An underlying cause of measurement discrepancies in determinations of osmotic characteristics in plant cells and tissues

Summary

It is frequently observed that liquid-exchange methods of measuring tissue water potential,Ψ, yield estimates further from zero than those obtained with vapour exchange methods, and that a similar situation exists with respect to plasmolytic methods of measuring vacuolar osmotic pressure,π, compared with estimates from expressed sap methods. Although a number of factors may contribute to this phenomenon, it is suggested that the underlying reason is one of membrane leakiness, in the liquid exchange methods, to the solutes commonly employed. Although the amount of solute uptake may appear to be small during the period of the determinations, the effective osmotic pressure of the external solution is reduced if there is a net volume exchange of water and solute, instead of a simple water exchange. Thus the solution which appears to be in equilibrium with the tissue is, in fact, more concentrated than the true equilibrium solution leading to a spurious estimate of Ψ or π.