Effects of superhydrophobic sand mulching on evapotranspiration and phenotypic responses in tomato (Solanum lycopersicum) plants under normal and reduced irrigation

Abstract Irrigated agriculture in arid and semi‐arid regions is a vital contributor to the global food supply. However, these regions endure massive evaporative losses that are compensated by exploiting limited freshwater resources. To increase water‐use efficiency in these giga‐scale operations, plastic mulches are utilized; however, their non‐biodegradability and eventual land‐filling renders them unsustainable. In response, we have developed superhydrophobic sand (SHS) mulching technology that is comprised of sand grains or sandy soils with a nanoscale coating of paraffin wax. Here, we investigate the effects of 1 cm‐thick SHS mulching on the evapotranspiration and phenotypic responses of tomato (Solanum lycopersicum) plants as a model system under normal and reduced irrigation inside controlled growth chambers. Experimental results reveal that under either irrigation scenario, SHS mulching suppresses evaporation and enhances transpiration by 78% and 17%, respectively relative to the unmulched soil. Comprehensive phenotyping revealed that SHS mulching enhanced root xylem vessel diameter, stomatal aperture, stomatal conductance, and chlorophyll content index by 21%, 25%, 28%, and 23%, respectively, in comparison with the unmulched soil. Consequently, total fruit yields, total dry mass, and harvest index increased in SHS‐mulched plants by 33%, 20%, and 16%, respectively compared with the unmulched soil. We also provide mechanistic insights into the effects of SHS mulching on plant physiological processes. These results underscore the potential of SHS for realizing food–water security and greening initiatives in arid regions.


| INTRODUC TI ON
Irrigated agriculture is a crucial contributor to global food security.
Of the total cultivated land only ~20% is irrigated, and yet it contributes a disproportionately high (30%-40%) amount to the global food production (World Water Assessment Programme, 2009). Globally, many arid and semi-arid regions have emerged as agricultural hubs by exploiting freshwater resources. For instance, in the last 50 years, irrigated agriculture footprints in India, China, and Brazil have expanded by 30% (Jain et al., 2020), 52% (Zhu et al., 2013), and 52% (Carvalho et al., 2020), respectively. Most arid regions are characterized by dry winds and high temperatures that lead to substantial evaporative and transpiration losses (Al-Naizy & Simonet, 2012;Balugani et al., 2017). These losses are compensated via irrigation and due to the giga-scale of agricultural operations, irrigation claims ~70% of annual freshwater consumption worldwide (Boretti & Rosa, 2019;Globalagriculture.Org;Koech & Langat, 2018;OECD. Org). Due to the slow recharge of groundwater in arid regions and additional constraints posed by environmental pollution, the climate change, and the increasing use of water in industrial and domestic contexts, ensuring food-water-climate security has emerged as one of the most formidable global challenges for the 21st century (Famiglietti, 2014;Scanlon et al., 2012;Steward et al., 2013;Watto, Bashir, & Niazi, 2018;Watto, Mugera, et al., 2018). Therefore,sustainable technologies for enhancing water-use efficiency in arid land agriculture are needed urgently.
When water is applied to the topsoil, it is lost via evaporation, transpiration, and percolation (Hillel, 2012;Sutanto et al., 2012).
Unlike evaporation and percolation, transpiration is crucial in the photosynthetic process and maintaining the optimal temperature required for plants' metabolic processes (Grill & Ziegler, 1998;Hetherington & Woodward, 2003). Transpiration thus cannot be reduced under the constraint of high crop productivity (without genetic engineering). Percolation could be reduced by applying a water barrier layer beneath the plant root zone, but this process is laborintensive and expensive (Nkurunziza et al., 2019). Thus, curtailing evaporative loss of water from the soil is the most prudent approach for enhancing water-use efficiency in arid lands.
Evaporation and transpiration constitute evapotranspiration (ET) (Hatfield & Dold, 2019;Rawitz & Hadas, 1994). The importance of transpiration is reflected in the water-use efficiency of plants, that is, the ratio of the total biomass (root and shoot) formed to the cumulative water transpired (Kadam et al., 2015), and the transpiration efficiency (TE), that is, the ratio of the shoot biomass formed to the cumulative water transpired (Vadez et al., 2014). The TE represents an aspect of water-use efficiency that depends on the water-conducting (i.e., hydraulic) potential of the plant to facilitate shoot physiological processes that enhance biomass under different soil water scenarios (Vadez et al., 2014). Therefore, TE is of interest in water-scarce environments where crops are most sensitive to soil moisture Sinclair, 2012). Transpiration also influences plants' reproductive efficiency, defined by the harvest index (HI), that is, the ratio of total yield to total vegetative biomass produced, pinpointing the plant resources invested in yield production (Porker et al., 2020;Unkovich et al., 2010).
As noted above, evaporation is the wasteful component of ET, which should be minimized, especially in arid regions. In this context, mulching refers to the application of a vapor diffusion barrier on the topsoil to curtail water evaporation, and it has been proven to enhance the soil moisture content and provide enhanced transpiration (Farzi et al., 2017;Moitra & Sarkar, 1996;Zhang et al., 2018), plant biomass, and yields (Mukherjee et al., 2010;Ramalan & Nwokeocha, 2000;Zhang, Xiong, et al., 2017). Mulching has also been demonstrated to enhance leaf chlorophyll content (Wang et al., 2015), photosynthesis (Niu et al., 2020;Zhang et al., 2019), andTE (Balwinder-Singh et al., 2011). It also improves plant root growth and root architectural and anatomical properties such as root diameter and late xylem vessel diameter, which improve water and nutrient uptake from the soil (Larsson & Jensen, 1996;Zhan et al., 2019).
Low-density polyethylene sheets of ~0.1 mm thickness have been used extensively for mulching in developed countries (Kasirajan & Ngouajio, 2012). Their application in the developing world is also on the rise, for example, consumption in China is set to exceed 2 million tons per year by 2024 (Liu et al., 2014;Wang et al., 2013).
Recent findings on the leaching of phthalates from plastic mulches into soils and their adverse effects on the soil quality and microbial activity over the long term are also concerning (Shah & Wu, 2020). Therefore, sustainable mulching technologies are needed.
We have recently developed a nature-inspired mulching technology--superhydrophobic sand (SHS) Mishra et al., 2017)--that is comprised of common sand grains or sandy soils coated with a nanoscale layer of paraffin wax. The micro-and nanoscale surface roughness of the sand grains (size distribution: 100-700 μm) and the hydrophobic nature of wax give rise to superhydrophobicity Arunachalam et al., 2020;Mahadik et al., 2020;Odokonyero et al., 2021). When laid on the topsoil with a sub-surface irrigation system, a 5-10 mm-thick SHS layer prevents capillary rise of water Domingues et al., 2018) and insulates the wet soil from solar radiation and dry air, thereby acting as a diffusion barrier for the water vapor (Gallo Jr et al., 2021). As water evaporation is arrested, more water is available in the plant root region, which boosts plant health in arid regions. In fact, our multi-year field trials of SHS on tomatoes (Solanum lycopersicum), wheat (Triticum aestivum), and barley (Hordeum vulgare) under arid land conditions in western Saudi Arabia have revealed significant enhancements in plant growth and yields .
Compared with plastic mulches, SHS is environmentally benign as it is made from sand/sandy soil and paraffin wax, and the latter is degraded by soil microbes (Marino, 1998;Roper, 2004). After about 9 months of SHS application, paraffin wax is degraded due to microbial activity and solar radiation and SHS gets incorporated in the soil, obviating land-filling; additionally, there are no detectable effects on soil microbial compositions . Despite these promising field results, quantitative insights into the effects of SHS on ET under varying irrigation scenarios as well as phenotypic traits responsible for yield enhancement are lacking.
In response, here, we quantify the effects of SHS mulching on tomato plants in growth chambers and pinpoint ET dynamics and phenotypic responses in terms of the following traits: plant height, stomatal conductance, stomatal pore size (aperture), leaf chlorophyll content, fruit yield, HI, TE, fresh mass, dry mass, xylem vessel, and root diameter. We investigate these effects of SHS mulching under normal (N) and reduced (R) irrigation scenarios, that is, 100% and 50% of pot capacity, respectively.

| Plants and SHS mulch
Tomato plants (S. lycopersicum), Seminis variety (St. Louis, Missouri, US), were purchased from local seed stores in Jeddah, Saudi Arabia. SHS was manufactured using common silica sand and paraffin wax following the optimized protocol detailed in our previous work Mishra et al., 2017). Briefly, sand grains with a size distribution ranging from 100 to 700 μm were utilized. First, paraffin wax was dissolved in an organic solvent (hexane) in a rotary evaporator and the sand was added. The mass ratio of wax: sand was 1:600. Our process led to a conformal coating of wax on each grain. The solvent was then recovered by decreasing the pressure and increasing the temperature and condensed for reuse, leaving behind SHS; resulting SHS grains had a 20 nm-thick wax coating.

| Plant growth conditions, treatments, and experimental design
Tomato seeds were sown in plastic trays using a potting mix from Stender AG and grown in growth chambers (Percival ® Scientific chamber, PGC-6L, Geneva Scientific LLC). After 4 weeks, the seedlings were transplanted into pots 1870 cm 3 in volume (15 cm top diameter × 10.5 cm bottom diameter × 14.5 cm height) containing approximately 2.4 kg of local sandy soil. The pots were watered via sub-surface irrigation to the following levels every 2 days: 100% pot capacity for N irrigation (i.e., the maximum soil moisture content after drainage of excess water from fully saturated potted soil) and 50% of this pot capacity for R irrigation. The irrigation level of each pot was determined gravimetrically. Thirty-two pots were prepared, including 16 with plants and 16 without plants, the latter of which were used to quantify evaporative losses.
The pots were separated into two groups: those mulched with the SHS and the unmulched ones. In each group, half of the pots were subjected to N irrigation while the other half was subjected to R irrigation.
The soil mulching entailed a 1.0-cm thick layer of SHS on the potting soil (approximately 265 g SHS/pot). Thus, there were four treatment combinations (SHS-N, unmulched soil-N, SHS-R, and unmulched soil-R), each with four replicates with and without plants. All pots were subjected to complete randomization in a 2 × 2 factorial design involving two experimental factors (i.e., soil mulching and irrigation regime), each with two levels . The growth chamber consisted of two levels (tier-1 and tier-2) with identical environmental conditions and pots were completely randomized in both tiers. During the growth period, nutrient solutions were applied to each pot every 2 weeks at the fol-

| Evapotranspiration
ET was partitioned into evaporation and transpiration by performing gravimetric measurements of pots every 2 days until the final harvest. The water loss from each pot was monitored, and water was added to compensate for the loss. The daily ET was recorded as the total water lost from each pot containing plants between the time of irrigation to the time of weighing as:

| Evaporation
Here, we consider that the evaporative loss of water from the pots with plants is the same as that in the pots without plants. The daily water loss through evaporation was then estimated from pots without plants as:

| Transpiration
The daily transpiration was determined as:

| Soil water content
Soil water content (SWC; i.e., the pot capacity) was determined gravimetrically. Potting soils each of known mass (i.e., mass of pot  (3) Transpiration = (ET − evaporation). plus soil) were fully saturated with water and allowed to drain freely overnight. After 16 h, the final weight of the pot assembly was determined, ensuring that no excess water was still dripping from the pot.
The difference between the initial and final pot weight represented the total SWC of the pot (i.e., 100% pot capacity). Thus, we maintained the SWC at a relatively constant level for both 100% or 50% pot capacity by compensating for the water lost through ET after each gravimetric measurement at 2-day interval, as has been done by prior researchers (Pellegrino et al., 2004;Ray & Sinclair, 1998).
Supplementary irrigation allowance of 50-200 g water/pot was added at different plant developmental stages so as to account for the effects of plant growth on the SWC determination.

| Stomatal conductance and leaf chlorophyll content
Leaf stomatal conductance was determined using an AP4 Porometer (Delta T, Cambridge, UK). Measurements were performed on three young but fully expanded leaves once a week (between 10:00 and 12:00), and the mean conductance for each treatment combination was calculated.

The leaf chlorophyll content index (CCI) was measured using a CCM-200
Chlorophyll Content Meter (Optic-Sciences, Inc.), with measurements being performed on three young but fully expanded leaves.

| Leaf stomatal pore and root anatomical features
Microscopic analyses of leaf stomata and root anatomic structures were performed on samples collected on the final date of harvest.
Two fully expanded upper leaves were cut from each plant (between 11:00 and 11:30) and immediately immersed in 70% ethanol inside 50 ml centrifuge tubes until they were analyzed. After 2 weeks, the preserved leaf tissues were washed with deionized water three times, added to 40 ml of concentrated sodium hypochlorite, and left to stay for approximately 4 h. The leaf tissues were re-washed with deionized water, and fresh 70% ethanol was again added onto the samples. This clearing process removed the chlorophyll from the leaves and rendered them a white visual appearance. The stomata of the cleared leaves were then observed using a digital microscope (Leica DVM6) equipped with the tools to measure the length and width of the stomatal pores. The total stomatal aperture area was calculated using the formula for the area of an ellipse, as where a and b represent the semi-major axis (or radius) and semi-minor axis (or width) of the ellipse, respectively.
Roots were washed by spraying tap water to remove soil debris; then root samples 10 cm in length from the growing tip were cut using a razor, washed in deionized water, and fixed in 70% ethanol until the time for microscopic analysis. Next, thin cross sections were made along the root using a sharp razor blade while holding the root protruding from a small polystyrene sheet in an upright posture.
The razor blade was perpendicular to the root axis. Cross sections were removed from the blade and put into a Petri dish of water. Two uniform cross sections of roots per plant were analyzed using digital microscope to examine the root anatomical structures. The diameter of xylem vessels and the total root diameter were quantified and compared between SHS mulched and unmulched plants.

| Plant growth, fruit yield, and biomass
The plant height was measured every week during the 98 days experimental period. Tomatoes were harvested and weighed at the time of harvest. During the final harvest, the shoots and roots of each plant were weighed, and then put in paper bags and oven-dried at 105°C for 4 days to determine their dry mass.

| HI and TE
After harvest, the HI was calculated for each plant as the ratio of total fruit yield (fruit mass per plant) to total fresh mass (shoot and roots) per plant, as: The TE of each plant was calculated as the ratio of dry shoot biomass in grams to the amount of water transpired in kilograms, as:

| Data analysis
Data analysis and graphical presentations were performed using Origin Pro Software (2019 version). A three-way analysis of variance (ANOVA) was used to analyze the effects of soil mulching, irrigation regimes, growth stage (i.e., time), and their interactions on ET. For the variables averaged across time (i.e., time-independent), a two-way factorial ANOVA was performed to determine the effects of soil mulching, irrigation regimes, and their interactions. In each analysis, mean comparisons were done using the Tukey test at the p < .05 level of statistical significance.

| Plant growth
Representative snapshots of plants grown under each condition (i.e., mulched and unmulched soils under N or R irrigation) at different weeks during their growth, from transplanting (week 0) to the final (4) Stomatal aperture area = × a × b, Total fruit yield Total fresh biomass .
fruit harvest (week 12), are shown in Figure 1. Overall, the plants grown in mulched soils were larger and appeared healthier than those grown in unmulched soil.

| Soil water content
Daily SWC corresponding to each irrigation level is detailed in Figure 2a. For both N and R irrigation scenarios (i.e., 100% and 50% pot capacity, respectively), the SWC was slightly increased during days 40-98 to compensate for the increase in plant size and water demand.
The fluctuations in SWC below the initial pot capacities observed at five occasions (i.e., between 30 and 50 days) represent 5 days when pot weights were recorded without adding water until the following day due to unavoidable circumstances. However, the plants were not severely stressed on these occasions. When averaged across both mulched and unmulched soils, the mean SWC was 0.62 ± 0.01 and 0.33 ± 0.01 kg/pot under N and R irrigation, respectively. Thus, we were able to maintain plant growth at 50% of the normal irrigation (i.e., the R irrigation strategy).

| Evapotranspiration
The daily ET significantly differed between the N and R irrigation scenarios and accounted for approximately 50% of the daily

| Soil evaporation and transpiration
Throughout the growth period, the daily evaporation was significantly lower from mulched soil than that in unmulched soil under both N and R irrigation, as shown in Figure 2c. Consequently, plants in mulched soil had higher daily transpiration than in unmulched soil under both irrigation scenarios, as shown in Figure 2d.

| Cumulative ET
There was no significant difference in the cumulative ET loss between mulched and unmulched soils under N irrigation; both had a mean cumulative ET of 13.9 ± 0.7 kg/plant over the 98 days of observation, as shown in Figure 3a. Under R irrigation, the cumulative ET was 13% higher in SHS-mulched soil than in unmulched soils (with mean values of 6.97 ± 0.57 and 6.06 ± 0.78 kg/plant, respectively).

| Cumulative soil evaporation and transpiration
In the mulched soils, the total cumulative evaporation under N and R irrigation accounted for 5% and 9% of the total ET while transpiration was 95% and 91%, respectively. In unmulched soils, evaporation under N and R irrigation accounted for 21.5% and 31.5% of the total ET whereas transpiration was 78.5% and 65.5%, respectively ( Figure 3b,c). Overall, SHS mulching suppressed evaporation and enhanced transpiration under N irrigation by 78% and 17%, respectively relative to the unmulched soils. These effects of SHS mulching were similar in percentage term to those under R irrigation but differed quantitatively as the amount of water under N was higher than in R irrigation.

| Plant height
Throughout the growth period, the maximum plant height in SHSmulched soil significantly exceeded that in unmulched soil by 9% (p = .005) and 11% (p < .001) under N and R irrigation, respectively ( Figure 3d). In particular, a more rapid increase in the plant height was observed during 10-45 days following transplanting; after 70 days, plant height started to level off.

| Dependence of ET on plant growth stage
Mean ET, evaporation, and transpiration during the days 0-30, 31-60, and 61-98 are compiled in Table 1. Overall, the mean ET, evaporation, and transpiration increased with time; significant differences were present in the mean ET and transpiration between each timeframe. However, no significant difference was found in the mean evaporation with time (p = .392), as evidenced by the three-way ANOVA results. Significant two-way interactions were present between mulching and irrigation regimes on transpiration (p = .03) and between irrigation regime and growth stage on ET (p < .001), evaporation (p = .04), and transpiration (p < .001).
F I G U R E 2 (a) Soil water content (SWC) in pots maintained at pot capacity for each treatment and irrigation regime, (b) daily changes in ET, (c) evaporation from the soil, and (d) transpiration between SHS mulched and unmulched soils under N and R irrigation strategies throughout plant growth. Each data point is a mean of four (n = 4) replicates; error bars are standard errors (±SE) of the means

| Effects of SHS mulching on physiological traits, biomass yield, and fruit yield
Our two-way ANOVA demonstrated the effect of SHS mulching was significant on evaporation and transpiration (p < .05) but not significant (p > .05) on the total ET (

| Main effects of soil mulching and irrigation regimes
We observed a 27.7% higher stomatal conductance for plants in

| Interaction effects of mulching and irrigation regimes
Significant interaction effects of mulching and irrigation were found on shoot fresh mass, shoot dry mass, and total fresh mass per plant. As shown in Figure 5a, shoot fresh mass was 27% higher in mulched soil than in unmulched soil under N irrigation (p < .001); whereas under R irrigation, the shoot fresh mass was higher in mulched soil by 13% than in unmulched soil (p = .041). In Figure 5b,  Abbreviations: df, degrees of freedom; ET, evapotranspiration; SHS, superhydrophobic sand.
increased the total fresh mass by 26.5% than in unmulched soils ( Figure 5c; p < .001); whereas under R irrigation, the total fresh mass per plant was 13.5% higher in mulched soil than unmulched soil (p = .037).

| Leaf stomatal status
Microscopy revealed that leaf stomatal pores for plants grown in mulched soils (Figure 6a-i and iii) were larger than those in unmulched soils (Figure 6a-ii and iv); the mean stomatal aperture (area) was 25% larger in mulched soils than unmulched soils regardless of the irrigation regime (p = .0019; Figure 6b-i). Furthermore, the mean stomatal aperture was 34.6% higher under N irrigation than R irrigation (p < .001; Figure 6b-ii).

| Root anatomical structure
Finally, root anatomical features were analyzed to determine the shoot-root feedback linkages with the observed trends in ET as a function of mulching and irrigation scenarios (Figures 7a-i-iv). The xylem vessel diameter was 21% larger for roots grown in mulched soil than unmulched soil (p = .001; Figure 7b-i). However, the difference in xylem diameter under differing irrigation scenarios was not statistically significant (p = .343; Figure 7b-ii). Additionally, total root diameter increased in mulched soil by 28% relative to the unmulched soil (p = .0177; Figure 7c-i), whereas it decreased from R irrigation to N irrigation by 20% (p = .0067; Figure 7c-ii).
To establish the correlation between our results described above, a regression analysis showed significant causal relationships between leaf stomatal conductance and stomatal aperture (Figure 8a), transpiration F I G U R E 4 Box plots showing effects of (i) soil mulching (SHS mulch vs.

| DISCUSS ION
Here, we draw together our experimental results to explain the mechanistic insights into the effects of SHS mulching on phenotypic responses in tomato plants. We start with the effects observed  unmulched soil, and (ii) under N and R irrigation. Each box represents the data distribution from 16 leaf samples (n = 16) with the median value along the mid-line. The white dot inside the box represents the mean value, the upper and lower sections of the box represent the 25% and 75% confidence intervals, respectively, and the whiskers on the box represent the 1.5 interquartile range. Percentage differences between treatments are presented along with the corresponding p-values derived from two-way ANOVA using p < .05 level of statistical significance F I G U R E 7 Cross sections of tomato roots from contrasting soil mulching and irrigation scenarios (a, i-iv) and main effects of soil mulching and irrigation regimes (b and c). Box plots showing the effects of (i) soil mulching and (ii) irrigation regimes on (b) root xylem vessel diameter and (c) total root diameter. Each box represents the data distribution from 16 root samples (n = 16) with the median along the mid-line. The white dot inside the box represents the mean value, the upper and lower sections of the box represent the 25% and 75% confidence intervals, respectively, the whiskers on the box represent the 1.5 interquartile range, and dots outside the box indicate outliers. Percentage differences between treatments are presented along with the corresponding p-values derived from two-way ANOVA using p < .05 level of statistical significance F I G U R E 8 Regression analysis showing the relationship between (a) leaf stomatal conductance and stomatal aperture, (b) stomatal conductance and total transpiration, (c) total fresh mass and total transpiration, (d) total dry mass and total transpiration, (e) total fruit yield per plant and total transpiration, and (f) total fruit yield per plant and TE. Each dot on the graph indicates an individual plant grown in SHS mulch or unmulched soil under N or R irrigation above-ground followed by those below-ground and then their interrelationships.
SHS mulching significantly reduced water evaporation from the soil and enhances SWC, which in turn promoted transpiration fluxes (Figures 2 and 3). The enhanced transpiration is attributed to the high stomatal conductance arising from the enlargement of the stomatal aperture in the mulched plants (Figures 4 and 6). This increase in stomatal pore sizes occurred through the active adjustment of the turgor pressure within their guard cells due to higher SWC (Blatt, 2000). Consequently plants' ability for gas-exchange (e.g., enhanced CO 2 uptake) and photosynthesis increased; these effects have been noted by other researchers (Bertolino et al., 2019;Condon et al., 2002;Putra et al., 2012).
SHS-mulched tomato plants had larger xylem vessels and root diameters than the unmulched plants (Figure 7), which is consistent with a previous study on the effects of plastic mulching on maize plants (Zhan et al., 2019). Larger xylem vessels and root diameter boosted the water-use efficiency, the TE, and the shoot biomass under N and R irrigation scenarios (Figures 4 and 5). These findings are also in agreement with previous studies on wheat plants (Kadam et al., 2015;Larson & Funk, 2016). Generally, large xylem vessels and root diameter can enhance the hydraulic conductivity of the soil water in the root zone (Bertolino et al., 2019;Comas et al., 2013;Niklas, 1985;Odokonyero et al., 2017), which has a direct influence on shoot responses (Odokonyero et al., 2017;Tardieu & Parent, 2017;Zarebanadkouki et al., 2016).
SHS mulching enhanced tomato plants' CCI, which contributed to higher fruit yield, biomass, and HI ( Figure 4). These effects necessitate robust synergism in root-shoot hydraulics to conduct nutrients such as nitrogen via the transpiration stream to the leaves.
Nitrogen availability in the soil and its uptake impact the total chlorophyll content in plant leaves (Bassi et al., 2018). Thus, the higher the CCI, the more efficient is the plant's photosynthetic capacity in mulched soil, leading to the increased growth, biomass, and yields (Condon et al., 2004;Haefele et al., 2009).
Next, we point to some observations that require further investigation to be fully explained. We observed a negative correlation between tomato fruit yield and TE (Figure 8f), although higher TE is known to increase crop yields in water-limited environments (Christy et al., 2018;Condon et al., 2004;Coupel-Ledru et al., 2016;Rebetzke et al., 2002;Sinclair, 2012;Vadez & Ratnakumar, 2016).
We consider that TE is a physiologically complex trait that depends on multiple physiological parameters, including photosynthesis, stomatal conductance, mesophyll conductance (i.e., the diffusion of CO 2 from sub-stomatal cavities to the sites of carboxylation in the chloroplasts) (Flexas et al., 2008;Flexas & Medrano, 2002), and other conditions that determine carbon balance and growth in plants (Natarajan et al., 2021). As a result, the conditions responsible for high TE in one situation may be associated with a low TE in another environment (Sinclair, 2012). Next, against common expectation, we did not observe significant differences in the TE of mulched and unmulched soils despite significant differences in the SWC, stomatal aperture and stomatal conductance. Instead, a lower stomatal conductance was associated with a higher TE under R irrigation than under N irrigation. This could be related to the drought avoidance mechanism whereby plants under R irrigation reduce their stomatal opening to optimize water use under the constraint of soil water deficit (Tardieu, 2005). Consequently, higher TE can be attained under R irrigation than under N irrigation as we observed in our study and also noted by others (Balwinder-Singh et al., 2011).
Lastly, we discuss the limitations of this study vis-à-vis realworld implications. Obviously, real arid conditions such as in western Saudi Arabia are significantly harsher than the conditions of the present controlled-environment study. Furthermore, unlike pots, there is no restriction to root growth and volume in the field (Ray & Sinclair, 1998;Ronchi et al., 2006;Wang et al., 2001); and evaporation from pots is affected by the potted area covered by leaves, unlike in the fields where plant area and density are not limited by space (Lu et al., 2018). Here, we draw the reader's attention to the main results of our 4-years-long field trials of SHS mulch on tomato (S. lycopersicum) plants . These experiments revealed that SHS led to significant reduction in the evaporation fluxes under N irrigation (30%-38%) and enhancement in the fruit yields (27%-72%), which is totally in agreement with the present study.
Therefore, insights furnished by this controlled-environment investigation into the interrelatedness of the various phenotypic responses are at least qualitatively relevant. In the future, we plan to quantify the effects of SHS mulching on root growth via noninvasive methods (Brown et al., 1991;Daly et al., 2015;Downie et al., 2012;Zhu et al., 2011), instantaneous leaf gas-exchange, and carbon isotope composition (δ 13 C) (Bednarz et al., 1998;Evans et al., 1986;Monneveux et al., 2006) toward a deeper understanding of above and below-ground processes in controlled environments and field settings.

| CON CLUS ION
We investigated the effects of SHS mulching on ET and plant phenotypic traits inside growth chambers under normal and reduced (50% of normal) irrigation scenarios. We found that irrespective of the irrigation regime, SHS mulching significantly suppressed evaporation and enhanced transpiration by 78% and 17%, respectively, compared with the unmulched soils (absolute numbers vary). Mulched plants had larger stomatal aperture (25%) and stomatal conductance (28%), and larger xylem vessel (21%) and root diameter (28%). These effects synergistically boosted rootshoot hydraulics. As a result, mulched plants had superior phenotypic features such as plant height (9%-11%), fruit yield (33%), total fresh mass (27%), and total dry mass (20%), relative to their unmulched counterparts. Taken together, these findings provide mechanistic insights into the beneficial effects of SHS mulching and underscore this technology's potential for achieving sustainable food security and greening initiatives in arid regions such as the Middle East and beyond.