Differential Redox Regulation and Antioxidant Dynamics in Tomato Fruits under Mercury Stress

The present study investigated the impact of mercury exposure on plant water status, oxidative stress markers, and antioxidant defense systems in leaves and fruits of tomato (Solanum lycopersicum L. cv. Micro-Tom). Plants were exposed to increasing concentrations of HgCl2 for 24 and 48 h. Mercury treatment led to a significant reduction in predawn leaf water potential, whereas other water-related parameters in both leaves and fruits remained largely unaffected. Oxidative stress was predominantly observed in leaves, as indicated by elevated hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels, while fruit tissues showed no significant accumulation of these oxidative markers. In contrast, fruits exhibited a marked activation of antioxidant defenses, including increased activities of superoxide dismutase (SOD) and catalase (CAT), along with concentration-dependent modulation of ascorbate-glutathione cycle enzymes (APX, MDHAR, DHAR, and GR) and their corresponding transcript levels. Alterations in the ascorbate pool, reflected by changes in reduced ascorbate (AsA) and dehydroascorbate (DHA), further indicated a dynamic regulation of cellular redox status in response to mercury exposure. Collectively, these findings demonstrate that mercury induces tissue-specific oxidative responses and rapidly activates antioxidant mechanisms in fruits, thereby contributing to the maintenance of redox homeostasis and protection against oxidative damage.