1130 THE LIGHT FACTOK. II. QUANTUM YIELD CHAP. 29 



Table 29.X 



Quantum Yield of Oxygen Evolution by Illuminated Chloroplast Suspensions 

 FROM Spinach (After French and Rabideau 1945) 



0.15 9.0 0.72 0.068 15 10 



0.15 3.15 0.71 0.0()4 16 5 



0.34 3.25 0.83 0.030 33 20 



0.39 3.3 0.88 0.022 46 20 



0.10 3.32 0.57 0.080 12 5 



0.10 3.04 0.51 0.037 27 20 



0.0G3 4.13 0.56 0.033 31 20 



0.103 4.18 0.545 0.013 78 20 



0.09 1.6 0.33 0.058 17 20 



0.045 2.8 0.47 0.031 38 5 



0.090 2.8 0.79 0.03G 28.0 5 



cantia (in the same set-up, a value of 7 = 0.092 was found for live Chlorella). 

 No clear change of 7 with light intensity was noticeable in the range used 

 (1400-6000 erg/cm. 2 sec, of which 33-72% was absorbed by the suspen- 

 sion). 



These quantum yields of the Hill reaction, although markedly lower 

 than the quantum yield of photosynthe-sis in vivo, were nearer the latter 

 than the (much higher) yields of the chlorophyll-sensitized photoxidations 

 in vivo {cf. Vol. I, page 513, and chapter 35). 



The considerable variability of the 7 values of the Hill reaction may be 

 caused, at least in part, by rapid deterioration of the material. Several 

 runs, made in succession with one batch, usually showed rapidly declining 

 yields. 



New measurements of the quantum yield of the Hill reaction, in whole 

 Chlorella cells and in chloroplasts from Phytolacca americana, were carried 

 out by Ehrmantraut (1951). They were made relative to the ethyl chloro- 

 phyllide-thiorea actinometer; two measurements of the quantum require- 

 ment of photosynthesis in carbonate buffer served as an additional, if 

 rough, check on the actinometer, since there is general agreement that this 

 requirement is I/70 = 10 =±= 2. The results (table 29. XI) are much more 

 consistent than those of French and Rabideau. They were further con- 

 firmed by five quantum yield measurements with quinone in whole Chlo- 

 rella cells using a monochromator (X 669 m^t) and a bolometer which gave, 

 for 1/7, the values: 10.2; 9.9; 9.3; 10.8; and 12.0 (average: 10.4). 



These values, obtained in acid medium (pH 6.5), throw indirect light 

 on the problem of the quantum yield of photosynthesis. Various kinetic 

 evidence points to the Hill reaction having both the primary photochemical 

 process and the rate-limiting dark reaction, in common with photosynthe- 



