Regulated deficit irrigation on ‘Cabernet Sauvignon’ grapes: impact on aroma and phenolic composition

Regulated deficit irrigation (RDI) is a technique aiming to address two main objectives: water resource savings and wine composition modification. Even though there is a large body of information that has been produced regarding the impact of RDI on grape and wine composition, there are still gaps to fill mainly because of inconclusive findings in several traits and site-specific optimization strategies that might be more suitable for high-end grapes. This study aims to understand the impacts of different levels of deficit irrigation on a large number of metabolites and other chemical parameters in grapes, from aroma to non-volatile compounds, in an industrial-scale trial. A three-year RDI experimental trial was implemented in a commercial vineyard of ‘Cabernet Sauvignon’ in the Maule Valley in Central Chile. Four RDI treatments were employed to replenish different portions of evapotranspiration (ET) from pea-size until harvest. These irrigation treatments were conceived as 100% ET, 70% ET, 50-100% ET (50% ET before veraison and 100% ET afterwards) and 35-100% ET (35% ET before veraison and 100% ET afterwards). The following parameters were measured: midday stem water potential (Ψstem), stomatal conductance (gs), grape growth, yield and must composition. Chemical analysis included GS-MS/MS for C13-norisoprenoids, monoterpenes, C6 alcohols and aldehydes, and methoxypyrazines; and HPLC for anthocyanin and low molecular weight phenols. RDI significantly affected yield, mainly due to reductions in cluster weight, berry size, and berry number. °Brix were found to be similar with no clear effect of the most restrictive RDI against the control. Significant differences were found in several secondary metabolites including higher concentrations of anthocyanin, tannins, and some specific flavonoids like catechin, quercetin, and myricetin in the most water-restricted treatments. Regarding aromas, RDI treatments increased the concentration of isobutyl-2-methoxypyrazine (IBMP), hexanal, 1,4 and 1,8-cineole, and linalool. Indicating that in the long run, RDI will concentrate most of the metabolites, either positive like anthocyanins and quercetins or potentially negative such as IBMP and hexanal.