Graduate Thesis Or Dissertation

Manganese absorption by barley roots

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  • An investigation of Mn uptake by five-day-old excised barley roots revealed that a metabolically-mediated process was involved. In short-term experiments, the rate of Mn absorption was comparable to that of the macronutrient cations. Like other metabolically absorbed cations, the Mn absorption rate was a direct function of the ambient concentration and the pH. An evaluation of mutual effects between Mn and other cations revealed several specific regulatory effects. Of the alkali cations studied, Li alone had a pronounced stimulatory effect while Na, K and Rb markedly reduced the absorption of Mn. The alkaline earth cations also exerted widely differing effects. Calcium appeared to promote the absorption of Mn, whereas Mg had a highly inhibitive effect. The combination of both Ca and Mg was even more inhibitory. Strontium apparently was without effect and Ba had a moderately depressive effect. Other polyvalent cations which were effective inhibitors of Mn absorption were Fe⁺⁺, Zn, Cu, Al and La. In contrast, Fe⁺⁺⁺ was virtually without effect. Manganese effectively blocked the absorption of Li and Mg, but greatly enhanced that of Na, K and Rb. These diverse regulatory effects and many others reported in the literature are explained by the following hypothesis: the cationic environment at the extracellular surface of the membrane is believed to control the specificity of the ion carrier. By attaching to critical activation sites, cations induce conformational changes in the carrier which modify its selective transport properties. The accessibility or affinity of transport sites for a given ion would depend on the particular configuration of the carrier. This mechanism, together with the mutual competition between some ions for the same transport site and the cationic maintenance of the cellular membranes, would explain most, if not all, of the regulatory effects exerted by cations on the ion absorption process.
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