1086 



THE LIGHT FACTOR. II. QUANTUM YIELD 



CHAP. 29 



generally requires an interpolation, illustrated by figure 29.1. The uncer- 

 tainty caused by this interpolation is unimportant for extended illumina- 

 tion periods, but can markedly affect the results obtained in short experi- 

 ments, for example, experiments of 5 or 10 minutes. Furthermore, short 

 illumination periods increase the importance of induction phenomena. 

 Warburg knew from his earlier work (c/. chapter 33) that an "induction 

 loss" {i. e., delayed onset of photosynthetic activity) can occiir after dark 

 periods of the order of several minutes, but he also knew that this effect 

 disappears if the light intensity is reduced considerably below the satura- 

 tion region {cf. fig. 33.8). Since in the quantum yield work very low 



10 



25 



30 



15 20 



TIME, mm. 



Fig. 29.1. Manometric determination of quantum jdeld (after Rieke 

 1939). Solid lines represent assumed course of photosynthesis and respira- 

 tion; dotted curve, the pressure changes read from manometer. KH is 

 interpolated yield of photosynthesis in 10 min. 



light intensities were used, Warburg assumed that induction can be neg- 

 lected. Subsequently, Emerson and Lewis (1939, 1941) found evidence of 

 an induction phenomenon of a different kind, consisting in photochemical 

 liberation of carbon dioxide during the first few minutes of illumination. 

 This carbon dioxide "gush" or "burst" does not disappear with decreasing 

 light intensity, and could be of particular importance in measurements in 

 very low light. 



An explanation of the carbon dioxide burst, suggested by Franck (1942), already 

 was described in Volume I (page 167). This theory suggests that the "burst" is caused 

 by the decomposition of the carbon dioxide-acceptor complex, ACO2, accumulated in 

 the dark. This decomposition follows the photochemical reduction of ACO2 to AHCO2, 

 and the reversal of this reduction by back reactions, e. g., in Franck's notation: 



,^^ «, , • .-.^ TT^, ,, forward reaction back reaction 



(29.2) lACOo + HChll :r^ >|AHC02 + Ch]} > 



light 



CO2 liberation „ 



HChl + AGO* } > HChl + CO2 



