Leaf tissue water relations and hydraulic properties of sclerophyllous vegetation on white sands of the upper Rio Negro in the Amazon region

The objective of this Study was to explore the leaf tissue water relations in terminal branches. as well as the relations between xylem structure and function of live selerophyllus species coexisting on white sands within the Amazon region. In these species, which possess costly leaves and thrive in art extremely nutrient-poor habitat, the preservation of leaf survival Would be of comparable importance to the preservation of xylem vessels. Three trees per species were tagged in the field for all measurements. Minimum leaf water potential (Psi) was -1.53 +/- 0.61 mid -0.94 +/- 0.10 MPa during rainless and rainy days, respectively. The Psi for turgor loss averaged -1.92 +/- 0.05 MPa. Therefore, minimum Psi was maintained within a safety range above the critical value for turgor loss. Xylem (K-x) and leaf (K-I) specific conductivity averaged 1.4 +/- 0.22 and 0.00033 +/- 0.000045 kg m(-1) s(-1) MPa-1, respectively. Water supply was favoured in species with higher vessel density, and all species depended on relatively less abundant larger vessels for water transport. This would be advantageous because leaves were unable to develop very negative water potentials in order to maintain transpiration. High transpiration rates may be restricted to a few hours daily so as to prevent cavitation of widest vessels.