Water balance in developing leaves of four tropical savanna woody species.

Water balance was studied in developing leaves of Curatella americana, Byrsonima crassifolia, Bowdichia virgilioides and Casearia sylvestris, four widespread tropical savanna evergreen woody species that renew their leaves during the dry season. Water potential components of leaves of different ages were estimated in the laboratory by determination of pressure-volume (PV) curves. Data from PV curves were used to help interpret the significance of variations in leaf water potential (ΨL) and stomatal conductance observed in leaves of different ages in the field.Initial osmotic potential at full turgidity as well as osmotic potential at the turgor loss point changed considerably during leaf development. Values of these water potential components for mature leaves were similar to those reported for temperate mesophytic woody species. Passive osmotic adjustment defined as the change in osmotic potential between full turgidity and turgor loss averaged only 0.3 MPa, much smaller than values for temperate mesophytic and drought hardy woody species. Since environmental conditions, especially soil moisture in the rooting zone, were essentially constant during leaf development, changes in leaf water potential components were not seasonal, but rather ontogenetic responses to diurnal water stress.In all species except C. sylvestris there were differences in ΨL between adjacent leaves exposed to the same environmental conditions, with immature leaves generally maintaining higher ΨL than mature leaves. The probable causes for this were 1) lower liquid flow resistance and 2) lower transpiration rates in younger leaves. At low transpiration rates liquid flow resistance was flux-dependent in all species, but became nearly independent of transpiration flux density (E) as E increased. This and their lower flow resistance permit high E to be sustained in developing leaves without excessive ψ drop across the leaf.During two typical dry season days E was high and even though complete or nearly complete turgor loss was sustained, these water deficits were not severe enough to cause complete stomatal closure. The possible relationship between a high stomatal conductance and the maintenance of a favorable carbon balance in these species is discussed.All of the water relations features observed suggested that in spite of their scleromorphic appearance, leaves of the four species under the conditions studied were adapted only to short term diurnal water deficits.