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Volume :22 Issue : 1 1995      Add To Cart                                                                    Download

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

Auther : FATMA EL-SHINTINAWY

 Botany Department, Faculty of Science, Tanta University, Tanta, Egypt

 ABSTRACT

On illumination of Chlamydomonas thylakoid membranes after a period of darkness, the intensity of ChI 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 01 phase. The different changes in the OlD phase upon the addition of both quinones demonstrate the predominance of the reduced Q1 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 Chiamydomonas thylakoid membranes, heat-treatment enhanced the initial Chl a fluorescence (F and plateau (Fe) levels reflecting the reduction of QA to Q 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 of the control. This leads to the assumption that heat-treatment converts the active centers of PS II into inactive centers in Chiamydomonas thylakoid membranes. The inactive centers are known to have an inefficient electron transfer to the lastoquinone pool and a poor water oxidation potential.

Abbreviations: DCBQ: 2,6-dichloro-p-benzoquinone; DMQ: 2,5-dimethyl-p-benzoquinone; PS II: photosystem II; PS I: photosystem I; QA: primary electron acceptor of photosystem II; QB: secondary electron acceptor of photosystem II. Ch1 a: chlorophyll a. ms: millisecond; PQ: plastoquinone; F: rate of emission; Ia: light flux, KF: rate constant of fluorescence.

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