Stability of thylakoid components under supra-high irradiance was studied with the cyanophyteSynechocystisPCC 6714. The activity of overall photosynthesis was quickly inactivated (T1/2=20 min) under supra-high irradiance (300 W m−2, white light). In parallel with the inactivation of photosynthesis, QA in PS II was also inactivated. Both inactivations were accelerated by chloramphenicol (CAP) addition. The reactivation of PS II required weak irradiation and was suppressed by CAP. However, PS I measured as P700 was very stable. The level of PS I measured as P700 was not significantly reduced by the irradiation for 12 h even in the presence of CAP while the level of Cytb559,component of PS II, was decreased markedly. The function of PS I before and after supra-high irradiation with CAP was examined by comparing sizes of P700 oxidation induced by a short flash, by a continuous light, and by determination of O2-and ferredoxin-reduction. No difference was observed in PS I actions before and after the irradiation treatment. These results indicate that the PS I complex is very tolerant of supra-high irradiation. However, the cells grown under supra-high irradiance contained much fewer PS I and PS II complexes than Cytb6−fcomplexes. The former levels were reduced to a half to one fourth of those before growth while the level of Cytb6−fcomplex was not reduced so much. A possible mechanism for changes in thylakoid composition under supra-high irradiation was disc
展开▼
