Foliar Spray of Stigmasterol Regulates Physiological Processes and Antioxidant Mechanisms to Improve Yield and Quality of Sunflower Under Drought Stress

Drought stress is an unavoidable challenge limiting plant production and quality. Stigmasterol is a potential compound for plant protection and improvement productivity under drought. Thus, the effects of using stigmasterol as exogenous treatment on improving growth and productivity of sunflower grown under drought were studied. A pot experiment was carried out at two summer seasons, using foliar treatment of stigmasterol 0, 100, 200, and 300 mg L−1 on sunflower plants under different irrigation levels 80% and 50% water irrigation requirement (WIR). Drought stress (50% WIR) provoked significant reductions in growth and yield components; the percentages of decrease in head diameter reached 26.55%, head circumference 26.05%, seed weight per plant 36.26%, and 100 seed weight 29.61%, via decreasing photosynthetic pigments and indole acetic acid while elevating hydrogen peroxide (H2O2), lipid peroxidation (MDA), membrane leakage, lipoxygenase activity, some antioxidant compounds, enzymes, and osmolytes. Stigmasterol has a promotive effect on growth and productivity of sunflower through improving photosynthetic pigments, indole acetic acid, non-enzymatic, enzymatic antioxidant, and osmolytes, while it decreased membrane leakage, H2O2, and MDA, thus, improving yield quality. Moreover, stigmasterol improves the economic importance of sunflower seed oil. About 200 mg L−1 of stigmasterol was the most effective concentration in improving yield parameters, as it causes 19.84% and 25.29% in seed weight per plant and 26.72% and 33.95% of 100 seed weight under 80% and 50% WIR, respectively. Stigmasterol improved growth and productivity of sunflower under normal water conditions and could overcome the reduced impact of drought by improving growth and development and different physiological attributes.