In situ localization of the sites of paraquat action

. Effects of paraquat were monitored by electron microscopic, cytochemical, and in vivo spectrophotometric procedures. Electron microscopy of paraquat‐treated pea leaf discs indicates that the chloroplasts are affected primarily with both thylakoid dilation and apparent lamellar fusions, resulting in a ‘honeycomb’ of lamellae. No ultra‐structural effects were noted when leaf discs were incubated in both DCMU and paraquat. Paraquat‐induced peroxide was located cytochemically along the stroma lamellae, on the ends of the grana stack and in the dilated thylakoids. Less destruction was noted in those samples where peroxide was precipitated by cerium chloride, indicating that peroxide or one of its products is one of the major causes of paraquat toxicily. In C 4 plants mesophyll plastids exhibited much more peroxide deposition than was detected in bundle sheath plastids. No plastid peroxide depositions were noted in a photobleaching mutant that is resistant to paraquat. In situ cytochrome f oxidation‐reduction was followed during paraquat treatments and resulted in a hastening of the dark re‐reduction of cytochrome f after only 1 h of treatment. This effect was prevented by DCMU or CeCI 3 . Electron transport, as measured by cytochrome f oxidation‐reduction, was completely obliterated by a 24 h paraquat treatment. These in situ studies on paraquat confirm previous studies that the primary effect in the light is on the plastid and involves peroxide or further products generated by peroxide.