A revised and unified pressure-clamp/relaxation theory for studying plant cell water relations with pressure probes: in-situ determination of cell volume for calculation of volumetric elastic modulus and hydraulic conductivity.

The cell-pressure-probe is a unique tool to study plant water relations in-situ. Inaccuracy in the estimation of cell volume (νo) is the major source of error in the calculation of both cell volumetric elastic modulus (ε) and cell hydraulic conductivity (Lp). Estimates of νo and Lp can be obtained with the pressure-clamp (PC) and pressure-relaxation (PR) methods. In theory, both methods should result in comparable νo and Lp estimates, but this has not been the case. In this study, the existing νo-theories for PC and PR methods were reviewed and clarified. A revised νo-theory was developed that is equally valid for the PC and PR methods. The revised theory was used to determine νo for two extreme scenarios of solute mixing between the experimental cell and sap in the pressure probe microcapillary. Using a fully automated cell-pressure-probe (ACPP) on leaf epidermal cells of Tradescantia virginiana, the validity of the revised theory was tested with experimental data. Calculated νo values from both methods were in the range of optically determined νo (=1.1-5.0nL) for T. virginiana. However, the PC method produced a systematically lower (21%) calculated νo compared to the PR method. Effects of solute mixing could only explain a potential error in calculated νo of <3%. For both methods, this discrepancy in νo was almost identical to the discrepancy in the measured ratio of ΔV/ΔP (total change in microcapillary sap volume versus corresponding change in cell turgor) of 19%, which is a fundamental parameter in calculating νo. It followed from the revised theory that the ratio of ΔV/ΔP was inversely related to the solute reflection coefficient. This highlighted that treating the experimental cell as an ideal osmometer in both methods is potentially not correct. Effects of non-ideal osmotic behavior by transmembrane solute movement may be minimized in the PR as compared to the PC method.