The size of the population of weakly coupled chlorophyll pigments involved in thylakoid photoinhibition determined by steady-state fluorescence spectroscopy.

On the basis of experiments with singlet quenchers and in agreement with previous data, it is suggested that a population of energetically weakly coupled chlorophylls may play a central role in photoinhibition in vivo and in vitro. In the present study, we have used steady state fluorescence techniques to gain direct evidence for these uncoupled chlorophylls. Due to the presence of their emission maxima, near 650 nm and more prominently in the 670--675 nm interval both chlorophylls b and a seem to be involved. A straightforward mathematical model is developed to describe the data which allows us to conclude that the uncoupled/weakly coupled population size is in the range of 1--3 molecules per photosystem.