Is osmotic potential a more appropriate property than electrical conductivity for evaluating whole-plant response to salinity?

Studies of whole-plant or crop responses to salinity often focus on yield or growth reduction in terms of solution ion concentration or electrical conductivity. The response functions describing salt stress may be better presented in terms of solution osmotic potential. We looked at the effect of increasing concentrations of NaCl and CaCl2, either alone or in equinormal combination, on three different plant species: bean (Phaseolus vulgaris L.), corn (Zea mays L.) and melon (Cucumis melo L.). Corn and melon were found to be relatively tolerant and beans more sensitive to salinity. When yield response was related to the electrical charge concentration of the salts, i.e. salinity was expressed in units of mequiv. L−1 or electrical conductivity, the stress effects of Na and Ca appeared to be of different magnitudes: plant growth was more sensitive to excess Na than to excess Ca and the effect of combined Na and Ca was intermediate. The effects of the two salts were, however, indistinguishable when salinity was expressed in terms of osmotic potential of the water. For all three species, the response curves of yield as a function of level of equipotential solutions of NaCl, CaCl2 or combinations of the two salts practically overlapped. Presentation and interpretation of the whole-plant salinity response in terms of osmotic potential would be beneficial in attempts to differentiate between the osmotic and toxic effects of salinity, in normalizing data sets and in increasing their relevance in practical applications.