Osmotic Water Stress: Mesophyll Saturation Deficit and Transpiration Rates of Tamarisk

Abstract

Estimates of the saturation deficit of the mesophyll cells of tamarisk plants indicate that salt accumulation at the evaporation surface of the stomatal cavity is small with increased salinity in the root substrate. Salt glands which pro- vide a mechanism for salt excretion prevent solute concentration at the evaporating surface which would reduce transpiration rates of this species. Transpiration rates were reduced by high saline content only at high vapor pressure deficits where other resistances within the plant may be affected by increasing transpiration rates beyond the transport capacity of the plant. Tarnarix pentandra Pall. exists and thrives under a wide range of saline conditions that prevent establishment and growth of other spe- cies. The ability of this plant to prosper under these conditions may be due to an adaptation for exuding salt through salt glands, thereby reducing solute concentration at the evaporating surfaces (Campbell and Strong, 1964 and Decker, 1961). Apparently, active salt glands are not associated with vascular bundles but are primarily desalting organs reducing salt concentrations at the surface of the leaf meso- phyll cells. However, this mechanism has not been isolated and its effect on transpiration rates under high saline conditions is unknown and may be a significant factor in regulating transpiration rates. As part of the effects of osmotic water stress on transpiration rates of tamarisk estimates of the mesophyll saturation deficit were deter- mined. The principle of the method was to estimate the equilibrium vapor pressure at which the net flux of water vapor between the plant and the surrounding air was zero (Whiteman and Koller, 1964). Ex- trapolation of the linear and curvilinear relationships between trans- piration and water potential to the abscissa (transpiration equals zero) 1 This work was performed in partial fulfillment of the requirement for a degree of Doctor of Philosophy at the University of Arizona and was supported by a con- tract with the U. S. Bureau of Reclamation.