Effects of Ion Activity and Sugar Polyalcohol Osmotica on Ion Uptake

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Abstract
Evangelou, V. P. and Wagner, G. J. 1987. Effects of ion activity and sugar polyalcohol osmotica on ion uptake.—J. exp. Bot. 38: 1637–1651. Theoretical and experimental data are presented here which show that ion activity—not just ion concentration—can be an important consideration in interpreting measurements of ion uptake from solution by plant tissues. It is also shown that sugar polyalcohols often used as osmotica in experiments using protoplasts, roots, vacuoles, etc. can greatly influence ion activity. The relationship between element concentration and ion activity is shown here to be biphasic—the degree being dependent on the type of anions and cations involved and ion concentration. Generally, the extent of biphasic behaviour increases with increasing ion valence (i.e. K + versus Mg2+ or Ca2+ versus Al3+) and this character can be prominent in the concentration range sometimes used to measure ion uptake by plants (i.e. 1 to 70 mol m⊟3). These observations can affect interpretation of ion uptake kinetics involving ions with valence greater than 1. High levels of sugar polyalcohols (i.e. 0·7 kmol m ⊟3 sorbitol or mannitol) significantly suppress ion activity and the nature and degree of suppression is dependent on ion type. The implications of this effect should be considered and quantified if ion uptake studies made in the presence of such osmotica are to be extrapolated to the simpler case of a water solution containing only inorganic salts.