Xylem cavitation and hydraulic control of stomatal conductance in Laurel (Laurus nobilis L.)

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 cavitation induced complete stomatal closure in air-dehydrated detached branches in less than 10 min. By contrast, a fine regulation of g(L) in whole plants 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 preventing 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 constantly wet soil. The stomatal response to increasing evaporative demand appeared 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.