Mitigation of Salt Stress in Tomato (Solanum lycopersicum L.) Through Sulphur, Calcium, and Nitric Oxide: Impacts on Ionic Balance, Nitrogen-Sulphur Metabolism, and Oxidative Stress

Background: In this study, hydroponic experiments were conducted to examine the roles of sulphur (S), calcium (Ca), and nitric oxide (NO) in alleviating salt stress (20 mM NaCl) in tomato (Solanum lycopersicum L.) seedlings. Methods: Analyses included Na+/K+ contents, inorganic nitrogen (nitrate, nitrite, ammonium), nitrogen- and ammonium-assimilating enzymes (NR, NiR, GS, GOGAT), sulphur-assimilating enzymes (ATPS, OASTL), protein content, ROS (O2∙−, H2O2), and in vivo NO visualization were conducted. Results: We observed that salt stress increased Na+, reduced K+, disrupted nitrogen and sulphur metabolism, elevated ROS, and decreased NO, causing oxidative stress and reduced enzymatic activity. Supplementation with potassium sulphate (40 µM), calcium chloride (30 µM), and sodium nitroprusside (SNP; 40 µM) mitigated these effects, enhancing enzymatic activities, restoring Na+/K+ balance, improving protein content, and lowering ROS. The protective role of NO was confirmed using inhibitors L-NAME (500 µM) and cPTIO (100 µM), which reversed SNP’s benefits and aggravated stress damage. Conclusion: Overall, S, Ca, and NO were found to synergistically improve salt stress tolerance by modulating ion homeostasis, nitrogen and sulphur metabolism, and oxidative balance, offering nutrient- and signal-based strategies to enhance tomato resilience under salinity.