1470 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



ments. Fig. 34.22 shows the dependence of the yield on flash energy at 

 saturating dark intervals; the "limiting yield" of Emerson and Arnold is 

 indicated by a horizontal dashed line. 



(3) The maximum yield per saturating flash depended on temperature 

 (compare fig. 34.22 with fig. 34.14). 



Tamiya and Chiba attributed the difference between their results and 

 those of Emerson and Arnold to the use of higher flash energies, and sub- 

 mitted that the light emission of condensor discharges, lasting only 10~^ 

 second, was insufficient for saturation. They pointed out that their own 

 results at low flash energies gave a picture similar to that of the earher 

 observers (fig. 34.23), with a saturation level independent of temperature 

 and reached after dark intervals of < 0.02 sec. at 25° C. and < 0.06 sec. 

 at 7° C. However, the maximum yield in fig. 34.23 is only about one quar- 

 ter that of Emerson and Arnold; with flash energies suflficient to equal 

 the latter (about 100 lux sec), Tamiya's curves show a strong dependence 

 of saturation on temperature. Furthermore, their explanation does not 

 apply to the observations of Weller and Franck, who used flashes, produced 

 by a 1000 watt Hg lamp and rotating sector, with energies up to 450 lux 

 sec, and nevertheless found (in agreement with Emerson and Arnold) no 

 dependence of the maximum flash yield on temperature (fig. 34.14). 



Tamiya (1949) suggested an interpretation of his experiments by a 

 kinetic scheme in which the effect on the flash yield of back reactions in the 

 primary photochemical apparatus was a function of temperature. This 

 required no change in the mechanism of fight saturation postulated by 

 Franck, but merely a change in the relative values of the several rate con- 

 stants. Both Franck's and Tamiya's picture can be illustrated by scheme 

 28.11 (p. 1037) and the reaction sequence (28.41, a-e). (It is irrelevant 

 for the kinetics whether the stabilizing reaction operates on the first reduc- 

 tion product of carbon dioxide, AHCO2 in scheme 28.11, or the first oxida- 

 tion product of water, AHO in the same scheme, or on some intermediate 

 oxidation-reduction system.) Franck, in order to account for the inde- 

 pendence of the maximum flash yield of temperature (observed by Emer- 

 son and Arnold, and confirmed by Weller and Franck), suggested (in es- 

 sence) that the relative values of k' (the rate constant of the back reaction 

 in scheme 28.11), k^ (rate constant of the photoproduct-enzyme complex 

 formation), and k[ (the rate constant of the transformation of this complex 

 and reactivation of the enzyme Eb) are such that all products of the flash 

 for which molecules of the enzyme Eb are available, are combined with 

 this enzyme before they are lost by back reactions (A;' <^ keE\); but that 

 by the time this first "batch" of photoproducts had been transformed, back 

 reactions have taken care of all the remaining flash products, and no "second 

 round" of the stabihzing reaction is possible {k' <C k'^. With this assump- 



