<p style="text-align: justify;"><strong>Aims</strong>: The impact of water deficit stress on vine shoot growth, berry weight, grape composition and overall vintage quality was investigated in Bordeaux vineyards. Methods for assessing water deficit stress were compared.</p><p style="text-align: justify;"><strong>Methods and results</strong>: Vine water status was assessed on three soil types during four vintages by means of stem water potential and carbon isotope discrimination measured on grape sugar. Regional water deficit was compared for a range of over 30 vintages by means of water balance modelling. It was shown that water deficit stress anticipated shoot growth slackening, limited berry weight and enhanced berry anthocyanin content. Berry sugar content was greatest when water deficit was mild. It was shown that stem water potential measurements and carbon isotope discrimination are accurate tools for assessing vine water status at plot scale. Seasonal water deficit at a regional scale can be correctly estimated by water balance models. Vintage quality in Bordeaux is determined by the intensity of water deficit stress rather than by the level of the temperatures.</p><p style="text-align: justify;"><strong>Conclusions</strong>: Vine phenology and grape ripening are highly dependent on water uptake conditions. Mild water deficit stress enhances grape quality for the production of red wines. Vine water status can accurately be assessed by means of stem water potential or carbon isotope discrimination measured on grape sugars. Quality losses through severe water stress can be avoided through the use of drought-adapted plant material, appropriate canopy management, yield reduction or the implementation of deficit irrigation.</p><p style="text-align: justify;"><strong>Significance and impact of the study</strong>: This study shows the key role of water deficits in the production of quality grapes for red wine production. Methods for assessing vine water status are compared and discussed. Among many existing methods, the accuracy of stem water potential, carbon isotope discrimination measured on grape sugar and water balance modelling are emphasized.</p>
Vine nitrogen status influence on S-cysteine conjugate precursors of volatile thiols, glutathione and phenolic content of the berries of Vitis vinifera L. cv. Sauvignon blanc have been assessed. Despite an increase in berry weight, the increase in nitrogen supply in the vine leads to higher cysteine precursor levels in grape juice. We also show that a late addition of nitrogen at berry set leads to a lower level of phenolic compounds in white grapes and to higher glutathione levels. Therefore, in white varieties, and particularly Sauvignon blanc, improving the nitrogen supply of the vine clearly increases its aromatic potential. However, an excess of nitrogen supply would increase the grape sensitivity to Botrytis cinerea and therefore decrease its quality.Résumé: L'effet du statut azoté de la vigne sur le potentiel aromatique de raisins de Vitis vinifera L. cv. Sauvignon blanc a été évalué. Sur une parcelle carencée en azote (modalité N), à cause d'une faible teneur en matière organique dans le sol, un supplément d'azote équivalent à 60 u/ha a été apporté (modalité L + N). Le stade tardif de l'apport, la nouaison, visait à limiter l'effet de l'azote sur les paramètres de la vigueur. Sur les deux modalités, l'alimentation en eau de la vigne était similaire et non limitante (potentiels tige > -0,65 Mpa tout au long de la saison). Il était important de vérifier l'absence de contrainte hydrique, car il a été montré que l'alimentation en eau influe sur la teneur en précurseurs aromatiques du raisin. Sur la modalité L + N, le rendement était équivalent à L et la surface foliaire primaire était similaire, mais la surface foliaire secondaire et le poids des bois de taille étaient plus forts. L'apport d'azote a provoqué une augmentation du poids des baies et de l'acidité totale du jus de raisin, à cause d'une plus forte teneur en acide malique, et une très forte augmentation de la teneur en azote assimilable. La teneur en sucres réducteurs n'était pas significativement modifiée. Les raisins des vignes, qui avaient reçu un apport d'azote, contenaient plus de précuseurs S-conjugués à la cystéine, plus de glutathion et moins de composés phénoliques. Cette constitution permet d'élaborer des vins avec une plus forte expression variétale. Il n'est cependant pas souhaitable d'avoir une alimentation en azote excessive des vignes, à cause d'une plus grande sensibilité des raisins à Botrytis cinerea.
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