Modification of Herbicide Binding to Photosystem II in Two Biotypes of Senecio vulgaris L

Abstract

The binding and inhibitory activities of 2 photosystem II inhibitors, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron [DCMU]) and 2-chloro-4-(ethylamine)-6-(isopropyl amine)-S-triazene (atrazine), were compared. Chloroplasts isolated from naturally occurring triazine-susceptible and triazine-resistant biotypes of common groundsel (S. vulgaris L.) showed the following characteristics. Diuron strongly inhibited photosynthetic electron transport from H2O to 2,6-dichlorophenolindophenol in both biotypes. Strong inhibition by atrazine was observed only with the susceptible chloroplasts. Hill plots of electron transport inhibition data indicate a noncooperative binding of 1 inhibitor molecule at the site of action for both diuron and atrazine. Susceptible chloroplasts show a strong diuron and atrazine binding (14C-radiolabel assays) with binding constants (K) of 1.4 .times. 10-8 M and 4 .times. 10-8 M, respectively. In the resistant chloroplasts the diuron binding was slightly decreased (K = 5 .times. 10-8 M), whereas no specific atrazine binding was detected. In susceptible chloroplasts, competitive binding between radioactively labeled diuron and non-labeled atrazine was observed. This competition was absent in the resistant chloroplasts. Triazine resistance of both intact plants and isolated chloroplasts of S. vulgaris L. is based upon a minor modification of the protein in the photosystem II complex which is responsible for herbicide binding. This change results in a specific loss of atrazine (triazine)-binding capacity.