Alternate methods of analysing water potential isotherms: some cautions and clarifications. II. Curvilinearity in water potential isotherms

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Abstract
The Hammel pressure bomb technique was used to obtain measurements of water potential, .PSI., and relative water content, R*, on single leaves of Populus ssp. [P. alba, P. deltoides, P. nigra and P. deltoides .times. P. nigra], Helianthus annuus and Fraxinus ornus. The data were plotted either as .PSI. vs. 1/R* or as 1/.PSI. vs. R* and analyzed by linear regression in the region where cell turgor was thought to be zero. In some cases the .PSI. vs. 1/R* transformation showed curvature in the zero turgor region whereas less curvature was present in the same region of R* in the 1/.PSI. vs. R* transformation. In theory, curvature can appear in the .PSI. vs. 1/R* transformation whenever a significant fraction of tissue water is contained in the apoplast and this was demonstrated by some of the data. The presence or absence of curvature in the 2 transformations can also be due to apoplast compressibility, the development of negative turgor pressure in living cells and the nonideality of the osmotic solutions in living cells. Linear regressions performed on curved data plots will lead to errors in the estimation of .pi.o, the osmotic pressure at full hydration and of other parameters derived from the analysis.