Predawn leaf water potential of grapevines is not necessarily a good proxy for soil moisture

Background: In plant water relations research, predawn leaf water potential (Ψ pd ) is often used as a proxy for soil water potential (Ψ soil ), without testing the underlying assumptions that nighttime transpiration is negligible and that enough time has passed for a hydrostatic equilibrium to be established. The goal of this research was to test the assumption that Ψ pd  = Ψ soil for field-grown grapevines. Results: A field trial was conducted with 30 different varieties of wine grapes grown in a single vineyard in arid southeastern Washington, USA, for two years. The Ψ pd and the volumetric soil water content (θ v ) under each sampled plant were measured multiple times during several dry-down cycles. The results show that in wet soil (θ v  > 0.146 m 3 m − 3 ), Ψ pd was significantly lower than Ψ soil for all 30 varieties. Under drought conditions (θ v  < 0.105 m 3 m − 3 ) Ψ pd lined up better with Ψ soil . There were differences between varieties, but these were not consistent over the years. Conclusion: These results suggest that for wet soils Ψ pd of grapevines cannot be used as a proxy for Ψ soil , while the Ψ pd  = Ψ soil assumption holds for dry soils.