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: <i>Y</i>_NA increased, whereas <i>Y</i>_I and Δ<i>I</i>/<i>I</i>_o decreased, and the P700 re-reduction half-time (t½) was prolonged (ANOVA, Tukey's HSD, <i>p</i> < 0.05), indicating pronounced acceptor-side over-reduction and impaired electron transport. PSII performance also declined (lower <i>F</i>_v/<i>F</i>_m and <i>PI</i>_ABS, higher Δ<i>V</i>_J; <i>p</i> < 0.05). NaHS pretreatment significantly alleviated these effects relative to the stressed control: PSI/PSII metrics partly recovered, net photosynthetic rate (<i>P</i>_n) and water-use efficiency (WUE) increased, H₂O₂ and MDA decreased, and SOD/POD/CAT activities rose (<i>p</i> < 0.05). Notably, NPQ_high correlated negatively with <i>Y</i>_NA (Pearson r < 0, <i>p</i> < 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.