Sources of Variation in Osmotic Potentials with Special Reference to North American Tree Species

Abstract

Abstract The purpose of this review is to discuss sources of variation in osmotic potentials (ψπ), with special reference to North American tree species. Lowering of osmotic potentials by solute accumulation (osmotic adjustment) has been reported in a wide variety of tree species during drought, but not in all species. Osmotic potentials at zero turgor for 37 tree species throughout the United States and Canada averaged (± s.e.) - 2.06 ± 0.13 MPa and - 2.54 ± 0.06 MPa for leaves or shoots under moist and dry conditions, respectively. The low standard error associated with the mean values suggest that, as a group, North American trees develop fairly similar osmotic potentials for a range of moisture conditions. Substantial solute accumulation often routinely occurs in developing leaves and in over-wintering leaves. Drought preconditioning can increase physiological activity in plants during subsequent drought by lowering ψπ. However, if drought is imposed too rapidly osmotic adjustment may not occur. Physiological plasticity in trees may cause variations in ψπ in responses to light regime and leaf canopy position. Light and nutrient regimes that promote high net photosynthesis should also promote the ability to osmotically adjust. However, stomatal responses and growth during water and nutrient stress may be unrelated to osmotic potentials because of species differences in rooting and water transport, guard cell turgor not being closely coupled with that of the bulk leaf, and changes in plant growth regulator concentrations. When examining changes in osmotic potentials using pressure-volume curves, it is important to rehydrate plant material to a water potential characteristic for that species at full hydration in the field. Moreover, as with most physiological measurements, sampling variation in light regime, canopy position, nutrient status, temperature, age, phenology, drought history, and diurnal timing of harvesting of the leaves and shoots should be minimized. For. Sci. 34(4):1030-1046.