12 



L.H.M. Duysens 



FE 890 ; upon receiving a quantum, FH S90 is bleached, yielding 

 F 890. Since energy transfer by induced resonance requires ab- 

 sorption of the energy receiving molecule in the region of the 

 fluorescence spectrum of the energy transferring molecule, the 

 transfer of energy from B 89O to FH 890 stops and the fluores- 

 cence yield of B 89O increases. For a simple energy transfer 

 model, if it is assumed that FH 89O does not fluoresce, a line- 

 ar relation can be derived between the inverse of the fluores- 

 cence yield and the increase in absorption. Experimentally, a 

 linear relationship is observed indeed, which supports the hy- 

 pothesis. fkX^ confirms the earlier suggestion of Wassink and 

 coworkers that bacteriochlorophyll fluorescence increases 

 because the "energy acceptor" of photosynthesis becomes exhaus- 

 ted. / gN 



In algae and chloroplasts, Kok^ 'observed a decrease in ab- 

 sorption at 700 m|i, which was in several aspects similar to the 

 bleaching at 89O m^ in pxirple bacteria. The bleaching could also 

 be brought about by mixtures of potaBsiumQferri-/and ferrocya- 

 nide, the E'q value being about 0.44^ '. Kok^^ 'also obser- 

 ved that the bleaching was brought about most strongly by light 

 absorbed by system 1, and concluded from indirect evidence 

 that the bleaching was reversed by light absorbed by system 2, 

 in a similar way as the oxidation-reduction shifts in cytochrome 

 C 420, Vredenberg (unpublished observations) confirmed this 

 conclusion directly. Thus F 700 is probably located between 

 systems 2 and 1. The high E'q value and other evidence indi- 

 cates that F 700 is located between C 420 and system 1. Fresu- 

 mably F 7OO is, as Kok suggested, the primary oxidant of system 

 1. Because of technical difficulties, it was so far not possi- 

 ble in our laboratory to establish a quantitative correlation 

 between the bleaching of FH 70O and the increase in fluores- 

 cence of chlorophyll aj^, i.e. the chlorophyll a of system 1. 

 The main difficulty was, that the fluorescence of chlorophyll 

 a^^ is snail compared to the fluorescence of the chlorophyll a 

 of system 2. There is, however, indirect evidence that chloro- 

 phyll a^ 3^?2| fluorescence and that changes in this fluores- 

 cence occur ^ * i in addition to changes in the fluorescence 

 yield of chlorophyll a^ and &2f there are also fluorescence 

 changes at 720 and in some species at 730 mji. The infrared 

 fluorescence is-primarily excited by blue light^ >-J»5>42; 

 (see also ^^* ^^ . 



Changes in the fluorescence yield of chlorophyll &2 and their 

 correlation with a primary pho tore due tant of system 2* 



The action spectrum for the excitation of the chlorophyll a 



