SOURCES OF VARIATION IN THE CARBON ISOTOPE SIGNATURE OF WINE GRAPE JUICE

Steven M. Jachym

doi:10.7273/000006943

The stable carbon isotope ratio (δ13C) – termed “delta c thirteen” – found in grape juice (Vitis vinifera L.) has gained attention as a marker of vine water status. Due to a unique ratio dependent on growing conditions, the δ13C can also be thought of as the grape juice’s “carbon signature.” Some proponents of this measure suggest the carbon signature is dependent wholly on the water status of the vine during ripening, while others state the carbon signature represents seasonal water stress; both theorize the signature may be useful in grower contracts – allowing a winemaker to price, accept, or reject loads dependent on perceived irrigation status. The objectives of this study were first, to explore the potential for water stress, nutrient management, and biotic stress to influence the δ13C of grape juice at harvest. And second, to make a recommendation as to the usefulness of juice δ13C at harvest as a representation of irrigation status throughout the season or solely during ripening. Accomplishing these objectives required leveraging existing field experiments in vineyards across eastern Washington. A drought stress experiment on two cultivars, Cabernet Sauvignon and Riesling, showed juice δ13C at harvest can be influenced by seasonal drought initiated at fruit set. A separate experiment on Cabernet Sauvignon revealed pre-ripening drought can influence the δ13C of berry juice collected in the pre-ripening period, but that influence may be overcome by irrigation during ripening. Third, three mineral nutrition experiments utilizing nitrogen and potassium revealed nitrogen treatments can manipulate δ13C in Syrah juice at harvest. Fourth, two observational experiments – one on the effects of powdery mildew foliar disease severity, the other on grapevine leafroll associated virus-3 positive versus negative vines – showed no difference between treatments in juice δ13C at harvest. Considering results, using the carbon isotope signature in grape juice solely as a marker for either seasonal irrigation practices or irrigation status solely during ripening is against industry interests, as juice δ13C is largely influenced by irrigation during the post-version (ripening) period, but may also be skewed by vineyard nitrogen management.

SOURCES OF VARIATION IN THE CARBON ISOTOPE SIGNATURE OF WINE GRAPE JUICE

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