Late-sown induced terminal heat stress impacts on wheat in Tarai region of eastern India: insights into biochemical, physiological, and yield responses

Abstract Wheat production is increasingly threatened by high-temperature stress. The Tarai belt of eastern India, a non-traditional wheat-growing region, remains understudied regarding heat stress impacts. This study evaluated five wheat cultivars under late-sowing-induced terminal heat stress (THS) condition, assessing physiological, biochemical, and agronomic traits and their interrelationships. Three late-sowing environments (LSE) were tested: 15-day (LSE-I), 30-day (LSE-II), and 45-day (LSE-III) delay, compared to timely-sown control (NSE). A 15-day delay had minimal impacts, whereas 30- and 45-day delays significantly reduced grain number per ear, ear length, and 1000-grain weight, resulting in 24% and 51% yield losses, respectively. Under LSE-II and LSE-III, substantial increases were recorded in flag leaf catalase (166–214%), peroxidase (191–227%), and proline (88–105%), while membrane stability index (MSI), relative water content (RWC), and chlorophyll index (SPAD) declined by 23–28%, 9–14%, and 7–17%, respectively. Prominent changes in antioxidant enzyme activities, proline, and phenol content were observed between anthesis and the soft-dough stage. Multivariate linear regression models indicated that yield was positively associated with RWC, MSI, SPAD, and canopy temperature, whereas enzymatic and secondary metabolite effects on yield were comparatively less pronounced. Among the cultivars, HD 2967 and K 0370 performed best under late-sown conditions, with HD 2967 showing the highest stability. Overall, the results highlight that sowing before 5 December and selecting suitable cultivars are essential for alleviating THS in the Tarai region of eastern India. RWC, MSI, and SPAD emerge as key physiological traits conferring resilience to THS, warranting their validation across a broader genotypic pool in future studies.