Background and Aims: Climate variation contributes to fluctuations in reproductive output, and spring temperature is thought to influence flower production in grapevines. For 3 years, we studied the influence of temperature from before budswell through to the appearance of individual flowers on reproductive development in field-grown Cabernet Sauvignon while minimising the influence of other microclimatic variables. Methods and Results: Dormant buds and emerging shoots were heated or cooled from before budswell until individual flowers were visible. Flower number per inflorescence was inversely related to pre-budburst temperature. Conversely, flower size, percent fruit set, and berry size increased with higher temperatures. Fruit set also increased as flower size and leaf area per flower increased; fruit set was erratic below 4 cm2 leaf area per flower. Berry mass and sugar content per berry increased with increasing flower size. Although yield per shoot varied threefold among treatments, differences in fruit composition were minor. Conclusions: Variations in early-season temperatures may alter substantially grapevine yield formation. The temperature effect may be a combination of direct effects on floral development and indirect effects arising from differences in shoot growth. Significance of the Study: This study shows that variations in temperature near budburst may be an important cause of large variations in grapevine yield.
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