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

In recent years alternative ways have been proposed to transform measurements of leaf water potential, Ψ, and relative water content, R*, in order to derive values of osmotic pressure at full turgidity in leaves and shoots, πo(when Ψ 0). Two types of transformations are usually considered: 1/Ψ versus R* and Ψ versus 1/R*, and linear regression is used to fit the data in the region where turgor is thought to be zero. It appears that when πo is estimated by linear extrapolation of 1/Psi; versus R* then apoplastic water might not influence the accuracy of πo but when the Ψ versus \/R* transformation is used apoplastic water causes an underestimate of πo. We examine the accuracy of the estimate of πo obtained from the two transformations when there are random errors in π, systematic errors in π, and when the osmotic solutions are non-ideal. The 1/Ψ versus R* transformation generally produces the best estimate of π0 by linear extrapolation.