Comparison between two methods of generating pressure—volume curves

Abstract. Pressure—volume (P—V) curves were generated on roots and shoots of coastal Douglas fir [ Pseudotsuga menziesii (Mirb.) Franco] seedlings using two procedures. In the first (Method A), samples were dehydrated inside a pressure chamber. Exuded stem sap was collected and weighed at successive pressure increases to derive the P—V curve. In the second method (Method B). excised samples were allowed to dry outside the pressure chamber by evapotranspiration. They were weighed periodically to determine sap loss and their corresponding balance pressures were determined in a pressure chamber in order to derive the P—V curve.
Estimates of volume averaged osmotic potential at full turgor and water potential at zero turgor which were derived graphically from the P—V curves, were different for each method. In general, estimates were more negative in Method A, by as much as 1.5 MPa in one case. Also, Method B did not record an osmotic adjustment in seedlings which were subjected to severe water stress while Method A did.