The effect of 4,6-dinitro-o-sec-butylphenol(DNBP) and several phenylureas, s-triazines, chlorinated phenoxyacetic acids and chlorinated benzoic acids on the photolytic activity (Hill reaction) of chloroplasts isolated from turnip greens (Brassica spp.) was studied. Photochemical activity was measured potentiometrically with ferricyanide as the electron acceptor.
For the various members of each herbicidal group close agreements were obtained between inhibitions expressed against the Hill reaction and their relative phytotoxicities, except for the chlorinated benzoic acids. Of the five phenylureas tested, 1-n-butyl-3-(3,4-dichlorophenyl)-1-methyrurea (neburon) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) were most inhibitory and 3-phenyl-1,1-dimethylurea (fenuron) was least inhibitory to the Hill reaction. 2-Chloro-4,6-bis(isopropylamino)-s-triazine (propazine) and 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) were the most inhibitory and 2-chloro-4,6-bis(diethylamino)-s-triazine (chlorazine) was the least inhibitory of seven s-triazines to the Hill reaction. DNBP inhibited the Hill reaction by 50 percent (I50) at 1.3 × 10-5 M. An I50 value of 5.1 × 10-4 M was obtained for 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) which was the most inhibitory of the chlorinated phenoxyacetic acids tested. 3,4,5-Trichlorobenzoic acid (3,4,5-TBA) was the most inhibitory chlorinated benzoic acid tested (I50 #x2A75; 2 × 10-3M).
I50 values were established and compared for chloroplasts isolated from turnip greens, corn and soybeans for diuron, simazine, DNBP, isopropyl N-(3,4-dichlorophenyl)carbamate (3,4-DCIPC), isopropyl N-(3-chlorophenyl)carbamate (CIPC), 2,4,5-T and 3,4,5-TBA. Chloroplasts from the three species were essentially equally sensitive to each of the seven herbicides. Hence, no evidence was obtained which would explain species tolerance or susceptibility.
When turnip-green chloroplasts were treated with inhibitory concentrations of diuron, simazine, DNBP, CIPC, 2,4,5-T and 3,4,5-TBA in the dark, the herbicides could be removed by washing without seriously impairing the photolytic activity of the chloroplasts. However, irreversible changes occurred when the chloroplasts were illuminated while in contact with the herbicides, i.e., photolytic activity was not completely restored when the chloroplasts were washed, except with simazine.