Darkness and the activity of diquat and paraquat on tomato, broad bean and sugar beet

A period of darkness after treatment increased the activity of diquat and paraquat on tomato, broad bean and sugar beet. The extent of the increase was dependent on the time of day (and year) of treatment, on the light quality and intensity before spraying and on the duration of the darkness. Damage from morning treatments, unlike those in the afternoon, increased rhythmically with increasing darkness. The rhythm showed a maximum about every 24 hr and continued through several days of darkness. The results underline not only the complexity of diquat and paraquat action but also the importance of strictly controlling experimental procedure when studying their mode of action.