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Melatonin (Mel) and hydrogen sulphide (H2S) have emerged as potential regulators of plant metabolism during abiotic stress. Presence of excess NaCl in the soil is one of the main causes of reduced crop productivity worldwide. The present investigation examines the role of exogenous Mel and endogenous H2S in tomato seedlings grown under NaCl stress. Effect of 30 µm Mel on endogenous synthesis of H2S was examined in roots of NaCl‐stressed (200 mm) tomato seedlings. Also, the impact of treatments on the oxidative stress markers, transport of K+ and Na+, and activity of H+‐ATPase and antioxidant enzymes was assessed. Results show that NaCl‐stressed seedlings supplemented with 30 µm Mel had increased levels of endogenous H2S through enhanced L‐cysteine desulfhydrase activity. Mel in association with H2S overcame the deleterious effect of NaCl and induced retention of K+ that maintained a higher K+/Na+ ratio. Use of plasma membrane inhibitors and an H2S scavenger revealed that Mel‐induced regulation of K+/Na+ homeostasis in NaCl‐stressed seedling roots operates through endogenous H2S signalling. Synergistic effects of Mel and H2S also reduced the generation of ROS and oxidative destruction through the enhanced activity of antioxidant enzymes. Thus, it is suggested that the protective function of Mel against NaCl stress operates through an endogenous H2S‐dependent pathway, wherein H+‐ATPase‐energized secondary active transport regulates K+/Na+ homeostasis.

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Potassium (K+) is one of the vital macronutrients required by plants for proper growth and blossoming harvest. In addition, K+ also plays a decisive role in promoting tolerance to various stresses. Under stressful conditions, plants deploy their defense system through various signaling molecules, including hydrogen sulfide (H2S). The present investigation was carried out to unravel the role of K+ and H2S in plants under NaCl stress. The results of the study show that NaCl stress caused a reduction in K+ and an increase in Na+ content in the tomato seedling roots which coincided with a lower H+-ATPase activity and K+/Na+ ratio. However, application of 5 mM K+, in association with endogenous H2S, positively regulated the Na+/H+ antiport system that accelerated K+ influx and Na+ efflux, resulting in the maintenance of a higher K+/Na+ ratio. The role of K+ and H2S in the regulation of the Na+/H+ antiport system was validated by applying sodium orthovanadate (plasma membrane H+-ATPase inhibitor), tetraethylammonium chloride (K+ channel blocker), amiloride (Na+/H+ antiporter inhibitor), and hypotaurine (HT, H2S scavenger). Application of 5 mM K+ positively regulated the ascorbate–glutathione cycle and activity of antioxidant enzymes that resulted in a reduction in reactive oxygen species generation and associated damage. Under NaCl stress, K+ also activated carbohydrate metabolism and proline accumulation that caused improvement in osmotic tolerance and enhanced the hydration level of the stressed seedlings. However, inclusion of the H2S scavenger HT reversed the effect of K+, suggesting H2S-dependent functioning of K+ under NaCl stress. Therefore, the present findings report that K+, in association with H2S, alleviates NaCl-induced impairments by regulating the Na+/H+ antiport system, carbohydrate metabolism, and antioxidative defense system.