Grapevine is the most cultivated fruit crop in the world. Its cultivation is devoted to obtain table grapes consumed as fruit, berries to be dried to obtain raisins, and grapes to be transformed in wine. To produce grapes for wine transformation, a recent approach is to manage cultivation to get the best quality. Among environmental factors, water deficit (WD) has been studied by several authors in its capacity to modify quantity and quality of production. In general a moderate WD improves grape quality, while if too severe the deficit impairs maturation and quality. To achieve this improvement, three irrigation strategies have been proposed recently: deficit irrigation (DI), regulated deficit irrigation (RDI), partial rootzone drying (PRD), and all of these have proved to be beneficial in improving grape quality and achieving a better water use efficiency. WD impacts grapevine physiology through several mechanisms: reduction of shoot growth, stomatal closure, photosynthesis reduction, abscisic acid (ABA) synthesis induction in roots and shoots. Photosynthesis is reduced, but initially this is associated with an increase in water use efficiency. Metabolomic and proteomic studies have shown a modification of gene expression and protein abundance/composition under WD: e.g. proline accumulation in leaves underwent a 2- or 3-fold increase, with an activation of both synthesis and degradation genes. In roots, a more intense suberification under WD has been observed, but also a differentiated modification of hydraulic conductivity of roots: some rootstocks increased their conductivity in early WD, while others decreased it, probably as aspects of different adaptive strategies. The reproductive cycle is modified by WD (fig. 1): berry size is reduced, skin : pulp ratio is often increased, primary and secondary metabolisms are modified. Amino acids are increased, sugars are in some cases increased or remains unaffected, titratable acidity is reduced or remain unaffected. Anthocyanin, proanthocyanidin, C13-norisoprenoid, and thiol concentrations are increased, while terpenes and esters are increased or remain unaltered probably depending on other environmental conditions. Secondary metabolites are crucial for grape and wine quality: under WD conditions some of them may improve their quality, thus benefiting the final product. In red cultivars, anthocyanins are generally increased with a moderate-severe WD; this is resulting from a more intense synthesis supported by an increased expression of genes related with anthocyanin synthesis. Proanthocyanidins (tannins) synthesis are rarely modified by WD, while stilbenes, nutraceutical components of wine, were observed to be more synthesised under water shortage. Genes codifying for wine aromatic compound precursors are induced in WD berries, and this results in wines with more fruity and less vegetal aroma. Therefore a moderate WD may be a very important and powerful tool to get a better grape and wine quality, together with a more rational water use in viticulture.
Keywords: gene expression, primary metabolism, secondary metabolism, Vitis vinifera, ripening