AbstractThe effects of bifenox (methyl 5‐(2,4‐dichlorophenoxy)‐2‐nitrobenzoate) and oxadiazon (5‐tert‐butyl‐3‐(2,4‐dichloro‐5‐isopropoxyphenyl)‐1,3,4‐oxadiazol‐2(3H)‐one on photosynthetic activity were investigated in isolated chloroplasts, and on respiratory activity in isolated mitochondria. The global effects of these chemicals were also investigated on cucumber cotyledon pieces. It was found that, in vitro, bifenox and oxadiazon acted on cotyledon pieces as typical diphenyl ether herbicides, causing complete pigment bleaching, even at low concentrations. In addition, bifenox and oxadiazon were shown to inhibit the photosynthesis process at the chloroplast level. At concentrations of up to 40‐50 μM, oxadiazon and bifenox were observed to inhibit fully the light‐dependent oxygen evolution of spinach class A chloroplasts, oxadiazon acting preferentially on electron transfer at the PS II level whereas bifenox acted on the photophosphorylation process. Comparison of the amounts of herbicide needed to inhibit photosynthesis and to cause bleaching of cucumber pieces leads to the conclusion that photosynthesis inhibition by bifenox and ox
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