Changes in fast chlorophyll a fluorescence transient in heat-treated Chlamydomonas reinhardtii thylakoid membranes

摘要

On illumination of Chlamydomonas thylakoid membranes after a period of darkness, the intensity of Chl a fluorescence passes through a characteristic sequence changes termed OIDPS transient. Addition of 2,5-dimethyl-p-benzoquinone quenched the DP rise only while 2,6-dichloro-p-benzoquinone addition quenched both the DP rise and the OI phase. The different changes in the OID phase upon the addition of both quinones demonstrate the predominance of the reduced Q_A~- component in the inactive photosystem Ⅱ centers. However, the effect of both quinones is a concentration dependence and DCBQ is more effective in intercepting the electrons from the inactive PS Ⅱ centers than DMQ. In Chlamydomonas thylakoid membranes, heat-treatment enhanced the initial Chl a fluorescence (F_I) and plateau (F_P) levels reflecting the reduction of Q_A to Q_A~-. The heated thylakoid membranes are sensitive to the addition of DMQ and DCBQ. Addition of DCBQ decreased the fluorescence to a level close to the steady-state of the control sample reflecting an effective electron flow. This in contrast to the DMQ treatment which did not reach (F_0) of the control. This leads to the assumption that heat-treatment converts the active centers of PS Ⅱ into inactive centers in Chlamydomonas thylakoid membranes. The inactive centers are known to have an inefficient electron transfer to the plastoquinone pool and a poor water oxidation potential.