Effects of lead and cadmium on duckweed Lemna minor L.

The presence of heavy metals (HM) in the waters is one of the most important pollution problem, since unlike most organic compounds, metals cannot be degraded or transformed by living organisms, therefore they accumulate in water and bottom sediments as well as in biota. Aquatic plants share the great potential to take up several HM and when these not target organisms are reached by pollutants, they may also be used as indicator species in the evaluation of the quality of the aquatic systems. Among them, the small floating aquatic monocotyledon Lemna minor L. has been used in phytoremediation and toxicity studies on HM, phenolics and herbicides, and this species can be considered a model plant for ecotoxicological studies (1 and references therein). Most of the studies have considered the plant response to the toxicity of single metal, while data on the effects of two or more metals are very few. The objective of this study was to determine the effect of lead and cadmium on L. minor plants. A laboratory screening with plants grown in medium polluted with 2.5, 5 and 10 ppm of Pb2+ and Cd2+ was set-up in order to detect the stress response of the macrophyte along seven days. The experiments were addressed to detect the effect of these HM on growth, pigments and phenolics content and on the activities of the antioxidant enzymes involved in the detoxification process, e.g. polyphenol oxidase (PPO; EC 1.14.18.1) and guaiacol peroxidase (G-POD; EC 1.11.1.7). A reduced growth rate was detected in the HM treated plants and also the chlorophylls and carotenoids amount decreased depending on the HM concentrations. Metal stress induced an enhancement of PPO and G-POD activities as well as of phenolic compounds production.