Effect of high and extreme high temperatures on berry tannin composition in Vitis vinifera cv. Shiraz

Climate change is leading to an increase in average temperature and in the frequency and severity of heatwaves, and is already significantly affecting grapevine phenology and berry composition. As weather conditions of Australian warm and hot grape growing regions evolve, flavonoids, for which biosynthesis depends on bunch microclimate, are expected to be affected. These compounds include flavan-3-ols and tannins which are important contributors to grape and wine quality. The aim of this project was to determine if berry tannin accumulation is sensitive to high temperature. While the synergistic effect of light and temperature on anthocyanins has been intensively examined, more knowledge is required for tannins as their metabolic pathway is yet to be fully elucidated, and their structural analysis is limited due to their complex nature.
Using a multi-disciplinary approach, the research focused on how high (>35 °C) and extreme high (>45 °C) temperatures impact on berry physiology, survival and detailed tannin composition. Temperature-related parameters (duration, intensity, day/night, phenological stages, levels and berry acclimation) were investigated across four glasshouse experiments, conducted on well-irrigated potted Shiraz grapevines. Bunch or whole-vine temperature was manipulated using experimental systems that prevented differences in canopy/bunch light exposure while allowing natural light and UV. The first two experiments used a factorial design to either investigate the effect of a single or several whole-vine heat treatments (+6 °C) or to assess the influence of high day (+8 °C; 45 °C) and night (+6 °C; 33 °C) bunch temperature, during berry development. Another experiment examined different intensities (35-54 °C) and durations (3-39 h) at mid-ripening (E-L 36). The last experiment combined the previous parameters that had affected tannins. Two additional assays evaluated the potential temperature impact on subsequent wine composition using wine-like extraction and micro-scale winemaking. Detailed tannin composition was primarily determined by liquid chromatography-tandem mass spectrometry after phloroglucinolysis, with complementary total tannin concentration (methyl cellulose precipitable assay). Tannin size distribution (gel permeation chromatography), together with primary and secondary metabolites (gas and liquid chromatography-mass spectrometry) were also analysed on key samples to provide a more comprehensive picture.

Providing berries were not damaged, tannin accumulation experienced just a short delay following high temperature exposure during early berry development. Differences were likely due to a combination of berry development disruption as well as a deregulation of some genes involved in tannin biosynthesis. Parameters most commonly impacted across all experiments were seed tannin size (increased) followed by tannin content (decreased) and skin galloylation (increased). Most differences were no longer evident by harvest, but if any, extractability was increased compensating for the decrease in berry phenolics. To complement compositional responses, berry survival thresholds were identified with green berries exhibiting visual damage for temperatures above 42-44 °C while red berries only started to necrose above 50 °C. In damaged berries, skin tannins were dramatically reduced while seed tannins were mostly preserved. With the experimental system used for this thesis, tannin accumulation showed an elastic response to high temperature and if berries were not shrivelled, quality was not impaired at harvest by the sole effect of temperature. The project provided enhanced knowledge on upper temperature limits for viable wine production, in turn informing critical timing for mitigation strategies.