The stoichiometry of the two photosystems in higher plants revisited |
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Authors: | Da-Yong Fan Alexander B. Hope Husen Jia Jan M. Anderson |
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Affiliation: | a Photobioenergetics Group, Research School of Biological Sciences, The Australian National University, Canberra, ACT 0200, Australia b State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China c School of Biological Sciences, Flinders University, Faculty of Science and Engineering, GPO Box 2100, Adelaide, SA 5001, Australia d Department of Chemistry, Faculty of Science, The Australian National University, Canberra, ACT 0200, Australia |
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Abstract: | The stoichiometry of Photosystem II (PSII) to Photosystem I (PSI) reaction centres in spinach leaf segments was determined by two methods, each capable of being applied to monitor the presence of both photosystems in a given sample. One method was based on a fast electrochromic (EC) signal, which in the millisecond time scale represents a change in the delocalized electric potential difference across the thylakoid membrane resulting from charge separation in both photosystems. This method was applied to leaf segments, thus avoiding any potential artefacts associated with the isolation of thylakoid membranes. Two variations of this method, suppressing PSII activity by prior photoinactivation (in spinach and poplar leaf segments) or suppressing PSI by photo-oxidation of P700 (the chlorophyll dimer in PSI) with background far-red light (in spinach, poplar and cucumber leaf segments), each gave the separate contribution of each photosystem to the fast EC signal; the PSII/PSI stoichiometry obtained by this method was in the range 1.5-1.9 for the three plant species, and 1.5-1.8 for spinach in particular. A second method, based on electron paramagnetic resonance (EPR), gave values in a comparable range of 1.7-2.1 for spinach. A third method, which consisted of separately determining the content of functional PSII in leaf segments by the oxygen yield per single turnover-flash and that of PSI by photo-oxidation of P700 in thylakoids isolated from the corresponding leaves, gave a PSII/PSI stoichiometry (1.5-1.7) that was consistent with the above values. It is concluded that the ratio of PSII to PSI reaction centres is considerably higher than unity in typical higher plants, in contrast to a surprisingly low PSII/PSI ratio of 0.88, determined by EPR, that was reported for spinach grown in a cabinet under far-red-deficient light in Sweden [Danielsson et al. (2004) Biochim. Biophys. Acta 1608: 53-61]. We suggest that the low PSII/PSI ratio in the Swedish spinach, grown in far-red-deficient light with a lower PSII content, is not due to greater accuracy of the EPR method of measurement, as suggested by the authors, but is rather due to the growth conditions. |
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Keywords: | BSA, bovine serum albumin Chl, chlorophyll DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethyl urea P700, photoactive Chl of the reaction centre of PSI EC signal, electrochromic signal EDTA, ethylenediaminetetraacetic acid EPR, electron paramagnetic resonance HEPES, N-(2-hydroxyethyl)piperazine-N&prime -(2-ethanesulfonic acid) PSI and PSII, photosystem I and II, respectively QA, QB, primary and secondary quinone acceptor in PSII, respectively YD, redox-active tyrosine D in PSII |
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