Climate shifts in south-eastern Australia: early maturity of Chardonnay, Shiraz and Cabernet Sauvignon is associated with early onset rather than faster ripening

Background and Aim: Early maturity of grape crops associated with recent warming is well documented but poorly understood. Here, we ask the question: is early maturity associated with early onset of ripening or with increased rate of sugar accumulation? Methods and Results: We used records of grape juice total soluble solids (TSS) for Chardonnay, Shiraz and Cabernet Sauvignon collected in commercial vineyards between 1995 and 2009. The combination of 15 vintages and three climatic regions yielded a range of average seasonal temperature between 15.5 and 20.5 degrees C. Using boundary functions of TTS versus time, we derived the onset and rate of increase in TSS, and the time of maturity set at 12 degrees Be for comparative purposes. Pooled across varieties, the rate of change in maturity was -9.8 +/- 0.94 degrees C/d. Shifts in onset accounted for 86% of the variation in time of maturity and the rate of ripening for the remaining variation. Conclusion: Early maturity associated with higher temperature (and related factors including higher radiation and higher vapour pressure deficit) is primarily driven by early onset of ripening under a wide range of production systems in south-eastern Australia. Significance of the Study: Viticultural practices aiming at delaying maturity to counteract the effect of high temperature, high radiation and high vapour pressure deficit are more likely to be successful if they target the onset of ripening.