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

The present study compares the binding and inhibitory activity of two photosystem II inhibitors: 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron DCMU]) and 2-chloro-4-(ethylamine)-6-(isopropyl amine)-S-triazene (atrazine). Chloroplasts isolated from naturally occurring triazine-susceptible and triazine-resistant biotypes of common groundsel (Senecio vulgaris L.) showed the following characteristics. (a) 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. (b) Hill plots of electron transport inhibition data indicate a noncooperative binding of one inhibitor molecule at the site of action for both diuron and atrazine. (c) Susceptible chloroplasts show a strong diuron and atrazine binding (¹⁴C-radiolabel assays) with binding constants (K) of 1.4 × 10⁻⁸ molar and 4 × 10⁻⁸ molar, respectively. In the resistant chloroplasts the diuron binding was slightly decreased (K = 5 × 10⁻⁸ molar), whereas no specific atrazine binding was detected. (d) In susceptible chloroplasts, competitive binding between radioactively labeled diuron and non-labeled atrazine was observed. This competition was absent in the resistant chloroplasts.