DIFFERENT FORMS OF CHLOROPHYLL A IN VIVO 

 AND THEIR PHOTOCHEMICAL FUNCTION 



Eugene Rabinovvitch and Govindjee 



Photosyntliesis Research Laboratory 



Department of Botany 

 University of Illinois, Urbana, Illinois 



Introduction 



From the study of the action spectra of chlorophyll fluorescence in 

 algae containing chfferent pigment complements, it has been con- 

 cluded that quanta absorbed by pigments other than chlorophyll a 

 are transferred by resonance to the latter pigment, with an efficiency 

 between < 20% (for some carotenoids) and > 90% (for chlorophyll 

 b and phycocyanin) . Comparison with the action spectra of photo- 

 synthesis suggested that these quanta contribute to photosynthesis 

 in proportion to the efficiency of their transfer — in other words, that 

 ultimately photosynthesis is sensitized only by excited molecules of 

 chlorophyll a, whether their excitation be due to direct absorption 

 or to resonance transfer. This simple and eminently plausible picture 

 is, however, unable to explain two phenomena: (I) The quantimi 

 yield of photosynthesis declines, in green algae (Chlorella) above 

 680 m^ (12, 13) (Fig. 1), and in red algae (Porphyridium cruentiim) 

 above 650 m^a (1) (Fig. 2) — i.e., in the region where light is absorbed 

 only by chlorophyll a. The action spectrum of chlorophyll a fluores- 

 cence shows a similar "red drop," and it seems that it, too, occurs 

 at considerably shorter wavelengths in red algae than in green algae 

 (8) . (II) The quantum yield of photosynthesis in the "red drop" 

 region can be brought up, often to the normal level of about 10 

 quanta per molecule O^, by simultaneous absorption of light of 

 shorter wavelengths (9-11, 11) . The action spectrum of this so-called 

 "second Emerson effect" follows the absorption curves of the main 

 accessory pigments (chlorophyll h in green algae, fucoxanthol in 

 brown algae, the phycobilins in red and blue-green algae) ; or, more 

 precisely, the curves showing the proportion of incident light absorbed 

 by these pigments. This suggests that excitation of chlorophyll a, 

 far from being the oidy mechanism of sensitization of photosynthesis, 



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