Berry splitting resistance of different grape cultivars

Abstract

Grape berry splitting results from excessive tensile stress that builds on the skin. The thin shell theory describes splitting resistance of grape berries as the maximum internal pressure at the point of splitting. Quantifying this relationship in genetically diverse Merlot, Syrah, Zinfandel, and Concord grapes enabled us to determine cultivar differences and understand developmental changes in splitting resistance. Berry clusters were collected, and berries were categorized by total soluble solids. Berries were mounted on injection needles, internal pressure was increased by injecting water into berries, and the pressure was recorded by a data logger. To determine which structure is enduring tensile stress, berries were soaked in solutions to soften or harden skin cell walls. Cuticle thickness was also examined by confocal microscopy. All cultivars shared the same developmental pattern; the resistance to splitting decreased rapidly after the onset of berry softening and before any visible color change. This suggests that berry softening is the critical phase when berries lose splitting resistance. Correlation analysis against soluble solids demonstrated Merlot had the highest splitting resistance among the four cultivars. Berry survival changed greatly in cell-wall manipulation solutions, suggesting that cell walls are stress-bearing structures in the skin. Merlot and Zinfandel berries had similar cuticle thickness, and Concord berries had a thicker cuticle than, but similar resistance to, Zinfandel and Syrah berries. These results suggest that cuticle thickness is insufficient to explain differences in splitting resistance among grape cultivars.