Investigation of the Physiological and Biochemical Effects of Brassinosteroid and Nitric Oxide in Two Wheat Varieties with Different Drought Tolerance

Drought, which causes oxidative stress by increasing the amount of reactive oxygen species (ROS) in plants, leads to a decrease in yield in bread wheat (Triticum aestivum L.). On the other hand, nitric oxide (NO) and brassinosteroids (BRs) are known to maintain cellular homeostasis under drought stress in wheat through various signal transduction pathways. This research focuses on the individual and combined effects of NO (Sodium nitroprusside, SNP) and BR (Epibrassinolide, EBL) on physiological and biochemical parameters in two wheat varieties with different drought tolerances: under polyethylene glycol (PEG)-induced and water-scarcity-induced drought stress. Drought stress decreased ascorbate peroxidase (APX), glutathione reductase (GR), and peroxidase (POX) activities in the drought-sensitive Kenanbey variety, while increasing superoxide dismutase (SOD), GR, APX, and CAT activities in the tolerant NKÜ Lider variety. Exogenous SNP and EBL treatments effectively mitigated drought-induced membrane damage and oxidative stress, leading to significant reductions in electrolyte leakage (EL), lipid peroxidation (TBARS), and ROS accumulation in both varieties. In the NKÜ Lider, under both PEG-induced osmotic stress and water-scarcity conditions, the combined SNP and EBL treatment increased root length (RL), biomass, dry weight (DW), and SOD isoenzyme band intensity, while reducing electrolyte leakage (EL), compared with the individual treatments. In the Kenanbey variety, under water-scarcity conditions, the combined treatment improved biomass, SPAD values, CAT and POX activities, and SOD isoenzyme band intensity, while reducing EL. Additionally, the dramatically reduced SPAD and water content in both drought treatments were increased to control plant levels with individual and combined treatments of SNP and EBL. These findings highlight the potential of NO and BR treatments as promising strategies to improve physiological and biochemical responses associated with drought tolerance in wheat varieties. These findings provide experimental evidence for the development of foliar regulators for drought tolerance in wheat seedlings.