Daily and Seasonal Osmotic Changes in a Tropical Treeline Species

The influence of environmental factors on osmotic propertiesin leaf tissue of Polylepis sericea, a widespread tropical treelinespecies in the Venezuelan Andes was studied under field andlaboratory conditions. Daily osmotic changes were determined with thermocouple psychrometers,and the degree of seasonal adjustments was estimated by analysisof pressure-volume (P-V) curves. Measurement of diurnal variationsin water potential components and soluble carbohydrates revealedosmotic adjustment between 0.6 and 1.0 MPa during the predawnhours. A linear relationship was found between minimum osmoticpotential and leaf temperature indicating that temperature maysignificantly affect the magnitude of the predawn osmotic potentialdrop. Analysis of P-V relationships indicated that there wasa seasonal adjustment, from the end of the wet season (November)to the middle of the dry season (February), of approximately0.5 MPa due probably to a net increase in osmotically activesolutes. Laboratory experiments revealed that lowering the preconditioningplant temperature decreased both the leaf osmotic potentialand the supercooling point and increased the leaf soluble carbohydratecontent. The adaptive significance of these mechanisms for trees growingat high elevations in tropical regions where freezing temperaturesmay occur any night of the year is discussed. Key words: Polylepis sericea, Osmotic adjustment, Cold resistance mechanisms.