1534 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



Franck (1945) determined small amounts of oxygen by the quenching of phos- 

 phorescence of dyes adsorbed on silica gel (page 851 ). Figure 35.8 shows the time course 

 of oxygen liberation by a chloroplast preparation, both in the absence and in the presence 

 of carbon dioxide. The oxygen liberation reaches a maximum immediately after the be- 

 gmning of illumination; after about 0.5 minute, it begins to decline. After a few min- 

 utes of this decline, a steady level is reached, and the oxygen liberation remains at this 



O 



X 



E 

 E 



UJ 



tr 



(/) 

 if) 

 UJ 



cc 

 a. 



3 4 5 



TIME, nninutes 



I 2 3 4 5 6 1 



TIME, minutes 



Fig. 35.8. Oxygen liberation by chloroplasts in light without added oxidant 

 (after Franck 1945). Oxygen concentration in nitrogen, after passage through 

 reaction vessel, determined by phosphorescence quenching method. Curves a, 

 with CO2; curves 6, without CO2. 



level for an hour or more. Even the initial rate of oxygen liberation is only 1.5% of the 

 rate of photosynthesis in vivo; the final, constant rate was only 0.3% of the latter. 

 The maximum rate depended strongly on the way the sample was prepared, the age of 

 the leaf, and other conditions. 



The pairs of curves compared in fig. 35.8 (ai,a2 and ^1,62) were obtained with aliquot 

 parts of the same chloroplast preparation, and the conditions differed only in the compo- 

 sition of the gas phase (pure N2 in 61,62, N2 + 20 mm. CO2 in 01,02). C02had no effect 



