Linear relation between leaf xylem water potential and transpiration in pearl millet during soil drying

Aims As soil dries, the loss of soil hydraulic conductivity limits water supply to the leaves, which is expected to generate a nonlinear relationship between leaf water potential (psi(leaf)) and transpiration (E). The effect of soil drying and root properties on psi(leaf) and E remains elusive. Methods We measured E and psi(leaf) of pearl millet for varying E and soil moisture using a root pressure chamber. A model of water flow in soil and plant was used to fit the psi(leaf) (E) relationship. Results The relation between psi(leaf) and E was linear at all soil moistures. The slope of psi(leaf) (E) increased with decreasing soil moisture due to the decreasing soil-root conductance. The fact that the relation remained linear also in dry soils and high E is surprising. Indeed, it indicates that the gradients in soil water potential (psi(soil)) were small, probably because of the large root surface (13.5 cm cm(-3)) active in water uptake. psi(leaf) at E = 0 was less negative than psi(soil), indicating a more negative osmotic potential in the xylem than in the soil. Conclusions We propose that the linearity between psi(leaf) and E and the high psi(leaf) (E = 0) compared to psi(soil) support transpiration in drying soils.