The Uptake and Translocation of Manganese by Plant Roots

Abstract

Manganese in plant cells can exist as a cation, in various complexée! states or can form metallo-proteins in which tightly bound Mn atoms probably produce an appropriate protein conformation. In addition a redox function for Mn seems likely in water splitting on the oxidant side of photosystem II The quantitative requirements for Mn in biochemistry are small and it seems probable that there is a great deal of Mn in cells which is not bound in this way. If there is a role for the free ion, it has not been clearly demonstrated. Against this micro-requirement it is strange to find that the system which transports Mn across the plasmamembrane of both roots and leaf tissue has a large capacity and, in the few reports which have appeared, a value for the Michaelis constant which seems high relative to the concentrations of free divalent Mn (MnII) found in most soils of neutral or basic pH. We are confronted with the paradox of a micronutrient for which there appears to exist a large capacity, rather sluggish transport system. It would be quite wrong, however, to suggest that the nature of this transport system is understood; even the physiology of Mn transport is inadequately described. The reader will see that most of the references in this chapter are more than 15 years old. Up to that time there had been a steady, but small stream of research papers dealing with Mn uptake by root and leaf tissue; since that time the stream has very nearly dried up. Serious research on Mn transport has not advanced much in a period in which nearly all of our contemporary thinking about the way in which ion transport is organised and energized has grown up. In reviewing the subject one can do little, therefore, but suggest what needs to be done to bring our understanding of Mn transport up to date.