Abstract
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 sub(A) super(-) 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 sub(1)) and plateau (F sub(p)) levels reflecting the reduction of Q sub(A) to Q sub(A) super(-). 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 sub(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.