Limitation of stomatal conductance by hydraulic traits: sensing or preventing xylem cavitation?

We tested the hypothesis that hydraulic conductance per unit leaf surface a rea of plant shoots (K-SL) determines the maximum diurnal stomatal conducta nce (g(L)) that can be reached by plants growing in the field. A second hyp othesis was tested that some xylem cavitation cannot be avoided by transpir ing plants and might act as a signal for regulating g(L) Eleven woody speci es were studied, differing from each other with respect to taxonomy, wood a natomy and leaf habit. Maximum diurnal g(L), transpiration rate (E-L) pre-d awn and minimum diurnal leaf water potential (Psi (pd) and Psi (min), respe ctively) were measured in the field. The critical Psi level at which stem c avitation was triggered (Psi (cav)) was measured on detached branches, usin g the acoustic method. A high-pressure flow meter was used to measure maxim um K-SL of 1-year-old shoots. Both g(L) and E-L were positively related to K-SL. The whole-plant hydraulic conductance per unit leaf area (K-WL) Of al l the species studied, calculated as the ratio of E-L to DeltaY(=Psi (pd)-P si (min)) was closely related to K-SL. In every case, Psi (min) (ranging be tween -0.85 and -1.35 MPa in the different species) dropped to the Psi (cav ) range or was <<Psi>(cav) (ranging between -0.71 and -1.23 MPa), thus sugg esting that some cavitation induced embolism could not be avoided. The poss ibility is discussed that some cavitation-induced reduction in K-SL is the signal far stomatal closure preventing runaway embolism. The lack of correl ation of g(L) to Psi (cav) is discussed in terms of the inconsistency of Ps i (cav) as an indicator of the vulnerability of plants to cavitation. Na di fferences in hydraulic traits were observed between evergreen and deciduous species.