ON THE LONG-WAVE DECLINE OF THE QUANTUM 



YIELD OF PHOTOSYNTHESIS IN THE RED ALGA 



FORPHYRIDIUM CRUENTUM 



J. B. Thomas and Govindjee 



Photosynthesis Research Laboratory 



Department of Botany 

 University of Illinois, Urbana, Illinois 



Introduction 



The quantum yield of photosynthesis, $ in the far red, instead 

 of remaining constant until a region is reached where chlorophyll a 

 no longer absorbs at all, declines with increasing wavelength (8, 9) . 

 Emerson and coworkers (cf. 3) found that the wavelength at which 

 this decline starts varies with the pigment composition of the photo- 

 synthetic system. They also observed that irradiation with light of 

 shorter wavelengths can restore the reduced quantum yield to its 

 full value (6, 7) . This "Emerson effect" was interpreted as evidence 

 of a need for cooperation in photosynthesis of an excited auxiliary 

 pigment with an excited chlorophyll a. It was noted (5) that the 

 amount of activation by auxiliary light depends on the intensity ratio 

 of auxiliary light and far red light. No effect occurred unless this 

 ratio exceeded a certain minimum. 



The published data do not prove whether, in the case of exclusive 

 excitation of chlorophyll a, the rate of photosynthesis becomes zero, 

 or has a certain small finite value. (The smallest value measured 

 by M. Brody and R. Emerson (2) with Porphyridium was </> = 0.01 

 in a narrow band centered at 700 ni/x) . The present study was in- 

 tended to answer this question. 



Methods 



We wanted to be able to vary the composition of the incident light 

 so that the proportion of energy absorbed by auxiliary pigments 

 could change in small steps. To this pinpose, an organism had to 

 be used in which the auxiliary pigments absorbed well outside the 

 red chlorophyll a band (which has a peak between 670 and 680 m/x). 



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