HYDRAULIC ARCHITECTURE, WATER RELATIONS AND VULNERABILITY TO CAVITATION OF CLUSIA-UVITANA PITTIER - A C-3-CAM TROPICAL HEMIEPIPHYTE

Clusia uvitana Pittier (Clusiaceae) is a tropical hemiepiphyte that ha s been shown to display a high plasticity in the expression of CAM in response to the environment. When water is available CO2 is taken up m ostly during the day. This study of the water relations and hydraulic architecture has revealed that leaf water potentials, Psi, ranged from -0.7 to -0.9 MPa and changed very little with time or water availabil ity. The absolute hydraulic conductivity of stem segments (K-h) and th e specific conductivity (K-s) were comparable to many other temperate and tropical species, but the leaf specificity conductivity (K-L) was 1/3 to 1/30 that of many other species. So stems supported high leaf a reas per unit of hydraulic conductivity. C. uvitana was very vulnerabl e to cavitation, reaching 50% loss of hydraulic conductivity at stem P si = -1.3 MPa. The species survives in spite of low K-L and high xylem vulnerability, because the CAM physiology insures low transpiration r ates and high ability to evade dehydration.