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 (K _SL) determines the maximum diurnal stomatal conductance (g _L) 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 g _L. Eleven woody species were studied, differing from each other with respect to taxonomy, wood anatomy and leaf habit. Maximum diurnal g _L, transpiration rate (E _L), pre-dawn and minimum diurnal leaf water potential (Ψ _pd and Ψ _min, respectively) were measured in the field. The critical Ψ level at which stem cavitation was triggered (Ψ _cav) was measured on detached branches, using the acoustic method. A high-pressure flow meter was used to measure maximum 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 all the species studied, calculated as the ratio of E _L to ΔΨ (=<I>Ψ</I><SUB>pd</SUB>-<I>Ψ</I><SUB>min</SUB>) was closely related to K _SL. In every case, Ψ _min (ranging between –0.85 and –1.35 MPa in the different species) dropped to the Ψ _cav range or was <Ψ _cav (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 K _SL is the signal for stomatal closure preventing runaway embolism. The lack of correlation of g _L to Ψ _cav is discussed in terms of the inconsistency of Ψ _cav as an indicator of the vulnerability of plants to cavitation. No differences in hydraulic traits were observed between evergreen and deciduous species.