FLUORESCENCE INDUCTION IN ALGAE AND CHLOROPLASTS

Abstract —The first phases of the fluorescence transient elicited by illumination of dark-adapted algae or isolated chloroplasts (biphasic rise φ_***v - φ_I - _P) have previously been shown to be controlled by two quenchers: Q , the primary acceptor of Photosystem 2 interacting with the secondary acceptor pool A ; and R , a non-photochemical quencher which goes into a non-quenching state as A is reduced.
The dependence of the kinetics of φ decay after illumination upon the redox state of A was studied. It is suggested that some of the centres are in a disconnected state, where electron transfer between Q and A is hindered, the amount of such centres being correlated to the reduction state of A . The implications of this hypothesis on the problem of the variation of the Q-A 'equilibrium constant' under different experimental conditions, and on Murata's 'weak light effect' are discussed.
The effect of 3-(3,4 dichlorophenyl)-1,1 dimethylurea (DCMU) on R is shown to depend upon the redox state of A . A DCMU-induced shift of the midpoint potential of R may account for this dependence.
Evidence is given suggesting that the transient reduction of A which occurs in algae during the φ_I-φ_P rise is controlled by an induction process on the acceptor side of Photosystem 1.