866 LIGHT AND LIFE 



Absorption by Accessory Pigments and their Relation to Chlorophyll a 



C. S. French has provided a review ol the relation to photosyn- 

 thesis ot the so-called accessory pigments, namely, the carotenoids, 

 phycobilins, and chlorophyll /;. It has become increasingly apparent 

 that these pigments are of much greater importance than was for- 

 merly thought to be the case. They are probably responsible for a 

 different chemical reaction to that mediated by chlorophyll a; and, 

 in the green plants, chlorophyll b appears to perform the same func- 

 tion as the more striking accessory pigments of the algae. The ex- 

 periments forcing this revision of classic viewpoints include the 

 demonstration that photosynthesis caused by one wavelength of light 

 is enhanced by addition of a second wavelength when given simul- 

 taneously (the so-called Emerson enhancement effect), or even when 

 given some seconds later; the analysis of chromatic transients; the 

 relative inactivity of chlorophyll a in red algae; and the occurrence 

 and activity of several different forms of chlorophyll a in green plants. 



The existence of different forms of chlorophyll n in living plants 

 is inferred entirely from the occurrence of several absorption peaks 

 in the red region of the spectrum, even though only one peak is 

 found for chlorophyll a in solution (at 662 m^i, in ethyl ether, for 

 instance). There is a characteristic absorption band at 682-684 niju, 

 in chlorophyll a in the living leaf just after it has formed from pro- 

 tochlorophyll (main band, 650 m^u.) . The 682-684 m^ band soon shifts 

 to 670-673 m/A in the mature leaf. There are other absorption bands 

 at 692-695 ni/x and 707 m^u.. The latter band has been found espe- 

 cially strongly developed in the flagellates Ochromonas and Euglena. 

 Chlorophyll b absorbs principally at 430 and 644 m/x in ether; caro- 

 tenoids, including fucoxanthin, absorb from 500 ny into the near 

 ultraviolet; phycoerythrin (red) and phycocyanin (blue), are strongly 

 fluorescent and may be identified by their fluorescent spectra as well 

 as their absorption spectra. 



With a nuiltiplicity of pigments present, the (juestion of which 

 ones function in jjhotosynthesis might, one would think, be answer- 

 able by comparing the action spectrum of photosynthesis with the 

 absorption spectra of individual pigments. Such comparison led 

 initially to tfie concept that chlorophyll a is the pigment of major 

 importance. Accessory pigments absorb light, but they were thought 

 merely to pass the energy on to chlorophyll a, absorbing at 695 m^, 

 before the chemical changes of photosynthesis began. It was there- 

 fore surprising to discover that light at 690-700 m^x, although ab- 



