Differential Effect of Cd2+ and Ni2+ on Amino Acid Metabolism in Soybean Seedlings

In 10-d-old soybean seedlings, the growth of roots and shoots was significantly inhibited at 50 and 100 M and more Cd²⁺, respectively, and by 50 M or more Ni²⁺. Although total protein content of roots exposed to 200 M Cd²⁺ or Ni²⁺ was similarly decreased compared to the control, the activity of nitrate reductase was much more inhibited by Cd²⁺. Ni²⁺-treatment (200 M) induced an accumulation of all free amino acids in roots associated with a decrease in alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities reflecting the accumulation of both alanine and aspartic acid, respectively. Cd²⁺-treatment (200 M) decreased the amount of all free amino acids. In addition, cysteine which is the main amino acid consisting the phytochelatin complexes constituted about 17.5 % of total free amino acids. The activities of both ALT and AST in Cd²⁺-treated roots were higher than in Ni²⁺-treated roots suggesting higher conversion of alanine and aspartate to pyruvate and oxaloacetate. Primary leaves excised from either Cd²⁺ or Ni²⁺-treated seedlings showed similar pattern of enzyme activities as roots.