The effect of redox potential on the kinetics of fluorescence induction in pea chloroplasts. II. Sigmoidicity

The shape of the fluorescence induction curve in chloroplasts inhibited by 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea has been determined at different redox potentials. At ?10 mV a monophasic and sigmoidal curve is seen which is transformed into an exponential curve when the potential is poised at ?150 mV. At this potential, the quencher with high midpoint, Q_H, is reduced but that with low midpoint, Q_L, is oxidized. Thus, a sigmoidal induction is observed during photoreduction of Q_L and Q_H but photoreduction of Q_L proceeds with exponential kinetics. A correlation between the relative proportions of Q_L and Q_H observed in redox titration and the sigmoidicity of induction is also seen upon depletion of Mg²⁺ and after alkalinization to pH 9.5. Several models are discussed to explain the relationship between Photosystem II interactions and Q heterogeneity.