72 PROBLEMS IN PHOTOSYNTHESIS 



d. H 9.2 {carbonate bujfer), compensated 



A'oj = 0.657 mm2 



i' = 5.4 /il quanta/min 



5 min / 

 1 5 min / + i' 

 1 5 min i 

 15 min / + i' 

 1 5 min i 



30 min / 



30 min / + i' 



xo, = hKo^ = 23.5 X 0.657 = 15.4 tx\ 

 1/95 = i't/xo, = 5.4 X 30/15.4 = 10.5 



Though these experiments were pubhshed in 1950, Bassham, Shibata and 

 Calvin (4) subsequently found that the quantum requirement of photo- 

 synthesis increases parallel with compensation. Franck (24), on theoretical 

 grounds only, arrived at the same conclusion. Five years after Warburg 

 and Burk had carried out their conclusive experiments, Emerson and Chalmers 

 (21) had to refute once again these erroneous views (see, however, the remarks 

 at the end of § 29). 



§ 29 The Significance of Carbon Dioxide Pressure 



The experiments described in the preceding paragraph clearly show that 

 the quantum requirement of photosynthesis in acid medium is 3 to 4, values 

 Warburg obtained nearly 40 years ago. It is of importance to note that in 

 alkaline medium (carbonate buffer) — the medium preferred by many other 

 investigators — much higher values are obtained, the quantum requirement 

 in alkaline medium being three times greater than in acid medium. In 

 order to explain this discrepancy, Warburg et al. (48) studied the influence 

 of the CO2 pressure in both media. They determined, in acid medium (pH 

 5) using the two-vessel method, the quantum yield at CO2 pressures varying 

 from 0.39 to 50 atm %. The maximum yield (0.342) was found at 5 atm % 

 CO2. At lower and higher pressures the quantum yield decreased (Fig. 29). 

 Similar behavior has been observed in alkaline medium (pH 8.8-9.5). 

 With the one-vessel method the quantum yields were measured at COo 

 pressures varying from 0.22 to 2 atm %. The highest quantum yield (0.304) 

 was obtained at 2.0 atm %. If the ip values calculated are inserted in the 

 curve of Figure 29, we see that — within the limits of experimental error — 

 the same curve is obtained as in acid medium with the two-vessel method. 



It is remarkable that the measurements in acid medium with the two-vessel 

 method and those in alkaline medium with the one-vessel m^ethod give the 

 same curve representing the dependency of the quantum yield on the CO2 

 pressure. This result is excellent evidence for the correctness of the two-vessel 

 method Warburg and Burk worked out several years ago. 



