Excitation energy distribution between the photosystems

If one considers the three major Chl-protein complexes of thylakoids (namely PS II, LHC and PS I, the former including the reaction centre and its Chi a antenna of the water-splitting photochemical reaction, the second containing Chi a 

Butler and Kitajima [116] have developed a model, the bipartite model for the latter situation, and Butler and Strasser [118] have provided a tripartite model to analyse the former situation (see also the review by Butler [119]). From the bipartite model, the following equations can be derived to analyse the fluorescence yield at F, and F.  

On the basis of these models, measurements of Fy and F at different ionic compositions of the medium, and of fluorescence excited either at 475 nm (a wavelength absorbed mainly by LHC) or at 435 nm (absorbed mainly by PS II and LHC) 

In agreement with previous results [121,122], Jennings [120] reported that upon removal of from the medium fluorescence quenching (of F ) is greater when excitation is at 475 nm than when 435 nm light is used. However, the decrease of Mg from 2.5 to 0.5 mM produced no change in the fluorescence ratio 

It seems therefore that Butler s bipartite model is adequate to describe the interactions of the Chl-protein complexes when the concentration of cations is above a level which ensures tight coupling of LHC and PS II, whereas the tripartite model is needed when cation concentration is so low as to cause uncoupling of these two complexes. The mechanism of this regulatory effect is unknown. 

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