RELATIONSHIP BETWEEN HEAVY-METAL ACCUMULATION AND TOXICITY IN SPIRODELA-POLYRHIZA (L) SCHLEID AND AZOLLA-PINNATA R BR

The accumulation of Cd, Cr, Co, Cu, Ni, Pb and Zn by Spirodela polyrhi za (L.) Schleid. and Azolla pinnata R. Br. was directly related to the concentration of metals in the medium during a 4 day exposure period. The hierarchy of metal accumulation was Ni > Zn > Co > Cu > Cd > Pb > Cr in S. polyrhiza and Ni> Zn > Co = Cd > Cu > Pb > Cr in A. pinnata. All metals inhibited relative growth rates of test plants in a concen tration-dependent manner. Depending on metal concentration in the medi um, the hierarchy of metal toxicity was Cd > Cu = Ni > Co > Cr > Zn > Pb for S. polyrhiza and Cd > Cr > Co > Cu > Ni > Pb > Zn for A. pinnat a. The present work showed that the sulphur affinity hypothesis cannot be applied to explain variabilities in toxicities of test metals. Whe n toxicity was evaluated in terms of metal concentration in plants the hierarchy of metal toxicity was Cr > Cd > Cu > Co > Ni > Pb > Zn for S. polyrhiza and Cr > Cd > Cu > Co > Pb > Zn > Ni for A. pinnata.