Do high temperature extremes impact berry tannin composition?

Abstract

Context and purpose of the study – Flavonoids, including flavonols, anthocyanins, and tannins, are important contributors to grape and wine quality, and their biosynthesis is strongly influenced by bunch microclimate. While the synergistic effect of light and temperature has been intensively examined on flavonoids in relation to bunch exposure, studies targeting the sole effect of high temperature have mostly focused on anthocyanins during the ripening period. With tannin biosynthesis starting around flowering, heatwaves occurring earlier in the grape growing season could be critical. Only a few papers report the impact of temperature on tannin synthesis and accumulation; to date, none have examined the effect of high temperature extremes which, in the context of climate change, relates to increases in heatwave intensity.

Material and methods – Three potted-vine experiments were conducted inside a UV-transparent glasshouse during the 2016-17 and 2018-19 seasons. Using fans blowing hot air onto individual bunches without affecting light exposure, several temperature-related parameters were tested on well-irrigated Shiraz vines. In order, these examined high day and/or night temperatures after fruit set (E-L 31, Coombes, 1995), day temperature intensities (Low: LT, High: HT and Very High: VHT) and durations (3 to 39 h) after véraison (E-L 36, ~10 °Brix), and high day temperature at two phenological stages (E-L 31 and/or E-L 36). Berries were sampled at regular intervals, peeled, ground, and skin and seed tannin composition individually analysed by LC-MS/MS after phloroglucinolysis.

Results – During Experiment 1, heat treatments were applied for three days (+8 °C) and/or three nights (+6 °C), with day maximum temperature reaching 44.8 °C and night maximum temperature reaching 32.8 °C. Berry size was immediately affected by day temperature, while skin tannin exhibited small differences with an increase in percentage of galloylation 15 days after the end of the treatment. During Experiment 2, LT, HT and VHT respectively reached a maximum of 37, 45, and 53 °C. VHT considerably impacted on berry physiology and composition, regardless of the treatment duration (12 or 30 h), leading to berry desiccation. Tannins extracted from the dried skin were significantly reduced with some flavan-3-ol subunits proportionally more degraded than others. While the effect on skin was substantial, seed tannins were only slightly affected. Night temperature at E-L 31 (Experiment 1) and day HT at E-L 36 (Experiment 2) affected other primary metabolites but not tannin composition. Experiment 3, conducted during the 2018-19 season, combined parameters for which tannin composition was affected during season 2016-17 to confirm observed trends.