Translocation of Calcium. Exchange versus Mass Flow

Abstract

In the field of plant nutrition there are many problems which require, for their solution, an understanding of the distribution of accumulated mineral salts between various plant parts. In their discussion of interrelations of salt accumulation with growth and development of the plant body, Steward and Sutcliffe (14) have provided a background for understanding distribution problems. But, they have concluded that despite the fact that the pattern of distribution is controlled and integrated, the method by which it is accomplished is totally unknown. In a discussion of upward movement of solutes, Biddulph (2) suggests that adsorptive forces residing in the biocolloids constituting the walls of the conducting channels might operate differentially to regulate the ascent of various ions towards their receiving tissues or organs. This suggestion could be justified from an experiment of Hewett and Gardner (8) showing an adsorption of zinc ions passing through sections of grape canes. Pursuing the matter further, Biddulph et al. (5) showed that labeled calcium did not ascend the stem of bean plants with the labeled water (THO) of the transpiration stream as would be expected in a mass flow system. Instead, the ascending calcium was shown to participate in exchange and deposition reactions. In short, the data indicated that calcium ascended the stem by an exchange mechanism. In recent years studies of solute uptake by roots and other tissues have been much emphasized, and the active phase of the process, as it relates to mineral accumulation in tissues, has been quite well described (12). However, agreement concerning the mechanics of uptake by roots of those ions destined for translocation to the shoots has not been attained. One of the questions which remains unanswered is the role of the ascending transpiration stream on both the uptake and release of nutrient ions into the vascular tissues (6,10, 13, and others). Since knowledge of the mechanisms of entry into the roots of translocatable ions is limited, the present experiments on the mechanism of ascent were designed in such a way that their interpretation would be independent of the mode of entrance. This was accomplished by studying the ascent of calcium, and the arresting of its ascent, w: thout interfering with the transpiration stream. The experiments were considered to be conclusive when, after arresting the ascent of calcium, it could be started again, at will, by introducing replacement ions of Ca, Sr, and MIg, but not with K. In this respect, the results displayed features which were consistent with exchange reactions, but which were wholly incompatible with mass flow.