OSMOTIC PRESSURE OF VACUOLAR SAP BY THE PLASMOMETRIC METHOD

Abstract

In the determination of the osmotic pressure of the vacuolar sap of a plant cell from the osmotic pressure of the solution which is just strong enough to cause plasmolysis or, if a number of cells is observed, is strong enough to cause plasmolysis of 50 per cent it is customary to multiply this osmotic pressure by the ratio of the volume of the cell at plasmolysis to the original volume in order to get the O.P. of the sap in the original state. Sometimes the change in the volume of a piece of tissue is taken as a measure of the change in volume of the cells. This procedure implies that the fractional decrease in the vacuole is the same as that of the cell and hence of the cytoplasm the volume of the wall will be neglected. This assumption is manifestly wrong when so-called Kappen plasmolysis occurs and the cytoplasm expands to fill the space between vacuole and wall. Clearly there would be no need to trouble about the possible falsity of this assumption in ordinary plasmolysis if the volume of the cytoplasm were negligible. In a spherical cell with a diameter of 40 , and a shell of cytoplasm 2 p thick on the average the volume of the cytoplasm is a third of that of the vacuole, and if only i p thick the fraction is onesixth fractions which may not be negligible. We have argued elsewhere (Briggs, I957) that many solutes which do not penetrate into the vacuole can enter readily into the cytoplasm. If the cytoplasm behaves as a fluid and its contents are subject to the same hydrostatic pressure as those of the vacuole then when the cell is in water the osmotic pressures of the cytoplasm and vacuole are equal. When the cell is placed in a solution such as the above the osmotic pressure of the cytoplasm will rise and its volume will increase at the expense of that of the vacuole and this will continue as the concentration of the external solution is increased and no plasmolysis will occur. For plasmolysis to occur the expansion of the cytoplasm must be limited since its osmotic pressure, finite when there is water outside, will continue to exceed the external osmotic pressure as the solute penetrates the cytoplasm. In other words the coefficient of elasticity of the cytoplasm must exceed zero so that when the cytoplasm expands an extra hydrostatic pressure will be exerted on the solution in the cytoplasm. To illustrate the consequences let us assume that the hydrostatic pressure on the cytoplasm extra to that on the vacuole is negligible when the cell is in water, that the volume of the vacuole is n times that of the cytoplasm when the cell is in water, and that when the cell is placed in a solution of a non-electrolyte the expansion of the cytoplasm results in an extra hydrostatic pressure on its contents equal to P.x/m, where P is the original osmotic pressure of the vacuole and of the cytoplasm and x is the ratio of the