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