Contrasting patterns of leaf water potential and gas exchange responses todrought in seedlings of tropical rainforest species

1. The leaf gas exchange and water potential (Psi (w)) responses to a soil drying cycle and to increasing atmospheric water vapour pressure deficit we re compared in seedlings of three tropical rainforest canopy species in a g rowth chamber. 2. Eperua falcata Aub. presented an anisohydric behaviour in relation to so il drought. The decrease in predawn Psi (w) at the beginning of the drying cycle was accompanied by a more pronounced decrease in midday Psi (w) Stoma tal closure occurred from a volumetric soil water content (Theta) of 0.1 m( 3) m(-3), which corresponded to approximately one third of the available so il water. 3. Diplotropis purpurea (Rich.) Amsh. displayed an isohydric behaviour. Mid day Psi (w) did not decrease - and the difference between predawn and midda y Psi (w) did not increase - over a large range of Theta (to less than 0.1 m(3) m(-3)). Stomatal conductance (g(s)) was more sensitive to soil drought than in Eperua. 4. A unique behaviour was found in Virola michelii: Heckel. Midday Psi (w) remained remarkably stable, and close to predawn Psi (w), over the drying c ycle as long as leaves did not dry, and g(s) was extremely sensitive to soi l drought. These results led us to postulate the existence of a homeostatic regulation of Psi (w) in Virola. 5. The three species differed in their stomatal sensitivity to atmospheric drought. Eperua was the least and Diplotropis the most sensitive. Virola, e ven though extremely sensitive to soil drought, displayed an intermediate s ensitivity of g(s) to atmospheric drought. 6. These findings suggest that the survival of Eperua, Diplotropis and Viro la in the tropical rainforest of French Guiana may depend partly on local h ydrological conditions.