Single and cumulative effects of whole-vine heat events on Shiraz berry composition

Aim: The aim of the study was to test the effect of a single heat event (HE), and the additive effects of repeated HEs at whole-vine level, on Shiraz berry composition, including detailed tannins. Methods and results: In a UV-transparent glasshouse, a system was developed to individually heat the above-ground parts of well-irrigated potted Shiraz vines without changing fruit and canopy light exposure. At the end of fruit set, and again prior to véraison, selected vines were heated to + 6 °C above ambient temperature for three consecutive days and nights in a combination of treatments to test the effect of a single HE and the additive effects of repeated HEs. A factorial design was used with four treatments (n=6): Control (C), heated at E-L 31 (HW1), heated at E-L 32 (HW2) and heated twice (HW1&2). Berries were sampled from fruit set until maturity at regular intervals, and primary and secondary metabolites, including detailed tannin composition, were analysed by GC-MS and LC-MS/MS, respectively. Temperatures (mean and maximum) inside the glasshouse were influenced by outside weather conditions and the 1st HE (HE1) was more intense than the 2nd (HE2). Photosynthesis was significantly decreased for the heated vines during HE1 where maximum temperature reached 45 °C, affecting both berry weight and titratable acidity (TA). HE2 was less intense with maximum temperature only reaching 40 °C, and had no effect on photosynthesis and less direct impact on composition. A few primary metabolites were affected by either HE1 or HE2 such as valine, leucine, pyruvic and lactic acids. Interactions between the two HEs were found for TA, malic acid and glucose at harvest. Skin tannin composition was significantly impacted by HE1, applied during the main biosynthesis period, but not by HE2. Epicatechin gallate terminal subunit concentration was the most impacted by heat. Seed physiology was also affected by HE1 and HE2 as well as seed tannin composition right after HE1. A small decrease in both total anthocyanins and total soluble solids at the end of véraison suggested that ripening was slightly delayed for HW1&2. Conclusions: Shiraz grapevines showed an elastic response to short heat stress between fruit set and véraison, with most impacts on physiology and composition observed during early post-treatment no longer evident by harvest. Significance and impact of the study: Increasing climate variability, with more frequent heatwaves, is a threat for viticulture in Australia where a large proportion of vineyards are located in already warm and hot regions. As grapevines may be exposed to abnormal high temperatures prior to véraison, knowledge on the effect of heat on metabolite biosynthesis occurring during this phase, such as tannins, is needed.