Exogenous Hydrogen Sulfide Alleviates Low Temperature and Fluctuating-Light-Induced Photoinhibition of Photosystem I in Morus alba Through Enhanced Energy Dissipation and Antioxidant Defense.

Low temperature combined with fluctuating irradiance frequently co-occurs and suppresses photosynthesis, with irreversible injury to photosystem I (PSI) recognized as a key constraint on growth and yield. To test whether exogenous hydrogen sulfide (H2S) mitigates this "cold-fluctuating light" stress in mulberry, we established six treatment combinations (room temperature controls, sodium hydrosulfide, and hypotaurine, each with or without low temperature plus fluctuating light). We quantified PSI/PSII photochemical properties, gas exchange, reactive oxygen species (ROS), and antioxidant enzyme activities. Under cold with fluctuating light, PSI was strongly inhibited: YNA increased, whereas YI and ΔI/Io decreased, and the P700 re-reduction half-time (t½) was prolonged (ANOVA, Tukey's HSD, p < 0.05), indicating pronounced acceptor-side over-reduction and impaired electron transport. PSII performance also declined (lower Fv/Fm and PIABS, higher ΔVJ; p < 0.05). NaHS pretreatment significantly alleviated these effects relative to the stressed control: PSI/PSII metrics partly recovered, net photosynthetic rate (Pn) and water-use efficiency (WUE) increased, H2O2 and MDA decreased, and SOD/POD/CAT activities rose (p < 0.05). Notably, NPQhigh correlated negatively with YNA (Pearson r < 0, p < 0.001), consistent with the notion that enhanced energy dissipation relieves PSI acceptor-side limitation. We propose that exogenous H2S stabilizes electron transport and supports carbon assimilation via a dual strategy-faster engagement of energy dissipation and activation of antioxidant defenses-highlighting its potential utility for managing stress in fruit crops under erratic early-season weather.