Integrated characterization of terminal heat stress resilience in linseed through agronomic, reproductive, and physiological responses

Terminal heat stress is a major climatic constraint affecting yield and seed quality in linseed ( Linum usitatissimum L.), particularly during the reproductive phase. To identify heat-tolerant genotypes and associated adaptive traits, 124 linseed genotypes were evaluated under optimum and delayed sowing conditions over two growing seasons, representing optimum and terminal heat-stress environments. Substantial genotypic variation was observed in seed yield, biological yield, capsules per plant, seeds per capsule, and other physiological parameters. Stress Susceptibility Index (SSI) and extent of yield reduction identified genotypes DLU-5, PULU-2, and PLU-2 as heat-tolerant, whereas KLU-30, DLU-15, and KLU-47 were highly susceptible. Comparative reproductive investigations of DLU-5 (tolerant) and KLU-30 (susceptible) demonstrated that DLU-5 preserved high pollen viability (70–75%), exhibited a substantial pollen load, and maintained the structural integrity of anthers and exine under terminal heat stress condition. In contrast, KLU-30 had lower pollen viability (30–40%) and showed substantial alterations in pollen morphology and ultrastructural integrity. Physiological responses further showed that tolerant genotypes sustained higher chlorophyll content, relative water content, and proline accumulation, along with lower electrolyte leakage, H 2 O 2 , and MDA levels under stress. These findings highlight the importance of reproductive resilience and efficient biomass allocation in linseed’s ability to withstand terminal heat stress, as well as its overall agronomic productivity. Future research should investigate the genetic basis of tolerant genotypes and validate them across diverse heat-stressed environments to accelerate breeding for enhanced heat resilience.