Altered light interception reduces grape berry weight and modulates organic acid biosynthesis during development

The response of grape berries at a cellular level to environmental change was explored with particular emphasis on physiological changes such as weight, sugar content, and the biosynthesis of organic acids. Three levels of light were used: highly exposed, moderately exposed, and light-excluding boxes (1% ambient with no change in temperature effect). Berry weight was significantly lower in light-excluding boxes than in exposed bunch treatments. Organic acid content and berry development were followed throughout the growing season. Light exclusion resulted in a significant reduction of both tartaric acid and oxalic acid compared with highly exposed fruit, suggesting that in this experiment, light irradiance influenced accumulation of these metabolites. In contrast, malic acid was broken down postveraison at a dramatically slower rate in light exclusion treatments. The sink properties of grape berries appear to change according to the light received by the bunch. These data imply that cluster shading significantly reduced berry size and suggest the role of organic acids as osmotica.