Abstract
Paraquat (10–30 μ m) exerted a dose-dependent and light-dependent toxicity on Chlorella sorokiniana. Paraquat was also seen to increase the superoxide dismutase content of these cells and to cause the appearance of a new electrophoretically distinct isozyme. Cells grown in the absence of paraquat contained one manganese-superoxide dismutase and two iron-superoxide dismutases, while the paraquat-grown cells contained an additional manganese-superoxide dismutase. Cells which were grown in the presence of 25 μ m Paraquat, and which therefore possessed elevated levels of superoxide dismutase, were resistant to 30 μ m Paraquat, whereas control cells were bleached and killed by this level of Paraquat. Electron micrography and chemical analysis revealed that Paraquat decreased the starch content of the cells and caused a failure of dividing cells to separate. It appears that Paraquat increases the photoproduction of O 2 − in C. sorokiniana and that an increase in the cell content of superoxide dismutase is an adaptive response which provides protection against this herbicide.
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