PHOTOCHEMICAL ALTERATIONS OF PHOTOSYSTEM-II INDUCED BY 2 DIFFERENT PHOTOINHIBITORY TREATMENTS IN ISOLATED-CHLOROPLASTS OF PEA - A THERMOLUMINESCENCE STUDY

Suspensions of chloroplasts were submitted to photoinhibitory treatments consisting of an illumination given either in the absence of electron acceptor, or in the presence of electron acceptor. Properties of two thermoluminescence bands: (1) the band at 32-35°C due to S 2 3Q-B recombination and (2) the low-temperature band (near -68°C) induced by an illumination at -80°C, were studied in the two types of photoinhibited material. In both photoinhibitory treatments the 32°C band was reduced in size in photoinhibited chloroplasts in parallel with decrease of rates of O2 evolution and kept the same shape and position in temperature. These results indicate that the PS II centers remaining active have retained quite normal properties compared to the control. However, in chloroplasts photoinhibited in the absence of acceptor, a high level of luminescence emission was observed after a large number of flashes (more than ten), compared to control chloroplasts. This difference demonstrates an increase in the reduction state of the plastoquinone pool after the photoinhibitory treatment performed when no exogenous acceptor is present. The low-temperature band was charged only by continuous light at -80°C (or in the range -100°C to -60°C) and in the literature it is known as the Zv band. The band was reduced in size in chloroplasts photoinhibited in the presence of an exogenous acceptor in parallel with the decrease of PS II activity. Conversely, in chloroplasts photoinhibited in absence of acceptor a new low-temperature band appeared which could be differentiated from the usual Zv band by its insensitivity to ethanol. This band increased with degree of PS II inhibition. This new ethanol-insensitive low-temperature band is proposed to result from recombinations of a charge pair generated in PS II centers photodamaged at the acceptor side. © 1990.