Abiotic stress effects on the grapevine (Vitis vinifera L.) canopy's carbon balance and metabolic profile of flavonoids in berries

Flavonoids are a very diverse family of phenylalanine-derived secondary metabolites sintetized by plant tissues for protection against environmental stresses, defense against herbivors or during senescence and ripening of different organs. Flavonoids in grape berries are critical in winemaking since they affect organoleptic properties and sensory attributes in wines such as color and astringency. They also are responsible for wine's potential to prevent coronary diseases and carcinogenesis in humans. Flavonoid biosynthesis in grape berries is affected by abiotic and biotic stress factors. The Carbon Nutrient Balance (CNB) and the Growth Differentiation Balance (GDB) hypotheses predict increased accumulation of phenylalanine-derived secondary metabolites under stress conditions such as moderate water and nutrient stresses that may limit growth more than carbon (C) assimilation at the whole plant level. It was hypothesized that enhanced C availability resulting from a more positive C balance at the whole canopy level would increase flavonoid concentrations in the skin of grape berries while maintaining similar ratios of different flavonoid groups as well as compounds within each group. This hypothesis was tested by: (a) parameterizing a biochemical model of photosynthesis under abiotic stress conditions, (b) coupling the parameterized single leaf photosynthetic model to a plant architecture and light interception model, (c) estimating whole canopy C balance, and (d) measuring changes in the relative and absolute concentrations of flavonoids in grape berry's skin at harvest in response to abiotic stresses. The model predicted a positive daily whole canopy C balance for all treatments. In spite of the fact that the daily C gain of non-irrigated vines was approximately 40% lower than that of the irrigated control, C surplus per unit biomass of fruit was greater for water stressed and non-fertilized vines. The diurnal C balance of the canopy was highly dependent on ambient temperatures. An ambient temperature of 30°C during the ripening period and moderate water stress maximized modeled canopy and berry/cluster C balance as well as flavonoid accumulation. Changes in specific flavonoids concentrations exceeded changes in their relative contents. Results obtained in this study were in agreement with predictions of the CNB and GDB hypotheses and have important implications for the optimization of management practices to increase fruit quality in hot climate viticulture.