THE OPTIMAL TEMPERATURE FOR THE GROWTH OF BLUEBERRY (VACCINIUM CORYMBOSUM L.)

Blueberry (Vaccinium corymbosum L.), a perennial evergreen or deciduous shrub, has recently been introduced into Southern China, where the subtropical climate is hot and humid in summer. Identifying the optimal growth temperatures and understanding the mechanisms of thermal stress on blueberry are not only critical to determining suitably growing areas in Southern China, but also significantly important for selecting and breeding new heat tolerance blueberry cultivars for adapting to subtropical climates. In this study, we examined the optimal temperature for the growth of six blueberry cultivars ('Bluecrop', 'Duke', 'Brigitta', 'Gulfcoasf,, 'O'Neal', and 'Blue Ridge') with four growth chambers where the temperatures were controlled at 25, 30, 35, and 40 degrees C, respectively. We found that initial increase in temperature dramatically enhanced the growth of four cultivars ('Bluecrop', 'Duke', 'Brigitta', and 'Blue ridge') through the warming effect, whereas this warming effect was substantially compromised with further increase in growth temperature, demonstrating an optimal temperature of 32.6, 30.4, 31.8, and 29.0 degrees C for the four cultivars respectively. By contrast, the aboveground, belowground, and total biomass of 'Gulfcoasf and 'O'Neal' were linearly declined with growth temperatures, indicating that elevating temperature above 25 degrees C had negative effects on blueberry growth. Meanwhile, we also found that the leaf photosynthesis, stomatal conductance, and transpiration of the six blueberry cultivars shared similar trends as plant growth in response to temperatures, suggested that leaf biochemical and photochemical processes affecting the optimal growth temperature of blueberry plants. Moreover, the temperature effects on blueberry growth was also attributed to the changes in the leaf number, leaf length and width, leaf biomass, as well as the leaf stomatal traits including density, openness, and spatial distribution pattern of stomata. In addition, high temperatures exceeding the optima also affected chloroplast structures through damaging grana lamella and stromal lamella as well as breaking chloroplast envelope. Our results suggested that the optimal growth temperature of blueberry was highly dependent on cultivars. Therefore, the optimal temperature found in this study can be used as an indicator in selecting and breeding new blueberry strains in adapting to high temperatures in subtropical China where the market demands for blueberry products have been skyrocketing.