Response of antioxidant defences to Zn stress in three duckweed species.

In the plants, Lemna gibba, Lemna minor and Spirodela polyrrhiza L., the effect of different concentrations of zinc (0.01, 0.05, 0.1, 0.5 and 1.5mgL(-1) Zn) applied for four day was assessed by measuring changes in the chlorophyll, protein, malondialdehyde (MDA), catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX) and guiacolperoxidase (GPX) activity of the plants. According to results, Zn contents in plants increased with increasing Zn supply levels. The level of photosynthetic pigments and soluble proteins reduced only upon exposure to high Zn concentrations. At the same time, the level of malondialdehyde (MDA) increased with increasing Zn concentration. These results suggested an alleviation of stress that was possibly the result of antioxidants such as catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX) as well as guaiacol peroxidise (GPOX), which increased linearly with increasing Zn levels. Cellular antioxidant levels showed an increase suggesting a defensive mechanism to preserve against oxidative stress given rise to by Zn. Besides, the proline amount in L. gibba, L. minor and S. polyrrhiza increased with increasing zinc levels. These conclusions proposed that L. gibba, L. minor and S. polyrrhiza are supplied with an efficient antioxidant mechanism against Zn-induced oxidative stress which saves the plant's photosynthetic machinery from damage. It is concluded that higher zinc levels cause oxidative stress in L. gibba, L. minor and S. polyrrhiza cells and may reason membrane damage through production of ROS and interferes with chlorophyll metabolism.