Comparative Triazine Effects Upon System II Photochemistry in Chloroplasts of Two Common Lambsquarters(Chenopodium album)Biotypes

Biotypes of several annual broadleaved weeds tolerant to 2-chloro-s-triazines have been recorded, including common lambsquarters(Chenopodium albumL.). The mechanism of resistance in common lambsquarter was based on the differential inhibition of the Hill reaction in chloroplasts by atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine]. Chlorophyll fluorescence and electron transport assays were used with isolated chloroplasts of atrazine-tolerant and susceptible biotypes, to determine the effect of atrazine and diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] on photosystem II (PS II) activity, differential atrazine penetration of the chloroplast envelope, and relative tolerance to chloro, methoxy, and methylthio triazines. Atrazine and diuron inhibited electron transport on the reducing side of PS II in susceptible biotype chloroplasts. In tolerant biotype chloroplasts only diuron inhibited electron transport whereas atrazine had only slight effects. There were no differences in the chloroplast membrane permeability to atrazine in the two biotypes. Chloroplasts of the atrazine-tolerant biotype of common lambsquarters were also tolerant to the other classes of triazines tested.