Heavy-Metal Accumulation by Citrus Roots

Abstract

Cu, Zn, Mn, and Fe were equally adsorbed from solns. by living and dead citrus roots, thus indicating that non-vital processes are involved in root accumulation of these elements. Competition for adsorption was found between certain metals when they were simultaneously present in soln., but previous adsorption of one metal did not interfere with the adsorption of another. Roots which first were allowed to adsorb Zn, Mn, or Fe were found to subsequently adsorb considerably more Cu in a given time than roots without this pretreatment. No converse effect was noted when Cu was the first ion adsorbed. Fe from Fe-EDTA was found to retard Cu adsorption to some extent when both were in a common soln. Cu accumulation was related to root protein. Color reactions indicated that the most intense accumulations of Cu and Zn were in the endodermis. The multilayered hypodermis also was an important accumulating tissue, whereas the metals were not detected in the epidermis or stele. Cu could be displaced from living roots with dilute acid. The mechanism of root adsorption of these metal cations is not unlike that with organic soils.