Antagonistic effects of α-tocopherol and α-tocoquinone in the regulation of cyclic electron transport around photosystem II

Abstract

α-Tocoquinone (α-TQ) and α-tocopherol (α-TOC) which cannot substitute for plastoquinone-9 (PQ-A) as an electron acceptor from photosystem II (PS II), influence the oxygen evolution activity of thylakoid membranes under continuous illumination. In the presence of the herbicide DCMU and the protonophore FCCP which stimulate cyclic electron transport around PS II, α-TQ decreased oxygen evolution whereas α-TOC enhanced it. The effects are attributed to a stimulation or an inhibition of cyclic electron transport around PS II by α-TQ and α-TOC, respectively. Results of flash light experiments on PS II preparations show that both α-TQ and α-TOC increased the d-parameter which describes the transition probability from the S3- to the S0-state of the oxygen-evolving complex, although to a smaller extent when PQ-A is added alone to the preparations. The initial S-state distribution in dark-adapted samples was changed only upon PQ-A addition and influenced neither by α-TQ nor by α-TOC supplementation. These effects indicate different kinds of interaction of PQ-A, α-TQ and α-TOC with the PS II components. α-TQ increased and α-TOC decreased the 'total miss' parameter both in the presence or absence of PQ-A. A possible site of interaction of α-TQ and α-TOC with the cyclic electron transport around PS II is suggested.