A Comparison of Osmotic Potential, Elastic Modulus, and Apoplastic Water in Leaves of Dryland Winter Wheat

Previous models which describe the water-content potential relationship for plant tissue do not account for die presence of apoplastic water in the tissue and its variation with water potential. This could cause errors in determination of tissue water parameters which are sensitive to plant water deficits. The theory was modified to account for apoplastic water fraction and used to determine the osmotic potential at full and zero turgor and tissue elastic modulus from moisture release curves for leaves of three winter wheat (Triticum aestivum L.) cultivars grown under moderately high and low stress environments in eastern Washington. Moisture release curves for individual leaves were obtained using a pressure chamber. Apoplastic water content was obtained independently from moisture release curves of frozen leaves using a hydraulic press. Osmotic potential at full and zero turgor showed significant responses to environment with leaves from the drier location having osmotic potentials 5 bars or more lower than leaves from the more moist site. Shifts in osmotic potential from wet to dry portions of the season were also about 5 bars. Apoplastic water fraction and elastic modulus were relatively insensitive to environment, averaging around 0.3 and 200 bars, respectively. Significant cultivar differences were found in tissue osmotic potential, but these did not correlate with performance of the cultivars in droughty environments. The study showed that apoplastic water in tissue, if not corrected for, markedly influences osmotic potential measurements using expressed cell sap since this water, which is essentially solute-free, dilutes the cell sap when the membranes are destroyed. The tissue water parameters studied provide further insight on physiological factors important in plant resistance to moisture stress.