[45] Measurement of spillover

This chapter discusses the measurement of spillover. At wavelengths greater than 680 nm, where the light absorbed by photosystem II is rate-limiting, a drop in the quantum yield for O2 evolution is observed (the red drop phenomenon). This effect does not occur when photosystem I is rate limiting. It is thought that, under these conditions, the excess excitation energy is transferred from photosystem II to photosystem I. This process is termed as “spillover.” The term spillover applies strictly to the case in which the energy absorbed by photosystem II is in excess and the photosystem II traps are closed. In this study, measurements of both chlorophyll a fluorescence and the quantum yields for electron transport reactions of the individual photosystems are used to monitor spillover. In addition, measurements of enhancement are also used. Chlorophyll a fluorescence at room temperature is measured. For most experiments, a chlorophyll concentration of 6.7 μg/ml is used. In this chapter chlorophyll a fluorescence at 77°K is also measured.