Combining leaf-to-fruit ratio manipulations with abscisic acid application adjusts sugar and anthocyanin concentrations in ripening berries

High temperatures (HT) usually increase berry sugar concentrations, while reducing organic acids and anthocyanin levels, thereby modifying wine quality and character. Viticultural practices such as leaf-to-fruit ratio (L/F) manipulation, combined with application of abscisic acid (ABA) can potentially be used to mitigate HT effects and adjust berry composition. In the present work, after confirming the effects of LF and ABA applications on berry composition over a six-year period, we studied the mechanisms underlying these effects on berry composition and ABA metabolism in ‘Cabernet Sauvignon’ fruiting-cuttings (i.e., Mullins’ vines). Reducing L/F significantly reduced berry sugar and anthocyanin contents, and slightly increased total organic acid content. ABA application increased anthocyanin concentration, and partially restored the coupling between sugar and anthocyanin accumulation under low L/Fs, without affecting the sugar/organic acid ratio. Several transcripts of the anthocyanin biosynthesis pathway (CHS2, CHS3, CHI, F3H, DFR, LODX, UFGT, MybA1 and MybA2) were less abundant under low L/F ratio, whereas some transcripts (CHI, F3H, F3’5’H, LODX, UFGT, MybA1 and MybA2) were upregulated after ABA treatment. ABA treatment had little effect on the transcript abundance of genes related to sugar accumulation, except SWEET10 in 12L plants. Carbon source limitation to clusters also had little effect on ABA biosynthetic genes, but decreased berry ABA concentration. In conclusion, our results show that ABA and sugar signaling synergistically interact to regulate the expression of anthocyanin biosynthetic genes and increase anthocyanin accumulation. Thus, combining L/F ratio manipulation with ABA applications may offer a fine-tuned way to reduce sugar concentration, while maintaining anthocyanin concentrations in grape berry, potentially offering a way to partially alleviate the warming effects of climate change.