Effects of soil water and nitrogen on drought resilience, growth, yield, and grain quality of a spring wheat

Abstract Drought imposes a significant challenge for crop production. However, little is known about the impact of drought priming and nitrogen (N) application and their interactive effects on drought resilience, yield, and grain quality in wheat. Spring wheat (cv. Stettler) was grown in plastic pots (25 cm diameter) with high, moderate, and low soil water levels and received N (added N) or without N (no N added), and subjected to acute drought for 10 days, then rewatering at the tillering stage. Canopy temperature, maximum efficiency of photosystem II, and normalized difference vegetation index were measured at 3-day intervals during drought-recovery periods to quantify drought resistance and resilience. Above-ground dry matter, straw dry matter, seed dry matter, harvest index, and grain N, phosphorus (P), and zinc (Zn) concentrations were determined. Both moderate- and low-water-grown plants had higher drought resistance than high-water-grown plants. The addition of N alleviated acute drought stress in high- and moderate-water-grown plants but exacerbated drought stress in low-water-grown plants. Both high and moderate water resulted in higher grain yields, but had a lower harvest index than low water. The highest and lowest grain N were observed in the low- and high-water-grown plants, respectively. The addition of N increased N and N:P in grains but decreased grain Zn:N. This study showed that moderate drought priming along with N application can improve drought resistance, yield, and grain quality. The results also indicated that canopy thermal imaging is a useful tool for high-throughput quantification of the drought resistance of wheat.


Introduction
Drought frequency and severity are significant challenges for crop production in many areas of the world, particularly in semi-arid to arid regions. Crop production systems must therefore be resilient to both the frequency and severity of drought to meet the increasing needs of global food production . Climate-smart crop cultivars and sustainable crop management practices can improve the resilience of crops and crop production systems in the face of climate change. A number of crop management practices, including plant stress priming (i.e., stress hardening) and precise nitrogen (N) application, are known to reduce the negative effects of drought on the growth and yield of wheat (Bicego et al. 2019;Cui et al. 2019;Wang et al. 2019). However, little is known about the impact of drought priming and N application and their interactive effects on drought resilience, yield, and grain quality in wheat (Huai 2017).
Recent advances in understanding of plant stress priming and stress memories have provided new insights for improving crop resilience ). Stress priming, also known as stress hardening, training, or conditioning, can induce a stress memory favorable for plants to be more tolerant to additional stress events in present or future generations (Fan et al. 2018;Wang et al. 2019). For instance, drought priming at the vegetative stage is reported to alleviate post-anthesis drought-induced yield loss in wheat primarily due to maintaining higher leaf water potential, photosynthetic rate, and antioxidative activities (Wang et al. 2014;Abid et al. 2016a;Cui et al. 2019;Tankari et al. 2021). Similarly, a number of studies show that N supply has the potential to alleviate drought-induced yield reduction in wheat by maintaining greater photosynthetic and metabolic activities even at low leaf water potential (Abid et al. 2016b). In contrast, it was reported that N application could increase drought stress in plants due to greater plant biomass and transpiration water loss and a lower root-shoot ratio (Shi et al. 2014). This clearly indicates that the effects of N application on drought sensitivity in wheat vary with drought severity, and thus it is important to examine the levels of soil water and N that maximize drought resilience in wheat.
Drought resistance and resilience are considered as important indicators for sustainable crop production in the face of climate change. Drought resistance is the ability of a plant or cropping system to remain unchanged during drought (Allison 2004;Mondal et al. 2016). It is usually calculated as the difference in plant performance measured between control and droughted plants at the end of drought stress (Vogel et al. 2012). Drought resilience is defined as the capacity of a plant or cropping system to absorb drought stress and how quickly it recovers upon rewatering/rainfall (Harrison 1979). It is, therefore, the degree of drought recovery and is generally calculated as the difference in plant performance measured between control and rewatered plants at the end of the recovery period (Vogel et al. 2012). Orwin and Wardle (2004) developed resistance and resilience indices to examine the resistance and resilience of soil microbes to heat stress, with an index of +1, indicating full resistance or resilience, and an index of −1, indicating no resistance or resilience. These indices were recently used to quantify soybean (Glycine max L.) resilience to drought under field conditions and revealed that soybean drought resilience was partly related to the recovery of photosynthetic traits (Elsalahy and Reckling 2022). In another study, the resilience index was applied to assess the effect of legume species diversity on drought resilience (Elsalahy et al. 2020).
The application of high-throughput plant phenotyping techniques is critical for the quantification of crop resilience to drought and for improving crop drought resilience through crop management practices and plant breeding. Infrared thermal imaging of crop plants is considered a useful high-throughput tool to determine canopy temperature (CT), plant water relationships, and drought resistance (Blum et al. 1982). Higher CT is an indicator of plant water stress as it has a strong negative association with leaf water potential, biomass production, and wheat grain yield (Blum et al. 1982;Zia et al. 2012;Bhandari et al. 2021). As plant N level and water use are tightly interdependent, thermal imaging has been used for evaluating plant N status and wheat grain yield (Guo et al. 2016). The use of leaf reflectance as a normalized difference vegetation index (NDVI) is commonly used for rapid assessment of crop biomass, plant N status, and grain yield estimation (Kizilgeci et al. 2021). Similarly, the chlorophyll fluorescence assay is rapid and widely used to examine plant photosynthetic efficiency under abiotic stress conditions (Murchie and Lawson 2013). While these high-throughput plant phenotyping techniques are extensively used to assess the drought tolerance, growth, and yield of crops, little attention has been paid to the highthroughput quantification of crop drought resistance and resilience. We applied these plant phenotyping techniques to quantify drought resistance and resilience in a spring wheat cultivar, as spring wheat is one of the most widely grown crops in Canada (Statistics Canada 2021). Plant biomass, grain yield, and grain quality are determined as performance indicators and are well known to be affected by soil water and N treatments (Saini and Westgate 1999;Wardlaw and Willenbrink 2000;Bicego et al. 2019). The objectives of the study were (i) to examine the impacts of drought priming and/or N addition on drought resilience, growth, yield, and grain quality of wheat, and (ii) to assess the suitability of high-throughput plant phenotyping techniques to quantify drought resilience in wheat.

Experimental design, treatments, and growth conditions
The study was designed as a randomized complete block with four replications and carried out at the Saskatoon Research and Development Centre of Agriculture and Agri-Food Canada, Saskatoon, SK, in a walk-in growth chamber. Fortyeight plastic pots (25 cm diameter and 18 cm tall) were filled up to 15 cm in height with a locally obtained mixture of 90% ww loam soil and 10% ww sand. The loam soil contained 40% sand, 35.5% silt, and 25.5% clay. Other characteristics of the loam soil were pH (1:2) = 8, organic matter = 3.35%, cation exchange capacity (CEC) = 34.6 meq per 100 g, electrical conductivity (EC) = 2.0 dS m −1 , NO 3 -N = 45.9 ppm, and available P (modified Kelowna) = 17.5 ppm. Urea was used as a source of N and 0.454 g of urea per pot was applied to 24 pots (i.e., added N = AN) and no urea was applied to the remaining 24 pots (i.e., no N = NN). Four wheat seeds (cv. Stettler) were sown after a third of the 48 pots were watered either to soil water levels of HW = high--80%-90%, MW = moderate--60%, and LW = low--40% field capacity. Seedlings were thinned to two plants per pot after stand establishment. At the tillering stage, four pots of the N and soil moisture treatments were exposed to acute drought by withholding water for 10 days followed by rewatering for another 10 days and the rest of the four pots were watered as per soil water treatments (i.e., control) from each soil water and N regime. The growth chamber was maintained at 25 • C during the day for 16 h, 16 • C during the night for 8 h, and a relative humidity of 70%. Thermal images of two plants per pot were recorded using a FLIRT640 camera (FLIR Systems AB, Sweden) at 3-day intervals over a 10-day acute drought and 10-day rewatering period. Thermal images of each plant were processed for mean CT on a desktop computer using FLIR Thermal Studio Suite (Teledyne FLIR, Wilsonville, OR, USA). Normalized difference vegetation index (NDVI) with a GreenSeeker (Tremble Sunnyvale, CA, USA) and maximum efficiency of PSII (F v /F m ) in leaves with a chlorophyll fluorometer (FluorPen FP-100; Photon Systems Instruments, Czech Republic) were recorded on two plants per pot at the same time thermal imaging was conducted. All wheat plants were moved to a greenhouse and grown with usual watering and intercultural operations until maturity after acute drought and rewatering treatments. Both plants from each pot were harvested at maturity. Seeds were separated from each plant and dried to a constant weight at 72 • C, and seed dry matter and above-ground dry matter per plant were recorded. The straw dry matter was determined by subtracting the seed dry weight from the above-ground dry matter. The harvest index (HI) was calculated as the ratio of seed dry matter to above-ground dry matter. Wheat plants were randomized at 3-day intervals in both the walk-in growth chamber and greenhouse to reduce the variability caused by growth conditions. Determination of total N, P, and Zn in wheat grains For the determination of grain NO 3 -N, approximately 0.5 g of dried and ground grain sample was added to 90 mL 0.01 mol/L CaCl 2 in a glass vessel. The solution was agitated on a reciprocating shaker for 30 min, and then filtered through a paper filter. Nitrate-N was measured by automated colorimetry after reduction by hydrazine and complexing with n-(1-naphthyl)ethylenediamine dihydrochloride. For total Kjeldahl N (TKN) analysis, approximately 1 g of dried and ground grain samples was digested with concentrated sulfuric acid and catalysts at 400 • C for 1 h. This converted organic N to ammonium. During analysis, a strong alkali converted ammonium to ammonia, which was captured in a weak boric acid solution and titrated with a standard acid to equilibrium. Total-N was calculated as a sum of NO 3 -N and TKN and reported as mg g −1 .
For the determination of total P and total Zn, approximately 0.5 g of dried and ground grain sample was digested in 10 mL of concentrated nitric acid in a glass vessel with microwave assistance. The digest was diluted with ultrapure water and analyzed with an inductively coupled plasma optical spectrometer. The concentration of total-P was reported as mg g −1 and the concentration of total-Zn was reported as μg g −1 .

Determination of drought resistance and resilience
The drought resistance and resilience of the wheat cultivar were quantified using CT, F v /F m , and NDVI recorded at 3-day intervals over drought and rewatering periods. Wheat drought resistance (i.e., plant functional ability to remain unchanged) was determined by minimum difference in CT/F v /F m /NDVI between droughted and control plants at the end of drought, while drought resilience (i.e., plant functional ability to absorb disturbance and recover) was determined by the minimum difference in CT/F v /F m /NDVI between droughted and control plants at the end of rewatering (Vogel et al. 2012). Moreover, drought resistance and resilience indices were quantified according to the equations below, as reported by Orwin and Wardle (2004): where D o is the difference in CT/F v /F m /NDVI between control and droughted plants at the end of stress; C o is the CT/F v /F m /NDVI ( • C) of the control plant, and D x is the difference in CT/F v /F m /NDVI between control and droughted plants at the end of recovery (rewatering). The value of the resistance and resilience index is bounded by −1 and +1. A resistance index value of +1 indicates that the stress had no detrimental effect on plants (high resistance), while a value of −1 indicates that the plant showed no resistance to the stress. Similarly, a resilience index value of +1 at any time point of the rewatering period indicates that the plant had full recovery (high resilience), while a value of −1 indicates that the plant showed no resilience to the stress (Orwin and Wardle 2004).

Statistical analysis
Data were checked for normality using the Shapiro-Wilks test, and a Box-Cox transformation was used to meet the statistical assumptions of linearity and normal distribution where necessary. Data were subjected to analysis of variance on each sampling date separately with the MIXED procedure using the nlme statistical package within R. In the analysis, soil water, N application, and acute drought and their interactions were considered fixed effects, and replication was considered a random effect. A Tukey's honestly significant difference was used to compare treatment means when the F-test showed significance at P < 0.05.

Canopy temperature
Soil water had a significant (P < 0.05) effect on CT during the acute drought period, with the highest and lowest CT recorded in LW-and HW-grown plants, respectively. Soil water had no significant effect on CT on most sampling dates during the drought recovery period. Nitrogen had no effect on CT over the drought and recovery periods. Acute drought significantly (P < 0.01) increased CT on all sampling dates during the drought period, while rewatering lowered CT in the plants exposed to acute drought compared with their corresponding control plants on most of the sampling dates. Acute drought also resulted in a higher relative increase in CT regardless of treatments at the beginning of stress. This was followed by a lower relative increase in CT in all plants except those grown with HW and without N, where CT continued to increase relative to their control plants as acute drought progressed (Fig. 1). On the other hand, there was a higher relative decrease in CT upon rewatering regardless of treatments, which continued to decrease until mid-way through the recovery period. A significant interaction (P < 0.01) between soil water and N was observed for the relative change in CT on the 7th day of acute drought. It was noted that N addition to either HW-or MW-grown plants alleviated acute drought stress, while N addition to LW-grown plants exacerbated acute drought stress in terms of relative change in CT (Fig. 2a). A significant interaction (P < 0.01) between soil water and N for relative change in CT was also noted on the 4th day of rewatering (Fig. 2b). The HW-grown plants that received N had a higher relative decrease in CT than those that did not receive N. Conversely, LW-grown plants that did not receive N had a higher relative decrease in CT than those that received N.

Drought resistance and resilience
Soil water had a significant (P < 0.05) effect on drought resistance in terms of CT on the 10th day of acute drought stress, with higher drought resistance in both MW-and LWgrown plants than HW-grown plants. Soil water had no effect on wheat resilience based on CT at the end of the recovery period. Also, soil water had no effect on drought resistance and resilience as determined by F v /F m and NDVI in wheat. The addition of N marginally (P = 0.07) improved drought resistance based on CT in wheat on the 10th day of acute drought stress but had no effect on wheat resilience to drought at the end of the recovery period. However, N addition had no effect on drought resistance and resilience as determined by F v /F m and NDVI in wheat. There was a significant (P < 0.01) interaction between soil water and N for drought resistance in terms of CT on the 7th day of acute drought stress (Fig. 3a). The HW-and MW-grown plants with N addition and the LW-grown plants without N showed equally higher drought resistance than the HW-grown plants without N and the LW-grown plants with N in terms of the minimum difference in CT between drought and control plants. A significant (P < 0.05) interaction between soil water and N was also observed for drought resistance on the 7th day of the acute drought period (Fig. 3b). Nitrogen addition contributed to plant drought resistance, which was dependent on the level of soil water since N supply resulted in higher drought resistance in HW-grown plants but decreased drought resistance in LW-grown plants. All treatments except HW-grown plants without N were fully drought-resilient on the 7th day of rewatering, although there were no differences among the treatments (Fig. 4a). Nitrogen addition decreased drought resilience in both HW-and LW-grown plants but increased drought resilience in MW-grown plants. There was no significant interaction (P = 0.11) between soil water and N for wheat resilience to drought determined by CT (Fig.  4b). There were also no significant interactions between soil Fig. 2. Interactive effects of soil water (i.e., high = HW, moderate = MW, and low = LW) and nitrogen (N; i.e., added N = AN, no N = NN) on % relative change in canopy temperature (CT) of a spring wheat (cv. Stettler) on the 7th day of acute drought stress (a) and the 4th day of rewatering (b). N = 4. Bars are mean ± SE. Bars with the same letters are not significant at P < 0.05. water and N for drought resistance and resilience as determined by F v /F m and NDVI in wheat.

Normalized difference vegetation index
The effects of soil water and N on NDVI were generally not significant (P > 0.05) at the beginning of acute drought stress. Drought had a significant (P < 0.05) effect on NDVI at the end of drought stress (10th day of acute drought), with a lower NDVI value in droughted plants than the well-watered control plants. There was also a significant (P = 0.02) interaction between soil water and N on the NDVI on the 10th day of acute drought stress (Fig. 5). The NDVI values were higher with than without N in both HW-and MW-grown plants. In contrast, the NDVI values were lower with than without N in LW-grown plants. There was no significant (P > 0.05) effect of soil water on plant NDVI responses on all sampling dates except on the 4th and 7th days of rewatering during the recovery period. Plants grown with both HW and MW exhibited significantly (P < 0.05) higher NDVI values than those grown with LW on the 4th and 7th days of rewatering. Nitrogen addition had no significant (P > 0.05) effect on plant recovery Fig. 3. Drought resistance of a spring wheat (cv. Stettler) grown with different levels of soil water (i.e., high = HW, moderate = MW, and low = LW) and nitrogen (N; i.e., added N = AN, no N = NN) and exposed to acute drought stress for 10 days in a walk-in growth chamber. Drought resistance of the plants was quantified by CT between drought and control plants (a) drought resistance index (b) on the 7th day of acute drought stress. N = 4. Bars are mean ± SE. Bars with the same letters are not significant at P < 0.05. from drought stress in terms of NDVI. There was a significant (P < 0.05) interaction between soil water and acute drought stress for NDVI on the 4th day of rewatering. All droughted plants grown with different levels of soil water showed full recovery in terms of NDVI compared with their respective control plants, but the extent of recovery varied among treatments (data not shown).
Dry matter, grain yield, and grain quality Soil water content had a highly significant (P < 0.01) effect on straw, above-ground dry matter, seed dry matter, and HI (Table 1). Both HW-and MW-grown plants were statistically similar and had higher straw, above-ground dry matter, and seed dry matter than LW-grown plants. The LW-grown plants displayed higher HI than HW-and MW-grown plants.
Overall, the addition of N significantly (P < 0.01) increased straw, above-ground, and seed dry matter. Acute drought significantly (P < 0.05) decreased straw dry matter, but increased Fig. 4. Drought resilience of a spring wheat (cv. Stettler) grown with different levels of soil water (i.e., high = HW, moderate = MW, and low = LW) and nitrogen (N; i.e., added N = AN, no N = NN) and exposed to acute drought stress for 10 days and rewatering for another 10 days in a walk-in growth chamber. Drought resilience of the plants was quantified by CT between drought and control plants (a) drought resilience index (b) on the 7th day after rewatering. N = 4. Bars are mean ± SE. Bars with the same letters are not significant at P < 0.05.

Fig. 5.
Interactive effects of soil water (i.e., high = HW, moderate = MW, and low = LW) and nitrogen (N; i.e., added N = AN, no N = NN) on NDVI of a spring wheat (cv. Stettler) on the 10th day of acute drought stress. N = 4. Mean ± SE. The same letters are not significant at P < 0.05. Table 1. Effects of soil water and nitrogen (N) on straw dry matter, above-ground dry matter (ADM), and seed dry matter of a spring wheat cultivar (Stettler) exposed to acute drought by withholding water for 10 days and rewatering for another 10 days in a walk-in growth chamber. HI. There were significant (P < 0.05) interactions between soil water and N application for straw, above-ground dry matter, and seed dry matter (Figs. 6a, 6b, and 6c). Nitrogen addition significantly (P < 0.01) increased straw, above-ground dry matter, and seed dry matter in both HW-and MW-grown plants, but not in the LW-grown plants. The significant interaction (P < 0.05) between N addition and acute drought was also noted for straw and above-ground dry matter. Soil water levels had significant (P < 0.01) effects on grain N and P concentrations and N:P and Zn:N in wheat grains ( Table 2). The highest grain N concentration and N:P were found in the LW-grown plants followed by the MW-grown plants, while the lowest grain N concentration was found in the HW-grown plants. Both HW-and MW-grown plants had higher grain P concentrations than the LW-grown plants. Plants grown with HW had higher Zn:N in grain than those grown with LW. Nitrogen addition significantly (P < 0.05) increased grain N concentration and N:P but decreased grain Zn:N regardless of soil water and drought treatments. Acute drought had no significant (P > 0.05) effect on the grain nutrient concentrations studied (Table 2). There were also no significant treatment interactions for grain nutrient concentrations.

Discussion
Effects of soil water, N application, and acute drought on CT and NDVI in wheat CT is an integrative signal that reflects plant water status as determined by an equilibrium between root water uptake and shoot transpiration (Blum et al. 1982;Berger et al. 2010). Our study showed that plants grown with LW and HW had the highest and lowest CT, respectively. The high CT in the plants under LW might be due to stomatal closure (Blum et al. 1982). The nonsignificant effect of N application on CT in wheat in our study might be due to the dominant effects of soil water and drought on CT. In contrast, Gao et al. (2019) reported that high stomatal conductance and leaf N were associated with increased water uptake from soils, leading to cooler canopies in wheat. Acute drought stress for 10 days significantly increased CT in wheat regardless of soil water and N treatments. A relatively cooler canopy in the MW-and LW-grown plants than the HW-grown plants is reflective of a clear drought adaptation in those plants regardless of N application. Our observed drought adaptation of wheat plants grown with MW and LW is consistent with other studies that reported higher drought tolerance in drought-primed plants due to the maintenance of higher leaf water potential, photosynthetic rate, and antioxidative activities in wheat (Wang et al. 2014;Abid et al. 2016a;Cui et al. 2019). The role of N addition in alleviating drought-caused yield reduction in wheat has been reported as both positive (Abid et al. 2016b) and negative (Shi et al. 2014). It is therefore mostly unknown to what extent soil water/drought along with N application can improve drought tolerance in wheat. Our results indicate that N addition alleviated drought in HW-and MW-grown plants but exacerbated drought in LW-grown plants in terms of relative change in CT on the 7th day of acute drought stress. Overall, plants were able to recover from drought-induced damage upon rewatering, as shown by the lower CT of the plants regardless of soil water and N treatments. However, the extent of drought recovery varied among the treatments. For instance, N addition led to greater drought recovery in HWgrown plants but not in LW-grown plants in terms of relative change in CT on the 4th day of rewatering. This suggests that the effects of drought priming and N addition are not simply additive, and the beneficial effect of N addition on drought tolerance and drought recovery is related to soil water levels.
NDVI is widely used as a proxy to monitor plant biomass, leaf area index, leaf greenness, and grain yield across temporal and spatial scales (Laidler et al. 2008;Thapa et al. 2019).
Our results indicate that soil water and N had no effect Fig. 6. Interactive effects of soil water (i.e., high = HW, moderate = MW, and low = LW) and nitrogen (N; i.e., added N = AN, no N = NN) on straw dry matter (a), above-ground dry matter (b), and seed dry matter (c) of a spring wheat (cv. Stettler) at maturity. The plants were exposed to acute drought stress for 10 days and rewatering for another 10 days. N = 4. Bars are mean ± SE. Bars with the same letters are not significant at P < 0.05. Table 2. Effects of soil water and nitrogen (N) on grain N, P, and Zn concentrations, and N:P and Zn:N ingrains of a spring wheat cultivar (Stettler) exposed to acute drought by withholding water for 10 days and rewatering for another 10 days in a walk-in growth chamber.  (Shepherd and Griffiths 2006;Cossani and Reynolds 2012;Mohammed et al. 2018).
Our results also showed that N addition increased NDVI in the MW-and HW-grown plants but decreased NDVI in the LW-grown plants on the 10th day of acute drought stress. Drought-induced reductions in NDVI in LW-grown plants could be due to reduced N uptake in low-moisture soils (Plett et al. 2020). Moreover, all droughted plants, regardless of N addition, showed full drought recovery, but the extent of the recovery varied among different levels of soil water in terms of NDVI on the 4th day of rewatering. This implies that soil water, but not N addition, was most critical for plant drought recovery in terms of NDVI.
Effects of soil water and N addition on drought resistance and resilience of wheat Drought resistance and resilience of wheat have been quantified using a method described by Vogel et al. (2012) and also using the equations developed by Orwin and Wardle (2004). High-throughput plant phenotyping techniques such as thermal imaging, chlorophyll fluorescence (F v /F m ), and canopy reflectance (NDVI) were applied to quantify drought resistance and resilience of wheat grown with different levels of soil water and N using the two different methods (Orwin and Wardle 2004;Vogel et al. 2012). Our results indicate that CT was more useful for quantifying wheat drought resilience compared with F v /F m and NDVI. Overall, MW-and LW-grown plants had higher drought resistance than HW-grown plants based on CT on the 10th day of acute drought stress, indicating a beneficial effect of drought priming on drought resistance in wheat (Abid et al. 2016a). The addition of N also marginally (P = 0.071) improved the drought resistance of wheat as determined by CT at the end of acute drought stress. Further analysis with the method used by Vogel et al. (2012) showed that N addition positively contributed to drought resistance in HW-and MW-grown plants, but not in LW-grown plants. The results obtained here are consistent with earlier reports, which showed an improved tolerance of wheat to moderate drought in terms of photosynthetic and antioxidative activities and grain yield under higher rates of N application (Abid et al. 2016b;Cui et al. 2019). Moreover, Zhong et al. (2019) demonstrated a decrease in drought resistance in a C 3 perennial grass when N was applied under severe drought. The drought resistance index as determined by CT according to the equation used by Orwin and Wardle (2004) indicated a clear contribution of N addition to drought resistance in wheat, with the magnitude of the contribution dependent on the level of soil water. This suggests that although this drought index was first introduced to quantify soil microbial resistance to heat stress (Orwin and Wardle 2004), it equally works to quantify drought resistance in plants. Our results on the drought index also show the beneficial effect of drought priming on acute drought resistance of plants grown under MW and LW. Wheat resilience to drought, as measured by CT according to the method described by Vogel et al. (2012) at the later part of the rewatering period, indicates that all the plants except those grown under HW and without N addition were equally drought resilient. While the results are marginally significant, N addition decreased resilience in terms of CT in both HW-and LW-grown plants, but increased drought resilience in MW-grown plants on 7th day of rewatering. The loss of drought resilience in the plants grown with N addition under HW could be as a consequence of irreversible drought-induced cellular damage due to low antioxidative activities resulting from a lack of drought priming (Abid et al. 2016a). Moreover, a decreased resilience to drought in the plants grown with N addition under LW might be due to irreversible cellular damage caused by severe water stress during the acute drought period.
Effects of soil water, nitrogen addition, and acute drought on dry matter, grain yield, and grain quality Soil water and N application greatly affect the growth, yield, and quality of wheat based on genotype, crop growth stage, and severity of drought (Ozturk et al. 2021). Our results indicate that both HW-and MW-grown plants displayed higher straw, above-ground, and seed dry matter than LWgrown plants regardless of N application, acute drought, and rewatering. A greater HI was noted in LW-grown plants than in both HW-and MW-grown plants. This suggests that higher dry matter partitioned to grains from stems and leaves in LW-grown plants during grain filling when the contribution of flag leaf photosynthesis decreased due to limited soil water. Furthermore, acute drought significantly decreased straw dry matter, but increased HI in wheat regardless of soil water and N addition, suggesting increased dry matter partitioning from stems and leaves to grains as flag leaf photosynthesis might be severely compromised during acute drought stress (Biswas and Jiang 2011;Abid et al. 2016a). Further analysis showed that N addition increased straw, above-ground, and seed dry matter in both HW-and MWgrown plants, but not in the LW-grown plants. These results agree with those on wheat drought resistance based on CT under different soil water and N treatments. This finding also suggests that preflowering drought stress and dry matter production are important determinants for yield even though stress is removed during the grain filling stage (Wang et al. 2014).
Availability of soil water can modulate carbon and N assimilation rates, alter uptake of other nutrients, and hence change grain protein content and other grain quality traits (Bicego et al. 2019). In our study, plants grown under LW and HW had the highest and lowest grain N concentrations, respectively. The high grain N concentration in the plants grown with limited soil water might be due to lower carbon assimilation and/or higher remobilization of N from vegetative tissues to developing grains, as evidenced by the lower above-ground dry matter production and higher HI. On the other hand, plants grown under HW had higher grain Zn:N than those grown under LW, suggesting that limited soil water could decrease Zn uptake from soils and its assimilation in grains. The addition of N significantly increased grain N concentration and N:P ratio, as found in previous studies in wheat (Chen et al. 2011;Long et al. 2017;Bicego et al. 2019).
Our study also showed that N addition significantly decreased the Zn:N ratio in grains. This implies that N addition can improve protein but lower Zn in wheat grains.

Conclusion
In this study, we found that wheat plants grown with LW had a higher CT than those grown with HW over the drought period. The addition of N was found to be beneficial to cope with drought stress for HW-and MW-grown plants, but not for LW-grown plants. This was further supported by the beneficial effects of N addition on drought resistance in HW-and MW-grown plants, but not in LW-grown plants. The advantage of drought priming on acute drought resistance based on CT was observed for the plants grown without N addition under LW. This indicates that optimization of soil water and N addition is essential to improving wheat drought resistance. The addition of N also increased straw dry matter, aboveground dry matter, grain yield, and grain N concentration, but decreased Zn:N in grains. This suggests that while N addition can improve yield and grain protein, it could lower Zn levels in wheat grains. Finally, according to our findings, thermal imaging appears to be the most useful high-throughput plant phenotyping technique to examine the drought resistance, growth, and yield of wheat.