Unexpected relationships between δ¹³C, water deficit, and wine grape performance

Water nutrition is crucial for wine grape performance. Thus soil investigation aims at characterizing spatial and temporal variability of available water. A possible strategy is to integrate monitoring and proxies of water availability. The carbon isotope ratio δ13C, measured in the alcohol of wine, is a promising tool to determine water stress during the vine growing season and vine performance. A research study was set up to evaluate the relationships between δ13C, soil water deficit, and wine grape viticultural and oenological performance. The trial was carried out for three years in the Chianti Classico wine production district (Central Italy), on not irrigated vineyards of a premium farm. The reference variety was Sangiovese. Eleven sites were chosen for vine monitoring and grape sampling. The performance parameters were alcohol and sugar content, sugar accumulation rate, mean berry weight, and extractable polyphenols. δ13C, stem water potential, and soil water deficit, as difference between soil water content, monitored during the veraison-harvest, and the standard wilting point, were measured. δ13C resulted directly related to stem water potential and soil water deficit, and showed absence to only moderate water stress. However, the relationship with viticultural and oenological results was contrary to the expectation, that is, the performance increased when the water stress decreased. The explanation was that the viticultural husbandry was so competing for the plants (high plant density, high pruning, weak rootstock, grass cover) that the effects of water stress on grape quality were magnified. In conclusion, δ13C cannot be directly used to estimate vine performance.