Role of Organic Acids in Sunflower Tolerance to Heavy Metals

Exposure of Helianthus annuus L. seedlings to Al3+, Cd2+ or Zn2+ resulted in a marked decrease of fresh and dry masses of the shoots and the roots. The increase of Al3+, Cd2+ or Zn2+ uptake was accompanied by a significant decrease of nitrate, phosphorus and K+ uptake. There was a significant increase of malic and citric acid contents in the shoots and roots of heavy metal-treated seedlings whereas the change in fumaric acid was insignificant. Al3+ and Zn2+ alone stimulated excretion of malic and citric acids to the rhizosphere. Addition of high concentrations of malic or citric acid alleviate to some extent the inhibitory effect of Al3+ and Zn2+ on plant growth.