1898 KINETICS OF PHOTOSYNTHESIS CHAP, 37D 



(section 4) that, whenever no dependence of the rate on CO2 is found in 

 the concentration region where the 7 = / [CO2] curve in fig. 37D.27 rises 

 steeply, this indicates that the cells are of poor photosynthetic efficiency. 

 In Gaffron's experiments, all cells appeared equally efficient when pro- 

 vided with sufficient carbon dioxide, but C02-adapted cells lost this effi- 

 ciency when CO2 declined below a few per cent, while bicarbonate-adapted 

 cells retained a high efficiency down to considerably lower CO2 concentra- 

 tions (provided sufficient bicarbonate was available as replacement) . 



Steemann-Nielsen (1953^'") measured the rate of oxygen liberation by 

 Chlorella as function of carbon dioxide concentration, using Winkler's 

 method of oxygen determination, at different light intensities (up to 9 

 klux). At 21° C, at 7 klux, the P = /[CO2] curve was horizontal be- 

 tween 0.05 and 1% CO2, and declined above 1% (C02-poisoning, cf. chapter 

 13, part B) ; the rate showed a decline also between 0.05 and 0.025%. At 

 0.6 klux, on the other hand, the rate of O2 liberation rose between 0.05 and 

 3% CO2, particularly clearly above 0.5%, in apparent support of the above- 

 mentioned paradox: "the lower the illumination, the greater the CO2- 

 requirement !" However, Steemann-Nielsen refused to believe that this 

 rise can represent a true dependence of P on [CO2] at low I, and suggested 

 that it results from a blocking effect of concentrated carbon dioxide on 

 respiration. True, no such effect could be detected when respiration was 

 measured in the dark; but Steemann-Nielsen suggested that it may occur 

 in light. (One could imagine, for example, that high CO2 concentration 

 favors photochemical "anti-respiration," i. e., the detouring of respiration 

 intermediates into the photoreductive cycle, as suggested by Franck and 

 others in the interpretation of some of Warburg's data; but Steemann- 

 Nielsen thought rather of the inactivation of a respiratory enzyme, in 

 analogy to the ideas of Calvin and his co-workers, mentioned in chapter 36.) 

 Steemann-Nielsen pointed out, in this connection, that Kok's and van der 

 Veen's observations (reported on p. 1114; cf. also Kok 1951) of a doubling 

 of the slope of P = /(/)-curves near the compensation point (interpreted 

 by them as evidence of the existence of a photochemical "anti-respira- 

 tion") had shown a clear-cut effect only in 5% CO2, and that only much 

 weaker, uncertain effects were noted in 0.2% CO2 (a relation not noted by 

 Kok). 



Steeman-Nielsen used these findings to re-interpret Warburg's findings 

 of a low quantum requirement at high C02-concentrations as a consequence 

 of a photochemical inhibition of respiration, which requires only little light 

 energy. 



(c) Carbon Dioxide Compensation Point 



In chapter 27 (section A) it was noted that "the carbon dioxide com- 

 pensation point has not been studied in the same systematic way as the 



