Studying heat waves effects on berry composition: first outlooks and challenges

Abstract

Cool-climate wine-growing regions nowadays face problems caused by climate change, such as drought spells and heat waves. These phenomena are expected to increase in intensity and frequency, raising the question of whether the wine style of said cool-climate regions is going to change. Many studies have shown secondary metabolites to be affected by abiotic stress, such as phenolics, anthocyanins and volatiles important for aroma. The question remains regarding short, intense drought spells, and the interaction this may have with heat stress, as drought and heat tend to naturally occur in unison. In our experiment, grapevines were grown and stressed for five days in a semi-controlled field setup. Potted vines were grown on lysimeter scales to record daily water loss per individual plant and impose drought stress at 25% field capacity, compared to non-drought stressed plants at 100% field capacity. Heat was applied using infrared heaters set to 800W at 30cm from the fruiting canes, increasing the cluster zone temperature by a mean +3.6°C, although some individual berries reached temperatures above 45°C. The heat treatment caused visible damage to grape clusters, with a portion of the berries becoming necrotic. At harvest, surviving berries were analysed for secondary metabolites (sugars, organic acids, phenolics, amino acids, monoterpenes, C6 alcohols) to assess changes in harvest quality in response to stress. Our data showed a weak correlation between changes in metabolite concentration and stress accumulation. Furthermore, the correlation coefficients were not significant once corrected for false discovery rate. Given that the sampled berries were the ones that survived the stress, our data suggests that the intensity and duration of the imposed stress was sufficient to cause serious problems for yield, but not necessarily quality, as neither primary nor secondary metabolites were significantly affected. Our efforts are currently exploring whether there is a temperature threshold which leads to changes in secondary metabolites even with a short-lasting stress, in order to determine possible mitigation strategies in future heat waves.