Quality of Merlot grapes as affected by different soil water availabilities

The amount of soil-available water directly affects grapevine water balance and the accumulation of different metabolites in the berries. Under conditions of moderate water stress, grapevines can maintain their physiological functions similar to watered plants. The aim of this trial was to investigate the relationship between water stress and phenolic accumulation in berries. Three-year-old ‘Merlot’ grapevines grafted on SO4 were grown in pots (80 L/pot), and subjected to different water regimes from véraison to maturation: control (C, 80% soil available water); moderate stress (M, 30% a.w.); severe stress (S, 15% a.w.). Soil water concentration was measured with TDR probes. Berry samples were collected weekly from véraison. Berries were weighed and the skins and seeds were separated from the flesh. Spectrophotometric analyses of total phenolics and anthocyanins in skins and seeds were performed. Localization of skin phenolics was carried out by transmission electron microscopy. M plants showed a behavior similar to C plants, but S plants revealed an unbalanced growth. Berry weight was reduced by water stress, but the concentration of phenolics increased. Under conditions of water stress, all the phenolics were less extractable from the berries. Electron microscopy revealed that phenolics were organized in little bodies in the vacuoles, but such structures were not observed in C-berry cells. From a physiological point of view, M plants demonstrated a good equilibrium in terms of growth, water balance and phenolic accumulation. Wine was made from these grapes in order to investigate phenolic extractability during fermentation. There was a lower extraction of phenolics from S grapes than from M and C grapes. Moderate stress appears to improve plant physiological responses and results in a good evolution of extractability of phenolics.