Alternative Methods of Analysing Water Potential Isotherms: Some Cautions and Clarifications

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Abstract
In recent years alternative ways were proposed to transform measurements of leaf water potential, .PSI., and relative water content, R*, to derive values of osmotic pressure at full turgidity in leaves and shoots, .hivin..pi.0 (when .PSI. = 0). Two types of transformations are usually considered: 1/.PSI. vs. R* and .PSI. vs. 1/R* and linear regression is used to fit the data in the region where turgor is though to be zero. When .hivin..pi.0 is estimated by linear extrapolation of 1/.PSI. vs. R* then apoplastic water might not influence the accuracy of .hivin..pi.0; but when the .PSI. vs. 1/R* transformation is used apoplastic water causes an underestimate of .hivin..pi.0. The accuracy of the estimate of .hivin..pi.0 obtained from the 2 transformations when there are random errors in .pi., systematic errors in .pi. and when the osmotic solutions are non-ideal is examined. The 1/.PSI. vs. R* transformation generally produces the best estimate of .hivin..pi.0 by linear extrapolation.