The H2(18O) enrichment of the leaf water of tropic trees: comparison of species from the tropical rain forest and the semi-arid region in Brazil.

The H2(18O) enrichment, delta, in the water of leaves from four Brazilian trees, was studied. In all trees the leaf water showed a periodic variation in delta, with a maximum in the early afternoon and a minimum around 6 a.m. In general delta was found to be either higher or lower than the stationary enrichment which is supposed to depend only on the relative atmospheric humidity. This effect is due to the slow response of the system to variations of the humidity. For a special case, where steady-state conditions could be anticipated, the kinetic enrichment was obtained to 20 +/- 3%, which agrees with theoretical predictions.