Coupled effects of soil drying and salinity on soil-plant hydraulics

A physical model interprets transpiration reduction under drought and salinity as a consequence of osmotic gradients at the root surface that cause a severe drop in leaf water potential. Salinity and soil drying are expected to induce salt accumulation at the root-soil interface of transpiring plants. However, the consequences of this on the relationship between transpiration rate (E) and leaf xylem water potential (psi(leaf-x)) are yet to be quantified. Here, we used a noninvasive root pressure chamber to measure the E(psi(leaf-x)) relationship of tomato (Solanum lycopersicum L.) treated with (saline) or without 100-mM NaCl (nonsaline conditions). The results were reproduced and interpreted with a soil-plant hydraulic model. Under nonsaline conditions, the E(psi(leaf-x)) relationship became progressively more nonlinear as the soil dried (theta <= 0.13 cm(3) cm(-3), psi(soil) = -0.08 MPa or less). Under saline conditions, plants exhibited an earlier nonlinearity in the E(psi(leaf-x)) relationship (theta <= 0.15 cm(3) cm(-3), psi(s)(oil) = -0.05 MPa or less). During soil drying, salinity induced a more negative psi(leaf-x) at predawn, reduced transpiration rate, and caused a reduction in root hydraulic conductance (from 1.48 x 10(-6) to 1.30 x 10(-6) cm(3) s(-1) hPa(-1)). The model suggested that the marked nonlinearity was caused by salt accumulation at the root surface and the consequential osmotic gradients. In dry soil, most water potential dissipation occurred in the bulk soil and rhizosphere rather than inside the plant. Under saline-dry conditions, the loss in osmotic potential at the root surface was the preeminent component of the total dissipation. The physical model of water flow and solute transport supports the hypothesis that a buildup of osmotic potential at the root-soil interface causes a large drop in psi(leaf-x) and limits transpiration rate under drought and salinity.