Absorption of 2,4-Dichlorophenoxyacetic Acid and 3-(p-Chlorophenyl)-1, 1-dimethylurea (Monuron) by Barley Roots

Absorption from culture solution of the herbicides 2, 4-dichlorophenoxyacetic acid (2, 4-D) and 3-(p-chlorophenyl)-1, 1-dimethylurea (Monuron) by excised barley (Hordeum vulgare L.) roots was studied to determine whether absorption was due to an active or a passive mechanism. Herbicide absorption was followed at low temperature, under anaerobic conditions, and in the presence of metabolic inhibitors and compounds of structure similar to that of the herbicide. Total absorption was divided into two phases, exchangeable and nonexchangeable herbicide, by washing the roots for 1 hour following absorption. Absorption of both exchangeable and non-exchangeable 2, 4-D appeared to depend on a supply of metabolic energy which suggests that an active mechanism may be involved. A possible conclusion is that 2, 4-D is absorbed by roots by an adsorption mechanism and that energy is required to maintain the integrity of the absorbing surfaces of the cell. In contrast, absorption of Monuron was independent of an energy supply. It is concluded that the bulk of the Monuron absorbed was taken up passively by diffusion.