The vulnerability of xylem conduits to cavitation theoretically determines the maximum flow rate of water through plants, and hence maximum transpiration (E), stomatal conductance (gs), and leaf area (A1. Field-grown Betula occidentalis with a favourable water supply exhibit midday xylem pressures (ψpx) approaching the cavitation-inducing range (–1·42 to – 2·2 MPa). We studied the ability of the stomata to prevent cavitation-inducing pressures when whole-plant hydraulic conductance per leaf area (k1 was reduced by making overlapping transverse cuts in the main stem. Controls were intact, or had only the phloem cut in the same pattern. Reducing k1 caused two responses: (1) variable gs or E with ψpx falling into the cavitation range causing up to 98% embolism and 100% leaf death, (2) decreased gs and E with ψpx remaining above the cavitation point and no leaf death or induction of cavitation. Shoots avoiding cavitation either produced new xylem and returned to control values of k1gs and E (experiments in June and July), or they showed continued decline in g, and E associated with increasing ψpx and eventual premature senescence of leaves (experiments in August). Whether embolism occurred after reducing k1 probably depended on the response time of stomata, and the proximity of ψpx to the cavitation range when the xylem was cut. Stomata probably responded indirectly to reduced k via small changes in leaf ψ root signalling was unlikely because of the constant rooting environment.

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