ROLE OF C1- IN PHOTOSYNTHESIS .2. EFFECT OF C1- UPON FLUORESCENCE

Cl- is necessary for noncyclic electron transport in isolated chloroplasts. In order to locate its functional site, the fluorescence of chloroplasts at 684 nm was examined with respect to the reduction/oxidation poise of the quencher compound located on the reducing side of Photoact II. The fluorescence yield in the absence of Cl- is lower due to a drop in the maximum fluorescence yield at saturating light intensity and a decrease in quantum efficiency. There is a good correlation between induction of the Cl- effect by heating or by increasing the pH of the suspending medium, and decline of the fluorescence quantum efficiency. The fluorescence yield kinetics in the absence of Cl- resemble more closely those obtained with added methyl viologen, which speeds electron transport, rather than those obtained with (dichloro-phenyl) methylurea or o-phenanthroline which block the oxidation of the quencher by Photosystem I. It is therefore thought that the site of Cl- involvement is on the oxidation or water-splitting side of Photosystem II and that the absence of Cl- decreases the electron flow rate from water to the quencher. © 1960.