Summary
In this short survey differences between species and varieties in the four major mechanisms that affect selective uptake of potassium and sodium to the plant within the root are considered. These include influx selectivity, K+/Na+ exchange at the plasmalemma, and selectivity at the tonoplast as well as at the symplasm-xylem boundary. The affinity of various plants for potassium influx in system 1 is rather uniform although varietal differences in barley have been observed. Differences are much more pronounced for sodium influx, for which Helianthus showed rather high and Fagopyrum rather low affinity. There is substantial variation between species in the efficiency of K+/Na+ exchange at the plasmalemma of cortical root cells; the three cereals Hordeum, Triticum, and Secale were highly efficient while K+/Na+ exchange in Atriplex, Helianthus and Allium was poor, even if the cytoplasmic sodium content was accounted for. Apparently there was no direct relation between salt tolerance and K+/Na+ exchange. The observed differences in the efficiency of K+-dependent sodium extrusion or K+/Na+ exchange were not due to the use of excised roots, they were observed also when roots of whole seedlings were investigated. At the tonoplast a 1∶1 exchange of vacuolar potassium for sodium has been observed in roots of Hordeum. By this exchange sodium ions are removed from the symplasm and potassium ions are recovered from vacuoles and thus made available for transport to the shoot. Indications for specific differences in this exchange have been observed; the exchange appears to be more efficient in Helianthus than in Hordeum roots. More comparative studies are needed here. At the boundary between symplasm and xylem vessels selectivity can be set up during xylem release of cations and there are reports that suggest a preference for sodium (Lycopersicum cheesemanii, Solanum pennellii, and Suaeda) and for varietal differences amongst tomatoes. Selectivity at this boundary, the plasmalemma of the xylem parenchyma cells was described in this paper by the selectivity ratio of transport that relates the rates of xylem transport to the cytoplasmic sodium and potassium concentrations. Based on this ratioAtriplex hortensis was shown to discriminate for sodium during xylem release while there was little selectivity in Hordeum and possibly some discrimination in favour of K+ in Allium roots. The data are shortly discussed in relation to salt tolerance and to the breeding of salt-tolerant crop varieties.
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Jeschke, W.D. Cation fluxes in excised and intact roots in relation to specific and varietal differences. Plant Soil 72, 197–212 (1983). https://doi.org/10.1007/BF02181958
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DOI: https://doi.org/10.1007/BF02181958