Foliar thiourea and potassium nitrate enhance physiological performance, antioxidant defense, and yield of heat-stressed wheat under field conditions

Terminal heat stress negatively affects wheat by disrupting physiological, biochemical, and agronomic traits. This study examined the effects of thiourea (50 µM) and KNO 3 (0.5%), applied individually or together, on a heat-sensitive wheat genotype (HS-240) grown under field conditions. Treatments were: T 0 (control), T 1 (thiourea), T 2 (KNO 3 ), and T 3 (thiourea + KNO 3 ). The combined treatment (T 3 ) improved key physiological traits, including photosynthesis rate (14.2 vs. 8.8 µmol CO 2 m - ² s - ¹), relative water content (83% vs. 65%), and chlorophyll stability (SPAD 42 vs. 28) compared to control. Biochemical defenses were also enhanced, with higher levels of proline, soluble sugars, total phenolics, and stronger antioxidant enzyme activities (SOD, CAT, POD, APX). Oxidative damage decreased, as indicated by lower electrolyte leakage and MDA, and canopy temperature depression increased (2.5 °C → 9.0 °C). These physiological and biochemical improvements translated into better crop performance, with higher grain yield (16 g vs. 12 g per plant), thousand-grain weight (42 g vs. 34 g), and harvest index (49%). Molecular analysis showed that T 3 strongly upregulated genes involved in stress tolerance, antioxidants, and yield regulation ( TaHSP17, TaHSP90, TaSOD, TaCAT, TaAPX, TaP5CS, TaDREB2, TaSUS, TaGW2, TaCKX2 ). Multivariate analyses (correlation, PCA, heatmap) confirmed positive associations between photosynthesis, antioxidant activity, and yield traits. Overall, the combined application of thiourea and KNO 3 enhances wheat heat tolerance through integrated physiological, biochemical, and molecular mechanisms, offering a practical and cost-effective strategy to sustain wheat productivity under high-temperature stress.