Maize is one of the main cereal crops worldwide, followed by wheat, which is a staple food in Egypt (OECD/FAO, 2020). It is either consumed freshly or indirectly as corn oil, starch, fructose, glucose, and livestock feed (Ranum et al., 2014). Owing to its low inherent production and high demand of maize, Egypt has become the fourth largest importer of maize in the world (OECD/FAO, 2020). The total maize production in Egypt is approximately 6.4 million tons, and 85,000 hectares of cultivated land is estimated for its production. Its consumption is approximately 16.1 million tons with a self-sufficiency ratio of 42% (Ibrahim et al., 2007). Thus, to minimize the gap between maize production and consumption, it is important to manage irrigation water more efficiently and enhance domestic maize production by following non-traditional procedures, such as growing in cultivated areas or planting high-yield varieties (Ouda et al., 2017).

In arid and semi-arid regions, water shortage particularly affects food security (Misra, 2014; Mancosu et al., 2015). Approximately 70–80% of the available freshwater is required for agriculture (Food and Agriculture Organization [FAO], 2017). Egypt depends on limited water resources from the Nile River (55 billion m³ of water/year) (MWRI, 2014). Over 95% of irrigated lands in Egypt are managed using surface irrigation (SI) systems, with a low irrigation efficiency of 45–50% (Osman et al., 2016; Abdou et al., 2021; Ali et al., 2021). Therefore, rational water governance seeks to reduce water losses and enhance crop productivity to withstand high evaporative demand. Assuming severe water supply shortages, a deficit and highly efficient irrigation strategy of drip irrigation (DI) system is highly recommended over the SI system to increase the benefits per unit of water.

The application of irrigation water below the evapotranspiration (ET) demand is known as deficit irrigation, which optimizes economic output when there is limited water supply (Pereira et al., 2012; Chai et al., 2016). Plants under deficit irrigation systems receive less irrigation water than the actual amount of water required at plant growth stages and/or during the total crop cultivation period (Badal et al., 2013). Hence, plants are exposed to water stress to some extent under deficit irrigation systems (Wakchaure et al., 2018; Parkash and Singh, 2020). A decrease in the ET rate of plants exposed to water stress results in severe water stress symptoms, such as leaf rolling, diminishing leaf area, and stunted growth and yield of plants (Wang et al., 2014). Drought stress induces several physiological, biochemical, and photosynthetic changes by regulating stomatal closure or controlling CO₂ flow into the mesophyll tissues (Yuan et al., 2016).

Approximately 20–25% of the maize cultivation areas are affected by drought globally (Shahrokhi et al., 2020). Sensitivity to drought stress can lead to dramatic fluctuations in maize yield, which is a common condition in Egypt (Gomaa et al., 2017). Maize exhibits distinct responses to water deficit depending on the developmental stage, crop tolerance level, and severity of the applied water stress treatments (Song et al., 2019; Cheng et al., 2021). The yield reduction due to water stress is primarily attributed to the disruption of physiobiochemical processes, inhibition of photosynthesis, and stunted plant growth and development (Song et al., 2019; Sharma et al., 2020; Cheng et al., 2021).

However, several studies have revealed that plants tend to accumulate various compatible solutes, such as soluble sugars and amino acids [e.g., proline (Pro)], as an adaptive tolerance strategy to increase salinity and/or drought tolerance (Giri, 2011; Semida et al., 2015; Arteaga et al., 2020). Pro is an essential amino acid that accumulates in various plant tissues, particularly in the leaves (Hayat et al., 2012). Proline accumulation plays an indispensable role in the regulation of cell osmosis (El Moukhtari et al., 2020). Additionally, under unfavorable conditions of water stress, Pro stabilizes membranes (Xia et al., 2020) and protects enzymes (Qamar et al., 2019; El-Nashaar et al., 2020), proteins, and macromolecules from denaturation (Agami et al., 2019). The effectiveness of exogenously applied Pro depends on the plant developmental stage and variety, Pro application rate, and the time of application (Semida et al., 2020). Excessive concentrations of free Pro can disrupt cell growth and protein functions. Hayat et al. (2007) observed that the foliar application of Pro (30 mM) negatively affects maize plants.

Using Pro as an antioxidant is an effective, unconventional, and inexpensive approach for mitigating the harmful effects of drought on maize production under different irrigation systems. Further studies are highly recommended to reduce the gap between water consumption and production of maize in arid and semi-arid regions. Therefore, this study aimed to elucidate the ameliorative effects of Pro at different nontoxic application rates on the morph-physiological and yield attributes of maize plants exposed to water stress. Furthermore, irrigation water requirements for maize plants grown under different irrigation systems and water scarcity conditions were determined.

Water stress adversely affects the growth and development of plants, particularly when the stress conditions remain constant. When plants are exposed to stress, they accumulate an array of metabolites, such as amino acids. Pro plays a highly beneficial role in plants exposed to various stress conditions. Besides acting as an excellent osmolyte, Pro has three fundamental biological roles in stress response, i.e., acting as a metal chelator, antioxidative defense molecule, and signaling molecule. Some reports; however, have indicated the toxic effects of Pro when applied exogenously at high concentrations. In this study, we aimed to (i) investigate the causes underlying the reduction in the growth and yield of maize plants induced by water stress and (ii) determine the crucial role of Pro in mitigating these negative effects at different nontoxic application rates of Pro to improve WP under water scarce conditions.

Our results demonstrated significant differences in the vegetative growth of maize plants in response to the two irrigation systems (Table 2). The DI system was relatively superior in the maize growth parameters, except for root weight, compared with the SI system. Similar to previous reports (Ghamarnia et al., 2013; Mahgoub et al., 2017), the observed increase in the root weight of plants under the SI system could be a result of the downward movement of irrigation water due to gravity, which promotes the penetration of roots through the soil profile to greater depths to extract water from the deeper layers. Maize plants exposed to water stress showed growth retardation. The inhibition of maize growth due to water stress was evident because of the decrease in the length, volume, and water potential of the root system, which disrupted the water extraction by the roots. This results in decreased cell division and elongation, leading to stunted plant growth. These results were consistent with those of previous studies (El-Samnoudi et al., 2019; Song et al., 2019; Sah et al., 2020; Cheng et al., 2021). However, the exogenous application of Pro not only provided osmoprotection but also enhanced the growth of the plants. The beneficial effect of Pro in enhancing maize growth characteristics under low irrigation levels might be attributed to its role in osmoregulation, and maintenance of the tertiary structure of proteins and enzymes, to help growing plants tolerate drought stress. These results were similar to those of the previous reports (Abdelaal et al., 2018, 2020; Qamar et al., 2019; El-Nashaar et al., 2020).

Additionally, drought adversely affected the plant water retention (RWC and MSI) and photosynthesis (Table 3). The decrease in water uptake under deficit irrigation regimes was associated with a decrease in leaf water potential. Drought alters the plant water status and stomatal functioning and inhibits chlorophyll biosynthesis, thus reducing the photosynthetic rates. Dehydration in the plant cell protoplasm could be considered as an effect of drought on the RWC of maize leaves. Similar observations were reported in a previous study (Xia et al., 2020). The foliar application of Pro enhanced the plant water status. This was evident by the reduction in water efflux under drought stress, which helped the cells maintain their cell turgor or osmotic balance. These results are consistent with those of another study conducted by Al-Khazrji et al. (2020). It is suggested that the exogenous application of Pro enhances membrane stability, thus preventing electrolyte leakage, as previously documented by Hayat et al. (2012).

Water stress further decreased the values of the physiological parameters in maize seedlings. This was evident by the disruption of plant photosynthetic potential, possibly due to the stomatal closure and/or metabolic destruction, such as the impairment of photosystem 1 (PSI) and photosystem II (PSII), which are chlorophyll-binding protein complexes (Ibrahim et al., 2019; Zheng et al., 2022). Furthermore, water stress increases the generation of reactive oxygen species (ROS), resulting in oxidative damage to plants and degradation of chlorophyll pigments (Hasanuzzaman et al., 2020; Mansoor et al., 2022). Alternatively, Pro may be responsible for scavenging ROS and other free radicals (Kaul et al., 2008). It may also protect plants from stress injuries by stabilizing membranes and proteins, allowing the transport of mitochondrial electrons, enhancing antioxidant enzyme activity, thus increasing stomatal conductance and facilitating higher CO₂ diffusion through leaves. This promotes higher photosynthetic capacity (Tùmová et al., 2018; Agami et al., 2019; Altuntaş et al., 2020; Semida et al., 2020; Tariq et al., 2021; Abdou et al., 2022). Hence, the foliar application of Pro can enhance the photosynthetic rate of maize under drought stress.

The decreased water uptake from the soil might be responsible for the detrimental effects of water stress on grain and biomass yields of maize (Table 4), resulting in the abovementioned inhibition in plant growth and development, disruption of photosynthetic pigments, and deficits in plant water content. Accordingly, a significant decrease in maize yield under water stress conditions was observed. The results are consistent with those of previous studies (Vazirimehr et al., 2014; Sah et al., 2020; Shah et al., 2020).

The present study demonstrated that the increase in the yield and yield components achieved by the application of Pro was higher than the reduction in the yield caused by drought stress, confirming that Pro treatment successfully compensated for the adverse effects of drought stress on the growth and yield of maize. Furthermore, the yield and yield components of maize under the DI system were higher than those under the SI system (Qamar et al., 2019; Abdelaal et al., 2020; El-Nashaar et al., 2020; Li et al., 2021), and WP increased consistently with increasing concentrations of exogenously applied Pro (Table 5). These results are consistent with those of the previous studies by Semida et al. (2015) and Semida et al. (2020) who found that pressurized irrigation systems via subsurface drip and/or surface drip increase the water-use efficiency and grain yield of maize compared with the SI system.

Notably, the monitoring of the salt distribution pattern in the soil profile (depth, 0–60 cm) based on different applied irrigation systems and treatments showed that the salt distribution pattern was different after plant harvest in the second season compared with the determined initial ECe values before maize planting. The results were consistent with those of the studies by Semida et al. (2015) and Ali et al. (2016) who reported that the highest salinity was observed in the deeper soil layers depending on the wetting front under the SI system. The lowest salinity was observed on the surface; however, under DI, salt accumulation increased on the soil surface.

With increasing water scarcity, well-designed deficit DI regimes and the exogenously applied antioxidant Pro can optimize the maize production and WP when the available water is insufficient to provide full irrigation. The DI system efficiently saved irrigation water input by 30% compared with the SI system. Among the tested irrigation levels, the full level (I₁₀₀) resulted in the highest grain yield. The moderate-deficit irrigation level (I₈₅) showed the maximum WP. The growth, physiological aspects, and grain yield of maize increased significantly after the foliar application of Pro to maize plants exposed to water stress. Therefore, using exogenous Pro at 4 mM for maize plants under the DI system and irrigating with 85% of their ET demand may be a promising agro-management strategy for improving the yield and WP of maize crops grown in arid and semi-aired regions, especially when water scarcity constraints the sustainability of maize production.

The original contributions presented in this study are included in the article, further inquiries can be directed to the corresponding author/s.

AE-AI, TA, YA, SA, KE-T, and NA: conceptualization. AE-AI, TA, YA, and NA: methodology. AE-AI, TA, SA, KE-T, and NA: formal analysis. TA, SA, KE-T, and NA: investigation and writing—original draft preparation. AE-AI, TA, YA, ME-S, HA, and NA: data curation. TA, ME-S, SA, KE-T, and NA: writing—review and editing. TA and HA: visualization. SA and KE-T: funding acquisition. All authors have read and agreed to the published version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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