Abstract
Photosynthetic reactions in isolated chloroplast fragments were investigated after they had been exposed to UV-light of 254 mμ, which is found to destroy the endogenous plastoquinone. The oxygen evolution is the most UV sensitive photosynthetic reaction. The ATP formation in the noncyclic and in the cyclic system with vitamin K5 as cofactor is less sensitive; the inactivation goes parallel to the destruction of the plastoquinone of the chloroplasts. Cyclic photophosphorylation with PMS as cofactor is diminished only after longer UV-irradiation. Photosynthetic reduction of NADP at the expense of an artificial electron donor system like DAD/ascorbate is not inactivated even after prolonged UV-exposure of the chloroplasts and therefore requires no plastoquinone. This indicates again, that the site of function of plastoquinone is close to the second light-reaction of photosynthesis.
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