Mapping plant water status to indirectly assess variability in grape flavonoids and inform selective harvest decisions

Abstract

Context and purpose of the study – Plant water stress affects grape (Vitis vinifera L.) berry composition and is variable in space due to variations in the physical environment at the growing site. Could we use water status maps as a sensitive tool to discriminate between harvest zones?

Material and methods – The study was carried out on 35experimental units placed on an equidistant grid within a 3.5 ha vineyard located in Sonoma County, Northern California. This drip irrigated vineyard was planted with Cabernet-Sauvignon on 110R, spur pruned and trained in two single high wires in a horizontally split canopy. The site was described through a digital elevation model, terrain analysis, NDVI and electrical resistivity maps. The natural variability of grapevine water stress was monitored by stem water potential (Ψstem), leaf gas exchange and δ13C of grape must at harvest. Geospatial analysis and clustering were used to differentiate the vineyard block into two management zones according to variability in water status.

Results – The two management zones were very distinct in water status and presented severe and moderate water stress. The average difference in Ψstem between the zones was of 0.2 MPa. Differences in stem water potential affected stomatal conductance, net carbon assimilation, and intrinsic water use efficiency that were different in all measurement dates. The two zones were selectively sampled at harvest for measurements of berry chemistry. Berry mass and yield per vine in the two water status zones were not different. A significant difference in total soluble solids (3.56 Brix) and in titratable acidity indicated a direct effect of water stress on ripening acceleration. Berry skin flavonol and anthocyanin composition and concentration were measured by C18 reversed-phased high-performance liquid chromatography (HPLC). Berry anthocyanins showed the highest differences between the two water stress zones. Dihydroxylated anthocyanins were more affected than the trihydroxylated ones, therefore, the ratio of the two forms increased. Flavonols were different in total amounts, but hydroxylation patterns were not affected. Proanthocyanidin isolates were characterized by acid catalysis in the presence of excess phloroglucinol followed by reversed-phase HPLC. Proanthocyanidins showed the least significant difference, although (+)-catechin terminal subunits were important predictors in a partial least square model used to summarize the multivariate relationships, predicting Ψstem or the management zone. The results highlight the importance of vineyard water status information for differential harvesting or direction to vineyard operators to modify irrigation management to equilibrate berry composition at harvest.