Afterglow of chlorophyll in vivo and photosynthesis

Abstract

Two pigment systems are involved in the afterglow of chlorophyll a-containing cells. Absorption in only one of these systems (promoting or “p” system) is effective in producing luminescence. If light is absorbed simultaneously by the other (quenching or “q” system), a decrease in luminescence results. Absorption in the “p” system is assumed to result in oxidation of water and reduction of a cytochrome. The afterglow is regarded as a chemiluminescence of chlorophyll excited by the recombination of photolysis products of water.

Absorption in the “q” system us assumed to oxidise cytochrome and reduce a pyridine nucleotide. In this way electrons are removed from the “p” system and recombination prevented. Each system provides about half the energy needed in complete photosynthesis.

Chloroplast luminescence is decreased by removal of electrons from the “p” system by addition of Hill reaction reagents.

The “p” system includes mainly “short-wavelength chlorophyll a” and the accessory pigments, the “q” system mainly “long-wavelength chlorophyll a”.

Indications are presented that energy for both systems is provided by reversible photooxidation of chlorophyll a. The necessary separation between electrons leaving and electrons entering chlorophyll is assumed to result from a difference in molecular orbitals of ground and excited states, inferred from the differences between reversible photooxidation and photoreduction of chlorophyll in vitro.

The function of pigment systems in photosynthetic bacteria is discussed in relation to those of green plants.