The relationship between delayed fluorescence and the H+ gradient in chloroplasts

1. The induction kinetics of delayed fluorescence have been studied in isolated chloroplasts and compared with the kinetics of H⁺ uptake. The slow phase of the delayed fluorescence rise, after replotting on a logarithmic scale, had the same half-rise time as H⁺ uptake.2. The kinetics of decay of the “state” filled during the slow phase of delayed fluorescence induction have been investigated by following the reappearance of the slow phase with increasing dark time after a prior period of illumination.3. The decay of the “state” filled during the slow phase was found to parallel the decay of H⁺ uptake under a variety of conditions in which the ionic environment was varied, or in the presence of ionophores or uncoupling agents.4. It is suggested that the slow phase of the delayed fluorescence induction occurs as a pH gradient develops across the thylakoid membrane, and that the pH gradient is equivalent to the “state”, the decay of which gave rise to a reappearance of the slow phase.