Effects of charge characteristics of Cu-chelates on the Cu uptake from the solution by italian ryegrass and red clover

Most of the studies on the effect of chelation on metal uptake are limited to anionic ligands such as EDTA. These synthetic chelating agents are known to depress the uptake of Cu, Zn, and Cd from nutrient solutions by plants (Harrison et aL 1984; Gregar and Lindberg 1986; Iwasaki et al. 1987b). Moreover, the existence of excess ligand can cause deficiency symptoms (Halvorson and Lindsay 1977). Dekock and Mitchell (1957) have pointed out that the residual charge on the chelate molecule is one of the factors controlling the uptake of metals by the roots. They have concluded that complexes without charge or with a single negative charge are taken up by the roots unlike those with two negative charges. However the effect of positively charged chelates has not yet been compared. The purpose of this study is to compare the effects of Cu-chelates with different charge (negative, neutral, and positive) on the Cu uptake by plants and to analyze the significance of their charges, especially the positive charge. We employed eth-ylenediaminetetraacetic acid (EDTA), ethylenediamine-N,N'-diacetic acid (EDDA), and triethylenetetramine (Trien), since these ligands form similar plane structures with Cu (Murakami 1965). EDTA and EDDA have four and two carboxylic groups, which are dissociated to give a tetravalent and divalent anion, respectively. These copper complexes usually have a net charge of –2 and 0. In contrast to EDTA and EDDA, Trien is an uncharged ligand, and the valence of the cupric cation remains unaffected by the complex formation (Murase and Ueno 1965). In the first experiment, Cu uptake from equimolar solutions of Cu and ligands was examined at various concentrations. These chelating agents, however, have different stability constants with Cu. Moreover, the changes in the solution pH during the experiment may lead to differences in the amount of unchelated Cu, which in turn