The mode of action of paraquat and diquat was studied by following some of the sequential events during the kill of flax cotyledon leaves by these two herbicides. After floating the cotyledon leaves on the herbicide solutions the first effects noticed included an inhibition of photosynthetic carbon dioxide uptake. The production of toxic products by the interaction of the bipyridyls with photosynthetic electron transport was thought to cease before major breakdown of the cell components was completed. Hydrogen peroxide production was demonstrated with isolated chloroplasts incubated with paraquat and diquat. During the time that photosynthetic electron transport continued there was a major rise in the level of malondialdehyde, a product of lipid peroxidation, and an increase in cell membrane permeability. Both occurred at a time when electron micrographs showed disruption of the cell tonoplast and plasmalemma. The bipyridyls are considered initiators of the kill process by indirectly effecting a loss of membrane integrity and a consequent loss of cellular organisation. The possible role of the vacuole acting as a lysosome is discussed with regard to the later breakdown of cell components. The interaction of paraquat and diquat, and other bipyridyls, benzyl viologen, morfamquat and triquat, with photosynthetic electron transport was investigated by studying the direct reduction of the bipyridyls and their subsequent reoxidation and also their inhibition of photosynthetic NADP reduction.
|Date of Award||1970|