Co-application of selenium and hydrogen sulfide confers PEG-induced drought stress tolerance in tomato plants by safeguarding photosynthetic apparatus and modulating proline metabolism and antioxidant systems.

Drought stress is a major environmental constraint that adversely affects plant growth, photosynthesis, and metabolic homeostasis. This study investigated the effects of co-application of selenium (Se; Na2SeO4; through soil) and hydrogen sulfide (H2S; NaHS; through foliage) on alleviating polyethylene glycol (PEG)-induced drought stress in tomato (Solanum lycopersicum L.) plants. Plants exposed to PEG (20 and 30%) significantly reduced plant growth, photosynthetic performance, chlorophyll-related indices, and membrane stability while increasing oxidative stress. However, the co-application of Se and H2S significantly improved net photosynthetic rate, stomatal conductance, chlorophyll fluorescence parameters, and photochemical efficiency under osmotic stress conditions. Moreover, spectral indices, including anthocyanin reflectance index 1 (ARI1), water band index (WBI), and Zarco-Tejada and Miller index (ZMI), were significantly enhanced, indicating improved water status and photosynthetic functionality. Co-application of Se and H2S treatment also strengthened antioxidant defense mechanisms by increasing the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POX), resulting in lower hydrogen peroxide accumulation, lipid peroxidation, and electrolyte leakage. Furthermore, the treatment promoted proline accumulation by modulating proline metabolism, thereby improving osmotic adjustment and stress tolerance. Correlation analysis further revealed strong positive associations among photosynthetic traits, antioxidant enzymes, and proline metabolism, while oxidative stress markers showed negative relationships with these protective responses. Overall, the co-application of Se and H2S collectively enhanced photosynthetic efficiency, antioxidant capacity, and proline metabolism, thereby improving tomato tolerance to PEG-induced drought stress. These findings highlight the potential co-application of Se and H2S as promising biostimulants for enhancing crop resilience under water-limited conditions.