Hierarchy of factors impacting grape berry mass: Separation of direct and indirect effects on major berry metabolites

Final berry mass, a major quality factor in wine production, is determined by the integrated effect of biotic and abiotic factors that can also influence berry composition. Under field conditions, interactions between these factors complicate study of the variability of berry mass and composition. Depending on the observation scale, the hierarchy of the impact degree of these factors can vary. The present work examines the simultaneous effects of the major factors influencing berry mass and composition to create a hierarchy by impact degree. A second objective was to separate the possible direct effects of factors on berry composition from an indirect effect mediated through their impact on berry mass. Vine water and nitrogen status of six blocks of Cabernet franc vines planted on sandy or gravelly soils were monitored over two years. Berries were analyzed from veraison to harvest. At each sampling date, fresh berry mass, berry seed mass and number, sugar content and concentration, and malic acid concentration were recorded. All studied factors significantly impacted final berry mass, but vine water status had the largest effect. The interaction between factors sometimes hid significant effects on berry compounds. Nevertheless, we showed by means of appropriate statistics that all factors had a direct impact on berry sugar and malic acid concentrations, although their order of impact varied with the metabolites considered. Conversely, the effect of factors significantly impacting berry sugar content is mediated through their impact on fresh berry mass