Relations between leaf and stem water potentials and stomatal conductance in three field-grown woody species

Simultaneous measurements of stomatal conductance (g(s)), and leaf (psi(leaf)) and stem (psi(stem)) water potentials were conducted from 0900 hours to early afternoon on field-grown trees of apple, grapevine and nectarine which received different irrigation rates. The correlations of g(s) with psi(stem) (r(2) = 0.85, 0.85, 0.79) was significantly higher than those with psi(leaf) (r(2) = 0.51, 0.61, 0.37) for apple, grapevine and nectarine, respectively. The high correlation between g(s) and psi(stem) has been explained through a qualitative analysis of a water transport model which takes into account the control of g(s) by root signals (Tardieu and Davies, 1993). The model suggests that g(s) psi(leaf), root water potential, root-signal intensity, and transpiration rate are linked in a feedback mechanism which leads to the high correlation of g(s) and psi(stem) and causes it to be better than that with psi(leaf).