A critical re-examination of pressure-volume analysis of conifer shoots: comparison of three procedures for generatingPVcurves on shoots ofPinus resinosaAit. seedlings

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Abstract
Prediction of water relations attributes for red pine (Pinus resinosa Ait.) derived from pressure-volume (PV) curves varied depending on which of three methods was used. The sap expression method entailed the enclosure of a shoot in a pressure chamber and expression of xylem sap by applying a constant selected pressure until sap flow ceased, at which point xylem water potential and shoot weight were measured. A sap expression PV curve was formed by aggregating pairs of water potential-weight measurements, each pair supplied by one of 25 shoots. The repeat pressurization method involved repeatedly measuring xylem water potential and shoot weight on a single shoot drying on a laboratory bench. Repeat pressurization PV curves were constructed from data provided by a single shoot. The composite method utilized single measurements of xylem water potential and shoot weight on 25-30 different shoots ranging in relative water content from about 1.0 to 0.5 achieved by bench drying. Composite PV curves were constructed from aggregate data supplied by a population of shoots. There was close agreement in all PV attributes generated using repeat pressurization and sap expression methods. In contrast, with the composite PV method, there was a fundamental difference in the slope of the linear region of the PV curves, causing osmotic potentials at full turgor and turgor loss to be more negative, and relative water content at turgor loss to be lower and symplast fraction to be higher. Comparison of composite and repeat pressurization PV curves over the same ranges in water content did not eliminate differences in derived water relations attributes. Differences in water potential isotherms related to the PV procedures used suggest that prolongedor repeated exposure to gas at high pressure may introduce errors in the estimation of water relations attributes.

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