Influence of Ioxynil on the Photo-reduction of Endogenous Plastoquinone by Isolated Chloroplasts of Vicia faba L.

Abstract

ALTHOUGH plastoquinone (a 2,3-dimethyl p-benzoquinone, with a C₄₅ side chain at C₅) was first isolated in 1946 by Kofler from alfalfa grass, it did not excite much interest until Crane¹ extracted the compound from the chloroplasts of green leaves. Plastoquinone (PQ) was first implicated as a participant in photosynthetic electron transport systems by Bishop². Recently, Redfearn et al.³ have shown plastoquinone to be an intermediary electron carrier in the photosynthetic system, being a part of the light-dependent water-splitting mechanism as outlined by Duysens et al.⁴. We have recently reported⁵ that ioxynil (4-hydroxy-3,5-diiodo-benzonitrile)⁶ inhibits photophosphorylation and the accompanying reduction of pyridine nucleotide by chloroplasts. The results reported have been improved in recent months to give 50 per cent inhibition of both processes at a concentration of ioxynil of 5 × 10⁻⁵ M. In an attempt to elucidate the action of ioxynil on the photosynthetic electron transport system, the ability of chloroplasts to photo-reduce endogenous plastoquinone was investigated.