Derivation of Pressure-Volume Curves by a Non-Linear Regression Procedure and Determination of Apoplastic Water

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Abstract
Data from pressure-volume (PV) analysis may be submitted to transformation I [i.e. leaf water potential (ψ1) versus inverse relative water content (1/R)] or to transformation II (i.e. 1/ψ1 versus R). This may cause an essential distortion of the error structure especially in transformation II due to the relatively large range which is to be covered by the 1/ψ1 ratio. Similarly, logarithmic transformation of leaf turgor potential (ψP) when deriving the sensitivity factor of elasticity (β) by linear regression from values of In ψp and 1/R may distort the error structure. In order to investigate the magnitude of the distortion effect on parameters derived from PV analysis by regression a non-linear regression procedure was compared with the common linear procedure when calculating ψp from β in the turgid region and leaf osmotic potential (ψP) in both the turgid and non-turgid region. As test plants we used field grown species of spring barley (Hordeum distichum L., cvs Gunnar and Alis). The results show that transformations and application of linear regression procedures distort the error structure of ψp more than the error structure of ψπ', which was only slightly affected. However, we recommend the use of the non-linear procedure in both cases. Furthermore, from PV analysis, obtained by thermocouple hygrometry on living and killed leaf tissue, respectively, we derived the mathematical basis for calculating the apoplastic water fraction (Ra). Ra was 0.15 at R= 1 and decreased with dehydration. The equations describing the relation between ψπ and R and between ψp and R were extended to take into account the apoplastic water fraction.

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