hydraulic conductivity and might lower ψshoot, which in turn could cause stomatal closure [42]. However, water deficit in 20 or 25°C-RZT plants was alleviated due to the larger root system [19].

## **4. Effects of root-zone temperature on NO3− uptake and assimilation**

NO3− is the major N source available in aerobic soils [43]. Once uptaken by root cells, NO3− can be redirected out of the root cell, either by extrusion into the external medium or by unloading into the xylem vessels to reach the aerial organs [29]. The third possible fate for NO3−, in roots as well as in leaves, is its uptake by the vacuole where it participates in the general osmoticum or serves as a reservoir to sustain the growth process when the external nitrogen supply becomes limiting [44].

with air temperature maintained at 22–24/18–20°C (day/night), for a period of 8 weeks. Their results showed that leaf N concentration increased with the increase of RZT from 12 to 36°C [51]. Cruz et al. grew Carob (*Ceratonia siliqua* L.) seedlings at different RZTs (10–40°C) with shoot temperature at 24 ± 1/20 ± 1°C (day/night). When nitrate (3 mM) was supplied as nitrogen source of nutrient solutions, they found that organic N concentration in roots increased with the increase of RZT from 10 to 40°C. Total N concentration in both shoot and toot increased with the increased RZT from 10 to 25°C. Investigators explained that increasing RZTs induced increased in ion-uptake rates, mainly nitrogen, might be responsible for the higher N content in plant. This situation increased also the demand for carbon in the root of Carob plant [52]. As discussed earlier, the amount of mineral nutrient especially nitrogen available to a plant is determined by the root morphological structure [3, 12, 13]. The amount of mineral nutrient uptake and transport also depend on transpiration rate as long-distance movement of nutrient

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For most higher plants, NO3− assimilation is the major pathway by which inorganic N is converted to an organic form [54]. The conversion of NO3− to NH4+, which can be directly used to synthesize organic N containing compounds, is a reduction process that occurs in two steps. NO3− is first reduced to NO2− in the cytosol by nitrate reductase (NR). NO2− is then translocated to the chloroplast where it is reduced into NH4+ by nitrite reductase (NiR). NR is NO3− inducible enzyme. Transcription of NR genes is induced by NO3− [54–56]. The reduction of NO3− could take place either in roots or in leaves or both [54, 56, 59]. Significant translocation of NO3− to the shoot would occur only when the net NO3− uptake rate was fast enough to

In our study of subtropical vegetable Nai Bai (*Brachyponera chinensis* L.) plant and Baby butterhead lettuce (*L. sativa* L.) plants, NO3− concentrations of leaf and root and maximum nitrate reductase activity (NRA) of leaf and roots were determined after 10, 20, and 30 days of

For Nai Bai plants, the leaf NO3− concentrations of either 25°C-RZT or A-RZT plants were similar at 10, 20, and 30 days after transplanting (DAT). However, the leaf NO3− concentrations of A-RZT plants were significantly lower than those of 25°C-RZT plants (**Figure 3A**). Regardless of growth stage, 25°C-RZT plants had significantly higher root NO3− concentrations than those of A-RZT plants (**Figure 3C**). Root NO3− concentrations of A-RZT remained constant at all growth stages (**Figure 3C**). However, 25°C-RZT plants had lower root NO3− concentrations at 10 DAT compared to those at 20 and 30 DAT (**Figure 3C**). At 10 DAT, leaf NRA of 25°C-RZT plants was significantly lower than those of A-RZT plants (**Figure 3B**). However, it increased from 10 to 20 DAT, and remained constant from 20 to 30 DAT. Leaf maximum NRA of A-RZT plants decreased from 10 to 20 DAT, and remained constant from 20 to 30 DAT. Therefore, at 20 and 30 DAT, maximum leaf NRA of 25°C-RZT plants was much higher than those of A-RZT plants (**Figure 3B**). Root maximum NRA of 25°C-RZT and A-RZT plants decreased from 10 to 20 DAT, and then remained constant to the 30 DAT. At 10 DAT, no significant difference in maximum root NRA was measured between 25°C-RZT and A-RZT plants. However, at 20 and 30 DAT, maximum root NRA of 25°C-RZT plants was significantly higher than those of A-RZT

through plants is predominantly by bulk flow in the xylem [53].

saturate the reduction process in the roots.

transplanting (**Figures 3** and **4**).

plants (**Figure 3D**).

The effects of RZT on NO3− and N contents in plants have been reported [45–47]. N contents were reduced in plants grown under high RZT [39, 48–50]. For instance, Du and Tachibana [48] grew cucumber (cv. "Sharp I") plants hydroponically at several RZTs: 25 (control), 30, 35, or 38°C, with shoot's temperature at 26/23 ± 3°C (day/night). Total N concentration in leaf was reduced as the RZT was raised to 35°C and to 38°C in particular. Similar results were obtained our research team with lettuce (cv. "Palma") plants [39]. Leaf organic N content was 32% lower in A-RZT plants than 20°C-RZT plants. Transfer of plants between these two RZTs altered leaf N content after 6 days. Leaf N content increased in A → 20°C-RZT plants, while decreased in 20°C → A-RZT plants. After 10 days of reciprocal RZT transfer, A → 20°C-RZT plants and 20°C → A-RZT plants had a similar leaf N content [39]. The results also showed tight temporal coupling of leaf N content, and light- and CO2- saturated photosynthetic O2 evolution rate throughout the reciprocal temperature transfers. He et al. [39] suggested that the decreased nutrient status of lettuce plants caused nonstomatal limitation of photosynthesis under high A-RZT conditions. In lettuce (cv. "Panama"), Tan et al., [12] reported that 20°C-RZT plants had higher leaf N concentrations on the basis of per unit dry weight compared with plants grown at A-RZT. Total shoot and root NO3 of 20°C-RZT plants were higher than A-RZT plants. 20°C → A-RZT plants suffered from a reduction of total mineral accumulation, and A → 20°C-RZT plants increased in total mineral accumulation [12]. In another study, Yeager et al. grew *Ilex crenata* Thunb. "Rotundifolia" plants in sand culture with the RZTs at 28, 34, or 40°C for 6 h daily. They found that root and shoot N accumulation (mg N/g dry weight) decreased when RZTs were increased from 28 to 40°C. These plants were fertilized twice daily with 500 mL of either 75, 150, or 225 mg N/L, to determine applied N rate on growth and N accumulation of "Rotundifolia" holly. The results showed that root and shoot N accumulation depended on RZT and the N rate. N accumulated by roots and shoots increased when the applied N concentration increased from 75 to 225 mg/L at each RZT. However, root and shoot N accumulation decreased with the root zone at 40°C compared with 28°C for 75 and 225 mg N/L applied. These data indicated that increased N fertilization rates would not alleviate growth reductions of holly caused by high RZT (40°C) [49].

In contrast, high RZT increased N content in plants has also been reported [49–51]. Johnson and Ingram exposed root systems of *Pittosporum tobira* Thunb plants to temperatures of 27, 30, or 40°C for 6 h daily for 7 months with air temperature around 30/24°C (day/night). They found that N level in leaf tissue (newly expanded leaves) was increased in plants at 40°C medium temperature [50]. This was supported by the work of Gosselin and Trudel who transferred the 10-week-old pepper (cv. "Bell Boy") plants to five different RZTs (12, 18, 24, 30, or 36 ± 2°C) with air temperature maintained at 22–24/18–20°C (day/night), for a period of 8 weeks. Their results showed that leaf N concentration increased with the increase of RZT from 12 to 36°C [51]. Cruz et al. grew Carob (*Ceratonia siliqua* L.) seedlings at different RZTs (10–40°C) with shoot temperature at 24 ± 1/20 ± 1°C (day/night). When nitrate (3 mM) was supplied as nitrogen source of nutrient solutions, they found that organic N concentration in roots increased with the increase of RZT from 10 to 40°C. Total N concentration in both shoot and toot increased with the increased RZT from 10 to 25°C. Investigators explained that increasing RZTs induced increased in ion-uptake rates, mainly nitrogen, might be responsible for the higher N content in plant. This situation increased also the demand for carbon in the root of Carob plant [52]. As discussed earlier, the amount of mineral nutrient especially nitrogen available to a plant is determined by the root morphological structure [3, 12, 13]. The amount of mineral nutrient uptake and transport also depend on transpiration rate as long-distance movement of nutrient through plants is predominantly by bulk flow in the xylem [53].

**4. Effects of root-zone temperature on NO3− uptake and assimilation**

nitrogen supply becomes limiting [44].

138 Plant Growth

reductions of holly caused by high RZT (40°C) [49].

NO3− is the major N source available in aerobic soils [43]. Once uptaken by root cells, NO3− can be redirected out of the root cell, either by extrusion into the external medium or by unloading into the xylem vessels to reach the aerial organs [29]. The third possible fate for NO3−, in roots as well as in leaves, is its uptake by the vacuole where it participates in the general osmoticum or serves as a reservoir to sustain the growth process when the external

The effects of RZT on NO3− and N contents in plants have been reported [45–47]. N contents were reduced in plants grown under high RZT [39, 48–50]. For instance, Du and Tachibana [48] grew cucumber (cv. "Sharp I") plants hydroponically at several RZTs: 25 (control), 30, 35, or 38°C, with shoot's temperature at 26/23 ± 3°C (day/night). Total N concentration in leaf was reduced as the RZT was raised to 35°C and to 38°C in particular. Similar results were obtained our research team with lettuce (cv. "Palma") plants [39]. Leaf organic N content was 32% lower in A-RZT plants than 20°C-RZT plants. Transfer of plants between these two RZTs altered leaf N content after 6 days. Leaf N content increased in A → 20°C-RZT plants, while decreased in 20°C → A-RZT plants. After 10 days of reciprocal RZT transfer, A → 20°C-RZT plants and 20°C → A-RZT plants had a similar leaf N content [39]. The results also showed tight temporal coupling of leaf N content, and light- and CO2- saturated photosynthetic O2 evolution rate throughout the reciprocal temperature transfers. He et al. [39] suggested that the decreased nutrient status of lettuce plants caused nonstomatal limitation of photosynthesis under high A-RZT conditions. In lettuce (cv. "Panama"), Tan et al., [12] reported that 20°C-RZT plants had higher leaf N concentrations on the basis of per unit dry weight compared with plants grown at A-RZT. Total shoot and root NO3 of 20°C-RZT plants were higher than A-RZT plants. 20°C → A-RZT plants suffered from a reduction of total mineral accumulation, and A → 20°C-RZT plants increased in total mineral accumulation [12]. In another study, Yeager et al. grew *Ilex crenata* Thunb. "Rotundifolia" plants in sand culture with the RZTs at 28, 34, or 40°C for 6 h daily. They found that root and shoot N accumulation (mg N/g dry weight) decreased when RZTs were increased from 28 to 40°C. These plants were fertilized twice daily with 500 mL of either 75, 150, or 225 mg N/L, to determine applied N rate on growth and N accumulation of "Rotundifolia" holly. The results showed that root and shoot N accumulation depended on RZT and the N rate. N accumulated by roots and shoots increased when the applied N concentration increased from 75 to 225 mg/L at each RZT. However, root and shoot N accumulation decreased with the root zone at 40°C compared with 28°C for 75 and 225 mg N/L applied. These data indicated that increased N fertilization rates would not alleviate growth

In contrast, high RZT increased N content in plants has also been reported [49–51]. Johnson and Ingram exposed root systems of *Pittosporum tobira* Thunb plants to temperatures of 27, 30, or 40°C for 6 h daily for 7 months with air temperature around 30/24°C (day/night). They found that N level in leaf tissue (newly expanded leaves) was increased in plants at 40°C medium temperature [50]. This was supported by the work of Gosselin and Trudel who transferred the 10-week-old pepper (cv. "Bell Boy") plants to five different RZTs (12, 18, 24, 30, or 36 ± 2°C) For most higher plants, NO3− assimilation is the major pathway by which inorganic N is converted to an organic form [54]. The conversion of NO3− to NH4+, which can be directly used to synthesize organic N containing compounds, is a reduction process that occurs in two steps. NO3− is first reduced to NO2− in the cytosol by nitrate reductase (NR). NO2− is then translocated to the chloroplast where it is reduced into NH4+ by nitrite reductase (NiR). NR is NO3− inducible enzyme. Transcription of NR genes is induced by NO3− [54–56]. The reduction of NO3− could take place either in roots or in leaves or both [54, 56, 59]. Significant translocation of NO3− to the shoot would occur only when the net NO3− uptake rate was fast enough to saturate the reduction process in the roots.

In our study of subtropical vegetable Nai Bai (*Brachyponera chinensis* L.) plant and Baby butterhead lettuce (*L. sativa* L.) plants, NO3− concentrations of leaf and root and maximum nitrate reductase activity (NRA) of leaf and roots were determined after 10, 20, and 30 days of transplanting (**Figures 3** and **4**).

For Nai Bai plants, the leaf NO3− concentrations of either 25°C-RZT or A-RZT plants were similar at 10, 20, and 30 days after transplanting (DAT). However, the leaf NO3− concentrations of A-RZT plants were significantly lower than those of 25°C-RZT plants (**Figure 3A**). Regardless of growth stage, 25°C-RZT plants had significantly higher root NO3− concentrations than those of A-RZT plants (**Figure 3C**). Root NO3− concentrations of A-RZT remained constant at all growth stages (**Figure 3C**). However, 25°C-RZT plants had lower root NO3− concentrations at 10 DAT compared to those at 20 and 30 DAT (**Figure 3C**). At 10 DAT, leaf NRA of 25°C-RZT plants was significantly lower than those of A-RZT plants (**Figure 3B**). However, it increased from 10 to 20 DAT, and remained constant from 20 to 30 DAT. Leaf maximum NRA of A-RZT plants decreased from 10 to 20 DAT, and remained constant from 20 to 30 DAT. Therefore, at 20 and 30 DAT, maximum leaf NRA of 25°C-RZT plants was much higher than those of A-RZT plants (**Figure 3B**). Root maximum NRA of 25°C-RZT and A-RZT plants decreased from 10 to 20 DAT, and then remained constant to the 30 DAT. At 10 DAT, no significant difference in maximum root NRA was measured between 25°C-RZT and A-RZT plants. However, at 20 and 30 DAT, maximum root NRA of 25°C-RZT plants was significantly higher than those of A-RZT plants (**Figure 3D**).

**Figure 3.** Leaf NO3 − concentration (A) and maximum NRA (B), root NO3 − concentration (C) and maximum NRA (D) of Nai Bai (*Brachyponera chinensis* L.) plant. Each point is the mean of five measurements of five different leaves from two different bins. Vertical bars represent the standard errors. Means with different letters above the columns are statistically different (*P* < 0.001) as determined by Tukey's multiple comparison test (unpublished data).

concentration in A-RZT roots (data not shown). Laurie and Stewart [60] grew cheakpea (*Cicer arietinum* L. ICARDA cultivar no. ILC 482) plants at high (40/25°C, day/night), and concentration in A-RZT roots, and moderate (25/17°C, day/night) temperature regimes. The results showed that high temperature (40/25°C) reduced shoot NRA (*in vivo*) but had little effect on

Baby butter head lettuce (*Lactuca sativa* L.) plant. Each point is the mean of five measurements of five different leaves from two different bins. Vertical bars represent the standard errors. Means with different letters above the columns are

−

Root Growth, Morphological and Physiological Characteristics of Subtropical and Temperate Vegetable...

concentration (C), and maximum NRA (D) of

http://dx.doi.org/10.5772/64824

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concentration (A) and maximum NRA (B), root NO3

statistically different (*P* < 0.001) as determined by Tukey's multiple comparison test (unpublished data).

On a total plant basis, high temperature growth shifted NRA from shoot to root, particularly in the young plant. Laurie and Stewart [60] also observed that there was a greater decline in leaf NRA with age [60]. This was in accordance with the present finding that leaf maximum NRA of 20°C-RZT Baby butterhead lettuce plants was highest in expanding young leaves (10 DAT) and lowest in fully matured leaves at 30 DAT (**Figure 4B**). Although leaf maximum NRA of A-RZT Baby butterhead lettuce plants decreased with the growth of plants, the decrease of NRA from 20 DAT to 30 DAT was not caused by the leaf age. Because the youngest fully expanded leaves were selected for the NRA analysis at both growth stages. Therefore, the repress of leaf maximum NRA in A-RZT plants at the late growth stage may be due to NR protein degradation caused by long-term supraoptimum RZT treatment. Although root maximum NRA was higher in A-RZT Baby butterhead lettuce plants compared to those of 20°C-RZT plants, the lower NO3− uptake and transport capacity (lower root and leaf NO3− concentration) still restricted NO3− reduction of A-RZT plants. This was evidenced by the lower

root NRA [57].

**Figure 4.** Leaf NO3

−

The leaf NO3− concentrations of 20°C-RZT were significantly higher compared to those of A-RZT plants, at different growth stages (**Figure 4A**). With prolonged growth to 30 DAT, leaf and root NO3− concentrations of 20°C-RZT plants increased (**Figure 3A**). However, there was no significant difference in leaf and root NO3− concentrations at A-RZT among the three growth stages (**Figure 4C**). Leaf maximum NRA in 20°C-ZT and A-RZT plants was highest at 10 DAT and decreased at 20 DAT and further decreased at 30 DAT (**Figure 4A**). Regardless of growth stage, leaf maximum NRA was significantly higher in 20°C-ZT than in A-RZT plants (**Figure 4B**). It was surprise to observe that root maximum NRA was many times higher in A-RZT plants than in 20°C-RZT plants, indicating that RZT altered the site of NO3− assimilation (**Figure 4D**).

The reduction of NO3− could take place either in roots, in leaves, or in both [57, 58]. For Baby butterhead plants, hot A-RZT treatment switched NO3− reduction from shoot to root, evidenced by the higher NRA in A-RZT roots than in leaves (**Figure 4D**) while NRA of 20°C-RZT leaves was much higher than that of roots (**Figure 4B**). However, this was not seen in Nai Bai plants (**Figure 3D**). High NO3− assimilation rate in the root may be due to low reduced N

Root Growth, Morphological and Physiological Characteristics of Subtropical and Temperate Vegetable... http://dx.doi.org/10.5772/64824 141

**Figure 4.** Leaf NO3 − concentration (A) and maximum NRA (B), root NO3 − concentration (C), and maximum NRA (D) of Baby butter head lettuce (*Lactuca sativa* L.) plant. Each point is the mean of five measurements of five different leaves from two different bins. Vertical bars represent the standard errors. Means with different letters above the columns are statistically different (*P* < 0.001) as determined by Tukey's multiple comparison test (unpublished data).

**Figure 3.** Leaf NO3

140 Plant Growth

(**Figure 4D**).

−

concentration (A) and maximum NRA (B), root NO3

cally different (*P* < 0.001) as determined by Tukey's multiple comparison test (unpublished data).

Nai Bai (*Brachyponera chinensis* L.) plant. Each point is the mean of five measurements of five different leaves from two different bins. Vertical bars represent the standard errors. Means with different letters above the columns are statisti-

The leaf NO3− concentrations of 20°C-RZT were significantly higher compared to those of A-RZT plants, at different growth stages (**Figure 4A**). With prolonged growth to 30 DAT, leaf and root NO3− concentrations of 20°C-RZT plants increased (**Figure 3A**). However, there was no significant difference in leaf and root NO3− concentrations at A-RZT among the three growth stages (**Figure 4C**). Leaf maximum NRA in 20°C-ZT and A-RZT plants was highest at 10 DAT and decreased at 20 DAT and further decreased at 30 DAT (**Figure 4A**). Regardless of growth stage, leaf maximum NRA was significantly higher in 20°C-ZT than in A-RZT plants (**Figure 4B**). It was surprise to observe that root maximum NRA was many times higher in A-RZT plants than in 20°C-RZT plants, indicating that RZT altered the site of NO3− assimilation

The reduction of NO3− could take place either in roots, in leaves, or in both [57, 58]. For Baby butterhead plants, hot A-RZT treatment switched NO3− reduction from shoot to root, evidenced by the higher NRA in A-RZT roots than in leaves (**Figure 4D**) while NRA of 20°C-RZT leaves was much higher than that of roots (**Figure 4B**). However, this was not seen in Nai Bai plants (**Figure 3D**). High NO3− assimilation rate in the root may be due to low reduced N

−

concentration (C) and maximum NRA (D) of

concentration in A-RZT roots (data not shown). Laurie and Stewart [60] grew cheakpea (*Cicer arietinum* L. ICARDA cultivar no. ILC 482) plants at high (40/25°C, day/night), and concentration in A-RZT roots, and moderate (25/17°C, day/night) temperature regimes. The results showed that high temperature (40/25°C) reduced shoot NRA (*in vivo*) but had little effect on root NRA [57].

On a total plant basis, high temperature growth shifted NRA from shoot to root, particularly in the young plant. Laurie and Stewart [60] also observed that there was a greater decline in leaf NRA with age [60]. This was in accordance with the present finding that leaf maximum NRA of 20°C-RZT Baby butterhead lettuce plants was highest in expanding young leaves (10 DAT) and lowest in fully matured leaves at 30 DAT (**Figure 4B**). Although leaf maximum NRA of A-RZT Baby butterhead lettuce plants decreased with the growth of plants, the decrease of NRA from 20 DAT to 30 DAT was not caused by the leaf age. Because the youngest fully expanded leaves were selected for the NRA analysis at both growth stages. Therefore, the repress of leaf maximum NRA in A-RZT plants at the late growth stage may be due to NR protein degradation caused by long-term supraoptimum RZT treatment. Although root maximum NRA was higher in A-RZT Baby butterhead lettuce plants compared to those of 20°C-RZT plants, the lower NO3− uptake and transport capacity (lower root and leaf NO3− concentration) still restricted NO3− reduction of A-RZT plants. This was evidenced by the lower total reduced N concentration of leaf and root of A-RZT plants compared to 20°C-RZT plants, especially at 20 and 30 DAT (data not shown). For Nai Bai plants, NO3− reduction took place both in leaves and roots. However, the major proportion (more than 80%) of total plant NO3− reduction occurred in leaves for both 25°C-RZT and A-RZT plants (**Figure 3C** and **D**). At very early growth stage (10 DAT), supraoptimum A-RZT did not decrease maximum NRA in leaves and roots of Nai Bai plants. Because of very low growth rate of the plant at this very early growth stage, the lower NO3− content in A-RZT plants did not limit NO3− reduction. As a result, both of total reduced N concentrations in leaves and roots of A-RZT Nai Bai plants remained at the similar levels to those of 25°C-RZT plants at 10 DAT (data not shown). However, at 10 DAT, sugar concentrations (especially, glucose, fructose, and starch) both in leaves and roots of A-RZT Nai Bai plants increased significantly compared to 25°C-RZT plants (data not shown). Sugar accumulation in roots may be considered as a general consequence of impaired growth [61]. These indicate that, for Nai Bai plants, carbohydrate metabolism is more sensitive to supraoptimum RZT than NO3− metabolism. Controversial results of effect of high RZTs on N metabolism in shoots and roots have been reported [38, 39, 59–61]. In our study, it was confirmed that RZTs significantly affect N metabolism both in leaves and roots of subtropical vegetable Nai Bai (*B. chinensis* L (**Figure 3**) and temperate vegetable Baby butter head lettuce (**Figure 4**).

RP 22/98 HJ, RP12/01, RI7/05 HJ, Ministry of Education, Singapore and Singapore Millennium

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[1] Malamy JE. Intrinsic and environmental response pathways that regulate root system architecture. Plant Cell, Environment. 2005; 28: 67–77, DOI: 10.1111/j.1365-3040.2005.

[2] He J. Mineral nutrition of aeroponically grown subtropical and temperate crops in the tropics with manipulation of root-zone temperature at different growth irradiances. Plant Stress. 2010; 4: 14–30, http://www.globalsciencebooks.info/JournalsSup/

[3] He J. Impact of root-zone temperature on photosynthetic efficiency of aeroponically grown temperate and subtropical vegetable crops in the tropics. In: Buchner Th. B. Ewingen NH, Editors. Theory and Applications in Energy, Biotechnology and Nanotechnology. Nova Science Publishers Inc.: New York; 2009. pp. 111–143, http://

[4] Hodge A, Berta G, Doussa C, Merchan F, Crespi M. Plant root growth, architecture and

[5] Kirk GJD, Van DL. Changes in rice root architecture, porosity, and oxygen and proton release under phosphorus deficiency. New Phytologist. 1997; 135: 191–200, DOI:

[6] Neumann G, Massonneau A, Martinoia E, Römheld V. Physiological adaptations to phosphorus deficiency during proteoid root development in white lupin. Planta. 1999;

[7] Xie Y, An S, Yao X, Xiao K, Zhang C. Short-time response in root morphology of *Vallisneria natans* to sediment type and water column nutrient. Aquatic Botany. 2005;

function. Plant Soil. 2009; 321: 153–187, DOI: 10.1007/s11104-009-9929-9.

208: 373–382, http://link.springer.com/article/10.1007%2Fs004250050572.

National Institute of Education, Nanyang Technological University, Singapore

trove.nla.gov.au/work/27991465?versionId=45343866. Ch 4.

Foundation (Project code: SMF-Farming System).

Address all correspondence to<EMAIL_ADDRESS>
**Author details**

**References**

01306.x/full.

10PS\_4\_SI2.html.

10.1046/j.1469-8137.1997.00640.x

81: 85–96, DOI: 10.1016/j.aquabot.2004.12.001.

Jie He

## **5. Conclusions**

Our studies showed that high A-RZT inhibited root elongation, branching, and hair formation but increased root diameter of subtropical and temperate vegetables grown in the tropics. However, cooling the RZ promoted root growth and development as well as shoot productivities of aeroponically grown temperate and subtropical vegetables in the tropics. Manipulation of rhizosphere environment can alter not only root morphology but also physiological characteristics of roots to meet changes in shoot water and nutrient demand in response to atmospheric high temperature. Hot A-RZT caused shoot water deficit of aeroponically grown plants with continual spraying nutrient solution due to the negative water balance between water uptake by the root system and water loss from the shoot. Thus, subtropical and temperate vegetables grown in the tropics greenhouse had experienced mid-day and permanent water deficit when they were grown at A-RZT. However, water deficit of C-RZT plants was alleviated due to the larger root system. Compared to subtropical and temperate vegetables grown at hot A-RZT, cooling the RZ enhanced NO3− uptake and its assimilation of shoots. However, effects of RZT on NO3− assimilation of roots depend on species. Adequate levels of nutrient, especially N of C-RZT plants, alleviated nonstomatal limitation of photosynthesis.

## **Acknowledgements**

All experiment work on subtropical and temperate vegetables grown aeroponically was supported by the Academic Research Fund (Grant reference numbers: RP10/95 HJ, RP 1/97 HJ, RP 22/98 HJ, RP12/01, RI7/05 HJ, Ministry of Education, Singapore and Singapore Millennium Foundation (Project code: SMF-Farming System).

## **Author details**

Jie He

total reduced N concentration of leaf and root of A-RZT plants compared to 20°C-RZT plants, especially at 20 and 30 DAT (data not shown). For Nai Bai plants, NO3− reduction took place both in leaves and roots. However, the major proportion (more than 80%) of total plant NO3− reduction occurred in leaves for both 25°C-RZT and A-RZT plants (**Figure 3C** and **D**). At very early growth stage (10 DAT), supraoptimum A-RZT did not decrease maximum NRA in leaves and roots of Nai Bai plants. Because of very low growth rate of the plant at this very early growth stage, the lower NO3− content in A-RZT plants did not limit NO3− reduction. As a result, both of total reduced N concentrations in leaves and roots of A-RZT Nai Bai plants remained at the similar levels to those of 25°C-RZT plants at 10 DAT (data not shown). However, at 10 DAT, sugar concentrations (especially, glucose, fructose, and starch) both in leaves and roots of A-RZT Nai Bai plants increased significantly compared to 25°C-RZT plants (data not shown). Sugar accumulation in roots may be considered as a general consequence of impaired growth [61]. These indicate that, for Nai Bai plants, carbohydrate metabolism is more sensitive to supraoptimum RZT than NO3− metabolism. Controversial results of effect of high RZTs on N metabolism in shoots and roots have been reported [38, 39, 59–61]. In our study, it was confirmed that RZTs significantly affect N metabolism both in leaves and roots of subtropical vegetable Nai Bai (*B. chinensis* L (**Figure 3**) and temperate vegetable Baby butter

Our studies showed that high A-RZT inhibited root elongation, branching, and hair formation but increased root diameter of subtropical and temperate vegetables grown in the tropics. However, cooling the RZ promoted root growth and development as well as shoot productivities of aeroponically grown temperate and subtropical vegetables in the tropics. Manipulation of rhizosphere environment can alter not only root morphology but also physiological characteristics of roots to meet changes in shoot water and nutrient demand in response to atmospheric high temperature. Hot A-RZT caused shoot water deficit of aeroponically grown plants with continual spraying nutrient solution due to the negative water balance between water uptake by the root system and water loss from the shoot. Thus, subtropical and temperate vegetables grown in the tropics greenhouse had experienced mid-day and permanent water deficit when they were grown at A-RZT. However, water deficit of C-RZT plants was alleviated due to the larger root system. Compared to subtropical and temperate vegetables grown at hot A-RZT, cooling the RZ enhanced NO3− uptake and its assimilation of shoots. However, effects of RZT on NO3− assimilation of roots depend on species. Adequate levels of nutrient, especially

All experiment work on subtropical and temperate vegetables grown aeroponically was supported by the Academic Research Fund (Grant reference numbers: RP10/95 HJ, RP 1/97 HJ,

N of C-RZT plants, alleviated nonstomatal limitation of photosynthesis.

head lettuce (**Figure 4**).

**Acknowledgements**

**5. Conclusions**

142 Plant Growth

Address all correspondence to<EMAIL_ADDRESS>
National Institute of Education, Nanyang Technological University, Singapore

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**Chapter 10**

**Provisional chapter**

**Molecular and Morphophysiological Analysis**

Drought is a major environmental stress factor that affects the growth and development of plants. Most of the physiological traits associated with drought tolerance are quantitative in nature. An important research strategy that has been widely used to deal with such complexity is to use molecular markers to identify quantitative trait loci (QTLs) in appropriate mapping populations. In response to drought brought about by soil water deficit, plants can exhibit either drought escape or drought resistance mechanisms, with resistance further classified into drought avoidance and drought tolerance. Drought escape is the ability of plants to complete the life cycle before severe stress arrives. Drought avoidance is the maintenance of high tissue water potential in spite of soil water deficit. Drought avoidance is consequence of improved water uptake under stress and the capacity of plant cells to hold acquired water that reduces water loss. Drought tolerance is the ability to withstand water deficit with low tissue water potential. Plant water status that includes leaf water potential, osmotic potential and relative water content (RWC) represents an easy measure of water deficit and provides best sensor for stress. Genomics‐assisted breeding (GAB) approaches, such as marker‐ assisted selection (MAS), can greatly improve precision and efficiency of selection in crop breeding. Molecular markers can facilitate indirect selection for traits that are difficult or inconvenient to score directly, pyramiding genes from different sources and combining resistance to multiple stresses. Conventional breeding for developing drought‐tolerant crop varieties is time‐consuming and labor intensive due to the quantitative nature of drought tolerance and difficulties in selection for drought tolerance. The identification of genomic regions associated with drought tolerance would enable breeders to develop improved cultivars with increased drought tolerance using marker‐assisted selection (MAS). This requires integration of knowledge from plant physiology and biotechnology into plant breeding. The availability of a large number of molecular markers, dense genetic maps and markers associated with traits and transcriptomics resources have made it possible to integrate genomics technologies

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Molecular and Morphophysiological Analysis**

**of Drought Stress in Plants**

**of Drought Stress in Plants**

Summy Yadav and Kamal Dutt Sharma

Summy Yadav and Kamal Dutt Sharma

http://dx.doi.org/10.5772/65246

into chickpea improvement.

**Abstract**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

## **Molecular and Morphophysiological Analysis of Drought Stress in Plants Molecular and Morphophysiological Analysis of Drought Stress in Plants**

Summy Yadav and Kamal Dutt Sharma Summy Yadav and Kamal Dutt Sharma

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65246

#### **Abstract**

(*Hordeum vulgare* L.). Journal of Experiment Botany. 1997; 48: 847–855. DOI: 10.1093/jxb/

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[61] Hansch R, Fessel DG, Witt C, Hesberg C, Hoffmann G, Walch-Liu P, Engels C, Kruse J, Rennenberg H, Kaiser W, Mendel RR. Tobacco plants that lack expression of functional nitrate reductase in roots show changes in growth rates and metabolite accumulation. Journal of Experimental Botany. 2001; 52: 1251–1258, DOI: 10.1093/jexbot/52.359.1251.

−

assimilation in herbaceous

48.4.847.

148 Plant Growth

physiol.org/content/131/3/1147.full.pdf.

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[59] Gojon A, Passard C, Bussi C. Root/shoot distribution of NO3

link.springer.com/article/10.1023/A%3A1000903504144

Drought is a major environmental stress factor that affects the growth and development of plants. Most of the physiological traits associated with drought tolerance are quantitative in nature. An important research strategy that has been widely used to deal with such complexity is to use molecular markers to identify quantitative trait loci (QTLs) in appropriate mapping populations. In response to drought brought about by soil water deficit, plants can exhibit either drought escape or drought resistance mechanisms, with resistance further classified into drought avoidance and drought tolerance. Drought escape is the ability of plants to complete the life cycle before severe stress arrives. Drought avoidance is the maintenance of high tissue water potential in spite of soil water deficit. Drought avoidance is consequence of improved water uptake under stress and the capacity of plant cells to hold acquired water that reduces water loss. Drought tolerance is the ability to withstand water deficit with low tissue water potential. Plant water status that includes leaf water potential, osmotic potential and relative water content (RWC) represents an easy measure of water deficit and provides best sensor for stress. Genomics‐assisted breeding (GAB) approaches, such as marker‐ assisted selection (MAS), can greatly improve precision and efficiency of selection in crop breeding. Molecular markers can facilitate indirect selection for traits that are difficult or inconvenient to score directly, pyramiding genes from different sources and combining resistance to multiple stresses. Conventional breeding for developing drought‐tolerant crop varieties is time‐consuming and labor intensive due to the quantitative nature of drought tolerance and difficulties in selection for drought tolerance. The identification of genomic regions associated with drought tolerance would enable breeders to develop improved cultivars with increased drought tolerance using marker‐assisted selection (MAS). This requires integration of knowledge from plant physiology and biotechnology into plant breeding. The availability of a large number of molecular markers, dense genetic maps and markers associated with traits and transcriptomics resources have made it possible to integrate genomics technologies into chickpea improvement.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Keywords:** abiotic stress, drought, physiological traits, conventional breeding, mark‐ er‐assisted selection, QTLs

avoidance (maintenance of tissue water potential) and drought tolerance [13]. Drought stress is the ability of plants to complete the life cycle before severe stress conditions arise. Drought avoidance is the maintenance of high tissue water potential under a soil water deficit. Improved water uptake under stress and the capacity of plant cells to hold acquired water reduces water loss leading to drought avoidance. Drought tolerance is the ability to withstand water deficit with low tissue water potential. Plants respond to water deficit using mechanisms of avoidance by improved root traits and by reducing water loss through reduced epidermal (stomatal and cuticular) conductance, reduced radiation absorption, and reduced evaporative surface (leaf area). Drought tolerance is the ability to withstand water deficit with low tissue water potential [14]. Plants under drought stress may survive by, among other mechanisms, main‐ taining cell turgor and reducing evaporative water loss by accumulating compatible solutes [15]. In recent years, much molecular information has been generated on the response of plants to environmental stresses. Plants respond to environmental stresses such as drought by the induction of both regulatory and functional sets of genes [16, 17]. Very little is known about the early events in the perception of stress signals [18, 19]. The common stress signaling pathways have been distinguished into abscisic acid (ABA) dependent and ABA independent [20, 21]. Most of the key genes in these pathways have been identified, such as transcription factors belonging to the class of dehydration responsive element‐binding protein (DREB)/C‐ repeat‐binding factor (CBF), ABA‐binding factor (ABF), Myelocytomatosis oncogene (MYC) and Myeloblastosis oncogene (MYB), including the identification of the stress‐responsive cis‐ elements ABA‐responsive element (ABRE) and dehydration responsive element (DRE). Downstream of the early signal perception events, signaling genes and molecules acting as

Molecular and Morphophysiological Analysis of Drought Stress in Plants

http://dx.doi.org/10.5772/65246

151

secondary messengers have been identified, revealing the role of Ca+

tolerance and/or resistance.

species (ROS) as secondary messengers. These regulatory mechanisms induce downstream functional genes, which are needed to establish new cellular homeostasis that leads to drought

Most of the physiological traits associated with drought tolerance are quantitative in nature. Genomics‐assisted breeding (GAB) approaches, such as marker‐assisted selection (MAS), can greatly improve precision and efficiency of selection in crop breeding [23]. Integration of genomics and breeding has a great potential for crop improvement. Molecular markers facilitate indirect selection for traits that are inconvenient to score directly (e.g., root traits, resistance to root knot nematodes), pyramiding genes from different sources (e.g., bringing together ascochyta blight resistance genes from different donors) and combining resistance to multiple stresses (e.g., resistance to fusarium wilt and ascochyta blight). Recent years have seen tremendous progress in the development of large scale genomic resources such as DNA‐ based molecular markers, comprehensive genetic maps, whole‐genome transcription profiling techniques to identify genomic regions and genes underlying plant stress responses [24]. These genomic tools will be useful to understand and access the diversity conserved in ex situ germplasm collections for crop improvement [25]. Thus, an understanding of drought stress and water use in relation to plant growth is of importance for sustainable agriculture. Con‐ ventional breeding for developing drought‐tolerant crop varieties is time‐consuming and labor intensive due to the quantitative nature of drought tolerance and difficulties in selection for drought tolerance [26]. Mapping of different genomes has been of interest to identify genomic

and reactive oxygen

## **1. Introduction**

Plant growth and productivity is adversely affected by nature's wrath in the form of various biotic and abiotic stress factors. Water deficit is one of the major abiotic stresses, which adversely affects crop growth and yield. Stress is an altered physiological condition caused by factors that tend to disrupt the equilibrium. Strain is any physical and chemical change produced by a stress [1]. Stress is used with various meanings, the physiological definition and appropriate term as responses in different environmental situations. If a factor deviates from its optimum does not necessarily results in stress. Stress is a constraint or unpredictable change imposed on regular metabolic patterns of growth results in injury, disease or aberrant physiology. Plants are mainly exposed to stresses such as drought, precipitation, salt, flooding, heat, oxidative stress and heavy metal toxicity. Drought stress occurs when the available water in the soil is reduced and atmospheric conditions cause continuous loss of water by transpiration or evaporation due to increase in temperature in nature. Drought stress tolerance is seen in almost all plants but its extent varies from species to species and even within species [2]. Conventional plant breeding attempts have changed over to use physiological selection criteria since they are time consuming and rely on present genetic variability [3]. Abiotic stresses tolerance is a complex trait, due to the interactions between stress factors and various molecular, biochemical and physiological phenomena affecting plant growth at different developmental stages [4]. High yield potential under drought stress is the target of crop breeders. In many cases, high yield potential can contribute to yield in moderate stress environment [5]. Drought stress leads to stomatal closure and limitation of gas exchange. Desiccation is much more extensive loss of water, which can potentially lead to maximum disruption of metabolism and cell structure and finally stops enzyme catalyzed reactions [6, 7]. Drought stress is characterized by reduction in water content, diminished leaf water potential and turgor loss, closure of stomata and decrease in cell enlargement and growth. Severe water stress may result in hampering photosynthesis, disturbing the overall metabolism and finally the necrosis of plant [8]. Water stress inhibits cell enlargement more as compared to cell division. Plant growth is reduced by affecting various physiological and biochemical processes, such as photosynthesis, respiration, translocation, ion uptake, carbohydrates, nutrient metabolism and growth promoters [9]. A better understanding of the morphophysiological traits can be used to create new varieties of crops to obtain a better productivity under drought conditions [10]. The reactions of plants to water stress differ significantly at various organizational levels depending upon intensity and duration of stress as well as plant species and its stage of growth [11]. A fundamental part for making the crops stress tolerant is to understand plant responses to different drought stress environments [12].

In response to drought brought about by soil water deficit, plants can exhibit either drought escape or drought resistance mechanisms, with resistance further classified into drought avoidance (maintenance of tissue water potential) and drought tolerance [13]. Drought stress is the ability of plants to complete the life cycle before severe stress conditions arise. Drought avoidance is the maintenance of high tissue water potential under a soil water deficit. Improved water uptake under stress and the capacity of plant cells to hold acquired water reduces water loss leading to drought avoidance. Drought tolerance is the ability to withstand water deficit with low tissue water potential. Plants respond to water deficit using mechanisms of avoidance by improved root traits and by reducing water loss through reduced epidermal (stomatal and cuticular) conductance, reduced radiation absorption, and reduced evaporative surface (leaf area). Drought tolerance is the ability to withstand water deficit with low tissue water potential [14]. Plants under drought stress may survive by, among other mechanisms, main‐ taining cell turgor and reducing evaporative water loss by accumulating compatible solutes [15]. In recent years, much molecular information has been generated on the response of plants to environmental stresses. Plants respond to environmental stresses such as drought by the induction of both regulatory and functional sets of genes [16, 17]. Very little is known about the early events in the perception of stress signals [18, 19]. The common stress signaling pathways have been distinguished into abscisic acid (ABA) dependent and ABA independent [20, 21]. Most of the key genes in these pathways have been identified, such as transcription factors belonging to the class of dehydration responsive element‐binding protein (DREB)/C‐ repeat‐binding factor (CBF), ABA‐binding factor (ABF), Myelocytomatosis oncogene (MYC) and Myeloblastosis oncogene (MYB), including the identification of the stress‐responsive cis‐ elements ABA‐responsive element (ABRE) and dehydration responsive element (DRE). Downstream of the early signal perception events, signaling genes and molecules acting as secondary messengers have been identified, revealing the role of Ca+ and reactive oxygen species (ROS) as secondary messengers. These regulatory mechanisms induce downstream functional genes, which are needed to establish new cellular homeostasis that leads to drought tolerance and/or resistance.

**Keywords:** abiotic stress, drought, physiological traits, conventional breeding, mark‐

Plant growth and productivity is adversely affected by nature's wrath in the form of various biotic and abiotic stress factors. Water deficit is one of the major abiotic stresses, which adversely affects crop growth and yield. Stress is an altered physiological condition caused by factors that tend to disrupt the equilibrium. Strain is any physical and chemical change produced by a stress [1]. Stress is used with various meanings, the physiological definition and appropriate term as responses in different environmental situations. If a factor deviates from its optimum does not necessarily results in stress. Stress is a constraint or unpredictable change imposed on regular metabolic patterns of growth results in injury, disease or aberrant physiology. Plants are mainly exposed to stresses such as drought, precipitation, salt, flooding, heat, oxidative stress and heavy metal toxicity. Drought stress occurs when the available water in the soil is reduced and atmospheric conditions cause continuous loss of water by transpiration or evaporation due to increase in temperature in nature. Drought stress tolerance is seen in almost all plants but its extent varies from species to species and even within species [2]. Conventional plant breeding attempts have changed over to use physiological selection criteria since they are time consuming and rely on present genetic variability [3]. Abiotic stresses tolerance is a complex trait, due to the interactions between stress factors and various molecular, biochemical and physiological phenomena affecting plant growth at different developmental stages [4]. High yield potential under drought stress is the target of crop breeders. In many cases, high yield potential can contribute to yield in moderate stress environment [5]. Drought stress leads to stomatal closure and limitation of gas exchange. Desiccation is much more extensive loss of water, which can potentially lead to maximum disruption of metabolism and cell structure and finally stops enzyme catalyzed reactions [6, 7]. Drought stress is characterized by reduction in water content, diminished leaf water potential and turgor loss, closure of stomata and decrease in cell enlargement and growth. Severe water stress may result in hampering photosynthesis, disturbing the overall metabolism and finally the necrosis of plant [8]. Water stress inhibits cell enlargement more as compared to cell division. Plant growth is reduced by affecting various physiological and biochemical processes, such as photosynthesis, respiration, translocation, ion uptake, carbohydrates, nutrient metabolism and growth promoters [9]. A better understanding of the morphophysiological traits can be used to create new varieties of crops to obtain a better productivity under drought conditions [10]. The reactions of plants to water stress differ significantly at various organizational levels depending upon intensity and duration of stress as well as plant species and its stage of growth [11]. A fundamental part for making the crops stress tolerant is to understand plant responses to different drought stress environments [12].

In response to drought brought about by soil water deficit, plants can exhibit either drought escape or drought resistance mechanisms, with resistance further classified into drought

er‐assisted selection, QTLs

**1. Introduction**

150 Plant Growth

Most of the physiological traits associated with drought tolerance are quantitative in nature. Genomics‐assisted breeding (GAB) approaches, such as marker‐assisted selection (MAS), can greatly improve precision and efficiency of selection in crop breeding [23]. Integration of genomics and breeding has a great potential for crop improvement. Molecular markers facilitate indirect selection for traits that are inconvenient to score directly (e.g., root traits, resistance to root knot nematodes), pyramiding genes from different sources (e.g., bringing together ascochyta blight resistance genes from different donors) and combining resistance to multiple stresses (e.g., resistance to fusarium wilt and ascochyta blight). Recent years have seen tremendous progress in the development of large scale genomic resources such as DNA‐ based molecular markers, comprehensive genetic maps, whole‐genome transcription profiling techniques to identify genomic regions and genes underlying plant stress responses [24]. These genomic tools will be useful to understand and access the diversity conserved in ex situ germplasm collections for crop improvement [25]. Thus, an understanding of drought stress and water use in relation to plant growth is of importance for sustainable agriculture. Con‐ ventional breeding for developing drought‐tolerant crop varieties is time‐consuming and labor intensive due to the quantitative nature of drought tolerance and difficulties in selection for drought tolerance [26]. Mapping of different genomes has been of interest to identify genomic locations of disease resistance genes and other yield‐related traits. Isolation and validation of genes underlying the QTL/genes for the traits of interest is an essential step to determine gene function. QTLs for drought tolerance have been identified for major important crop species such as rice, maize, wheat, barley, sorghum, pearl millet, soybean and chickpea. These QTLs were identified for many important traits which include yield and yield‐related traits under drought stress conditions, physiological responses including water‐soluble carbohydrates, carbon isotope ratio, osmotic potential, chlorophyll content, flag leaf rolling index, grain carbon isotope discrimination, relative water content, leaf osmotic potential, osmotic adjust‐ ment, chlorophyll and chlorophyll fluorescence parameters to drought stress, flowering time, root traits. Major QTLs contribute to the traits with higher phenotypic variation. These QTLs, after validation in desired germplasm, can be used for introgressing drought tolerance from the donor genotypes into less drought‐tolerant cultivars or breeding lines (recipient parents) avoiding transfer of undesirable or deleterious genes from the donors (linkage drag).

moisture availability, where the growing season is shorter and terminal drought stress predominates. In field‐grown clones of Robusta coffee, leaf shedding in response to drought stress revealed that drought‐sensitive clone has greater extent of leaf shedding [37]. Time of flowering is a major trait of a crop adaptation to the terminal drought and high temperature environments. Short‐duration varieties to be developed to minimize yield loss from terminal drought, as early maturity helps the crop to avoid the period of stress [38]. However, yield is generally correlated with the length of crop duration under favorable growing conditions, and

Molecular and Morphophysiological Analysis of Drought Stress in Plants

http://dx.doi.org/10.5772/65246

153

The ability of plants to maintain relatively high tissue water potential despite a shortage of soil‐moisture; mechanisms for improving water uptake, storing in plant cell and reducing water loss confer drought avoidance is referred to as drought avoidance. different mechanisms for drought avoidance are being reported in different plant species which include maintenance of turgor through increased rooting depth, efficient root system and increased hydraulic conductance and reduction in water loss through reduced epidermal (stomatal and lenticular) conductance, reduced absorption of radiation by leaf rolling or folding and reduced evapora‐ tion surface (leaf area). In crops, high root biomass has been of interest because the more the roots, the more their efficiency in absorption of water. This gives a plant more advantage in times when less moisture is available in the soil. A positive correlation between root system sizes and resistance to water stress has been found in several crops and many breeding attempts have focused on obtaining cultivars with larger root systems [40]. Saxena [41] has developed a chickpea cultivar with a greater degree of drought tolerance from combining large root traits of ICC4958. Similarly, Krishnamurthy et al. [42] has reported that large root biomass in a minicore collection of ICRISAT chickpea germplasm had high correlation with drought tolerance. Root system size is a complex trait since it is determined by intrinsic genetic factors and modulated by numerous environmental cues such as nutrient and moisture availability in the soil [43]. He also noted that smaller leaf surface was also a desirable trait related to drought tolerance. Plants with small leaf surface (pinnules) have shown to experience reduced water loss [41]. Glaucousness or waxy bloom on leaves helps with maintenance of high tissue water potential and is therefore considered as a desirable trait for drought tolerance [44, 45]. Varying percentage of glaucousness in wheat led to increased water‐use efficiency, but it has minimal affect on total water use or harvest index. Determination of leaf temperature indicated glaucous leaves were 0.7°C cooler than non‐glaucous leaves and had a lower rate of leaf senescence [43]. It was also suggested that a 0.5°C reduction in leaf temperature for 6 h per day was sufficient to extend the grain‐filling period by more than three days. However, yield advantages are likely to be small as many varieties already show some degree of glaucousness.

The ability of plant to withstand water‐deficit with low tissue water potential is referred as drought tolerance. A balance between maintenance of turgor and reduction in water loss helps plants to survive drought stress conditions [46]. Plants can combat drought stress by mainte‐

any decline in crop duration below the optimum would tax yield [39].

**2.2. Drought avoidance**

**2.3. Drought tolerance**

## **2. Drought stress improvement**