Accumulation and localization of cadmium in Echinochloa polystachya grown within a hydroponic system

Phytoremediation is a technology for extracting or inactivating pollutants. Echinochloa polystachya [(H.B.K.) Hitchcock] (Poaceae) is a fast-growing perennial grass that is common in tropical areas and is often found in oil-polluted soils that contain high concentrations of heavy metals. However, its tolerance to heavy metals, and its ability to accumulate them, has yet to be investigated. Here we test the hypothesis that E. polystachya is able to accumulate high concentrations of cadmium (U). Plants were grown hydroponically with different levels of Cd2+ (0, 0.25, 1, 2, 10, 50, and 100 mg L-1), and were found to be tolerant to Cd2+ at all levels. No metal-toxicity symptoms were observed at any Cd2+ level. Root and leaves Cd concentrations were 299 +/- 13.93 and 233 +/- 8.77 mg kg(-1) (on a dry weight basis), respectively. Scanning electron microscopy showed the inclusion of Cd within the xylem; this result was confirmed by energy dispersive X-ray spectrometry. Leaf tissues also accumulated Cd, especially within the bulliform cells of the epidermis. We conclude that E. polystachya is a hyperaccumulator of Cd. While data for other metals are not yet available, E. polystachya shows promise in the phytoextraction of Cd from polluted tropical sites. (c) 2006 Elsevier B.V. All rights reserved.