Vulnerability to cavitation of leaf minor veins: any impact on leaf gas exchange?

Vulnerability to cavitation of leaf minor veins and stems of Laurus nobilis L. was quantified together with that of leaflets, rachides and stems of Ce ratonia siliqua L. during air-dehydration of 3-year-old branches. Embolism was estimated by counting ultrasound acoustic emissions (UAE) and relating them to leaf water potential (Psi (L)). The threshold Ti, for cavitation wa s less negative in L. nobilis than in C. siliqua according to the known hig her drought resistance of the latter species. Leaf minor vein cavitation wa s also quantified by infiltrating leaves with fluorescein at different dehy dration levels and observing them under microscope. Distinct decreases in t he functional integrity of minor veins were observed during leaf dehydratio n, with high correlation between the two variables. The relationship betwee n leaf conductance to water vapour (g(L)) and Psi (L) showed that stomata o f L. nobilis closed in response to stem and not to leaf cavitation. However , in C siliqua, A decreased in coincidence to the leaf cavitation threshold , which was, nevertheless, very close to that of the stem. The hypothesis t hat stem cavitation acts as a signal for stomatal closure was confirmed, wh ile the same role for leaf cavitation remains an open problem.