Statewide relationships between water potentials, gas exchange and δ13c of grape musts in California. Implications for use in precision viticulture

Abstract

Context and purpose of the study– The measurement of carbon isotopic discrimination of musts (δ13C) at harvest is an integrated assessment of water status during ripening of grapevine. It is an alternative to traditional measurements of water status in the field, which is crucial for understanding spatial variability of plant physiology at the vineyard scale, proven useful for delineation of management zones in precision viticulture. The aim of this work was to attune the method for the first time to California conditions across a range of areas and cultivars with different hydric behavior, and to evaluate its efficiency in delineating management zones for selective harvest in commercial vineyards.
Material and methods – The experiment was performed in 91 experimental units located at four different locations across the State, planted to three different table and wine grape cultivars (Crimson Seedless, Cabernet Sauvignon, Merlot) whose hydric behavior ranged from isohydric to anisohydric, and in between. Leaf gas-exchanges and stem water potentials (Ψ) were measured routinely in each experimental unit, and the δ13C at harvest. At one of the locations, δ13C and water potentials were measured on an equi-distant grid, spatialized and clustered to compare their efficiency in the differentiated the vineyard block into two distinct zones having grapes with different flavonoid composition.
Results – A significant and direct relationship was evident between δ13C and average stem water potential (R2 = 0.72), stomatal conductance (R2 = 0.66) and net carbon assimilation (R2 = 0.62) measured throughout the season. Differences between the cultivars were small, independently from their reported hydric behavior and it was possible to pool all of them together. This was also true in crossed relationships between stem water potential, stomatal conductance, and net carbon assimilation that were not able to clearly discriminate between the reported hydric behaviors. A unique state-wide calibration was therefore developed between δ13C and plant water status. Simulation exercise demonstrated that variability in slope and R2 of the δ13C ~ Ψ regression can be caused by comparison of discrete measurements (Ψ) of water status to a continuous measurement (δ13C), and that apparent variability decreased with increasing sampling points of the discrete measurement (Ψ).The use of δ13C was then tested in a precision viticulture context. The management zones obtained by δ13C and stem water potentials were similar at 72% and allowed to separate the harvest in two pools, having statistically different grape composition (soluble solids, organic acids and anthocyanin profiles). Our results provided evidence that δ13C discrimination was a reliable and repeatable assessor of plant water status in vineyard ecosystems useful for delineation of management zones in precision viticulture.