KINETIC ANALYSIS OF THE FLUORESCENCE INDUCTION IN 3‐(3,4‐DICHLOROPHENYL)‐1,1‐DIMETHYLUREA POISONED CHLOROPLASTS

Abstract— The size of the area over the fluorescence rise curve of chloroplasts is a measure of the total number of quanta utilized in photosystem II during the fluorescence induction, while the growth of the area reflects the progress of photochemical events. In the presence of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU), the growth kinetics of the area are affected by the reoxidation of the primary acceptor Q^‐ with stored oxidizing charges on the donor side of system II.

At low light intensities, a slow component of this back reaction may limit the steady state fluorescence emission. At higher intensities, however, the fluorescence rise is limited solely by photochemical events, although fast thermochemical reactions like the immediate recombination of photochemically separated charges may affect the efficiency of the photochemistry.

A kinetic analysis of the area growth at moderate light intensities revealed that it occurred in two first order phases which were described by the rate constants k_α and k_β. The biphasic nature suggested a sequential two‐electron reduction of the primary acceptor Q, or the presence of two different types of photochemical centers in system II. The rate constants were light intensity dependent. They also were affected by changes in pH, by an addition of NH₂OH, or by a preillumination with short flashes prior to addition of DCMU. It is suggested that the pH of the medium, the presence of NH₂OH, and the flash induced state S_n of the water splitting enzyme, control the values of k_α and k_β by changing the rate constants of electron carrier interactions in the reaction center complex, with a resulting modification of the frequency of back reactions between the primary donor and the primary acceptor.