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

Abstract

We tested the hypothesis that hydraulic conductance per unit leaf surface area of plant shoots (KSL) determines the maximum diurnal stomatal conductance (gL) that can be reached by plants growing in the field. A second hypothesis was tested that some xylem cavitation cannot be avoided by transpiring plants and might act as a signal for regulating gL. Eleven woody species were studied, differing from each other with respect to taxonomy, wood anatomy and leaf habit. Maximum diurnal gL, transpiration rate (EL), pre-dawn and minimum diurnal leaf water potential (Ψpd and Ψmin, respectively) were measured in the field. The critical Ψ level at which stem cavitation was triggered (Ψcav) was measured on detached branches, using the acoustic method. A high-pressure flow meter was used to measure maximum KSL of 1-year-old shoots. Both gL and EL were positively related to KSL. The whole-plant hydraulic conductance per unit leaf area (KWL) of all the species studied, calculated as the ratio of EL to ΔΨ (=Ψpd-Ψmin) was closely related to KSL. In every case, Ψmin (ranging between –0.85 and –1.35 MPa in the different species) dropped to the Ψcav range or was <Ψcav (ranging between –0.71 and –1.23 MPa), thus suggesting that some cavitation-induced embolism could not be avoided. The possibility is discussed that some cavitation-induced reduction in KSL is the signal for stomatal closure preventing runaway embolism. The lack of correlation of gL to Ψcav is discussed in terms of the inconsistency of Ψcav as an indicator of the vulnerability of plants to cavitation. No differences in hydraulic traits were observed between evergreen and deciduous species.