Reduction Of Pheophytin In The Primary Light Reaction Of Photosystem II

The photochemical reduction of pheophytin and bacteriopheophytin has been shown in vitro [l-3] . In reaction centers of photosynthetic bacteria bacteriopheophytin a [4-91 and bacteriopheophytin b [ 10, 1 l] act as an intermediary electron carrier between bacteriochlorophyll dimer and the ‘primary’ electron acceptor, a complex of ubiquinone and Fe. When the ubiquinone is in the reduced form the photoaccumulation of reduced bacteriopheophytin can be observed [5-111. In various species of green plants 1.5-2.3 molecules of pheophytin have been found per 100 molecules of chlorophyll [ 121 . In photosystem II of green plants the photoreduction of the primary electron acceptor, Q (plastoquinone), is accompanied by a blue shift of absorption bands at 54.5 nm and 685 nm [13,14] which can belong to a bound or aggregated form of pheophytin in reaction centers of photosystem II [ 141. The photoreduction of pheophytin may be observed in photosystem II preparations from pea chloroplasts at 20°C [ 151. In this work a reversible reduction of pheophytin in the primary light reaction of photosystem II in pea subchloroplast particles at redox potentials (Eh) from -50 mV to -550 mV (when Q is in the reduced form) is demonstrated. This photoreaction is observed at -170°C as well as at 20°C and is accompanied by a 2-3 fold decrease in the chlorophyll fluorescence yield.