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Summary The leaf gas exchange and water potential (Ψ w ) responses to a soil drying cycle and to increasing atmospheric water vapour pressure deficit were compared in seedlings of three tropical rainforest canopy species in a growth chamber. Eperua falcata Aub. presented an anisohydric behaviour in relation to soil drought. The decrease in predawn Ψ w at the beginning of the drying cycle was accompanied by a more pronounced decrease in midday Ψ w . Stomatal closure occurred from a volumetric soil water content (Θ) of 0·1 m 3 m −3 , which corresponded to approximately one third of the available soil water. Diplotropis purpurea (Rich.) Amsh. displayed an isohydric behaviour. Midday Ψ w did not decrease – and the difference between predawn and midday Ψ w did not increase – over a large range of Θ (to less than 0·1 m 3 m −3 ). Stomatal conductance ( g s ) was more sensitive to soil drought than in Eperua . A unique behaviour was found in Virola michelii : Heckel. Midday Ψ w remained remarkably stable, and close to predawn Ψ w , over the drying cycle as long as leaves did not dry, and g s was extremely sensitive to soil drought. These results led us to postulate the existence of a homeostatic regulation of Ψ w in Virola . The three species differed in their stomatal sensitivity to atmospheric drought. Eperua was the least and Diplotropis the most sensitive. Virola , even though extremely sensitive to soil drought, displayed an intermediate sensitivity of g s to atmospheric drought. These findings suggest that the survival of Eperua , Diplotropis and Virola in the tropical rainforest of French Guiana may depend partly on local hydrological conditions.
Bonal et al. (Wed,) studied this question.