Thomas Groenveld, Charles Obiero, Yingxue Yu, Markus Flury, and Markus Keller*
*Washington State University, 24106 N. Bunn Road, Prosser, WA, 99350 (mkeller@wsu.edu)

Accurate and representative measurements of soil moisture are challenging because of the high spatial variability in soil under field conditions. Leaf water potential (Ψ_leaf) is a function of the soil water potential (Ψ_soil) and the vapor pressure deficit (VPD) and is controlled by the stomata. Predawn leaf water potential (Ψ_pd) is often used as a proxy for Ψ_soil, assuming that there is no transpiration at night and that enough time has passed for a hydrostatic equilibrium to be established. Despite numerous publications that indicate there is a disequilibrium between Ψ_pd and Ψ_soil and potential causes for this, models that are based on the equilibrium assumption continue to be published, including models for irrigation scheduling and several functions that divide plants, including grape varieties, into different levels of isohydricity. A field trial was conducted to test the assumption that Ψ_pd = Ψ_soil. Thirty different winegrape varieties were grown in a single experimental vineyard in southeastern Washington, where the Ψ_pd and the soil water content under each sampled plant were measured multiple times during two dry-down cycles. The Ψ_soil was calculated from the soil water content by means of retention curve data. At high soil water content, Ψ_soil was consistently greater than Ψ_pd for 24 out of 30 varieties, but the difference decreased as the soil dried. These results suggest that the Ψ_pd = Ψ_soil assumption is only met for dry soils, while correction factors are required for wet soils. The difference between Ψ_soil and Ψ_pd may reflect the water potential gradient from the bulk soil to the leaves due to nighttime transpiration.

Funding Support: Washington State Department of Agriculture Specialty Crop Block Grant Program and Washington State Grape and Wine Research Program