Effects of copper concentration on mineral nutrient uptake and copper accumulation in protein of copper-tolerant and nontolerant Lotus purshianus L.

Abstract

One copper-tolerant and one copper-sensitive inbred line of Lotus purshianus L. derived from a copper mine waste site in Northern California and one inbred line of the same species derived from a pasture next to the mine waste were examined for the effects of excessive copper concentrations on mineral nutrient uptake and accumulation of copper in protein fractions. Plants were grown from seeds for a period of 24 days in a modified Hoagland nutrient solution culture supplemented with 3, 6, and 10 μ M copper as copper sulfate. The basal nutrient solution without copper amendment was used as the control treatment. The uptake of Cu found in the roots was 100 times or more than that in the leaves. The root tissue copper concentrations reached a plateau under 6 μ M copper treatment. The leaf tissue copper concentrations increased with the increase of copper concentration in the solution culture. No difference in pattern of copper uptake was detected between the copper-tolerant and nontolerant plants. The effects of excessive copper concentrations caused reduction of Ca uptake in the leaf tissue and P uptake in both the root and leaf tissues, and no difference was found between the copper-tolerant and nontolerant plants. Increased tissue copper concentration caused greater reduction of Fe, Mn, and Zn uptake in the nontolerant plants than in the tolerant plants; this difference may be important for the growth of the tolerant plants under conditions of excessive copper concentrations. Protein extracted from the roots and leaves of both the copper tolerant and nontolerant plants was subjected to Sephadex G-75 column separation. Two major peaks of protein fractions were detected. Under low (normal level) copper concentration treatment, the copper-tolerant and nontolerant plants had similar Cu/protein ratios. However, under higher copper concentration challenged conditions the copper-tolerant plant had a considerably greater Cu/protein ratio (peak II protein) than the nontolerant plants. The amino acid composition of the copper-rich protein fraction (peak II) extracted from both the tolerant and nontolerant plants demonstrated a high asparate (about 25%) content. The contents of glutamate, cystine, and glycine were about 11, 2.5, and 10%, respectively, and the rest of the amino acids were in a range of 2 to 6%. This pattern of amino acid composition is different from the amino acid composition of the phytochelatin metallothionein-like proteins found in copper-tolerant plants which are very high in cysteine. Instead, the copper rich protein found in the L. purshianus resembles the copper-binding protein found in spinach which has high acidic amino acids and asparate content. More studies are needed to characterize this copper-binding protein and discover its possible role in copper tolerance of L. purshianus.