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.