Combination of plant and soil water potential monitoring and modelling demonstrates soil-root hydraulic disconnection during drought

In grapevines and other deep-rooting plants, heterogeneous drying from the surface to deeper soil layers drives water extraction by roots. Modelling and measurements have shown that dry soils, especially with sandy texture, create abrupt water potential gradient in the rhizosphere. At the scale of the thin contact between the soil and roots, the hydraulic continuity could be lost at the soil-root interface in dry soil conditions. This study aimed to understand how the multiscale interactions between soil and roots affect grapevine root water uptake and water potential. Using a physically-based model, implementing rhizosphere and root system hydraulic properties, and loss of soil-root hydraulic continuity in dry soil conditions, we quantified belowground hydraulic conductances and their impact on grapevine root water uptake and water potential in different soil types with vertical hydraulic properties heterogeneity. Soil-root hydraulic disconnection prevented the plant from feeling the dry shallowest soil horizons avoiding very negative trunk water potentials, and moved water uptake towards deeper wet soil horizons. The main belowground hydraulic bottleneck of soil-plant system during drought is soil-texture dependent, with the rhizosphere limiting root water uptake in the sandy subplot, and the root system in the loamy subplot. By highlighting the key roles of rhizosphere hydraulics, root hydraulics and hydraulic disconnection on root water uptake and plant water status, in different edaphic conditions, this study enhanced our mechanistic understanding on soil-root water relations in soil water limited conditions.