Oxidative stress enzyme activity in Lemna minor L. exposed to cadmium and lead

Duckweed (Lemna minor) is an aquatic plant used in phytotoxicity tests for xenobiotic substances. This study assessed whether ascorbate peroxidase (APX), guaiacol peroxidase (GPX) and superoxide dismutase (SOD) in L. minor may be used as bioindicators of stress caused by heavy metals. The plants were cultivated on Knopp's medium spiked with cadmium (0.001 - 1.0 mM) and lead (0.01 - 1.0 mM). Cadmium showed higher toxicity than lead in L. minor. At lower lead doses (0.01 - 0.1 mM) growth was even slightly stimulated. Both heavy metals (0.001 - 0.05 mM Cd, 0.01 - 0.5 mM Pb) brought about chlorosis and modified the enzymes of the antioxidative system. GPX showed the highest increase in response to increased cadmium content in the medium and cadmium bioaccumulation. APX activity was affected slightly, but SOD activity was not correlated with cadmium or lead accumulation. Of the antioxidative enzymes analyzed in L. minor test plants, only GPX proved useful as a biochemical stress indicator of heavy metal pollution.