Four Vitis vinifera L. cultivars grown at five locations throughout California were studied to determine the relationships among temperature and vapor pressure deficit (VPD) and leaf water potential (Ψl) measured under clear skies at midday (solar noon) or in some instances midmorning to midafternoon. Stem water potential (Ψstem) was also measured on several occasions. Vines were irrigated at 100% or greater of measured or estimated vineyard evapotranspiration, and deficit or nonirrigated vines were included for comparison. Temperature and VPD were determined at the time of measurement. The highest and lowest values of Ψl measured on well-watered grapevines were −0.51 and −1.15 MPa, respectively. Leaf and stem water potentials were linearly related to VPD and ambient temperature. The coefficient of determination was greater for the relationship between Ψl and VPD (R2 = 0.74) than ambient temperature (R2 = 0.58). Based on the regressions, estimates of Ψl at a VPD of 2 and 5 kPa for fully irrigated grapevines would be −0.65 and −0.89, respectively, while those of Ψstem at the same VPDs would be −0.37 and −0.57 MPa, respectively. Leaf water potential of water-stressed vines was less responsive to VPD or temperature when Ψl values ranged from −1.2 to −1.45 MPa. The values of Ψl and Ψstem as a function of VPD or temperature could serve as baselines indicating whether grapevines are fully irrigated or not water stressed under the environmental conditions found in semiarid grapegrowing regions.
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