Sodium Hydrosulfide Modulates Membrane Integrity, Cation Homeostasis, and Accumulation of Phenolics and Osmolytes in Zucchini Under Nickel Stress

In this study, the role of hydrogen sulfide (H₂S) in growth and metabolism of zucchini (Cucurbita pepo L.) was explored in a hydroponic culture system under both normal and nickel (Ni) stress conditions. Exposure of zucchini seedlings to 50 mg/l Ni(NO₃)₂ reduced root elongation and dry weight of roots and aerial parts. These reductions were attributed to Ni accumulation and depletion of essential bivalent cations in tissues. Excessive Ni resulted in the induction of oxidative stress, as evidenced by increased levels of electrolyte leakage (EL) and hydrogen peroxide (H₂O₂) content in roots and leaves. Moreover, Ni stress increased the endogenous H₂S content in parallel with the accumulation of proline, soluble sugars, phenolics, and flavonoids in roots and leaves. Exogenous application of 100 µM sodium hydrosulfide (NaHS) as a donor of H₂S singly or associated with Ni solution significantly improved zucchini growth but the impacts of NaHS were more pronounced in Ni-stressed plants than control seedlings. Notably, NaHS amplified the endogenous H₂S content and maintained Ni and the homeostasis of cation in roots and aerial parts of Ni-stressed seedlings. Applying NaHS significantly lowered the accumulation of proline, whereas it elevated the content of soluble sugars in roots and leaves of Ni-stressed seedlings. Furthermore, applying NaHS decreased the EL and H₂O₂ content which were correlated to the enhancement of the phenolic and flavonoids content as well as 1,1-diphenyl-2-picrylhydrazyl radical and hydroxyl radical scavenging power in roots and leaves. By contrast, adding hypotaurine (HT), as a H₂S scavenger, weakened the beneficial effect of NaHS on all above-mentioned traits that further confirmed the signaling role of H₂S in regulating growth, membrane integrity, the content of phenolics and osmolytes, and mineral homeostasis under both control and Ni stress conditions.