Mechanism of Paraquat Resistance in Hordeum glaucum. I. Studies With Isolated Organelles and Enzymes

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Abstract
The mechanism of resistance to paraquat was studied in a biotype of the important and widespread weed, Hordeum glaucum Steud. (barley grass). Experiments with isolated chloroplasts and protoplasts from resistant and susceptible plants established that resistance is not due to any change in the active site of paraquat binding. No difference was found in the permeability of paraquat through the plasmalemma or chloroplast envelope membranes. As paraquat catalyses the production of excited oxygen states, the capacity of varying enzymic means of detoxifying these destructive species was examined. No differences between leaf tissue of resistant and susceptible plants were found in the activities of superoxide dismutase, catalase or peroxidases. These experiments establish that, if paraquat enters the symplast of the resistant biotype, it will exert its herbicidal effect in the normal way. Thus the resistant plants must have the ability to prevent paraquat from entering the symplast.