Selective destruction of the strongly dichroic red‐shifted chlorophyll form (C709 nm) in photosystem I (PSI) trimers from Spirulina, by either non‐selective high intensity illumination (photobleaching) or incubation with low concentrations of Triton X‐100 is accompanied by changes in the circular dichroism spectrum of the same amplitude and of opposite sign at 677 nm. The data are interpreted in terms of a dimeric chlorophyll structure with excitonic bands at these two wavelengths. Similar photobleaching experiments with PSI‐200 from maize also suggest the presence of bulk antenna/red form excitonic interactions.

In spinach thylakoids, the quenching of the singlet excited state in the photosystem II antenna by m‐dinitrobenzene does not change the rate of the light induced degradation of the D1 reaction centre protein and offers only limited protection against photoinhibition itself. These results are discussed in terms of the role of non‐photochemical quenching as a photoprotective strategy.