Correlations among Predawn Leaf, Midday Leaf, and Midday Stem Water Potential and their Correlations with other Measures of Soil and Plant Water Status in Vitis vinifera

A study was conducted to compare three measurements of determining water status of grapevines (Vitis vinifera L.) in the field. Predawn leaf water potential (ΨPD), midday leaf water potential (Ψl), and midday stem water potential (Ψstem) were measured on 'Chardonnay' and 'Cabernet Sauvignon' grapevines grown in Napa Valley, California late in the 1999 growing season. Both cultivars had been irrigated weekly at various fractions (0, 0.5, and 1.0 for 'Chardonnay' and 0, 0.5, 0.75, and 1.5 for 'Cabernet') of estimated vineyard evapotranspiration (ETc) from approximately anthesis up to the dates of measurements. Predawn water potential measurements were taken beginning at 0330 HR and completed before sunrise. Midday Ψl and Ψstem measurements were taken only between 1230 and 1330 HR. In addition, net CO2 assimilation rates (A) and stomatal conductance to water vapor (gs) were also measured at midday. Soil water content (SWC) was measured in the 'Chardonnay' vineyard using a neutron probe. Values obtained for ΨPD, Ψl, and Ψstem in this study ranged from about -0.05 to -0.8, -0.7 to -1.8, and -0.5 to -1.6 MPa, respectively. All three measurements of vine water status were highly correlated with one another. Linear regression analysis of Ψl and Ψstem versus ΨPD resulted in r 2 values of 0.88 and 0.85, respectively. A similar analysis of Ψl as a function of Ψstem resulted in an r 2 of 0.92. In the 'Chardonnay' vineyard, all three methods of estimating vine water status were significantly (P < 0.01) correlated with SWC and applied amounts of water. Lastly, ΨPD, Ψl, and Ψstem were all linearly correlated with measurements of A and gs at midday. Under the conditions of this study, ΨPD, Ψl, and Ψstem represent equally viable methods of assessing the water status of these grapevines. They were all correlated similarly with the amount of water in the soil profile and leaf gas exchange as well as with one another. vine YPD among three watering treatments but no differences in Ψl were found when measured at 1000 and 1600 HR. They concluded that ΨPD better reflected soil water availability than Ψl. van Zyl (1987) concluded that ΨPD detected the onset of water stress in grapevines earlier and more accurately than Ψl.