Background and Aim: We tested the hypothesis that elevated temperature decouples anthocyanins and total soluble solids (TSS) in developing berries and explored the effects of fruit load and water supply on the putative decoupling.
Methods and Results: We established three 22 factorial experiments where two thermal regimes (elevated temperature, control) where combined with varieties (Shiraz, Cabernet Franc), fruit loads (bunch removal, control) or water regimes (irrigated, water deficit). The relationship between anthocyanins and TSS had a lag phase where TSS increased with no change in anthocyanins and an approximately linear phase where both traits increased in parallel for TSS > 11.4°Brix. Mean residuals in the linear phase were 0.065 mg/g in controls compared with −0.064 mg/g in the heated treatment, hence demonstrating the thermal decoupling of anthocyanins and TSS across varieties fruit loads and water regimes (P < 0.0001). Delayed onset of anthocyanin accumulation at elevated temperature explained the decoupling. Water deficit increased the anthocyanin : TSS ratio.
Conclusion: Elevated temperature decoupled anthocyanins and sugars in berries by a relative shift in onset rather than rate of accumulation of these berry components.
Significance of the Study: Warming trends are likely to disrupt the anthocyanin : sugar ratio in berries with consequences for colour : alcohol balance in red wines. Water deficit established shortly before veraison could partially restore these ratios.