ACCUMULATION AND TRANSPORT OF NICKEL IN RELATION TO ORGANIC-ACIDS IN RYEGRASS AND MAIZE GROWN WITH DIFFERENT NICKEL LEVELS

Difference in Ni tolerance/accumulation in plant genotypes might be us ed to identify or develop plants for remediation of high Ni soils. Rye grass was shown to be more sensitive to Ni toxicity and accumulated mu ch more Ni in shoots than maize. The objectives of this study were to examine the relationship of organic acids to Ni accumulation and xylem transport of Ni in ryegrass (Lolium perenne L.) and maize (Zea mays L .). The results showed that accumulation of Ni in shoots was 5 to 7 fo ld higher in ryegrass than in maize grown at 20 to 80 mu M Ni, whereas Ni concentration in ryegrass roots was only 1 to 2 fold higher at 0.1 to 40 mu M Ni and 1.5-fold lower at 80 mu M Ni than that of maize roo ts. Xylem transport rates of Ni increased with increasing Ni supply fo r both species, and were about 2 to 7 times higher in ryegrass than in maize. Shoot concentrations of citric, malic, oxalic and cis-aconitic acids increased at Ni levels above 20 mu M, and were about 2 to 6 tim es higher in ryegrass than in maize. Whereas, maize roots accumulated greater amount of malic, oxalic, and cis-aconitic acids than ryegrass roots, especially at Ni levels of 40-80 mu M. The rate of Ni exudation by roots in the two species was significantly correlated with root Ni concentrations. It could be concluded that high Ni accumulation in sh oots was closely related to high xylem transport rates of Ni and that the accumulation of organic acids, citric and malic acid in particular . A high root exudate rate of Ni and the enhanced accumulation of orga nic acids, malic acid in particular, in roots might be among the impor tant factors which are associated with the tolerance of crops to toxic Ni levels.