Leaf photosynthetic and morphological traits underlie heat stress resilience in northern highbush blueberry (Vaccinium corymbosum L.)

Northern highbush blueberry ( Vaccinium corymbosum L.) is a temperate fruit crop increasingly exposed to heat stress as heatwaves become more frequent. This suggests the need to characterize thermotolerance traits to facilitate sustainable production. We hypothesized that variation in leaf photosynthetic temperature responses, thermotolerance, and morphology underpin genotypic differences in heat stress resilience in V. corymbosum . Leaf photosynthesis temperature response curves and chlorophyll fluorescence-based thermotolerance curves (LT 50 ) were measured at three phenological stages (bloom, mid-ripening, post-ripening) in ten genetically diverse V. corymbosum genotypes. Leaf morphological traits were also assessed at mid-ripening. Significant genotypic variation was observed in the photosynthesis temperature optimum ( T Opt ) and LT 50 at all stages, and the leaf thermotolerance threshold of V. corymbosum was established at 43.1-47.2 °C, lower than values reported for many other plant species. To validate the functional relevance of these traits, four genotypes spanning the range of T Opt and LT 50 were evaluated during a natural heatwave. Genotypes with the highest T Opt and LT 50 maintained greater photosystem II efficiency and gas exchange under heat stress, whereas genotypes with the lowest values were more susceptible. Most genotypes exhibited long-term seasonal acclimation of photosynthesis from bloom to mid-ripening, while only the resilient genotype showed evidence of short-term acclimation of A Opt during the heatwave. Leaf size and color were positively associated with LT 50 , linking structural traits with thermotolerance. Collectively, these results demonstrate that photosynthetic T Opt , LT 50 , and leaf size and color determine heat stress resilience in V. corymbosum , providing mechanistic insight into plant responses to episodic heat stress. • We established the leaf thermotolerance threshold of 43.1-47.2 °C in V. corymbosum . • V. corymbosum displayed genotypic diversity in leaf T Opt and LT 50 . • Leaf T Opt , LT 50 , and E contributed to heat resilience following a heatwave. • The heat-resilient genotype displayed short-term thermal acclimation to the heatwave. • Darker green leaves with larger leaf area predicted a higher LT 50 .