Nitric oxide and hydrogen peroxide crosstalk enhances photosynthesis and nitrogen assimilation in tomato and brinjal under Cr(VI) stress

Heavy metal contamination, particularly with hexavalent chromium [Cr(VI)], is a growing environmental concern that severely impairs plant growth, photosynthesis, and nitrogen metabolism, posing a threat to crop productivity and food security. This study investigates the individual and interactive roles of hydrogen peroxide (H₂O₂) and nitric oxide (NO) in alleviating Cr(VI) toxicity in tomato (Solanum lycopersicum) and brinjal (Solanum melongena). The exposure to 20 µM Cr(VI) for seven days significantly reduced shoot/root length, biomass, photosynthetic pigment levels, and the activity of nitrogen-assimilating enzymes (NR, NiR, GS, GOGAT), with brinjal showing greater sensitivity than tomato. Supplementation with H₂O₂ or NO significantly mitigated these effects, improving growth parameters by up to 34% and restoring pigment content and nitrogen metabolism. Inhibitor-based assays revealed that H₂O₂ functions upstream of NO in the signaling cascade, as NO scavengers (c-PTIO, L-NAME) abolished H₂O₂-induced recovery, while ROS inhibitors (NAC, DPI) did not affect NO’s benefits. The novelty of this study lies in its demonstration of a hierarchical H₂O₂–NO signaling pathway that modulates nitrogen metabolism and antioxidant defense under Cr(VI) stress. These findings suggest a hierarchical H₂O₂–NO signaling mechanism that mitigates oxidative and metabolic damage under Cr(VI) stress. Among treatments, the application of SNP showed the most effective amelioration of Cr(VI) toxicity. This research provides a promising physiological strategy to enhance metal stress tolerance in solanaceous crops. Future work may explore the gene regulatory networks associated with this crosstalk for broader agricultural applications.