Introduction

Wheat is one of the food crops that are produced, consumed, and stored most often in the world. It is now cultivated all over the world and is the main source of food. It is a staple food in many nations, providing over one-third of the world’s population with more than half of their calories and protein. One of the most affordable cereal crops, wheat is linked to higher human nutritional values (Marcek et al. 2019). According to estimates, in order to feed the world’s 9 billion people by 2050, wheat production must rise by 60% worldwide (Borisjuk et al. 2019). Changes in global precipitation patterns due to climate change are predicted to increase the frequency of droughts, which would exacerbate yield depression. One of the main factors restricting wheat yield is water stress, which has a negative impact on global food availability (Hussain et al. 2019). Drought episodes have the potential to significantly reduce yield and pose a serious threat to the sustainability of crop production Zahoor et al. (2017). Significant wheat yield losses of up to 29% have been attributed to drought (Daryanto et al. 2016). Herein, it is imperative to investigate the different wheat cultivars sensitivity to drought, especially as the frequency of droughts becomes more extreme as climate change intensifies. Drought has adverse effects on enhancement, growth, and wheat quality (Wasaya et al. 2021).

Plant adaptations to drought vary greatly according to plant species, developmental stage, stress intensity, and duration (Jaleel et al. 2008). Plant morphological and physiological parameters, including leaf area, plant height, relative water content, leaf water potential, and chlorophyll contents, are severely impacted by drought (Salim et al. 2021). The generation of reactive oxygen species poses a threat to plant cells because it can damage cell organelles, such as nucleic acids, metabolic enzymes, chloroplasts, mitochondria, membranes, and lipids (Hasanuzzaman et al. 2020). Oxidative stress induced by drought causes plant abnormalities in biochemical and physiological strategies that lead to plant cell death (Mittler 2002). Photosynthesis is mostly affected by drought due to the fact that it damages the process of photosynthesis and results in stomatal closure (Li et al. 2021).

According to Ghobadia et al. (2013), different plant species respond to water stress in different ways. Regarding the evacuation mechanism, the plant finishes its life before a drought occurs. Plants compete with drought in tolerance mechanisms through stomatal closure and reduced transpiration rates. In drought-tolerant mechanisms, plants maintain an excellent distribution of total assimilates under drought by maintaining the shoot/root ratio and increasing photosynthetic pigment for a meritorious distribution of all assimilates (Ashfaq et al. 2016).The amount of chlorophyll varies during drought conditions and serves as a measure of the photosynthetic capacity of plant cells. Plant tolerance to drought is significantly influenced by carotenoids content. Drought reduces the amount of proteins available for harvesting light and inhibits the synthesis of chlorophyll a and b (Bojovi and Stojanovi 2005). Biochemical and physiological criteria are encouraged to facilitate fast and easy screening of highly drought-tolerant cultivars. Herein, cultivar evaluation under controlled conditions could be one of the most incredible techniques for selecting the most gorgeous drought-tolerant cultivars (Salim et al. 2021).

Developing and selecting new cultivars that are adapted to drought is one of the best strategies to combat it. In addition to morpho-physiological criteria, a drought index is a useful tool for identifying cultivars that are drought tolerant. It depends on yield losses under stressed versus non-stressed conditions. Our aim was to assess four Egyptian bread wheat cultivars (Giza 171, Sakha 95, Misr 1, and Misr 3) drought tolerance based upon morpho-physiological criteria, stress index, as well as yield and yield attributes.

Materials and Methods

In 2020/2021 and 2021/2022, two consecutive seasons, field trials were carried out at the Ismailia Agricultural Research Station, Ismailia Governorate (Lat. 30° 35′ 30″ N, Long. 32° 14′ 50″ E, 10 m above the sea level), Egypt under sprinkler irrigation. For the experiment, four bread wheat cultivars, Misr 1, Misr 3, Giza 171, and Sakha 95 were acquired from the Wheat Research Department, Field Crops Research Institute, Agricultural Research Center, Egypt (Table 1).

Table 1 Origin, pedigree, and selection history of the studied wheat cultivars
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Before planting and during preparing the seed bed, soil specimens were collected from the top layer (0–30 cm). The mechanical and chemical properties of the samples were examined (Table 2). The monthly mean values of the relative humidity, minimum, and maximum temperature were computed based on daily records of the meteorological conditions (Fig. 1).

Table 2 Analysis of the cultivated soil, both mechanically and chemically, before the two studied seasons
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Fig. 1
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The experimental site’s meteorological conditions during the two studied seasons; Tmin minimum Temperature, Tmax maximum temperature, Tavr average temperature, RH relative humidity

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Irrigation treatments were done immediately after the completion of germination: 50% of field capacity for drought and 100% of field capacity for control. Three replicates of a split-plot design were employed, with irrigation treatments and cultivars assigned to the main and sub-plots, respectively. Thirty kg P2O5 fed−1 was applied at seed bed preparation, 24 kg K2O fed−1 was applied to plants during the tillering stage in two equal doses within ten-day intervals, and 120 kg N fed−1 was applied in five identical dosages. During heading and at the harvest stage, ten samples were gathered from each treatment to estimate the growth parameters and three samples were gathered for the biochemical assays.

Leaf area is equal to length × Breadth X 0.75 (Quarrie and Jones 1979).

Leaf area index is equal to leaf area/land area covered by plant.

Specific leaf area is equal to leaf area/dry mass (Beadle 1993).

Degree of succulence is equal to water amount/leaf area (Delf 1912).

Shoot distribution is equal to fresh mass/length (Arduini et al. 1994).

Shoot density is equal to dry mass/length (Arduini et al. 1994).

Flag leaf relative water content (RWC) was estimated using Schonfeld et al. (1988).

Flag leaf saturation water deficit (SWD) was adopted by Schonfeld et al. (1988).

The membrane stability index (MSI) was adopted according to Sairam et al. (2002).

Chlorophylls and carotenoids Chlorophylls and carotenoids were estimated according to Jichtenthaler and Buschmann (2001).

Nutrients Uptake

The whole plants with spikes during heading and plant shoots with leaves and grains at maturity were oven dried at 75 °C and then crushed. In sulfuric acid and hydrogen peroxide, 0.5 g of dried crushed material undergoes digestion (Yang et al. 2011). For K estimation, the digested specimens were determined on the flame photometer model Jenway PFP 7. N uptake and P uptake were read using the auto-analyzer spectrophotometer model Tecator 5032–5032-controller, FIAstar 5010 analyzer, and 5017 sampler.

Yield Analyses

The heading date was determined based on field observations by counting the days from sowing to 50% of flowering for the experimental units.

The maturity date was calculated by counting the days from sowing to the plant’s physiological maturity.

Spike length (cm) was determined at the time of maturity from the spike’s base to its highest point.

Spikes number/m2 were computed for plants from the middle lines on the square meter once they reached full maturity.

The spike weight was measured on the sensitive balance.

Grains number/spike were computed by taking the mean number of grains in ten spikes, which was obtained by manually cutting these ears.

Hundred kernel weights (g) were counted randomly and weighed using the sensitive scale.

Biological yield (g/m2) was calculated using the ten center lines, i.e., distance within rows was 20 cm (2 × 3 m) for each testing unit, and was extracted as g/m2.

Grain yield (g/m2) was determined by weighing the grain after it had been separated from the straw and was extracted as g/m2.

Straw yield (g/m2) was estimated as g/m2.

Harvest index = grain yield / biological yield.

Stress Indexes

The stress sensitivity index (SSI) was estimated based on Fisher and Maurer (1978).

The stress tolerance index (STI) was calculated according to Kristin et al. (1997).

Yield Stability Index (YSI) was estimated according to Bouslama and Schapaugh (1984).

Mean productivity (MP) was adopted by Hossain et al. (1990).

Tolerance intensity (TOL) was estimated according to Rosielle and Hamblin (1981).

Statistical Analysis

Utilizing analysis of variance (ANOVA) methodology, the gathered data were statistically assessed. According to Steel et al. (1997), the least significant difference (LSD) test (p ≤ 0.05) was utilized to compare the significant differences among treatment means using CoHort/CoStat software, Version 6.311, with distinct letters point to noticeable variations between treatments at p ≤ 0.05. The analysis of the main components was carried out using Microsoft Excel, version 2010.

Results and Discussion

Changes in Growth Vigor

Changes in Shoot Growth Vigor

Drought is one of the environmental elements that have a big impact on plant growth and development which exert notably remarkable effects on plant life (Iqbal et al. 2023a). Morphological traits were proven to be important factors in wheat yield control. According to the current results in Table 3 and Fig. 2, water stress resulted in a discernible decline in fresh and dry shoot masses, shoot length, number of tillers, shoot density, and shoot distribution during heading for all four wheat cultivars in both seasons. These results showed conformity with the findings of Ahmad et al. (2022) who claimed that the adverse effects of drought on wheat plants caused a significant decrease in morphological features. The findings thus support Munns (2002) theory that the detrimental effects of water stress on shoot growth may be attributed to stress decreasing plants’ capacity to absorb water, which leads to a rapid decline in growth rate and a series of metabolic alterations that are similar to those brought on by water stress. Also, a decrease in photosynthesis as well as disruptions in mineral uptake, protein synthesis, and/or carbohydrate metabolism could be the cause of the decrease in shoot growth during a drought (Tawfik et al. 2006).

Table 3 Effect of water treatment, wheat cultivars, and their interaction on shoot growth vigor of wheat cultivars at heading during the two studied seasons
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Fig. 2
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Effect of di-interaction between irrigation and cultivars on shoot growth vigor of wheat cultivars at heading during the two studied seasons. The standard error of the mean (n = 10) is shown by vertical bars. Distinct letters point to noticeable variations across treatments at p ≤ 0.05

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Cultivars also differed significantly with respect to shoot growth criteria in both the first and second seasons, recording the highest values in Sakha 95 compared to Giza 171, Misr 1, and Misr 3. All parameters related to shoot growth, with the exception of shoot density, were significantly impacted by the combination of cultivars and irrigation. Sakha 95 wheat cultivar recorded the maximum values of the above-mentioned criteria under both stressed and unstressed conditions; in contrast, the minimum values of shoot growth parameters were demonstrated for Misr 3 in both study seasons. Hence, all four cultivars had the same behavior in terms of drought tolerance by decreasing shoot growth (Fig. 2).

Changes in Flag Leaf Growth Vigor

Flag leaves of wheat are normally considered the main organ of photosynthesis. Hence, specific area, leaf area index, and degree of succulence are important adaptive indicators of stress tolerance. Results showed in Table 4 and Fig. 3 that drought caused a noticeable reduction in all flag leaf growth parameters; flag leaf fresh weight, dry weight, leaf area, degree of succulence, and leaf area index during heading for all four wheat cultivars in both seasons. This is explained by the fact that less water uptake means lower water content in growing leaves, which is manifested by lower leaf relative water content and a higher leaf saturation water deficit (Table 5 and Fig. 4). The present findings align with Nezhadahmadi et al. (2013), who reported that the adverse impacts of drought on wheat plants led to a notable decline in their morphological characteristics and productivity. Also, Mubeen et al. (2013) reported that higher irrigation produced higher leaf area and the other leaf parameters.

Table 4 Effect of water treatment, wheat cultivars, and their interaction on flag leaf growth vigor of wheat cultivars at heading during the two studied seasons
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Fig. 3
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Effect of di-interaction between irrigation and cultivars on flag leaf growth vigor of wheat cultivars at heading during the two studied seasons. The standard error of the mean (n = 10) is shown by vertical bars. Distinct letters point to noticeable variations across treatments at p ≤ 0.05

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Table 5 Effect of water treatment, wheat cultivars, and their interaction on flag leaf relative water content (RWC), flag leaf saturation water deficit (SWD), and flag leaf membrane stability index (MSI) of wheat cultivars at heading during the two studied seasons
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Fig. 4
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Effect of di-interaction between irrigation and cultivars on flag leaf relative water content (RWC), flag leaf saturation water deficit (SWD), and flag leaf membrane stability index (MSI) of wheat cultivars at heading during the two studied seasons. The standard error of the mean (n = 3) is shown by vertical bars. Distinct letters point to noticeable variations across treatments at p ≤ 0.05

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These findings support the theory put forth by Ghanem et al. (2019) that a decrease in growth may be an adaptive feature that helps plants withstand drought by allowing them to sequester energy and assimilates that would otherwise be needed for shoot and leaf growth into molecules that act as barriers to the drying out process. This decrease may also be explained by the ways in which drought modifies plant–water relations, lowers plant water content (Table 5 and Fig. 4), induces osmotic stress, inhibits cell division and expansion, and, ultimately, decreases total plant growth (Tables 3 and 4; Figs. 2 and 3).

Cultivars also differed significantly with respect to flag leaf growth criteria, recording the highest values in Sakha 95 and the lowest values in Misr 3 in the first and second seasons. Furthermore, in both study seasons, there was an interaction effect between irrigation and cultivars on every flag leaf growth criterion with the exception of the leaf area index and degree of succulence (Fig. 3). In this case, it seems that the Sakha 95 wheat cultivar is the most resilient to mitigate the negative effects of drought on all growth criteria related to shoot and leaf area. Conversely, the least tolerant cultivar was Misr 3.

Changes in Water Relations

Estimating water relations is a crucial step in any study on drought tolerance in plants. The current study’s Table 5 and Fig. 4 show that, for all four wheat cultivars, water stress resulted in a significant increase in SWD of the wheat flag leaf during heading and a further decrease in RWC% when compared to control plants. The observed decrease in RWC was closely related to the increase in SWD, which was associated with a significant decrease in almost all shoot criteria (the number of tillers, shoots biomass, and shoots length). Therefore, a decrease in RWC under drought indicates a decrease in plant cell turgor pressure, leading to growth retardation. Our findings are consistent with those of Ahmad et al. (2022), who reported a significant reduction in wheat plant RWC due to drought. In water-stressed plants, the growth of drought-tolerant genotypes results in higher relative water content (Ahmadizadeh 2013). Variations in water uptake between cultivars and water loss could be the cause of the decrease in RWC in leaves. Cultivars also differed significantly with respect to RWC and SWD, whereas more RWC and less SWD were recorded by Sakha 95, which outperformed Giza 171, Misr 1, and Misr 3 in the first and second seasons. Therefore, a high RWC indicates a drought-tolerant cultivar.

The combination between irrigation and cultivars showed that Sakha 95 was the most tolerant cultivar; it described the highest values of RWC and the lowest value of SWD under stress and non-stress conditions, followed by Giza 171, Misr 1, and Misr 3, the least tolerant cultivars with the lowest values of RWC and the highest values of SWD in both study seasons (Fig. 4). Herein, RWC and SWD can serve as an effective tool for selecting wheat cultivars with excellent drought tolerance. Therefore, Sakha 95 and Giza 171 were noticed to be superior cultivars in drought tolerance, as they reduced leaf water loss under drought.

Change in Membrane Stability Index (MSI)

The cell membranes are considered the most important cellular targets commonly encountered under various stress conditions (Levitt 1980). The degree of membrane damage reflects the plant’s tolerance to drought. The extent of membrane damage illustrates the plant’s drought tolerance. In regards to this, the membrane stability index (MSI) is a widely used indicator of plant membrane stress. Data stated in Table 5 and Fig. 4 showed a significant decrease in MSI% for the four wheat cultivars during heading compared to the control ones in both seasons. These results are in accordance with Zayed et al. (2023). Water stress increases the generation of free radicals that cause lipid peroxidation of biomembranes, reflecting stress-induced tissue damage (Harinasut et al. 2003). Also, drought stress impairs the stability of the cell membrane, which exacerbates damage to the cell wall (Mubarik et al. 2021). Moreover, membranes are a major site of cellular and organelle injury (Candan and Tarhan 2003), as reactive oxygen species can react with unsaturated fatty acids to cause peroxidation of essential membrane lipids in plasma lemma or intracellular organelles (Smiroff 2005), leading to leakage of cell contents and cell death (Basu et al. 2021) and, consequently, reduced membrane stability.

Cultivars also differed significantly with respect to MSI, Sakha 95 recorded the maximum MSI compared to Giza 171, Misr 1, and Misr 3 in the first and second seasons. As shown in Fig. 3, the interaction between irrigation and cultivars had a significant effect on MSI in both study seasons. The combination of irrigation and cultivars showed that the Sakha 95 wheat cultivar depicted the maximum MSI value under stressed and non-stressed conditions, followed by Giza 171, Misr 1, and Misr 3, which showed the minimum MSI value, which means that Sakha 95 was the most tolerant cultivar to water stress and Misr 3 was the least tolerant cultivar. These results showed conformity with Salim et al. (2021) who stated that when comparing drought-tolerant genotypes to non-tolerant genotypes, there were notable increases in MSI.

Changes in Pigment Content

Photosynthesis is a crucial process in the life cycle of crops Iqbal et al. (2023b). Leaf pigments are an important trait related to environmental interactions (Peremarti et al. 2014). Furthermore, chlorophyll content has been established as an indicator to assess a plant’s tolerance to drought (Flexas et al. 2002). The data from this work showed that drought significantly reduced the total pigment, Chl a + b, Chl a/b, Chl b, and Chl a of all four wheat cultivars during heading in both seasons. At the same time, drought increased the carotenoid production of four wheat cultivars during the heading under drought (Table 6 and Fig. 5). Moreover, a decrease in photosynthetic pigments may be linked to a decrease in shoot growth during a drought (Table 3 and Fig. 2). Drought inhibits photosynthesis by destroying the photosynthetic system, breaking down the machinery that makes chlorophyll, and reducing the uptake of nutrients from the soil and their translocation within plants (Sikuku et al. 2010). Additionally, it has been reported that drought damages the membranes of thylakoid (Rana et al. 2017), which has a negative impact on the synthesis of chlorophyll as well as the distribution and accumulation of photoassimilates (Medrano et al. 2002).

Table 6 Effect of water treatment, wheat cultivars, and their interaction on plant pigments at heading during the two studied seasons
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Fig. 5
figure 5

Effect of di-interaction between irrigation and cultivars on carotenoids, Chl b, and Chl a of wheat cultivars at heading during the two studied seasons. The standard error of the mean (n = 3) is shown by vertical bars. Distinct letters point to noticeable variations across treatments at p ≤ 0.05

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Regarding the pigment content of flag leaves, cultivars varied greatly as Sakha 95 recorded the higher levels of total pigment, chlorophyll a + b, chlorophyll a/b, and chlorophyll a, while Giza 171 and Misr 1 and Misr 3 recorded lower levels in the first and second seasons. These results showed conformity with Ahmad et al. (2022) who stated that when comparing drought-tolerant wheat genotypes to non-tolerant genotypes, there were notable increases in pigments content. Additionally, a more pronounced decrease is observed in wheat genotypes susceptible to drought (Lv et al. 2018).

Hence, drought shortens the plant’s life span, causing accelerated senescence that leads to pigment degradation. In addition, chlorophylls have been considered an indicator to assess plant drought tolerance, where a decrease in chlorophyll as a result of drought is considered a marker of chlorophyllase-induced oxidative stress injury (Hosseinzadeh et al. 2018).

Compared to the control value, water stress significantly increased the levels of carotenoid in the current study. Carotenoids are thought to play a critical protective role as a photodynamic effect, and they can also function as photoreceptors (Lawlor 1989). According to Frank and Cogdell (1995), carotenoids play several important functions in the plant cell such as light harvesting through singlet status energy transmission, photoprotection through chlorophyll triplet status annealing, singlet oxygen scraping, excess energy dissolution, and plastid structural stability. Hence, Sakha 95 and Giza 171 were the most drought-tolerant wheat cultivars, as they enhanced carotenoids biosynthesis under water stress.

Moreover, there was an interaction effect between irrigation and cultivars on the total pigment, Chl a + b, Chl a/b, Chl b, and Chl a (Table 6). The combination of irrigation and cultivars showed that the Sakha 95 wheat cultivar had the highest Chl a, Chl b, carotenoids, and total Chl values. Moreover, Misr 3 wheat cultivar had the lowest values in the both seasons. Thus, it indicates that Sakha 95 was the most drought-tolerant cultivar (Fig. 5).

Changes in Days to Heading and Days to Maturity

According to Motzo and Giunta (2017), one of the most critical variables in determining grain yield for wheat cultivars under specific environmental conditions is the length of phenological growth. There were differences between cultivars in terms of how long it took from emergence to heading. The present results unequivocally demonstrated that heading (days) and maturity (days) in both seasons were significantly reduced as a result of drought (Table 7 and Fig. 6). Therefore, the decrease might be due to a reduction in the life span of plant leaves under drought and thus accelerated senescence. This might be due to more development and growth as well as greater duration to produce spike. These outcomes agree with those of Iqbal et al. (2016), who found that irrigation treatments had a significant impact on the number of days it took for heading or maturity. Also, our findings align with those of Poudel et al. (2020), who observed a significant reduction in the number of days associated with wheat cultivars due to drought. Moreover, Blum (2010) demonstrated that early flowering, or the “earliness feature,” is a mechanism for escaping drought, particularly in late developmental stages.

Table 7 Effect of water treatment on wheat cultivars and their interaction on heading, maturity, number of spikes/m2, spike length, and spike weight of wheat cultivars during the two studied seasons
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Fig. 6
figure 6

Effect of di-interaction between irrigation and cultivars on days to heading, days to maturity, no. of spikes/m2, spike length, and spike weight of wheat cultivars during the two studied seasons. The standard error of the mean (n = 3) is shown by vertical bars. Distinct letters point to noticeable variations across treatments at p ≤ 0.05

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Cultivars also varied significantly with regard heading (days) and maturity (days), with the Sakha 95 wheat cultivar having more days to heading and maturity, followed by Giza 171, Misr 1, and Misr 3 in the first and second seasons. Herein, it has been proven that drought shortens the vegetative growth period by speeding up the growth stage and significantly reducing heading (days) and maturity (days) particularly, in less drought-tolerant cultivars.

Moreover, there was an interaction between irrigation and cultivars on heading (days) and maturity (days). The interaction indicated that Misr 3 was the earliest cultivar, taking 68.67 and 66.67 days to heading as well as 121.67 and 118.66 days to maturity under unstressed and stressed conditions, respectively, while Sakha 95 was the most delayed cultivar, taking 73.66 and 67.66 days to heading as well as 127.66 and 121.67 days to maturity under unstressed and stressed conditions, respectively (Table 7 and Fig. 6).

Changes in Yield and Yield Components

Yield is an outcome of the combination of plant metabolic responses, and any variable that affects the metabolism at any stage of plant growth and development might have an impact on the yield. Grain yield is the most important selection criterion for drought tolerance because it is the most essential financial attribute of wheat plants (Ali et al. 2023). Data in Tables 7 and 8; Figs. 6 and 7 showed that drought clearly revoked all yield and yield attributes: spike length, number of spikes/m2, spike weight, 100-kernel weight, grain number/spike, harvest index, biological yield/m2, grain yield/m2, and straw yield/m2 of all four wheat cultivars in the first and second seasons. This reduction could attributed to less total source strength, which is the result of the product of the total leaf area and the average photosynthetic efficiency of that leaf area, affects crop yield (Table 4 and Fig. 3). These results aligned with Sallam et al. (2019) and Ahmad et al. (2022) who noted a reduction in wheat yield and yield-related traits under water stress conditions.

Table 8 Effect of water treatment on wheat cultivars and their interaction on grain numbers/spike, 100 kernel weight, biological yield, grain yield, straw yield, and harvest index of wheat cultivars during the two studied seasons
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Fig. 7
figure 7

Effect of di-interaction between irrigation and cultivars on crop yield, grain yield, straw yield, and harvest index of wheat cultivars during the two studied seasons. The standard error of the mean (n = 3) is shown by vertical bars. Distinct letters point to noticeable variations across treatments at p ≤ 0.05

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Also, drought-induced dehydration led to protein denaturation, ROS release, and a reduction in plant biomass, which in turn reduced wheat yield and all of its characteristics (Keyvan 2010). Moreover, Investigation has shown that longer grain-filling times, higher chlorophyll contents, longer turgor, or a combination of these may contribute to improved yield and yield-related characteristics in wheat and barley under drought (Li et al. 2009).

Regarding wheat cultivars, the studied four wheat cultivars showed great and significant differences in all measured yield attributes and yield, as well as harvest index, in both study seasons. Sakha 95 wheat cultivar was found to have significantly higher values of all traits, including spike length, number of spikes/m2, spike weight, 100-kernel weight, grain number/spike, harvest index, biological yield/m2, grain yield/m2, and straw yield/m2. Furthermore, it proved to be more drought-tolerant than the other tested cultivars. These results aligned with those of Ahmad et al. (2022) who pointed out Faisalabad 2008, one of the eleven wheat genotypes tested showed significantly higher levels of all traits and was also shown to be more drought tolerant than the other genotypes.

Grain yield was lowered by drought, which resulted in fewer and lighter grains (Table 8 and Fig. 7). Consistent with these findings, Karim et al. (2018) reported that drought suppresses photosynthesis and, in turn, limits the amount of assimilates that reach the grain, resulting in a significant decrease in the weight of individual grains and, consequently, in yield. Also, Abid et al. (2018) indicated that grain development is the most important stage of wheat growth beneath drought since grain size is dictated by drought-induced disruption of the duration and rate of grain filling, which diminishes yield. Additionally, cellular proliferation can be inhibited by drought stress, which reduces plant size and yield (Iqbal et al. 2023a).

The reduced number of grains may be caused by the short spike length under drought (Table 7). Dogan et al. (2016) found that grain weights were influenced by multiple genes and varied by genotype and environment. The reduction in 100 kernel weight could potentially be attributed to insufficient water uptake and limited photosynthetic transformation in the plant, causing an early maturity and shriveled kernels. Moreover, leaf abscission, which can cause plant shoots to lose weight and reduce biological yield, may be the cause of the harvest index’s decline under water stress.

Our results in Table 8 and Fig. 7 cleared that harvest index was reduced under drought in all four wheat cultivars in both seasons. These results matched those of Maqsood et al. (2002), who found that increased irrigation led to a greater harvest index.

Based on all of the aforementioned findings, it is possible to link the decrease in morphological criteria (Tables 3 and 4; Figs. 2 and 3), plant water relations (Table 5 and Fig. 4), membrane characteristics (Table 5 and Fig. 4), and photosynthetic pigment (Table 6 and Fig. 5) to the reduction in grain yield of stressed wheat plants. Furthermore, a restricted photosynthetic translation within the plant is linked to the decreased grain yield during droughts, accelerating maturity, and resulting in shriveled grains. Also, the poor remobilization of assimilates to the grains results in small-sized grains. Additionally, drought stress decreased yield by reducing the size of the grains during the initial phases of reproductive growth and the number of grains during the development of grains (Youldash et al. 2020). Moreover, the grain yield reduction under drought was due to the reduction in flag leaf growth vigor and photosynthetic pigments (Table 6 and Fig. 5).

Moreover, in both study seasons, there was a significant interaction between irrigation and cultivars that affected harvest index, spike weight, biological yield, grain yield, and straw yield. Moreover, in Sakha 95, all the previously listed parameters were maximized under both stressed and non-stressed conditions, as opposed to Misr 3, where the minimum yield and yield components were recorded for the first and second seasons.

Changes in Grain and Straw Nutrients (N, P, and K)

Our findings showed that, when compared to control values, NKP uptake was significantly enhanced by grain and straw under drought in the first and second seasons (Table 9 and Fig. 8). On the contrary, the increment in grain and straw NKP might be due to improved utilization of applied fertilizers. Moreover, in order to meet plant needs and encourage plant growth, belowground roots mainly take up water and nutrients from the soil; this greatly increases crop yields (Iqbal et al. 2023c). The recorded increase in NPK uptake under drought stress might be an adaptive feature to raise the cellular osmotic pressure and help to maintain more water. Plants may have accumulated K contents in developing ears for osmotic adjustments because awns have stomata and chloroplasts and can photosynthesize; consequently, as demonstrated by our results, abridged K contents were perceived in straw at maturity (Fig. 8).

Table 9 Effect of water treatment on wheat cultivars and their interaction on nitrogen, phosphorus, and potassium uptake in grain and straw of wheat cultivars during the two studied seasons
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Fig. 8
figure 8

Effect of di-interaction between irrigation and cultivars on nitrogen, phosphorus, and potassium uptake in grain and straw of wheat cultivars during the two studied seasons. The standard error of the mean (n = 3) is shown by vertical bars. Distinct letters point to noticeable variations across treatments at p ≤ 0.05

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All wheat cultivars in this study showed significant genetic variation in drought tolerance with respect to grain and straw NKP, Sakha 95 wheat cultivar showed significant superiority over Giza 171, Misr 1, and Misr 3 in both seasons. Higher NPK contents in Sakha 95 than in the other tested cultivars may be the reason for its superior performance and tolerance under drought stress in the present investigation.

As shown in Table 9, there was an interaction effect between irrigation and cultivars on grain and straw N contents at the heading. The combination between irrigation and cultivars showed that Sakha 95 had the highest values, while Misr 3 had the lowest values under stressed and non-stressed conditions in both seasons. Thus, it was indicated that Sakha 95 was the most drought-tolerant cultivar and Misr 3 was the least drought-tolerant one.

Drought Tolerance Indices

Data in Table 10 showed that the high values of yield stability index (YSI), mean productivity (MP), and stress tolerance index (STI), while the low values of tolerance intensity (TOL) and stress sensitivity index (SSI) were recorded for Sakha 95, Giza 171, Misr 1, and Misr 3, respectively, in the first and second seasons. High YSI index values indicated potential yield under drought (Ladoui et al. 2020). The results of Ladoui et al. (2020), who stated that high TOL values indicated potential yield under non-stressed conditions, are consistent with our findings. High cultivar tolerance to stress was indicated by a stress-sensitive index (SSI) value of less than one (Choukan et al. 2006). Our results closely matched those of Guendouz et al. (2012), who discovered that the best indicators for predicting tolerance were MP and STI.

Table 10 Drought tolerance indices: sensitivity index (SSI), mean productivity (MP), tolerance intensity (TOL), stress tolerance index (STI), and yield stability index (YSI) of four Egyptian bread wheat cultivars during the two studied seasons
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Finally, high MP, STI, and YSI values, as well as low SSI values, indicated yield potential and drought tolerance. Based on these results, Sakha 95 > Giza 171 > Misr 1 > Misr 3 in terms of drought tolerance.

Conclusion

According to the results of the current field study, the wheat cultivars reacted differently to the drought. Yield and yield attributes (spike length, number of spikes/m2, spike weight, grain number/spike, 100-kernel weight, grain yield/m2, straw yield/m2, biological yield/m2, and harvest index), morphological criteria (shoot and the flag leaf growth vigor), and physiological parameters (leaf relative water content, leaf membrane stability index, photosynthetic pigments, and NPK uptake) were all beneficial. On the basis of morpho-physiological criteria, all four wheat cultivars had better yield and tolerance to drought, where Sakha 95 and Giza 171 were superior cultivars in drought tolerance. Moreover, drought tolerance indices (SSI, MP, STI, and YSI) showed that Sakha 95, outperformed Giza 171, Misr 1, and Misr 3 in drought tolerance. These results could be applied by plant breeders to design breeding programs that produce elite cultivars that are more genetically drought tolerant and confer adaptation to arid environments. Therefore, additional investigation is needed to find out how well the screened cultivars perform at molecular level. Based on the findings of the current field study and according to Egypt’s agro-ecological conditions, farmers should be advised to cultivate Sakha 95, and Giza 171 on drought-prone soils.