Modulation of Grapevine Berry Development and Chemical Composition by Solar Irradiance

In arid regions, where solar irradiance levels are high, insolation may become a limiting factor for plant development. Absorption of excess light energy by plant tissues may increase their temperature, impact metabolic processes, and form reactive oxygen species. Compared to leaves, fruits have a limited ability to regulate their temperature and dissipate this excess of energy. Their defense mechanisms therefore involve the accumulation of compounds that filter solar irradiance and protect against oxidation and dehydration. As a result, the effect of solar irradiance and fruit temperature on the composition of wine grapes has received much attention in recent years. However the complexity of the metabolic processes involved in grape response to light, and the lack of accurate micrometeorological measurements in field trials, have, thus far, hindered the predictability of grape compositional consequences given a set of meteorological conditions. We unraveled the dynamics of metabolite changes in the grape associated with the modification of solar irradiance conditions. We integrated high- resolution micrometeorological measurements with seasonal, spatial, and diurnal analyses of the grape metabolic profile. We found that grape chemical composition readily responded to changes in solar irradiance, involving the preferential accumulation of sugar alcohols, amino acids, and flavonols, at the expense of sucrose, malate, flavan-3-ols and anthocyanins, respectively. Our results also revealed that the variability in grape composition within a cluster is a function of the variability of solar irradiance and the light-response curve of the specific metabolite group. Last, via the analysis of diurnal metabolite changes, we found an indirect effect of the sun path on fruit sucrose levels, affecting the translocation of assimilates to the fruit. Conversely, the grape insolation pattern directly affected sugar metabolism and the levels of amino acids and phenylpropanoids in the fruit. Our study shows that the spatiotemporal solar regime in a vineyard is a major factor driving spatial variability and diurnal fluctuations in fruit composition. Taken together, we conclude that precise solar irradiance management is a central element of improving fruit quality and homogeneity and mitigating the detrimental consequences of warm and arid conditions.