LIGHT, PIGMENTS, AND PHOTOSYNTHESIS 



C. S. French 



Department of Plant Biology, Carnegie Institutioji of Washington 



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Introduction 



Chlorophyll has long been known to be the major light-absorbing 

 pigment in plants. Somehow it converts its captmed energy into 

 chemical form. About 1880, Engelmann foiuicl that the red and blue 

 phycobilin pigments of some algae also used their absorbed light for 

 photosynthesis. Fmthermore, the light absorbed by some carotenoids 

 is photosynthetically active. The name "accessory pigments" has 

 become associated with all photosynthetically useful pigments other 

 than chlorophyll. The nature of these accessory pigments and the 

 evidence for their participation in photosynthesis has been discussed 

 by Blinks (5), in an article with a full bibliography. Some of the 

 properties of accessory pigments and some recent experiments con- 

 cerning the way in which they are used in the process of photosynthesis 

 will be reviewed here. 



The general idea has been that accessory pigments help the plant 

 to catch more of the incident energy because they can take up green 

 light, which is poorly absorbed by chlorophyll. The accessory pig- 

 ments can transfer their absorbed energy to chlorophyll a. This 

 transfer process is detected through the emission of chlorophyll fluores- 

 cence by plants exposed to light of wavelengths that are absorbed 

 almost entirely by the accessory pigments (12, 13, 29, 59) . Energy 

 transfer has been considered the mechanism by which the accessory 

 pigments function in photosynthesis. Thus the carotenoids, the 

 phycobilins — phycoerythrin and phycocyanin — and chlorophyll b have 

 been thought of as accessory only in the sense of extra light-gatherers 

 that funnel the light into the usual channel of photosynthesis via 

 chlorophyll a. 



Several experiments of the last few years lead to a different idea 

 of the function of accessory pigments. The accessory pigments now 

 seem to have a much greater importance than had previously been 

 realized. Apparently they are responsible for another chemical reac- 



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