Aspects of tissue water relations and seasonal changes of leaf water potential components of evergreen and deciduous species coexisting in tropical dry forests.

This study compared the tissue water relations and seasonal changes in leaf water potential components of an evergreen tree,Morisonia americana, and two evergreen shrubs,Capparis verrucosa andC. aristiquetae, with two deciduous trees,Humboltiella arborea andLonchocarpus dipteroneurus, and the deciduous vineMansoa verrucifera. All these species coexist in a tropical dry forest in Venezuela. Leaves of the evergreen species are sclerophyllous, while those of the deciduous species are mesophytic. Leaf area to leaf weight ratios of fully mature leaves were about 75 and 170 cm2 g-1 in evergreen and deciduous species, respectively. Seasonal fluctuations of leaf water content per unit of dry weight, water potential, and turgor pressure were smaller in evergreen than in deciduous species. The analysis of tissue water relations using pressurevolume curves showed that evergreen species could develop a higher leaf turgor and lose turgor at lower leaf water potentials than deciduous species. This was related to a lower osmotic potential at full turgor in evergreen (≃-3.0 MPa)_than in deciduous (≃-2.0 MPa) species, rather than to the elastic properties of leaf tissue. The volumetric modulus of elasticity was 14 MPa in evergreen compared with 7-10 MPa in deciduous species. Thus, leaf characteristics are important in determining the drought resistance of evergreen species of this tropical dry forest.