Ben-Min Chang, Yun Zhang, and Markus Keller*  
*Washington State University, 24106 N. Bunn Rd., Prosser, WA 99350 (mkeller@wsu.edu)

Grape berry-splitting results from cuticle failure due to excessive tensile stress on the skin. Split berries expose flesh cells to the atmosphere. Without the protection of the hydrophobic cuticle, splitting leads to dehydration and increases susceptibility to pathogen infection. If the resistance to splitting could be measured, varietal differences could be better understood. The objectives of this research were: (1) to determine varietal differences, and (2) to understand developmental changes in resistance. Based on a thin shell theory, we simplified splitting resistance as the maximum internal pressure at the time a berry splits. This study further investigated the resistance of Merlot, Syrah, Zinfandel, and Concord. Berry clusters were collected, and berries were categorized by total soluble solids. Five berries from each cluster were sampled and mounted on injection needles. An apparatus was designed to inject liquid into berries and to accumulate internal pressure. The hydraulic pressure in the berry and berry expansion were recorded automatically by a data logger. Cuticle thickness was examined by confocal microscopy. The resistance to splitting decreased greatly after the onset of berry softening in all varieties, suggesting that berry splitting is unlikely to be observed before softening. Among the four varieties, Merlot had the highest resistance to internal pressure. Correlation analysis against soluble solids confirmed that Merlot is more splitting-resistant throughout ripening than the other tested varieties. Merlot and Zinfandel had similar cuticle thickness, thus cuticle thickness was in-sufficient to explain the superior resistance of Merlot. Moreover, Concord berries had a thicker cuticle than, but similar resistance to, Zinfandel and Syrah berries.

Funding Support: Chateau Ste. Michelle Distinguished Professorship in Viticulture