QUANTUM YIELD 1967 



and "dim," the level of [A], established in dim hght, will be too low to sup- 

 port the higher rate of photosynthesis during the bright half-cycles, and this, 

 too, may cause respiration intermediates to rush into the emptied photosyn- 

 thetic mechanism. In both cases — that of transition from dark to light, 

 and that of transition from dim to bright — a "priming" of the CO2 cycle 

 is necessary, and, while this "autophotocatalytic" adjustment is effected, 

 respiration intermediates are reduced with a relatively low quantum re- 

 quirement. This picture does not immediately explain why respiration 

 appears enormously increased in the first minute after the return into dark 

 (or dim) conditions; nor why a value approaching 1 is obtained for the 

 quantum requirement if the gas exchange at the beginning of the first 

 minute of "dim light" is subtracted from that at the beginning of the first 

 minute of "bright light" (cf. fig. 33. 6A). Details of Franck's interpreta- 

 tion of these transient phenomena have been given in chapter 33 (part C). 

 In essence, Franck assumes that the induction caused by shortage of A 

 does not affect AO2 in the same way as ACO2. In other words, the ratio 

 AO2/ACO2 is variable during the minute light-minute dark cycle (while 

 Warburg and Burk's calculations are based on the assumption of a constant 

 QpC^l). Franck thinks that the on-the-minute manometric readings lacked 

 the precision needed to assert the constancy of Qp, and that the curves of 

 Burk, Warburg et at. — from which the "one quantum process" was derived 

 ■ — are, for this reason, not rehable enough to calculate quantum yields for 

 fractions of a minute, even by averaging five or ten measurements. Doubts 

 on this account were expressed also by Brown (1953). 



A similar suggestion was made hy Schenk (1952), but rejected by Warburg (1952) 

 with reference to the constant value of Qp = 1 obtained by averaging the results of the 

 two-vessel experiments. 



Franck's interpretations do not cover the subsequently published papers 

 of Warburg et al. (1953, 1954). As stated before, these extremely inter- 

 esting papers bring the maximum yield controversy onto a new plane. In 

 the light of the past history, they require, first of all, independent experi- 

 mental confirmation — in respect to both the I/7 and the Qp values; this 

 will be awaited with the greatest interest. 



(e) Quantum Yield of Hill Reaction 



The quantum requirement of the Hill reaction (cf. chapter 29, section 4) 

 was again measured by Wayrynen (1952) and by Warburg (1952). Wayry- 

 nen used sugar beet chloroplasts with ferricyanide as oxidant and measured 

 the rates potentiometrically. A quantum requirement of 8.3 was calcu- 

 lated by extrapolation to zero light intensity (the light curves of Hill reac- 

 tion bend early, cf. chapter 35, page 1620). This value, which is in satis- 



