The possible link between stomatal conductance (g(L)), leaf water potential
(Psi(L)) and xylem cavitation was studied in leaves and shoots of detached
branches as well as of whole plants of Laurus nobilis L. (Laurel). Shoot c
avitation induced complete stomatal closure in air-dehydrated detached bran
ches in less than 10 min. By contrast, a fine regulation of g(L) in whole p
lants was the consequence of Psi(L) reaching the cavitation threshold (Psi(
CAV)) for shoots. A pulse of xylem cavitation in the shoots was paralleled
by a decrease in g(L) of about 50%, while Psi(L) stabilized at values preve
nting further xylem cavitation. In these experiments, no root signals were
likely to be sent to the leaves from the roots in response to soil dryness
because branches were either detached or whole plants were growing in const
antly wet soil. The stomatal response to increasing evaporative demand appe
ared therefore to be the result of hydraulic signals generated during shoot
cavitation. A negative feedback link is proposed between g(L) and Psi(CAV)
rather than with Psi(L) itself.