The Role of Water and Atmospheric CO2 on the δ13C Value of Sugars of Grape Must

Climatic parameters influence the δ13C value of sugar in grape must. With the aim of investigating this dependence, grape must samples were collected from two viticultural Italian areas (Oltrepò Pavese, Lombardia region and Illasi–Mezzane, Veneto region), which share similar soil mineralogical compositions. Water uptake by the plant is the primary factor affecting the δ13C values of sugar: the greater the water availability, the lower the δ13C value. This is supported by a correlation between the δ13C values and the climatic water balance (BICc), which is defined as the difference between daily rainfall and crop evapotranspiration. Pre-harvest atmosphere was also sampled at both sites to determine its concentration and δ13C value. Using the Farquhar model, enrichment factors and εCO2-sugar were calculated. A moderate correlation was found between cumulative rainfall and the associated values of the enrichment factor: approximately 60% of the variation in sugar δ13C can be attributed to water uptake and to the δ13C values of atmospheric CO2. Rainfall alone showed an even stronger correlation with εCO2-sugar, suggesting that water availability is the dominant factor influencing the sugar δ13C.