Inhibition of the Uptake and Long-Distance Transport of Calcium by Aluminium and Other Polyvalent Cations

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Abstract
Aluminium, scandium, and iron inhibit the uptake of calcium by week-old barley plants from acid culture solutions (pH 4.0–4.2). The inhibition by scandium can be detected when its ratio to calcium is 1:1000. The onset of the inhibition may be quit rapid and will persist for at least. 24 h in the absence of the polyvalent cation. The inhibition caused by 25 and 50 μM aiuminium sulphate may be overcome if the calicum chloride concentration in the medium is raised to 15mM, but in this situation aluminium still inhibits root growth by more than 50 per cent. Elution experiments show that polyvalent cations reduce the amount of calcium held in the water free space (WFS) and the Donnan free space (DFS) but increase both the exchangeable and absorbed chloide content of the root. Aluminium-treated roots transported much less calcium to the shoot system than untreated plants. Autoradiographs showed that this difference was reflected in a greatly reduced labelled-calcium concentration over the tissues of the stele. By contrast the non-exchangeable fraction of labelled calicum in the cortex was similar in both treatments. Autoradiographs of 46Sc showed that it was restricted to the epidermis and outer rank of cortical cells from whence it controls calicum movement throughout the root. A theory to account for this control is outlined.

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