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

BackgroundIn plant water relations research, predawn leaf water potential (& psi;(pd)) is often used as a proxy for soil water potential (& psi;(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 & psi;(pd) = & psi;(soil) for field-grown grapevines.ResultsA 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 & psi;(pd) and the volumetric soil water content (& theta;(v)) under each sampled plant were measured multiple times during several dry-down cycles. The results show that in wet soil (& psi;(soil) > - 0.14 MPa or relative extractable water content, & theta;(e) > 0.36), & psi;(pd) was significantly lower than & psi;(soil) for all 30 cultivars. Under dry soil conditions (& psi;(soil) < - 0.14 MPa or & theta;(e) < 0.36) & psi;(pd) lined up better with & psi;(soil). There were differences between cultivars, but these were not consistent over the years.ConclusionThese results suggest that for wet soils & psi;(pd) of grapevines cannot be used as a proxy for & psi;(soil), while the & psi;(pd) = & psi;(soil) assumption may hold for dry soils.