Yun Zhang,* Joel C. Perez, and Markus Keller
*Irrigated Agriculture Research & Extension Center, Washington State University, 24106 N. Bunn Rd., Prosser, WA 99350 (yunzhang@wsu.edu)

Berry transpiration is important for both berry growth and ripening. Genetically diverse Vitis cultivars were used to investigate developmental changes in and factors determining berry transpiration. Transpiration rates were measured on both whole attached clusters (cluster chamber method) and on detached berries (weight loss method). Results obtained using these two methods were in good agreement. The chamber method also demonstrated that detaching berries did not alter their transpiration. The contribution of the rachis to whole cluster transpiration was minor. Berry transpiration rate highly correlated with diurnal changes in vapor pressure deficit, and thus with ambient temperature and relative humidity. Transpiration rate and, to a lesser extent, cuticular conductance, peaked when the berry skin color was red/purple (~13 Brix) and then declined with further ripening. Berries of different cultivars shared similar developmental patterns of transpiration and cuticular conductance, with minor differences in their absolute values. When berry transpiration was artificially reduced in the field, sugar accumulation slowed down yet cracking incidence increased. During late-season (≥18 Brix), berry transpiration accounted for a daily loss of 2 to 5% of berry weight across 11 cultivars. These results showed that berry transpiration was determined by both environmental factors (air temperature and relative humidity) and cultivar-specific factors (primarily berry surface area and cuticular conductance). Moreover, berry transpiration facilitated ripening and may partially contribute to late-season weight loss.

Funding Support: Washington State Grape & Wine Research Program, Washington State University, Chateau Ste. Michelle Distinguished Professorship, Rhone Rangers