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

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 (Pn). 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.