Responses of photosynthesis to high temperature stress associated with changes in leaf structure and biochemistry of blueberry (Vaccinium corymbosum L.)

Blueberry (Vaccinium corymbosum L.) is a deciduous flowering shrub, which originates from North America. In recent years, Blueberry has been introduced into subtropical China where featured with high temperature climates. Understanding the thermal endurance ability of different blueberry cultivars is necessary for heat-tolerant cultivars selection and plantation in subtropical China. We conducted a comparative study on six cultivars including 'Bluecrop', 'Duke', 'Brigitta', 'Gulfcoasr, 'O'Neal', and 'Blue Ridge' at different temperatures of 25/20, 30/25, 35/30, or 40/35 degrees C (day/night) to examine the changes of growth, physiology, and biochemistry in blueberry plants. We found that the biomass of blueberry plants generally shared bell-shaped curves with increasing growth temperatures, indicating different optimal growth temperature for each cultivar. Meanwhile, the six blueberry cultivars also shared different heat endurance abilities that the appearances of 'Duke' and 'Blue Ridge' featured with wilted plants and defoliated leaves, but the other four cultivars (`Bluecrop', 'Brigitta', 'Gulfcoase, and 'O'Neal') present better appearances under high temperature stress. The good performances of the four heat tolerant cultivars in response to high temperature stress may benefit from the higher transpiration rates, which can dissipate much more heat through leaf transpiration than the two heat sensitive cultivars ('Duke' and 'Blue Ridge'). Thus, the heat tolerant cultivars suffered less from high temperature stress can be attributed to the higher efficiency of heat dispersion, which may be explained by the changes in the distribution pattern of stomata and the total stomatal aperture area per leaf area (SAAI) through altering the stomatal density as well as the aperture size and shape of individual stoma. These findings suggested that thermal tolerant cultivars may improve their heat dispersing efficiency through regulating stomatal traits to protect harmful impacts on chloroplast structure and function, and membrane thermostability from high temperature stress as evidenced by the intact chloroplasts as well as the higher F-v / F-m and electrolyte leakage than the two heat sensitive cultivars.