QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1113 



ments of Warburg and Burk can be duplicated by strict adherence to their 

 specifications. However, the results obtained in this way are not only of 

 low precision (as revealed by wide scattering) , but, what is more important, 

 contain a systematic error. 



The differential manometer experiments of Emerson and Lewis (1941) 

 had been far more precise than either the experiments of Warburg, Burk 

 et al. (1948-1950), or the experiments which Emerson and co-workers made 

 in 1949-1950 under conditions closely imitating those of Warburg and 

 Burk. It seems that the most reliable of the presently available data on the 

 quantum yield remain those derived from these older measurements.* 



Manometric quantum yield measurements have also been reported by 

 Kok (1948, 1949). He considered the loss of light by scattering in thin 

 suspensions as a lesser experimental difficulty than the large respiration, 

 the wide variation of local light intensity, and the intermittency of illumi- 

 nation inevitable in strongly agitated, dense suspensions. He therefore 

 worked with Chlorella suspensions that absorbed only 30-40% of the in- 

 cident light (yellow sodium light), and used an Ulbricht sphere for the meas- 

 urement of absorption. He found practically linear light curves up to re- 

 markably high incident intensities — sometimes as high as 20 times the 

 respiration-compensating light! (Compare chapter 28, section A2). 

 Quantum yield deteraiinations were made by Kok in four different ways: 

 (1) by measuring the carbon dioxide exchange only, oxygen being absorbed 

 by chromous chloride in the side arm of the Warburg vessel ; (2) by measur- 

 ing the oxygen exchange only, carbon dioxide being absorbed, in the usual 

 way, in carbonate buffer ; (3) by measuring both the carbon dioxide and the 

 oxygen exchange by the two vessel method, and (4) by measuring the net 

 exchange in a single vessel, and assuming Qp = 1.09. 



The quantum requirements, I/7, were calculated from the slope of the 

 straight, ascending section of the light curves, thus avoiding explicit use of a 

 respiration correction. [The underlying assumption is, of course, that the 

 respiration, R, is the same at all light intensities at which a straight line is 

 obtained for the function P — R = /(/)]. Kok found the so-calculated 

 efficiencies to depend on the age of the suspension (c/. fig. 28.13). (This 

 probably means, primarily, dependence on the freshness of the culture 

 medium.) The yields were almost independent of the temperature and the 

 light intensity used in the cultivation of the algae. They were about 20% 

 higher in acid media (water, cultvu'e liquid, or phosphate buffer) than in 

 alkaline carbonate buffers. Lowering the oxygen pressure to 0.25% had 

 no effect on the quantum yield, and the same seemed to be true of changing 

 the temperature from 10 to 20 or 30°C. 



The 1/7-values obtained by the four methods ranged (apart from a 



* New results by Warburg and Burk (1951 '■2) pertain not so much to the question 

 of the quantum yield of photosynthesis, as to that of the mechanism of utilization of the 

 quanta. They will be described in chapters 36 and 37. 



