Asymmetric warming effect on the yield and source:sink ratio of field-grown grapevine

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Using large-scale open-top chambers to approximate the thermal regimes of the Barossa Valley in the next decades, we measured yield and vegetative growth of grapevines in three factorial field experiments combining two thermal regimes (elevated temperature vs. control) with: two water regimes (experiment 1), two source:sink ratios generated by manipulation of fruit load (experiment 2) and four varieties (experiment 3). Water regime, source:sink ratio and varieties are sources of variation typically embedded in climate change studies using time series.Yield of controls ranged from 1.2 to 10.5kgvine−1. Warming had asymmetric effects on fruit yield and fruit/pruning weight ratios: yield response to elevated temperature varied from 46% reduction to 177% increase in relation to controls, and the response of fruit/pruning weight ratio varied from 56% reduction to 145% increase. Yield response to temperature was primarily explained by the response of bunch number (r2=0.92, P<0.0001). The effect of elevated temperature on fruit/pruning weight ratio was closely related to the response of fruit yield (r2=0.79, P<0.0001) and unrelated to the response of pruning weight (P=0.18). Elevated temperature reduced concentration of starch in trunks but not in roots.We conclude that the asymmetric responses of yield and fruit/pruning weight ratios resulted from seasonal influences on warming effects, whereby the largest responses of these traits were mediated by the promotion of bunch formation following relatively low temperatures in the preceding season. This highlights the need to assess warming effects in the light of both the background temperature and the distribution of thermally contrasting seasons in particular locations. Lower source:sink ratio and reduced starch concentration in trunks are two important elements of the carbon balance of the vine with long-term physiological and viticultural implications.