Hydrogen Sulfide Mitigates Manganese-Induced Toxicity in Malus hupehensis Plants by Regulating Osmoregulation, Antioxidant Defense, Mineral Homeostasis, and Glutathione Ascorbate Cycle

Manganese (Mn) is a toxic metal element that adversely affects plant growth. Hydrogen sulfide (H2S) is considered an important signaling molecule with significant potential in alleviating various abiotic stresses. However, there is limited information available on the role of H2S in alleviating manganese stress in plants. In this study, the effects of exogenous H2S and its scavenger, homocysteine thiolactone (HT), on the physiological and biochemical parameters of Malus hupehensis var. pingyiensis seedlings were evaluated. Our results show that H2S treatment significantly alleviates growth inhibition and oxidative damage induced by manganese stress in Malus hupehensis seedlings, primarily by enhancing antioxidant enzyme activity and up-regulating the ascorbate-glutathione (ASA-GSH) cycle. H2S treatment increased photosynthetic pigment content and helped maintain osmotic balance in leaves, thereby enhancing key gas exchange parameters and mitigating manganese-induced suppression of photosynthesis. H2S treatment enhanced the absorption of Ca, Mg, Fe and Zn under manganese stress, significantly reduced manganese accumulation in Malus hupehensis seedlings, and modulated the transcriptional expression of MTPs, facilitating the transfer of manganese to the leaves. Thus, H2S reduces oxidative damage and promotes growth under Mn stress, highlighting its important role in plant stress tolerance.