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Effects of Atrazine, Cyanazine, and Procyazine on the Photochemical Reactions of Isolated Chloroplasts

The effects of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], cyanazine {2-[[4-chloro-6-(ethylamino)-s-triazin-2-yl] amino]-2-methylpropionitrile}, and procyazine {2-[[4-chloro-6-(cyclopropylamino)-1,3,5-triazine-2-yl] amino]-2-methylpropanenitrile} on the photochemical reactions of isolated pea (Pisum sativum L. ‘Progress #9 Dwarf’) chloroplasts were studied. Atrazine, cyanazine, and procyazine inhibited electron transport but did not uncouple photophosphorylation. The primary site of inhibition for all three herbicides was on the reducing side of photosystem II; the electron transfer step between the primary electron acceptor (Q) and the plastoquinone pool of the electron transport chain is suggested as the site of action of all three herbicides. The amount of inhibition of electron transport observed after addition of herbicide to isolated chloroplasts was time-dependent for cyanazine and procyazine but not for atrazine. This was apparently due to a slower partitioning of cyanazine and procyazine from the aqueous phase of the reaction solution into the highly hydrophobic environment within the chloroplast membrane. Treatment of the thylakoid membranes with detergent reduced the time-dependent inhibitory nature of cyanazine and procyazine, and the ability of atrazine to block electron transport. A photosystem II-dependent electron transport assay and a chlorophyll fluorescence induction assay were used to determine the inhibitory potentials of atrazine, cyanazine, and procyazine. After allowing for differences in the rate of membrane penetration, I50 values of approximately 2 × 10−7 M were determined for each of the three herbicides.