Delayed fluorescence and the high-energy state of chloroplasts.

Abstract

The relation between the “high energy state” of photophosphorylation and delayed fluorescence has been studied in spinach chloroplasts. In the presence of an electron acceptor or cofactors, the kinetics of the rise in intensity of light emitted 1 millisecond after illumination showed two distinct phases,‐a rapid phase complete in less than 0.1 s, and a sigmoidal slow phase with a halfrise time of about 0.3 s. The maximal intensity and time course of emission were found to vary with electron flow and the degree of coupling. With nigericin or with NH4CI, the slow phase of the intensity rise was abolished but the rapid phase was unaffected. With valinomycin the rapid phase was substantially inhibited, but the extent of the slow phase was increased to give the same maximal intensity as observed in the absence of ionophore. With carbony cyanide p‐trifluoromethoxy‐phenylhydrazone (FCCP), or a combination of valinomycin and nigericin, both phases were substantially inhibited. This spectrum of sensitivities has been interpreted as showing that the slow phase depends on the pH component, and a part of the rapid phase on the electrical component, of the electro‐chemical H+‐gradient. Addition of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethyl‐urea always inhibited emission. A mechanism relating delayed fluorescence to the “high energy state” is suggested. The photochemical act of System II is envisaged as occurring between acceptor and donor sites on opposite sides of the thylakoid membrane and in equilibrium with pools the mid point potentials of which are dependent on the pH of the phase. It is shown that in such a system the energy conserved in the oxido reductive poise and in the electrochemical H+ gradient are additive and that both contribute to a decreased activation energy for delayed fluorescence. By dissipating the H+ gradient, the activation energy requirement is increased so that the intensity of emission falls. It is concluded that other mechanisms for emission also operate, in particular one dependent on electron flow as such.