The aim of the present study was to develop and test an extended theory of the pressure-clamp technique that would allow the estimation of additional cell water-relation parameters using the pressure probe. It was assumed that intercellular water transport (vacuole → vacuole) occurs via a composite 'membrane' comprising the tonoplast, cytoplasm, plasma membrane, plasmodesmata and cell wall and that solute transport across the micropipette tip of the probe is dominated by convection. The extended theory allows the simultaneous estimation of cell volume (V), effective cell-sap osmotic pressure (σΠ0), compositemembrane hydraulic conductance (AL(P)) and the 'instantaneous' volume change (ν0) at the start of the pressure clamp. With an estimate of cell-sap osmotic pressure (Π0), the weighted-average reflection coefficient (σ) of the composite membrane may also be determined from an endosmotic pressure-clamp experiment. In principle, the cell volume before the clamp (V0) can be estimated as V0 = V + ν0, and the cell volumetric elastic modulus (ε) can be estimated as ε = -V0(ΔP/ν0), where ΔP (
