Zero field resonance and spin alignment of the triplet state of chloroplasts at 2 degrees K.

Microwave induced transitions in zero magnetic field have been observed in the photoinduced triplet of chloroplasts treated with dithionite by monitoring changes in the intensity of the 735 nm fluorescence band at 2 degrees K. Similar results were obtained with chloroplasts treated with hydroxylamine plus 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination. The zero field parameters are D = 0.02794 +/- 0.00007 cm-1, E = 0.00382 +/- 0.00007 cm-1, i.e. equal to those of monomeric chlorophyll a to within the experimental error. The photoinduced triplet appears to be linked to Photosystem II. This indicates that the low temperatures 735 nm fluorescence band of chloroplasts is at least partly due to Photosystem II.