Biochemical responses of sorghum and maize to the impacts of different levels of water deficit and nitrogen supply

During the 2015–2016 growing season, a field experiment was conducted to investigate the impact of different irrigation regimes (low and severe water stress treatments) and nitrogen levels on the biochemical responses of sorghum and maize. Moisture environments were determined based on the maximum allowable depletion of available soil water (ASW), with depletion levels of 55–60% and 85–90% of ASW selected as the low and severe water stress levels, respectively. Nitrogen fertilizer was applied via drip fertigation at three stages (28, 43, and 55 days after sowing) in the form of Urea (N: 45%), at a rate of 37.5 kg ha−1 N for each stage. Under severe water stress, both crops showed a decrease in chlorophyll and carotenoid content, relative water content (RWC), and maximum quantum efficiency of photosystem II (Fv/Fm). Additionally, severe water stress led to an increase in antioxidant enzyme activity, proline, malondialdehyde (MDA), other aldehydes (Alds), and hydrogen peroxide, ultimately resulting in a 42% reduction in crop yield. Light intensity also increased under severe water stress, particularly in the middle and bottom of the plant canopy, indicating a decrease in the available leaf area for receiving light. Nitrogen application effectively reduced the production of oxygen free radicals, mitigated cell membrane damage, decreased the activity of antioxidant enzymes and osmolytes, and increased the activity of photosynthetic pigments, Fv/Fm, and RWC, resulting in an increase in crop yield of up to 18%. Compared to sorghum, maize exhibited lower antioxidant activity, proline content, chlorophyll, carotenoid, RWC, Fv/Fm, stress tolerance index, and higher concentrations of MDA, Alds, and H2O2, indicating its higher sensitivity to water deficit stress. Additionally, even under low water stress treatment, maize failed to produce an acceptable dry matter yield.