1. 1. In isolated chloroplasts with intact envelopes strong fluorescence quenching upon prolonged illumination with red light is accompanied by an absorbance increase. Both effects are reversed by uncoupling with cyclohexylammonium chloride.
2. 2. The fluorescence quenching is reversed in the dark with kinetics very similar to those of the dark decay of chloroplast shrinkage.
3. 3. In intact leaves under strong illumination with red light in CO2-free air a low level of variable fluorescence and a strong shrinkage response are observed. Carbon dioxide was found to increase fluorescence and to inhibit shrinkage.
4. 4. Under nitrogen, CO2 caused fluorescence quenching and shrinkage increase at low concentrations. At higher CO2 levels fluorescence was increased and shrinkage decreased.
5. 5. In the presence of CO2, the steady-state yield of fluorescence was lower under nitrogen than under air, whereas chloroplast shrinkage was stimulated in nitrogen and suppressed in air.
6. 6. These results demonstrate that the fluorescence yield does not only depend on the redox state of the quencher Q, but to a large degree also on the high-energy state of the thylakoid system associated with photophosphorylation.
Abbreviations: DCMU, 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea