Biochar coupled with KNO₃ improves tomato yield and quality by modulating photosynthesis, nitrogen metabolism and...

Extreme drought is the major limiting factor for global tomato production. Biochar and potassium nutrition (KNO₃) can improve plant drought tolerance; however, the regulatory mechanisms underlying their combined application in alleviating drought-induced damage in tomatoes remain unclear. This study investigated the interactive effects of biochar and KNO₃ on the physiological responses, yield, and fruit quality of tomato under drought stress. Drought stress significantly inhibited biomass accumulation, leading to a 33.8% reduction in fruit yield. Compared with the drought-only treatment (B0K0), the combined application of biochar and KNO₃ increased root hydraulic conductivity and leaf water potential, elevated relative leaf water content by 10.2%, and promoted photosynthetic pigment synthesis, resulting in a 45.0% increase in net photosynthetic rate (P_n). Furthermore, the combined regulation increased the K⁺/Na⁺ ratio to 3.29-fold that of B0K0, and upregulated the expression of superoxide dismutase gene SlSOD1, peroxidase gene SlPOD1, and catalase gene SlCAT1 by 2.03-, 2.25-, and 2.65-fold, respectively, thereby enhancing reactive oxygen species scavenging capacity. Under biochar application, an appropriate concentration of KNO₃ maintained relatively high activities of key nitrogen metabolism enzymes, effectively repairing drought-induced metabolic disorders. Foliar spraying of 100 mmol L^-1 KNO₃ (K2) combined with biochar increased tomato fruit yield by 35.2%, whereas the 50 mmol L^-1 concentration (K1) increased soluble sugar and vitamin C content by 25.1% and 17.0%, respectively. Comprehensive drought tolerance evaluation demonstrated that the combination of biochar with K1 was the optimal approach for synergistically enhancing tomato yield and fruit quality. This study provides a solid theoretical basis for efficient tomato production in arid environments.