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 (psi(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 (k1, 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 k1. At this time, there was no difference in leaf psi(p-xylem) between injected shoots and controls, and leaf psi(p-xylem) was not significantly different from the highest cavitation-inducing pressure (psi(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 psi(p-xylem) below psi(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.