QUANTUM YIELD MEASUREMENTS BY THE MANOMETRIC METHOD 1003 



inquiry that they noticed the carbon dioxide "burst" (1040, 1041), and 

 suggested that failure to recognize it could have been responsible for the 

 large pressure changes observed, and the high quantum yields calculated by 

 Warburg and Negelein. 



Figure 20.3, taken from Emerson and Lewis (1041), shows the rate of 

 pressure changes as observed in minute-to-minute measurements in two 

 vessels with different liquid : gas ratio. It indicates that upon illumination, 

 an initial gush occurs, which lasts, in the light of (ho particular intensity 

 used, for about throe minutes, and then gives place to a more or less steady 

 rate of pressure change. In dark, too, the steady rate of gas consumption 

 is not established at once; after rapid, irregular variations, a rather ex- 

 tended, slow decrease in the rate is observed; subsequent experiments 

 have shown that it takes about one hour for the rate of gas uptake to be- 

 come quite steady. 



The quantum yields of Warburg and Negelein were obtained by averag- 

 ing the pressure changes over ten minute periods, which included the time 

 during which the gas burst could have occurred, according to figure 20.3A. 

 Obviously, values of the quantum yield calculated in this way could be de- 

 ceptive, and higher averages could result from the use of periods shorter 

 than ten minutes. 



By comparing the curves obtained with two vessels, Emerson and Lewis 

 (1040, 1041) sought information as to the relative role of oxygen and car- 

 bon dioxide in the gas burst, and in the extra gas consumption in darkness. 

 They found (fig. 20. 3B) that both were due to carbon dioxide and not to 

 oxygen; the absorption and liberation of the latter (solid line in fig. 20. 3B) 

 showed only minor disturbances, which could perhaps be attributed to 

 uncertainties in the evaluation of the measurements. 



The factors that Warburg and Negelein, Rieke and Emerson and Lewis 

 have described previously as indispensable for the realization of the highest 

 quantum yield were found by Emerson and Lewis to affect mainly or ex- 

 clusively the carbon dioxide gush. 



The carbon dioxide concentration in the medium affected the quantity of carbon di- 

 oxide taken up in the dark, and therefore also the amount of the gas released in the light. 

 This offered an explanation of the observation that large concentrations of carbon di- 

 oxide (such as 5%) were needed to obtain high quantum yields. (The light curves for 

 different values of the parameter [COa]— c/. figures 28.1 to 28.5— indicate that the latter 

 should be without influence at such low light intensities.) 



A similar consideration applies to the role of temperature. It is known {cf. Figs. 

 28.6-28.8, and chapter 31) that at low light intensities, when the photochemical process 

 proper limits the over-all rate of photosynthesis, changes in temperature have no in- 

 fluence on the maximum quantum yield. Thi' observed effect of temperature contra- 

 dicted this experience. Now, it became likely that the effect of temperature was due to 

 its influence on the carbon dioxide uptake in the dark and its subsequent disengagement 

 in the light, and not on photosynthesis itself. 



