Grape Berry Transpiration Is Determined by Vapor Pressure Deficit, Cuticular Conductance, and Berry Size

Developmental changes and factors determining grape berry transpiration were investigated in three genetically diverse Vitis cultivars. Transpiration rates were measured on whole clusters, using a custom-designed cluster chamber, and on individual berries by weighing detached berries over time. Results obtained using the two methods were in good agreement. The chamber method verified the assumption that detaching berries does not alter their transpiration and also showed that rachis transpiration was minor compared to whole cluster transpiration. Berry transpiration fluctuated with vapor pressure deficit which was the main determinant of the driving force. The transpiration rate per berry and, to a lesser extent, the cuticular conductance, peaked when berries were red/purple (similar to 13 Brix) and then declined with further ripening. Due to the decline in cuticular conductance during late ripening, the positive linear relationship between berry transpiration rate and surface area weakened after berries ripened. Despite the similar developmental patterns of berry transpiration and cuticular conductance, Concord (V. labruscana) berries consistently had much lower cuticular conductance than Merlot and Syrah (V. vinifera) berries. These results showed that berry transpiration was determined by both external factors (air temperature and relative humidity) and cultivar-specific internal factors (primarily berry surface area and cuticular conductance).