NaHS-driven attenuation of hexavalent chromium toxicity: interplay of chromium sequestration and the ascorbate-glutathione cycle.

Hexavalent chromium (Cr (VI)) is highly toxic and severely impairs plant growth and productivity. This study evaluated the role of exogenous hydrogen sulfide (H2S), supplied as sodium hydrosulfide (NaHS), in mitigating Cr (VI) stress in lemongrass (Cymbopogon flexuosus) grown in chromium-rich overburden soil (OBS) from the Sukinda chromite mine, Odisha, India. We hypothesized that H2S alleviates Cr (VI) toxicity by restricting Cr bioavailability and strengthening the glutathione (GSH)-mediated redox cycle. Cr (VI) stress reduced root and shoot length by 58 % and 62 %, whereas NaHS supplementation improved them by 40 % and 52 % compared with OBS. Chromium accumulation in plant tissues decreased under NaHS, indicating restricted uptake or enhanced sequestration. Antioxidant analyses revealed significant enhancement of enzymatic activities with NaHS in garden soil, APX (39 %), GR (100 %), MDHAR (43 %), and DHAR (191 %). In contrast, OBS alone suppressed APX (19 %) and MDHAR (18 %) but increased GR (20 %) and DHAR (29 %), reflecting oxidative stress adaptation. NaHS in OBS (T4) markedly reduced oxidative damage, lowering H2O2 (41 %), MDA (38 %), and EL (45 %), while nearly restoring RWC (2.8 %). However, Inhibition of glutathione biosynthesis with L-buthionine sulfoximine (BSO) diminished these protective effects, confirming the crucial interplay between H2S signalling and GSH metabolism. These findings establish that exogenous H2S mitigates Cr (VI) toxicity in lemongrass by modulating the GSH-mediated redox cycle, reducing chromium accumulation, and enhancing antioxidant defences, highlighting H2S as a promising strategy for improving plant tolerance and phytoremediation efficiency in Cr-contaminated mining soils.