Berry sugar accumulation and softening dynamics in a slow ripening genotype and its response to hormone application

The onset and rate of sugar accumulation in wine grapes are important traits for wine style and quality attainment. As sugar accumulation rates increase due to the consequences of a rapidly changing environment, an uncoupling between the concentration of sugar and other compounds contributing to wine quality is often observed. Management practices can mitigate accelerated sugar accumulation only to a certain extent, while exploration of natural variants with fruit ripening traits of interest represents a more permanent solution. In this study, differences between two full siblings with extreme phenotypic variation in fruit ripening were investigated. Sugar accumulation, berry growth and softening over three years showed an unprecedented ripening delay of over 65 days. Hypothetical causes for the ripening failure were tested through a series of experiments. Leaf physiology was not impaired in the slow ripening genotype, and severe cluster thinning did not impact ripening time or rate. Abscisic acid treatments promptly triggered sugar accumulation, and treatment efficacy was dependent on the timing and dose applied. These results suggest a failure in the signaling process involved in ripening initiation, a first step towards a fundamental characterization of mechanisms of slow ripening that could aid selection of improved grape genotypes more suited to future conditions due to climate change.