Mitigation of Nickel Stress-Induced Toxicity in Odontarrhena inflatum and Erysimum cheiri by Modulating Ion Homeostasis and Antioxidant Defense System

Nickel (Ni) is considered a micronutrient for plants; however, in high concentrations, Ni becomes toxic and hazardous to plants, humans, and animals. High levels of Ni inhibit seed germination, reduce chlorophyll content, and cause oxidative stress to plants. Hydrogen sulfide (H₂S) can help mitigate the N-toxicity. While the modulation of plant tolerance to heavy metal stresses by H₂S is a known phenomenon, the regulatory effects of H₂S on plant growth, physiology, and associated mechanisms under Ni concentrations remain elusive and not fully understood. In this study, the effects of H₂S supplementation (0, 100, and 200 μM) on the growth, Ni accumulation, and antioxidant responses of Odontarrhena inflatum (a Ni- accumulator) and Erysimum cheiri under different levels of Ni stress (0, 80, 160, and 320 µM) were investigated in a hydroponic experiment. The results showed a significant decline in growth parameters, photosynthetic pigment content, and mineral uptake in O. inflatum and E. cheiri subjected to high levels of Ni stress. However, H₂S application significantly enhanced the photosynthetic pigments content, plant growth, and nutrient uptake. Nickle accumulation and hydrogen peroxide (H₂O₂) content increased considerably under Ni stress while H₂S supplementation significantly reduced Ni and H₂O₂ levels. Furthermore, Ni stress increased the concentration of cysteine (Cys), soluble protein, and proline and enhanced the activities of antioxidant enzymes while H₂S supplementation further increased these parameters, countering Ni toxicity. These results provide a comprehensive evaluation of the significant role of H₂S in Ni-stressed O. inflatum and E. cheiri by reducing Ni accumulation, and oxidative stress and promoting growth and metabolic characteristics. In conclusion, H₂S plays a crucial role in mitigating Ni toxicity in plants. By enhancing growth, reducing oxidative stress, and improving nutrient uptake, H₂S supplementation offers a promising strategy for alleviating the detrimental effects of heavy metal stress in plants. This study underscores the potential of H₂S as a beneficial treatment for improving plant resilience in environments contaminated with heavy metals.