FLUORESCENCE INDUCTION IN ALGAE AND CHLOROPLASTS

Abstract

Abstract—The first phases of the fluorescence transient elicited by illumination of dark‐adapted algae or isolated chloroplasts (biphasic rise φ***v ‐ φI ‐ 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 φI‐φP rise is controlled by an induction process on the acceptor side of Photosystem 1.