*Article* **Simulation and Evaluation of the Radiation Dose Deposited in Human Tissues by Atmospheric Neutrons**

**Ernesto Ortiz 1,2,\*, Blanca Mendoza 1,3, Carlos Gay 2, Victor Manuel Mendoza 2, Marni Pazos 2 and Rene Garduño 2**

**Abstract:** The evaluation of the radiation dose (RD) deposited by atmospheric neutrons in human tissues is of vital importance due to the potential damages that over exposure to this radiation may cause to human health. The goal of this work was to obtain the RD that atmospheric neutrons with energy from 1 to 1000 MeV deposit in tissues of the human body (blood, adipose, bone and brain) as a function of both altitude and latitude. With the help of the Geant4 software, we developed a numerical simulation that allowed us to reach our goal; atmospheric neutron fluxes were obtained from the Excel-Based Program for Calculating Atmospheric Cosmic-Ray Spectrum (EXPACS). We found that the RD deposited by atmospheric neutrons increases with the increase in altitude and latitude, e.g., for an altitude of high mountain (4 km), the RD is increased ∼19 times; while, for an altitude of commercial flights (10 km), the RD is increased ∼156 times; in both cases, regarding the RD at sea level. We also found that, in the range of energies from 1 to 100 MeV, the RD deposited in the bone tissue sample is considerably lower that the RD deposited in the blood, adipose and brain tissue samples. On the other hand, for the range of energies between 200 and 1000 MeV, the RD deposited in the bone tissue sample is considerably greater that the RD deposited in the blood, adipose and brain tissue samples.

**Keywords:** atmospheric neutrons; radiation dose; passengers and flight crew; cosmic rays

#### **1. Introduction**

The atmospheric air showers are initiated by high-energy primary cosmic rays that enter the atmosphere isotropically from outer space, producing a large number of secondaries particles in a serie of successive collisions with target nuclei of the atmospheric constituents [1]. The primary cosmic rays which consists predominantly of protons, alpha particles and heavier nuclei are influenced by the galactic, the interplanetary, the magnetospheric and the geomagnetic magnetic fields while approaching the Earth [2].

In addition, the incoming cosmic rays with energies below about 20 GeV are modulated by the interplanetary magnetic field which is embedded in the solar wind [3], the expanding magnetized plasma generated by the Sun, which decelerates and partially excludes the lower energy cosmic rays from the inner solar system. There is a significant anticorrelation between solar activity and the intensity of the cosmic rays with the energies already mentioned.

Atmospheric neutrons are secondary particles resulting from interactions of primaries with nuclei of air constituents. It is useful to distinguish low-energy neutrons with energies less than about 10 MeV from high-energy neutrons. Low-energy neutrons are mostly

**Citation:** Ortiz, E.; Mendoza, B.; Gay, C.; Mendoza, V.M.; Pazos, M.; Garduño, R. Simulation and Evaluation of the Radiation Dose Deposited in Human Tissues by Atmospheric Neutrons. *Appl. Sci.* **2021**, *11*, 8338. https://doi.org/ 10.3390/app11188338

Academic Editors: Roberta Sparvoli and Francesco Palma

Received: 22 June 2021 Accepted: 31 August 2021 Published: 9 September 2021

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

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

evaporation products of excited nuclei and manifest an isotropic angular distribution. High-energy neutrons can be produced by hadronic collisions but also in charge exchange reactions of leading particles at very high energies, the angular distribution of high energy neutrons is anisotropic [2]. In addition, when high-energy particles collide with the atoms in the aircraft material, producing a local shower of particles, including electrons, lighter ions, gamma rays, and neutrons that are dangerous to human health [4].

Because the neutrons are produced by charged particles they inherit and also exhibit both latitude and longitude effects. Furthermore the flux of the low-energy neutron component is modulated by the solar cycle; in addition, strong solar flares may cause significant neutron fluxes in the atmosphere [2], sporadic enhancements of solar cosmic ray fluxes are also caused by coronal mass ejection, in these events the particles are accelerated to energies of hundreds of MeV up to several GeV [5,6].

The secondary energetic particles interacting with the human body may potentially cause an increase in cancer risk as the dose equivalent exposure increases, particularly in passengers and flight crew [7]. High RD may cause the breaks, potentially lethal to the cell by damaging the DNA strands, while low RD of ionizing radiations seems to have carcinogenic effects, even after years or decades, both in the exposed individuals and in subsequent generations [8]. The atmospheric neutron component of this complex radiation field, in particular, holds special interest in the cancer research community. At aircraft altitude (flight level around 10 km) of the total radiation dose deposited by secondary particles, the contribution of neutrons is around 40% [9].

The potential negative effect of atmospheric neutrons on human health is the motivation to carry out the present work, in this, we developed a numerical simulation using Geant4 software, with which, we calculated the radiation dose (RD) deposited by atmospheric neutrons in various tissues of the human body as function of the fluxes of these particles for various altitudes and latitudes; however, in all these geographical points we have maintained the same longitude (0◦), since these will allow us to show the great variations of the RD as a function of altitude and latitude.

In this work, we have focused on the RD that atmospheric neutrons deposit in human tissues, if the readers want to know the effects of charged particles, they can read for example [6,8,10].

#### **2. Materials and Methods: Setup of the Simulation**

Excel-Based Program for Calculating Atmospheric Cosmic-Ray Spectrum (EXPACS) calculate cosmic ray fluxes of neutrons, protons, muons, electrons, positrons, and photons nearly anytime and anywhere in the Earth's atmosphere [11].

In Figure 1, we show the atmospheric neutron spectra for various altitudes obtained with EXPACS for geographical points with a latitude of 0◦ and a longitude of 0◦. These data were calculated for the year 2020, period of minimum solar activity.

**Figure 1.** Atmospheric neutron spectra for various altitudes at geographical points with a latitude of 0◦ and a longitude of 0◦. See text for more details.

In Figure 2, we show the atmospheric neutron spectra for various latitudes obtained with EXPACS for geographical points with an altitude of 10 km and a longitude of 0◦. These data were calculated for the year 2020, period of minimum solar activity.

**Figure 2.** Atmospheric neutron spectra for various latitudes at geographical points with an altitude of 10 km and a longitude of 0◦. See text for more details.

The Geant4 software is a toolkit for the simulation of the passage of particles through matter. It is used for a variety of applications domains, including high-energy physics, astrophysics and space science, medical physics and radiation protection [12]. Geant4 is used extensively in medical physics applications such as particles beam therapy, microdosimetry and radioprotection. The basic extensibility of the toolkit has facilitated its expansion into new user domains, such as biochemistry, material science and non-destructive scanning.

For this simulation we used as source, fluxes of 5 × 105 neutrons with kinetic energies between 1 and 1000 MeV. We also used tissues of blood (G4\_BLOOD\_ICRP, *ρ* = 1.06 g/cm3), adipose (G4\_ADIPOSE\_TISSUE\_ICRP, *ρ* = 0.95 g/cm3), bone (G4\_BONE\_CORTICAL\_ICRP, *ρ* = 1.92 g/cm3) and brain (G4\_BRAIN\_ICRP, *ρ* = 1.04 g/cm3) as samples obtained from Geant4 material's database, with a spherical geometry with a radius of 5 cm. These samples are tissues equivalent reported by The International Commission on Radiological Protection (ICRP). The ICRP is the primary body in protection against ionising radiation, it is a registered charity and is thus an independent non-governmental organisation created by the 1928 International Congress of Radiology to advance for the public benefit the science of radiological protection [13].

With the help of Geant4 software, we got the RD that neutrons deposit in the different sample tissues as a function of neutron's energy, the neutrons were injected isotropically to each of the samples. We arbitrarily choose geographic points, all of them with the same longitude (0◦), at these points, we calculated the RD that neutrons deposited in the samples for different altitudes and different latitudes.

To obtain the RD in the tissues, we used the spectra shown in Figures 1 and 2 to calculate the neutron flux for each energy of interest, in addition, these fluxes were calculated for periods of one hour over the spherical area of the samples, later we multiplied the neutron flux and the RD deposited per neutron, for each energy, finally we added all products to obtain the RD deposited in the tissues.

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

When calculating the RD that neutrons deposit in the different sample tissues, under the conditions mentioned in the previous section, we obtained the statistical error less than 1% for each calculated value. Since statistical errors are relatively small, we decided not to point them out in Figure 3 that we present in this section.

In Figure 3, we show the RD that neutrons deposit in the samples as a funtion of their kinetic energy. In this figure, we can see three sections with different behavior of the RD deposited in the samples as a function of the energy of the neutrons; first, for energies of 1 to 20 MeV the RD curves show an increase with the energy of the neutrons; second, for energies of 20 to 500 MeV the RD curves are approximately flat; finally, for greater energies at 500 MeV the RD curves show an increase with the energy of the neutrons.

**Figure 3.** Radiation dose deposited by 5 × 105 neutrons to each energy in the tissue samples. This result was obtained by a numerical simulation.

In Figure 3, we can also observe that, neutrons with energy less than 200 MeV deposited a less RD in the bone sample compared with the RD deposited in the other samples, whereas that, the RD deposited in the adipose sample is greater than the RD deposited in blood and brain samples; this is due to the chemical composition of the tissues, for adipose tissue the dominant element is carbon, for blood and brain tissues is oxygen and for bone tissue are oxygen together with calcium. Neutrons with energy equal or greater than 200 MeV deposited a higher RD in the bone sample compared with the RD deposited in the other samples, this occurs because the cross section of the neutron varies with the kinetic energy of this particle.

Considering the RD deposited by the neutrons in the bone tissue sample as a reference, in the range of energies from 1 to 100 MeV, the RD deposited in the blood, adipose and brain tissue samples are 167.9%, 182.8% and 172.3% respectively; while, for the range of energies between 200 and 1000 MeV, the RD deposited in the blood, adipose and brain tissue samples are 81.0%, 80.8% and 80.5% respectively.

The neutron fluxes as a function of the altitude and the geographical latitude were calculated under the conditions mentioned in the previous section. In Figure 4, we show the RD that neutron with energies between 1 and 1000 MeV deposited in the tissue samples for an altitude between 0 and 10 km above sea level. To avoid overlapping of the curves, we have multiplied them by a factor shown in their respective labels.

In Figure 4, we can see that the RD deposited in the tissue samples is minimal at sea level and increases with height, the reason for this is that the neutron flux increases with increasing height. All curves have the same behavior. From the data obtained from the simulation and considering as reference the RD deposited by neutrons at 0 km above sea level, for an altitude of 4 km, corresponding approximately to an altitude of high mountain, the RD is increased ∼19 times, while, for an altitude of 10 km, corresponding to an altitude of commercial flights, the RD is increased ∼156 times.

**Figure 4.** Radiation dose deposited in blood, adipose, brain and bone tissue samples by atmospheric neutrons with energies between 1 and 1000 MeV, as a function of the altitude for a latitude of 0◦ and a longitude of 0◦. This result was obtained by a numerical simulation, the RD was calculated for a period of one hour in spherical samples with a radius of 5 cm. See text for more detail.

In Figure 5, we show the RD that neutron with energies between 1 and 1000 MeV deposited in the tissue samples as a function of the latitude for an altitude of 10 km above sea level and a longitude of 0◦. To avoid overlapping of the curves, we have multiplied them by a factor shown in their respective labels. We can see that the RD deposited in the tissue samples is lower in equatorial zones (∼0◦); the above is due to the fact that in these zones the rigidity cutoff is maximum, containing lower flux of cosmic rays and consequences a lower production of atmospheric neutrons. In polar zones (∼80◦), the flux of cosmic rays is greater due to the low-rigidity cutoff which implies a greater production of atmospheric neutrons and consequently a higher RD deposited in the tissue samples. The RD in polar zones is approximately 4.3 times higher than the RD in equatorial zones for an altitud of 10 km.

**Figure 5.** Radiation dose deposited in blood, adipose, brain and bone tissue samples by atmospheric neutrons with energies between 1 and 1000 MeV, as a function of the latitude for a altitude of 10 km and a longitude of 0◦. This result was obtained by a numerical simulation, the RD was calculated for a period of one hour in spherical samples with a radius of 5 cm. See text for more detail.

The statistical errors of the data presented in Figures 4 and 5 are dominated by the propagation of the statistical error that is generated when calculating the neutron flux from the energy spectra, therefore, they present variations with the altitude and the latitude. We also found that statistical errors calculated as a percentage are the same for all tissue samples used.

The statistical errors of the RD deposited in each tissue sample of Figure 4 are 5.0%, 4.9%, 4.9%, 4.8%, 4.9%, 5.1%, 5.4% and 5.7% for altitudes of 0, 1, 2, 3, 4, 6, 8 and 10 km above

sea level, respectively, while for the data in Figure 5 are 5.7%, 5.7%, 5.7%, 5.6%, 5.4%, 5.2%, 5.0%, 5.0% and 5.0% for latitudes of 0◦, 10◦, 20◦, 30◦, 40◦, 50◦, 60◦, 70◦ and 80◦, respectively.

#### **4. Conclusions**

In this work, and with the help of Geant4 software, first, we calculated the RD that neutrons with energy between 1 and 1000 MeV deposit in blood, adipose, bone and brain tissue samples; second, we calculated the RD that these particles deposit in the tissue samples as a function of different altitudes and latitudes, in both cases, for a longitude of 0◦.

When we considered neutrons with a flat spectra (5 × 105 neutrons for each energy) and as a reference the RD deposited by them in the bone tissue sample, we found that in the range of energies from 1 to 100 MeV, the RD deposited in the blood, adipose and brain tissue samples is greater by 67.9%, 82.8% and 72.3% respectively. On the other hand, for the range of energies between 200 and 1000 MeV, the RD deposited in the blood, adipose and brain tissue samples is lower by 19.0%, 19.2% and 19.5% respectively.

When we obtained the atmospheric neutron spectra from [11]; we calculated the RD deposited by these particles in different human tissues as a function of both altitude and latitude and for a longitude of 0◦. The results are shown in Figures 4 and 5. In general, we found that the RD deposited by atmospheric neutrons increases with the increase in altitude and latitude, e.g., for an altitude of 4 km (altitude of high mountain), the RD is increased ∼19 times; while, for an altitude of 10 km (altitude of commercial flights), the RD is increased ∼156 times, respectively; in both cases, regarding the RD at sea level.

**Author Contributions:** Conceptualization, E.O. and B.M.; Investigation, E.O.; Methodology, E.O.; Software, E.O.; Supervision, B.M. and C.G.; Writing—original draft, E.O.; Writing—review & editing, B.M., C.G., V.M.M., M.P. and R.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Posdoctoral Grant Programa de Becas Posdoctorales de la Universidad Nacional Autónoma de México.

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

**Informed Consent Statement:** Not applicable, for studies not involving humans or personal data.

**Data Availability Statement:** Data are available, contact the corresponding author.

**Acknowledgments:** The authors thank The European Organization for Nuclear Research (CERN) for Geant4 software.

**Conflicts of Interest:** The authors declare no conflict of interest. The sponsors had no role in the design, execution, interpretation, or writing of the study.

#### **References**


*Article* **Innovative Strategies for the Use of Reflective Foils for Fruit Colouration to Reduce Plastic Use in Orchards**

**Patrick Hess 1 , Achim Kunz 2 and Michael M. Blanke 1,\***

2 Campus Klein-Altendorf, University of Bonn, 53359 Rheinbach, Germany<EMAIL_ADDRESS>
**\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** (1) *Background*: Plastic in fruit orchards represents an environmental issue due to large CO2eq emissions associated with its production from fossil fuel and disposal (often incineration). (2) *Materials and methods*: Apple cv. "Braeburn Hillwell" trees on M9 rootstocks under a hail net were used at Campus Klein-Altendorf (CKA), Germany (50 ◦N) in 2018. In order to reduce the use of plastics to improve the red colouration of fruit particularly under hail nets, three alternatives to the current use of reflective mulch in each alleyway between the tree rows were explored, with uncovered grass alleyways as control. About 2800 colour measurements were done in the four weeks prior to harvest on 720 attached fruit below and above 1 m height in the field, and ca. 6900 additional colour measurements were conducted at harvest. (3) *Results*: The underlying regulatory mechanisms contrasted between the diffusive reflection of the white woven ground cover (such as LumilysTM or ExtendayTM) in the alleyways and aluminium foil under the trees with regular (straight) light reflection. Good fruit colouring and a plastic reduction were achieved (a) through spreading the white woven ground cover in every other row, and (b) through substituting the white ground cover with aluminium foil (80% recycled). Both methods can reduce greenhouse gas (GHG) emissions (75–110 kg CO2eq/ha for the first option a). (4) *Conclusion*: Plastic use in fruit orchards can be reduced by multiple use of the material in the same or several years, spreading it in every other row or substituting it by another reflective material, a relevant step towards an environment-friendly sustainable horticulture.

**Keywords:** anthocyanin; ExtendayTM; fruit colouration; light reflection; LumilysTM; plastic recycling; reflective films; resource conservation; sustainable horticulture

#### **1. Introduction**

Criticism concerning food and fruit production at all stages of the supply chain includes unnecessary waste from the field to wholesale, retail, restaurants and the consumer [1–4]. The excessive use of plastic is criticised due to its fossil-fuel based, non-renewable nature, which can constitute a major source of GHG emissions due to production, pollution, waste and possibly micro plastics as important aspects for a sustainable, environment-friendly horticulture.

As a result of climate change and associated changes in the weather system, such as an increase in hailstorms, protective hail nets have become more widespread [3]; the combination of the resultant lack of light under the hail net [5] and warmer autumn temperatures [6] prevents the (red) colouration in many fruits and in many locations worldwide, from Brazil to Chile, Washington DC to Ontario and Bonn to Bologna [3]. The consumer (and trade), however, demands red-coloured fruit. In the consumer's perception, the red colour is associated with a ripe fruit, sweetness and good taste [3,7].

Hence, the objective of the present study was to investigate possible alternatives or modifications to the use of reflective mulch for colouration of fruit such as apricot, apple, (Anjou) pear, (red) grape berry, peach, persimmon (kaki), etc. In other words, the stimulation of the anthocyanin synthesis [8–10] for sustainable cultivation of these fruit

**Citation:** Hess, P.; Kunz, A.; Blanke, M.M. Innovative Strategies for the Use of Reflective Foils for Fruit Colouration to Reduce Plastic Use in Orchards. *Sustainability* **2021**, *13*, 73. https:// dx.doi.org/10.3390/su13010073

Received: 11 November 2020 Accepted: 14 December 2020 Published: 23 December 2020

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

**Copyright:** © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/).

crops. Apple was chosen here due to its widespread cultivation in many countries with temperate climate zones throughout the world [4]. With ca. 80 million metric tons annual production (FAOSTAT [11]), it counts as the third largest fruit crop worldwide.

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

#### *2.1. Location, Fruit Trees, Colour Measurement and Maturity Indexing*

Nine-year-old cv. 'Braeburn Hillwell' apple trees on M9 rootstocks were cultivated at Campus Klein-Altendorf (CKA), University of Bonn, Germany (50 ◦N). The trees were trained to slender spindles and planted N-S to optimise light conditions (Jackson and Palmer, 1972) [12]. Fruit colour was measured with a portable i1 Pro (Xrite, Michigan, USA) in the lower third (<1 m tree height) and middle third (<1 m) of the trees, since fruit in the shaded lower parts of the tree canopy (Figures 1 and 2) are particularly prone to poor colouration. In these lower and middle parts of the tree, five fruits were marked in the west and five in the east tree periphery. Colour was repeatedly measured four times per fruit, i.e., every 90◦ around the fruit equator, on these still attached fruits and marked spots, resulting in 2800 values, and averaged for Figure 3, while colour values measured on the down-facing side of the fruit under 1 m height are presented in Figure 4. The second round of colour measurements was on harvested fruit using automated machine grading in an MSE 2000 (Greefa, Geldermalsen, Holland) with dedicated single fruit evaluation [13]. The 100 fruit samples for maturity indexing at two dates comprised ten apples per plot and treatment, i.e., five fruits from the east side and five fruits from the west side of the tree, and included the assessment of starch degradation, sugar content and fruit firmness, using standard methods [5].

**Figure 1.** Different approaches using (**a**) (recycled) aluminium foil directly under the trees (left), or (**b**) Lumilys woven textile both after 6 weeks exposure, both at the end of the experiment before harvest.

**Figure 2.** Pictorial representation of moving Lumilys mulch within the same row ("relocation") in autumn 2018.

**Figure 3.** Effect of plastic (closed circles) and aluminium (triangles) reflective mulches or chemical measures (Biostimulant StimplexTM) relative to the control (open circles) **on red colour formation (anthocyanin synthesis) at the fruit equator** as a decline in ◦hue colour angles for apple cv. 'Braeburn Hillwell' at Campus Klein-Altendorf in 2018 (n = 100 colour measurements per treatment and date plus SEs at the 5% error level).

**Figure 4.** Hue colour angle of the **bottom side of apples below 1 m height** exposed to aluminium foil (triangle), LumilysR reflective (plastic) mulch, LumilysR in alternating tree rows (closed circles), the biostimulant StimplexR, control/grass (open circle) and relocation P.1 and P.2 for the four measurement dates (n = 10/per treatment and measurement day plus SEs at the 5% error level for StimplexTM control, relocation P2 and P1 and aluminium foil).

#### *2.2. Reflective Mulches and Biostimulants*

Reflective white woven textile mulch type LumilysTM (100 g/m2 ; Beaulieu Textiles, Belgium) [14] was spread on 30 August 2018, i.e., ca. five weeks prior to the anticipated harvest (traditional way of using reflectors), with grassed alleyways without ground cover as control. The 2.6 m wide white polypropylene mulch cloth leaves a ca. 50 cm wide gap for soil respiration/aeration and water uptake close to the tree trunk.

The innovative alternatives to the white reflective groundcover in every row included (a) spreading only every other row (thereby saving 50% of the plastic) (Figure 2) and (b) the substitution of the (diffuse) reflective mulch by aluminium foil (type Profissimo®, dm pharmacies, Karlsruhe, Germany) (regular reflection) under the trees, leaving a 2.3 m wide gap on the grassed alleyway 1; the foil used was made from 80% recycled aluminium. The biostimulant Stimplex®/Acadian®, based on extracts of the brown algae (*Ascophyllum nodorum*; Acadian Seaplants Co., Dartmouth, NS, Canada), was our third alternative (Table 1).

**Table 1.** Experimental design to stimulate fruit colour at Klein-Altendorf in 2018.

Notes: n.a-not applicable.

The white woven reflective mulch (LumilysTM) was moved ca. every ten days in the fruit colouration period of six weeks before the harvest. In this relocation trial, the reflective mulch was spread in the alleyway either on the west side or the east side of the apple trees.

#### *2.3. Statistics*

The experiment comprised 80 apple trees. Each treatment consisted of ten trees plus one border tree, and the plots were replicated. A hundred and forty maturity tests were conducted at two dates on 10 apples per plot and treatment, i.e., five fruits from the east and five fruits from the west side of the tree, and averaged. A total of 2800 colour angle measurements were performed in the orchard on 560 attached fruits below and above 1 m height (Figure 3). All 5,515 apple fruits in the experiment were subjected to automated grading for colour (Figure 4), ca. 700 apples per treatment. Data were checked for normal distribution and homogeneity of variance, and then subjected to an analysis of variance (ANOVA) using Excel's statistical function and SEs at the 5% error level presented in Figures 3 and 4.

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

Repeated non-destructive colour measurements on the attached fruit (Figure 3) show the decline in the hue colour angle during fruit maturation, i.e., progressive development of red (anthocyanin) colour formation of the apple cv. 'Braeburn Hillwell' fruit. With about 63 ◦hue, the least red colouration was observed with the relocation of the reflective mulch within the same row and the uncovered grassed as control (60 ◦hue; Figure 3).

*3.1. Development of Fruit Colouration on the Tree and Effect of Fruit Position in the Canopy on Colouration*

*The best fruit colouration (55* ◦*hue) was observed with reflective mulch every other row/alleyway ("alternating rows"; Figures 2 and 3) and the aluminium foil under the trees (53* ◦*hue; values averaged over the east and west side and over <1 m and >1 m*).

Both polypropylene mulch relocations in plot 1 and plot 2 (Figure 2) and their use in every other row ("alternating") provide a plastic reduction of 50%. The colour of the apple fruit from relocation plot 1 (56 ◦hue) and relocation plot 2 (62 ◦hue) differed drastically, particularly in the period during 28 September to 8 October, with an apparent discrepancy between the decrease in the ◦hue colour angle of 9 ◦hue for apple in plot 1 and the 3 ◦hue for relocation plot 2 (Figure 3). In plot 1 the relocation of the reflective sheets within the same row had the strongest colouration effect, and the fruit were exposed longer to Lumilys reflective mulch in the alleyway on the east side than in relocation plot 2. *The reflective mulch (Lumilys) appeared more effective on the less sunny east side of the apple tree rows with north-south planting, especially in late autumn with a decreasing solar angle,* which has not been shown before, to our knowledge. Fruit on the *west* side of the trees benefited from sufficient PAR and UVB radiation for colouration (anthocyanin synthesis).

The lower, more shaded and green side of the apple fruit, particularly in the bottom of the tree canopy below 1 m, is severely affected by the lack of light. *This bottom side of the fruit below 1 m in the tree canopy benefited most, in terms of the best red colour, from the aluminium foil directly under the tree (70.4* ◦*hue) and LumilysR in every other ("alternating") alleyway (70.8* ◦*hue)* (Figure 4).

When the chemical biostimulant StimplexTM was applied, however, the colouration on the lower side of apple fruits was not improved (99 ◦hue), relative to the untreated control (grass; 89.4 ◦hue; Figure 4), and hence appeared unsuitable for this purpose.

The use of aluminium foil under the tree would mean a 100% reduction of plastic for fruit colouration in the orchard. Other (organic) alternatives like fresh wheat straw, which conveniently becomes available concomitantly, and bio-degradable white sports field paint failed in previous studies in our location in the Northern hemisphere [8], as well as evaporative cooling [15] and other biostimulants [15,16].

#### *3.2. Colour Assessment—Automated Grading of Harvested Fruit*

The colour threshold for the machine grading of all ca. 6900 harvested apple fruits was set to >50% red peel colouration for the classification as premium fruit. Reflective mulch every in other row ("alternating") had the largest share, with 81% in this sorting category (>50% surface red), followed by the aluminium foil, with 80% in relation to the traditional design of the reflective mulch in every row (75%). Where Lumilys mulch was relocated three times within the same row to save 50% of plastic, fruit from relocation plot 1 gained a similar hue value as Lumilys, with 73%, but that from relocation plot 2 exhibited a much lower value, with only 59%, i.e., less red colour (Figure 2). This discrepancy between fruit colour in plot 1 and plot 2 is in line with the results of the field colour measurements with attached fruit (Figure 1a). The control with uncovered grassed alleyways showed only 65% fruit in this premium colour category, as with the biostimulant Stimplex (Figure 5). The results of the colour sorting (Figure 5) correspond to the results of the colour angle measurements in the field (Figure 3), which also showed the best red fruit colour in both the application of the reflective mulch (LumilysR in alternating tree rows and the aluminium foil directly underneath the trees). The results of the colour measurements of attached apple fruits in the field (n = 3600 measurements) and of the automated sorting machine of ca. 6900 fruits show that the aluminium foil directly underneath the trees leads to apples with a better red colouring, thereby substituting 100% of plastic mulch by (recycled) aluminium. The reflective mulch (Lumilys) in the alleyway of every other row ("alternating") provided an equally good red colour, thereby reducing the plastic mulch input in the orchard by 50%.

**Figure 5.** Effect of colour enhancing measures on the portion of apples with more than 50% red colour on their surface, with SDs at the 5% error level from machine sorting results (n = 6900).

#### *3.3. Maturity Indexing of Apple Fruit*

The maturity indexing showed that the biostimulant StimplexTM accelerated ripening (Table 2) and advanced fruit maturation by one week, causing it to ripen as early as 4 October 2018. The red colour of the apples was bright red, and they retained their firmness (Table 4). An early harvest could lead to an increase in the sales price for apples of an early variety (such as "Gala") at an early location [13] (Table 2). All other treatments with reflective mulches had no influence on fruit maturity and, above all, maintained firmness as a pre-requisite for storability.

**Table 2.** Maturity indexing (Streif index) of apples on 4 October 2018 (n = 10 per treatment).

In Table 2, grey colour background represents metal, blue plastic and green a chemical alternative.

#### *3.4. Economics*

The additional proceeds, as a result of the colour improvement, were calculated by taking the percentage change of the apples with more than 50% red colour as compared to the control. The price difference between fruits with > 50% red colour, which sell at an average of 0.45 €/kg (€ 0.35–0.55), and those with less than 50% red colour, which sell at an average of 0.08 €/kg, was multiplied by yields of 30 t/ha, 40 t/ha and 50 t/ha. The results show that the largest financial gain was from well-coloured fruit from trees with reflective mulch in every other alleyway (row), followed by aluminium foil (under the tree) and then Lumilys in every alleyway/row ("traditional" approach) and change P. 1. These results confirm that Lumilys in alternating alleyways/rows and aluminium foil represent possible alternatives to spreading reflective mulches like LumilysR in every row (Figure 1a).

The calculation is based on the cost for LumilysR reflective mulches (2700 m/ha), at 50 cent/m2 (plus 240 €/ha hooks), and the recycled aluminium foil, at 51 cent/m2 (plus 328 €/ha sandbags), which hardly differed. The biostimulant StimplexR was more economical due to the lower cost of 151 €/ha for two product applications (2 × 4 L/ha) and a low workload. Annual costs were based on a 10-year lifespan for Lumilys and a single or double use of the (recycled) aluminium foil.

A financial gain as a difference between gross income and cost was seen in the reflective mulch in every other alleyway/row ("alternating"), aluminium foil (under the tree), Lumilys and change P. 1, as compared to control/grass. Based on a yield of 50 t/ha, a financial gain was achieved with (i) LumilysR in every row 1091 €/ha), (ii) LumilysR in every other row ("alternating") (2899 €/ha), and (iii) aluminium foil (1388 €/ha), while the triple re-location of the LumilysR foil saves 50% in material costs but is not profitable due to the insufficient red colour of the apples for marketing as premium fruit (Table 3).

**Table 3.** Financial gain of employing the four alternatives for fruit colouration.

/ no colour improvement relative to the untreated control, red background indicates financial losses, green financial gains.

#### *3.5. Sustainable Apple Colouration—Economic and Ecological Aspects*

Current criticism in the public and media targets the excessive use of plastic in the fruit supply chain [4,17–20]. The present study has shown that the use of plastics for fruit colouration could be decreased in three ways. Both the plastic mulch (Lumilys) in every other row and aluminium produced apple fruit with sufficient fruit colouration for economic marketing as premium fruit in class I and saved 50% or 100%, respectively, of plastic input in the orchard. The traditional use of plastic mulch in the alleyway in every row [21,22] is associated with 2700 m rolls × 2.6 m width × 0.1 kg/m2 ⇒ 700 kg polypropylene/ha. Using the emission factor of 2.2–3.2 kg CO2eq/kg PP (PlasticsEurope, Brussels, Belgium, 2005) [23], this is equivalent to GHGs of 1.5–2.2 t CO2eq/ha for full orchard coverage (100%). A reduction in the use of plastic mulch in every other row (or translocation) would hence save 0.75–1.1 t CO2 eq/ha in the material alone, with a multiple use, for ca. 10 years, of 75–110 kg CO2 eq/ha/year.

For one hectare, 108 kg of aluminium foil is required to substitute the plastic mulch in every row. Aluminium production requires a high energy input of 13–16 kWh/kg aluminium (Hydro Inc., Chicago, IL, USA, 2016) [24]. Hence, it often takes place where energy is cheaply available, adding long-distance transport to the final validation. We used aluminium for the first time, since recycled material became available, which contains 80% recyclat and requires only 20% of new aluminium for its production. Based on the German electricity grid emission factor of 0.6 kg CO2 eq/kWh, the GHG emissions associated with the production of primary aluminium are equivalent to 7.8–9.6 kg CO2 eq/kg aluminium. Per acreage, with a need for 108 kg aluminium per hectare, this amounts

to 821–1037 kg CO2eq/ha. Since the employed aluminium foil was made of 80% recycled aluminium with ca. 10% GHG of the primary aluminium (Hydro.com 2016), this results in 223–290 kg CO2eq/ha. However, using aluminium foil made from 100% recycled aluminium would reduce the value to 82–104 kg CO2eq/ha (Table 4). Even using the aluminium foil twice could significantly reduce the CO2 load to between 112 and 145 kg CO2 eq/ha. A combination of 100% recycled aluminium foil with a 2-year use would reduce the CO2 burden to 41–52 kg CO2 eq/ha. The decisive factor is that the use of the 100% recycled aluminium foil is associated with less CO2 emissions, which makes its use more sustainable.

**Table 4.** Greenhouse gas (GHG) emissions associated with different scenarios of employing mulches for fruit colouration.

\* range originates from 7.8–9.6 CO2eq/kg aluminium (Hydro.com, 2016)]; plastic in blue, metal in green background colour.

Both results of the colour measurements in the field (Figure 3) and machine grading (Figure 5) show the large potential for colour improvement in situations with colouration problems on the East side (morning sun, weak and short) and in the lower part (below 1 m) of the tree canopy (Figure 3).

This environmental benefit is largely caused by the fact that the aluminium foil was not laid out over the entire alleyway, due to its regular reflection property [25]. At present, the single use of the aluminium foil in an orchard protected by a hail net may be extended to two years, looking at the clean stage of the material at the end of its use in the orchard under our conditions (Figure 1a). The aluminium foil employed consisted of 80% recycled aluminium and may be used for another year, depending on its contamination. Meanwhile, there are also aluminium foils made of 100% recycled aluminium, which is sustainable. The possibility of recycling the aluminium foil used depends on the degree of contamination, and it may even be recycled again.

Despite its origin from fossil fuel, the use of reflective polypropylene (PP) mulches such as LumilysTM is sustainable, when used repeatedly (7-years warranty—estimated 10-year lifespan) and properly disposed of. At the waste collection points, the plastic material is washed, shredded and then melted—the PP pellets/granules are the source of recycled plastics such as boxes or flower pots [17].

#### **4. Conclusions**

The sustainability aspect is judged here on the basis of a sufficient red colour of the apples, which determines their sales price and orchard (bio-)economy. Short-lived biodegradable plastic mulches [4] are not suitable for this purpose, if fossil-fuel based. The use of aluminium foil made of 80–100% recycled aluminium is associated with considerably lower CO2 emissions than new aluminium, with its high energy demand. Both the recycled aluminium foil with single (or duplicate) use, and the Lumilys plastic mulch,

a material with a long lifespan, used twice a year over several years and/or in every other row, results in ecologically and economically sound horticulture and could be further improved by recycling the fairly clean white single pure plastic polypropylene.

**Author Contributions:** Conceptualization, M.M.B.; methodology, A.K.; investigation, P.H.; resources, A.K.; writing—draft preparation, P.H.; writing—review and editing, M.M.B.; supervision, M.M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** We thank Simon John von Freyend for revising the English and HJ Wiesel for the professional cultivation of the experimental fruit trees at Campus Klein-Altendorf (CKA) and MDPI for the invite to this article and for waiving the APC.

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

#### **References**