Cyclic electron flow under saturating excitation of dark-adapted Arabidopsis leaves

The rate of cyclic electron flow measured in dark-adapted leaves under aerobic conditions submitted to a saturating illumination has been performed by the analysis of the transmembrane potential changes induced by a light to dark transfer. Using a new highly sensitive spectrophotometric technique, a rate of the cyclic flow of approximately 130 s(-1) has been measured in the presence or absence of 3-(3,4-dichloro-phenyl)-1,1-dimethylurea (DCMU). This value is approximately 1.5 times larger than that previously reported [Proc. Natl. Acad. Sci. U. S. A. 99 (2001) 10209]. We have characterized in the presence or absence of DCMU charge recombination process (t(1/2) approximately 60 micros) that involves P(700)(+) and very likely the reduced form of the iron sulfur acceptor F(X). This led to conclude that, under saturating illumination, the PSI centers involved in the cyclic pathway have most of the iron sulfur acceptors F(A) and F(B) reduced. In the proposed mechanism, electrons are transferred from a ferredoxin bound to a site localized on the stromal side of the cytochrome b(6)f complex to the Q(i) site. Two possible models of the organization of the membrane complexes are discussed, in which the cyclic and linear electron transfer chains are isolated one from the other.