Genetic Variability, Heritability and Trait Associations of Morpho‐Physiological and Yield‐Related Traits in Rapeseed ( <scp> <i>Brassica napus</i> </scp> L.) Under Terminal Heat Stress: Implications for Heat‐Tolerant Breeding

ABSTRACT Terminal heat stress during reproductive development severely constrains yield formation in rapeseed ( Brassica napus L.), yet effective selection criteria for heat adaptation remain insufficiently defined. In this study, field evaluations conducted over two growing seasons were used to characterize the physiological, morphological and yield‐related responses of commercial rapeseed genotypes exposed to terminal heat stress. Heat stress disrupted photosynthetic stability, plant water relations and reproductive development, resulting in reduced yield performance. Canopy temperature and stomatal conductance consistently emerged as key physiological indicators of heat adaptation, exhibiting relatively high heritability and reflecting a greater capacity to maintain plant water status and photosynthetic activity under stress conditions. Morphological traits such as plant height and number of siliques per plant, which also showed substantial genetic variability and strong associations with seed yield, were identified as reliable selection criteria for breeding programs. Multivariate analyses enabled clear discrimination among genotypes and revealed contrasting physiological and yield‐related response patterns to heat stress. Although none of the genotypes exhibited complete tolerance, Hyola 61 and Hyola 50 demonstrated comparatively better adaptation and more stable performance under heat stress, whereas Agamax and Hyola 401 were consistently susceptible. Overall, the findings highlight the urgent necessity of developing heat‐adapted rapeseed genotypes in the context of ongoing global warming, emphasizing that effective improvement of heat tolerance requires selection for integrated physiological stability and reproductive resilience, supported by multiyear evaluations to account for strong genotype × environment interactions in heat‐prone environments.