The influence of darkness on the uptake and movement of diquat and paraquat in tomatoes, sugar beet and potatoes

SUMMARY Uptake of diquat and paraquat was doubled and occasionally quadrupled when tomatoes were darkened after treatment. The increase was not directly related to the duration of darkness because, after a time, uptake decreased. Three possible explanations for this decrease are considered, namely: diquat exudation from the leaves, downward movement into the roots and the complexing of diquat in plant tissue. Evidence was against exudation from leaves and downward movement into the roots. Diquat activity also depends on its degree of movement in the plant but the possible influence of other factors is discussed when treatments are made in the afternoon or in the morning after a period of reduced light intensity. In a CO2‐free atmosphere, uptake and activity increased in light and were comparable to those in darkness. Uptake is therefore not impeded by phyto‐toxic effects of diquat but possibly by the presence of a high concentration of solutes or by photosynthetic intermediates.