Interactive effects of drought and high radiation on photosystem II performance and redox homeostasis in soybean (Glycine max L.)

Photosystem II (PSII) is extremely sensitive to drought and high light stress, both of which are expected to increase under changing climate conditions. We studied two soybean cultivars, JS335 and Kalitur, under drought stress, high radiation, and their combination. Chlorophyll fluorescence indicated decline in quantum yield with increasing light intensity and duration, particularly beyond 900 µmol photons m⁻²s⁻¹. JS335 showed better recovery from photodamage compared to Kalitur. The combined drought and high radiation treatment reduced chlorophyll content and relative water content with increased malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) after 15 days. Antioxidant enzymes responded dynamically, with a gradual increase in catalase activity from day 3 and ascorbate peroxidase reaching its maximum on day 9. These results indicate that combined stress enhanced damage to PSII, while greater antioxidant defense and effective Reactive Oxygen Species (ROS) management in JS335 form the basis of its superior tolerance, providing worthwhile characteristics for improving soybeans under stressful conditions.