Adapting the Priestly-Taylor Index as a Physiological Stress Indicator in Vineyard Agrosystems

Abstract Seasonal management of plant water status and the accompanying physiological responses are critical aspects of viticultural production. Presently, grapevine (Vitis vinifera, L.) water status is measured via in-season measurements of stem water potential or post-season analysis of must carbon isotope ratios, with the former limited by reliance on laborious measurements and the latter providing information post-season. Therefore, there is a gap in reliable, real-time measurements of plant water status. Technological advances in surface renewal measurement in vineyards have provided an economical and reliable method for measuring actual evapotranspiration of a vineyard. This experiment utilized surface renewal calculations to derive a novel index of grapevine water stress, the Priestly-Taylor index (β-index), and related it to measurements of stem water potential, leaf-gas exchange, and must carbon isotopes from three vineyards with differing irrigation strategies over two growing seasons. The sensible heat flux, latent heat flux and net radiation varied across these vineyards and affected the actual vineyard evapotranspiration measured. Likewise, the β-index was different across these vineyards and ranged from 1.7 to 2.1 in the Sacramento Valley of California to 0.5 to 1.2 in the Napa Valley of California. The β-index was related to stem water potential, net carbon assimilation and stomatal conductance (r2 = 0.42, r2 = 0.45, r2 = 0.33, respectively). Results indicated that the β-index was an indicator of real-time vineyard water status and a proxy for physiological responses in vineyards. The coupling of atmospheric controls on evapotranspiration with plant physiological responses makes β a powerful tool for irrigation management in large scale agrosytems.