Cucumber (Cucumis sativus L. var. Chicago Pickler), lettuce (Lactuca sativa L. var. Buttercrunch), and millet (Panicum miliaceum) seeds were exposed for seven days to six or more concentrations of cadmium (Cd), chromium (Cr VI), copper (Cu), manganese (Mn), nickel (Ni), and zinc (Zn). The responses of the germinating seeds, root lengths, and plant heights were compared. The procedure employs seeds exposed in pouches which are held in a vertical position to aqueous solutions of the six test metals at 22 ± 2°C in the dark.
Cucumber and lettuce are test species recommended by the U.S. EPA (FIFRA), U.S. FDA, and OECD for environmental assessments. Millet, a riverine species of the Midwest, has been used by the Illinois State Water Survey to evaluate environmental effects of chemicals and waste water effluents.
The endpoints chosen were the IC50 (concentration causing 50% inhibition of root elongation or of plant heights, in comparison to the control) value and the NOEC (No-Observed-Effect Concentration) value. Both endpoints were determined after 5 and 7 days exposure. Root elongation was the most sensitive measure of inhibition. Lettuce was the most sensitive species to chromium and cadmium and equally sensitive as cucumber and millet to the other four metals. Inhibitory effects were usually greater after 7 days exposure than at 5 days.
The results from this study are compared with those obtained by the Illinois State Water Survey, which used test solutions prepared from the same stock and exposed seeds from the same lot as those used in our trials in 10 cm disposable petri dishes containing Whatman No. 1 filter paper. IC50 values obtained in our study differed significantly from those obtained by the Illinois State Water Survey study. The reasons for these differences will be discussed in the paper.