**5.6 Spectrophotometric assays**

For the determination of absorbance spectra, the procedure of Bricker and coworkers (2002) was employed. First, purified PSII particles were assayed for chlorophyll concentration by methanol extraction, and samples were diluted to 20 g/ml chlorophyll in a solution of 50 mM MES-NaOH, pH 6.5, 10 mM MgCl2, 5 mM CaCl2, 50 mM NaCl, 0.04% -D-dodecyl maltoside, and 25% glycerol. After performing a baseline correction on a Cary 100 dualbeam spectrophotometer (Varian, Inc.), the samples were either oxidized with a few crystals of potassium ferricyanide, for the reference cuvette, or reduced with a few crystals of sodium dithionite, for the sample cuvette. After a 10-minute dark incubation, reduced minus oxidized difference spectra were collected by scanning the samples from 540-580 nm for the cytochrome-specific spectra, and from 400-800 nm for the overall absorbance spectra.
