A CRITICAL REEXAMINATION OF PRESSURE-VOLUME ANALYSIS OF CONIFER SHOOTS - COMPARISON OF 3 PROCEDURES FOR GENERATING PV CURVES ON SHOOTS OF PINUS-RESINOSA AIT SEEDLINGS

Prediction of water relations attributes for red pine (Pinus resinosa Ait.) derived from pressure-volume (PV) curves varied depending on whi ch of three methods was used. The sap expression method entailed the e nclosure of a shoot in a pressure chamber and expression of xylem sap by applying a constant selected pressure until sap flow ceased, at whi ch point xylem water potential and shoot weight were measured. A sap e xpression PV curve was formed by aggregating pairs of water potential- weight measurements, each pair supplied by one of 25 shoots. The repea t pressurization method involved repeatedly measuring xylem water pote ntial and shoot weight on a single shoot drying on a laboratory bench, Repeat pressurization PV curves were constructed from data provided b y a single shoot, The composite method utilized single measurements of xylem water potential and shoot weight on 25-30 different shoots rang ing in relative water content from about 1.0 to 0.5 achieved by bench drying. Composite PV curves were constructed from aggregate data suppl ied by a population of shoots, There was close agreement in all PV att ributes generated using repeat pressurization and sap expression metho ds, In contrast, with the composite PV method, there was a fundamental difference in the slope of the linear region of the PV curves, causin g osmotic potentials at full turgor and turgor loss to be more negativ e, and relative water content at turgor loss to be lower and symplast fraction to be higher, Comparison of composite and repeat pressurizati on PV curves over the same ranges in water content did not eliminate d ifferences in derived water relations attributes. Differences in water potential isotherms related to the PV procedures used suggest that pr olonged or repeated exposure to gas at high pressure may introduce err ors in the estimation of water relations attributes.