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

## TL;DR

Exogenous hydrogen sulfide helps mulberry trees withstand cold and fluctuating light by improving photosynthesis and reducing oxidative stress.

## Contribution

This study demonstrates that low-dose hydrogen sulfide mitigates cold and fluctuating light stress in mulberry through enhanced energy dissipation and antioxidant defenses.

## Key findings

- Hydrogen sulfide treatment increased photosynthesis and water-use efficiency under stress.
- H2S reduced oxidative stress and activated antioxidant enzymes in mulberry leaves.
- Enhanced energy dissipation and antioxidant defenses stabilized electron transport in photosystem I.

## Abstract

Sudden cold snaps, often accompanied by rapid fluctuations in sunlight, disrupt leaf light-harvesting mechanisms. In mulberry trees, this combination weakens the photosynthetic apparatus, decreases photosynthetic and water-use efficiency, and increases oxidative stress. To investigate potential mitigation, we tested the effect of a small, safe dose of hydrogen sulfide (H2S), a naturally occurring gas in plants. Young mulberry plants were exposed to low temperatures and fluctuating light, with or without a one-time leaf spray that gradually releases H2S. Under stress, photosynthesis declined, and oxidant levels increased. H2S treatment, however, enhanced photosynthesis and water-use efficiency, reduced oxidant accumulation, and activated endogenous protective enzymes. Two mechanisms were observed: excess light energy was dissipated as heat, and antioxidant defenses were enhanced. These effects stabilized energy flow and improved carbon utilization under stress. The results indicate that low-dose H2S could serve as a practical strategy for mitigating the impact of erratic spring weather on fruit tree yield.

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.