EXPERIMENTS ON THE HERBICIDAL ACTION OF 1,1 '-ETHYLENE-2,2 '-DIPYRIDYLIUM DIBROMIDE

Light was shown to increase the rate of kill obtained with I,I'-ethylene-2,2'-dipyridylium dibromide, without being essential for herbicidal action. The light effect, which was observed only in green tissues, increased with intensity up to about Io k-lux, and was inhibited by N-(4-hlorophenyl)-N,N'-dimethyl ureaor the absence of oxygen, but not by potassium cyanide or in a carbon-dioxide-free atmosphere. The herbicide was also shown to stimulate and inhibit respiration. The stimulation was transitory, and appeared to depend on a continuous supply to the tissues, while the inhibition did not. These observations are interpreted, consistently with chemical evidence, to show that the reduction of the herbicide to its free radical form is an essential step in the sequence of toxic reactions. It is supposed that energy for reduction comes from the photolysis of water in the light, and from respiration in the dark. Once formed, the radical may initiate chain reactions in the presence of oxygen which end in the degradation of essential cytoplasmic components.