Physiological and nutritional mechanisms underlying chloride-induced drought resistance in tomato

Chloride (Cl^-), long regarded as a micronutrient or salinity-associated ion, is now recognized as a beneficial macronutrient in higher plants. This study evaluated whether Cl^- supplied at macronutrient concentrations enhances drought resistance in tomato (Solanum lycopersicum L.) through improved physiological performance and nutrient efficiency. Tomato plants were grown under greenhouse conditions under well-watered (CTR) or water-deficit (WD) regimes, receiving either micronutrient (SP) or macronutrient (CL) Cl^- supply. At macronutrient levels, Cl^- alleviated drought-induced growth inhibition, maintaining both vegetative and reproductive biomass and promoting adaptive root growth. CL-treated plants exhibited enhanced leaf water status, reduced stomatal conductance (g_s) without compromising net photosynthesis rate (A_N), and increased intrinsic water use efficiency (WUEᵢ) during WD. Cl^- also preserved PSII efficiency, maintained chlorophyll levels, reduced lipid peroxidation (MDA), and enhanced total antioxidant capacity. Nutritionally, Cl^- improved nitrogen and cation (K⁺, Ca²⁺, and Mg²⁺) utilization efficiencies under drought conditions. Principal component analysis integrating physiological and nutritional traits under WD revealed that CL plants maintained a coordinated and functionally integrated response associated with improved water status, photosynthetic efficiency, and nutrient use. Overall, these findings indicate that Cl^- acts as a true macronutrient enhancing drought resilience in tomato through coordinated effects on osmotic adjustment, nutrient optimization, and oxidative stress mitigation. Incorporating Cl^- into fertilization programs may therefore represent a cost-effective and sustainable strategy to improve crop WUE and productivity under water-limited conditions.