A STUDY ON THE FLUORESCENCE INDUCTION CURVE OF THE DCMU-POISONED CHLOROPLAST

The fluorescence induction curve of the DCMU-poisoned chloroplast can be resolved into three kinetically different phases, a rapid sigmoidal phase (alpha) followed by two slower exponential phases (beta and gamma), by using a mathematical analysis method previously described (Hsu, B.D., Lee, Y.S. and Jang, Y.R. (1989) Biochim. Biophys. Acta 975, 44-49). Evidence is given which suggests that the three phases originate from different types of Photosystem II (PS II). In order to disclose the nature of the three phases, fluorescence inductions are measured with chloroplasts subjected to various modifying treatments. It is found that only the rapid alpha-phase responds to the depletion of Mg2+, the addition of ferricyanide and the variation in excitation light intensity. On the other hand, the slower beta- and gamma-phases are accelerated by adding electron donors like hydroxylamine. Inhibition of oxygen evolution by alkaline pH shift or Tris wash results in a partial conversion of the alpha-phase to the beta- and gamma-phases. The results are explained by the hypothesis that the alpha-phase represents the major 'normal' PS II centers, while the beta- and gamma-phases represent two minor groups of 'abnormal' PS II centers with low quantum efficiencies and which are rate-limited by their slow electron donation systems.