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 Ψ(pd) = Ψ(soil) for field-grown grapevines. RESULTS: A field trial was conducted with 30 different cultivars 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 (Ψ(soil) > − 0.14 MPa or relative extractable water content, θ(e) > 0.36), Ψ(pd) was significantly lower than Ψ(soil) for all 30 cultivars. Under dry soil conditions (Ψ(soil) < − 0.14 MPa or θ(e) < 0.36) Ψ(pd) lined up better with Ψ(soil). There were differences between cultivars, 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 may hold for dry soils.