Abstract

During low transpiration conditions, xylem water may nevertheless experience positive pressures (i.e. root pressure) and exude from cut surfaces. This is commonly attributed to coupled movement of water and solutes across root cell plasmalemmas, despite a free energy difference between soil and exudate that is often in the wrong direction. This paper presents a simple model, consisting of three homogeneous phases and two heterogeneous interfaces, which suffices to reproduce much of the qualitative behaviour of this phenomenon. The three phases are: (i) a large soil water compartment of constant composition; (ii) a fixed-volume xylem compartment; (iii) and separating them, an elastic symplastic compartment. The two interfaces are: (i) symplast : soil, idealised as plasmalemma exhibiting active uptake of solutes from soil to cell plus a water flux proportional to the free energy gradient; and (ii) symplast : xylem, idealised as plasmalemma exhibiting active uptake of solutes from xylem to symplast plus two parallel water fluxes, one driven by free energy gradient and one by hydrostatic pressure difference. This model behaves appropriately (i) under transpiration-related hydraulic tension in the xylem, (ii) during desiccation stress produced by hyperosmotic soil water, and (iii) in the face of applied pressure, which inhibits exudation from a cut surface.