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

Abstract

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 (LT₅₀) 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 LT₅₀ 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 LT₅₀ were evaluated during a natural heatwave. Genotypes with the highest T_Opt and LT₅₀ 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 LT₅₀, linking structural traits with thermotolerance. Collectively, these results demonstrate that photosynthetic T_Opt, LT₅₀, and leaf size and color determine heat stress resilience in V. corymbosum, providing mechanistic insight into plant responses to episodic heat stress.