Light Into Life: Delayed Fluorescence as a Probe of Primary Processes in Photosynthesis.

Several interrelated problems in photosynthesis are studied with prompt and delayed fluorescence. In one study, we use bundle sheath cells enriched in photosystem I as samples. By using far red light and electron transport inhibitors, we confirm that delayed fluorescence is a photosystem II product. Our results may also indicate that there is no photosystem II in maize bundle sheath cells. We show that although photosystem I produces no delayed fluorescence, preillumination with far red light can affect the delayed fluorescence produced by a later blue flash. This effect is altered by agents that affect cyclic electron transport. We show that the delayed fluorescence produced by a microsecond flash increases linearly with flash strength, but that an enhancement of the delayed fluorescence produced by a preilluminating flash can increase as the square of the flash strength. This enhancement effect saturates at a lower light level than the delayed fluorescence from the preilluminating flash itself does. We interpret our results to suggest that there are two subpopulations of photosystem II centers, with different optical cross sections and delayed fluorescence yields.