Limitation of transpiration by hydraulic conductance and xylem cavitation in Betula occidentalis

The extent to which stomatal conductance (g_s) 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), g_s, and xylem pressure (Ψ_p-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 g_s. The k measurement was expressed per unit leaf area (k_l, leaf-specific conductance). Leaves measured within 2 d of shoot injection showed reduced g_s and E relative to non-injected controls, and both parameters were strongly correlated with k_l At this time, there was no difference in leaf Ψ_p-xylem between injected shoots and controls, and leaf Ψ_p-xylem was not significantly different from the highest cavitation-inducing pressure (Ψ_p-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 g_s and E values to the control range. This was associated with a decrease in leaf Ψ_p-xylem below Ψ_p-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.