Abstract
Sunburn is a physiological disorder that reduces grape quality and vineyard yield. It is the result of excessive sunlight and high temperatures. As climate change continues to increase air temperatures, reports of sunburn damage in vineyards worldwide are becoming more frequent. Grapes produce secondary metabolites (carotenoids, polyphenols and aroma compounds) to counter photooxidative stress and acclimate to higher radiation environments. This study evaluated changes in these compounds in during ripening when grapes were exposed post-flowering (ED) and at véraison (LD), and compared them to a nondefoliated control (ND). ND contained more α-terpineol and violaxanthin, and the defoliated treatments contained more zeaxanthin, β-carotene, C6 compounds and flavonoids. ED berries adapted better to higher-light environments, displayed larger changes in secondary metabolite concentrations and lower levels of sunburn damage than LD berries did. The composition of berries with increasing sunburn damage was evaluated for the first time. Berries with no damage had the lowest concentrations of flavonoids and oxidized glutathione, and the highest concentrations of chlorophyll and α-terpineol. As damage increased, destruction of photosynthetic pigments, increase in polyphenols and loss of aroma compounds were evidenced. A significant effect of temperature and developmental stage on grape composition was also observed. This study provides a holistic overview of changes in secondary metabolites experienced by grape berries when exposed to excessive light, how these vary along development and how they affect sunburn incidence.
Highlights
Introduction iationsCanopy management practices such as defoliation are commonly performed in cool viticultural regions to increase air flow, improve spray penetration, decrease disease pressure, reduce vine yield and improve fruit composition [1]
There were no significant differences between bunch number, bunch weight, yield or most basic chemistry parameters (TSS, titratable acidity (TA), pH and yeast assimilable nitrogen (YAN)) between treatments at either vineyard at harvest (Supplementary Table S2)
Significant differences were observed only between vineyards: berry weight, TA and Total soluble solids (TSS) were higher at BAL while pH and YAN were higher at CUM
Summary
Canopy management practices such as defoliation are commonly performed in cool viticultural regions to increase air flow, improve spray penetration, decrease disease pressure, reduce vine yield and improve fruit composition [1]. By opening the bunch area, radiative exposure at the grape level is increased, and the sudden exposure of shaded fruit (without prior acclimation of the tissue) might lead to photooxidative damage and sunburn, especially when ambient temperature is excessive [2]. Sunburn damage adversely affects grape (and wine) composition, leading to significant economic losses of up to 50%. Solar radiation is critical to the development of plants and fruits, the loss of shade can result in excessive sunlight and the production of reactive oxygen species (ROS).
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