1940 KINETICS OF PHOTOSYNTHESIS CHAP. 37D 



4. Maximum Quantum Yield (Addendum to Chapter 29) 



The "quantum yield controversy" was followed in Vol. II, Part 1, 

 (p. 1104), through 1950, when Warburg, Burk and co-workers (1949, 1950) 

 reported minimum requirements of 2.5-5 quanta per molecule oxygen for 

 the photosynthesis of ChloreUa in phosphate buffers equilibrated with 5% 

 carbon dioxide. In most of these experiments, the yield was determined 

 from the increment of gas exchange, caused by the addition (for a period of 

 5-20 min.) of measured red light ("bright" period) to unmeasured white 

 background illumination ("dim" period). The beam was narrow, the sus- 

 pension very dense, and the shaking very rapid, so that each ChloreUa cell 

 went through rapid and wide fluctuations of light intensity during each 

 bright period, in addition to the slower alterations caused by the changes 

 from "dim" to "bright" and back. 



The ratios —1/Qp (= ACO2/AO2), derived from two- vessel experi- 

 ments, showed no deviations from —1 (in excess of ±20%) which would 

 indicate "one-sided" induction phenomena (CO2 exchange without corres- 

 ponding O2 exchange, or vice versa). 



A methodological criticism of these experiments by Nishimura, Whit- 

 tingham and Emerson (1951) also was reported in Part 1 of Vol. II (p. 

 1109). They concluded that the observations of Warburg and Burk can 

 be reproduced, but that the method is of low precision and subject to system- 

 atic error. The magnitude of the latter could not be estimated, but its 

 direction Avas that of an increase in apparent efficiency.* 



One of the points in this controversy was the occurrence and relative 

 importance of the "carbon dioxide burst" which Emerson and co-workers 

 (chapter 29, section 1, and chapter 33, section 3) made responsible for some 

 of the high yields calculated by Warburg and co-workers. Emerson and 

 co-workers suggested (p. 1111) that when Warburg and co-workers found 

 manometric curves without any induction bends, this was due to acciden- 

 tal, approximate compensation of the physical lag of the manometer by the 

 carbon dioxide burst. It was important in this connection to check the 

 reality of the burst by an independent method — if possible, one permitting 

 unambiguous identification of the gas responsible for the pressure changes. 

 Brown and Whittingham (1954) were in fact able to observe the burst, 

 by mass spectroscopy, and identify it as due to C02-molecules. It was 

 noted in both ChloreUa and Scenedesmus; in agreement with Emerson's 

 findings, it increased strongly with the partial pressure of carbon dioxide, 

 and with the length of the dark period preceding illumination. 



With 5% CO2, the effect of the burst could be noted even in repeated 



(1 minute dark, 1 minute light) cycles. When cells were exposed to C(13)- 



*Pirson, Krollpfeiffer and Schaefer (1953) supplied a thorough discussion of the re- 

 liability of the two-vessel technique, with equal and unequal voluniea of suspension. 



