UC Davis

The pressure chamber method has been widely used for decades to measure water potential in plant tissues. In this method, air pressure around the tissue being tested is increased at a constant rate, and the pressure required to visually observe the initiation of sap efflux from the tissue (the endpoint pressure) is used as the measure of the tissue’s water potential. We developed a novel system for quantitatively measuring the volume of sap efflux over time, and hence for more objectively determining the endpoint pressure. In this system, sap is absorbed through a membrane under hydrostatic tension and the sap volume is measured directly. We used the system to test the influence of different pressurization rates and regimes on pressure chamber measurements of water potential in greenhouse-grown grape leaves. We found that increasing pressurization rates resulted in progressively lower apparent water potential compared to a control (slow) pressurization rate. While the underlying causes are not entirely clear, a time lag for sap flow to be detected appears to be the likely cause. Increases in temperature were also associated with increases in pressurization rates, and such temperature effects have been suggested in the literature as causing real (not apparent) changes in water potential. However, direct measurement of leaf temperatures inside the pressure chamber did not support this hypothesis. The application of split pressurization regimes however, where a high initial rate is followed by a slow rate for endpoint determination, showed consistent endpoints, essentially the same as the control method, regardless of the initial rates used, and hence these can be used to accomplish both accuracy as well as time efficiency in pressure chamber measurements. Further investigation and data from more species will be needed to test the generality of these conclusions.