Limitation of transpiration by hydraulic conductance and xylem cavitation in Betula occidentalis
The extent to which stomatal conductance (gs) was capable of responding to reduced hydraulic conductance (k)and preventing cavitation-inducing xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air-injection technique. From 1 to 18 d after shoot injection we measured midday transpiration rate (E), gs, and xylem pressure (Ψp-xylem) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2–3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and gs. The k measurement was expressed per unit leaf area (kl, leaf-specific conductance). Leaves measured within 2 d of shoot injection showed reduced gs and E relative to non-injected controls, and both parameters were strongly correlated with kl At this time, there was no difference in leaf Ψp-xylem between injected shoots and controls, and leaf Ψp-xylem was not significantly different from the highest cavitation-inducing pressure (Ψp-cav) in the branch xylem (-1.43 ± 0.029 MPa, n=8). Leaves measured 7–18 d after shoots were injected exhibited a partial return of gs and E values to the control range. This was associated with a decrease in leaf Ψp-xylem below Ψp-cav and loss of foliage. The results suggest the stomata were incapable of long-term regulation of E below control values and that reversion to higher E caused dieback via cavitation.