Effect of soil moisture availability on Vitis vinifera cv. Merlot: from leaf water potential to grape composition

Grape phenolics are a relevant part of grape quality, and their metabolism in the berry may be modified by environmental factors. To determine the influence of different water regimes on the polyphenolic composition of berries, research was conducted for two years (2000 and 2001) on three-year-old Vitis vinifera cv. Merlot potted vines, comparing three water supply levels: (1) control (C), at ~80% of available water (aw); (2) moderate stress (M), at ~30% aw; and (3) severe stress (S), at ~15% aw. The treatments were applied in both years from veraison to fruit maturity. Predawn leaf water potential was reduced only in the S treatment in 2000, while in 2001 both M and S treatments had a lower value. Berry weight was reduced in M and S treatments, but no differences were observed in sugars, pH, and berry titratable acidity. Total skin and seed polyphenols were increased in S vines, but a probable increase of the structural complexity of phenolic compounds inside the berry (higher degree of polymerization) caused a lower extractability in winelike solution. Anthocyanins, which are monomers, were more concentrated and also more extracted in S berries. Water-stressed Merlot grapes (both S and M) will benefit from a longer maceration because a higher concentration of polymerized polyphenols could be extracted, stabilizing color and improving the mouthfeel properties of the resulting wines.