EFFECT OF ZN ON PROTECTIVE ENZYM ES ACTIVITY,OSMOLYTES CONTENT AND　CA2+ D ISTRIBUTION IN LEAVES OF NYM PHO IDES PELTATUM

Zinc (Zn) is an essentialmicroelement for normal growth and development of plants at low concentration; how2ever, it is also an important environmental pollutant due to anthropogenic pressure as well. In the present study, Nymphoides peltatum, a rooted2floating aquatic macrophyte, was cultivated with elevated concentration of Zn (0, 5, 10, 15 and 20mg/L) for 9d respectively. The response of activities of superoxide dismutase(SOD) and peroxidase(POD), osmolytes(proline and soluble sugar) to Zn stress was investigated and Ca2+ ultrastructural distribution in leaf cells was examined with the cytochemicalmethod of calcium antimonate precipitation. The results indicated that Zn decreased SOD activity by 11. 46% -75. 22% and the inhibition action reached significant level (R = -0. 8642, p 2+ mainly distributed within intercellular space and vacuole, some calcium deposits could be randomly found in cytoplasmic matrix and nucleus under normal conditions, when different dose of Zn was added into the culture solution, and the calcium level in these compartments lowered while that in cytoplasm increased remarkably, especially bigger particles of calcium deposits appeared at the inner aspect of plasmalemma and cell nucleus, which might be attributed to the opening of the calcium channels in plasmalemma and tonoplast and the loss of activity of the Ca2+ pump, this, in turn further enhanced the calcium level of cell interior. The rise of Ca2+ level in cytoplasm and cell nucleusmaybe related to the changes of a series of physiological and biochemical processes. The dense distribution of Ca2+ in inner aspect of plasmalemma and in cell nucleusmight lead to the damage to the plant cell or the death in the end. Based on observations in the present investigation, it can be concluded that several defense systems are activated simultaneously when affected by Zn, including the induction of stress enzymes (POD), increase of synthesis or content in osmolytes (proline and soluble sugar) and changes in the distribution and content of loosely-bound Ca2+.