NEW FLASHING LIGHT EXPERIMENTS 1475 



results (including the temperature dependence of the flash yield); but, 

 quantitatively, this interpretation encounters the same difficulty as was 

 mentioned above in discussing Tamij^a's own theory: as a general rule, 

 one would expect an alternative "ceiling" on the maximum rate to become 

 effective only if it is lower than the usual one {cf. fig. 34.13) ; while the Wel- 

 ler-Franck theory can account for Tamiya's findings only if one credits 

 Tamiya's cells with a much higher than usual content of the (normally 

 limiting) "finishing" enzyme (Eb), and a content of active "preparatory" 

 enzyme (Ea?) just sufficient to give the same maximum rate in constant 

 Ught as is usually permitted by Eb (since Tamiya's algae showed, as men- 

 tioned above, a normal rate of photosynthesis in saturating steady light). 



Another, as yet vague, explanation of Tamiya's results would be to relate them to the 

 "oxygen bursts" described in Chapter 33, and suggest that perhaps with dark intervals 

 of > 0.1 sec. (needed to exceed the Emerson- Arnold maximum flash yield) a large part 

 of the respiration intermediates, accumulated during the dark periods, is reduced in 

 light, producing an oxygen burst and increasing the apparent yield of photosynthesis. 

 In other words, one could suggest that light flashes efi"ectively inhibit respiration during 

 the whole period of exposure to flashing illumination. However, no such effect was 

 noticed by Brown (1953) in mass spectrographic study of respiration in light {cf. chapter 

 37D). Also, one does not see offhand why the same effect should not have been pro- 

 duced also by instantaneous discharge flashes (after dark intervals of the same length). 

 Refuge could be taken to the great variability which "transients" show in dependence 

 on the metabolic history and nature of the cells ; but the effects of dark intervals of the 

 order of one second on flash yield crop up somewhat too regularly for such an explana- 

 tion. 



We can refer in this connection also to the findings of Gilmour et al. on 

 chloroplast suspensions, and their kinetic interpretation in terms of a 

 "reservoir" in which energy-rich photoproducts can be stored, to be re- 

 leased for "finishing" after the flash. These authors suggested a mecha- 

 nism of the filling of the reservoir which makes it ineffective in instantaneous 

 flashes, but operative in flashes of the same integral energy but longer dura- 

 tion {cf. section 7 below). 



An even more radical departure from the conclusions reached in the earlier flashing 

 light experiments was suggested by Burk and coworkers (1951, 1952, 1953). They as- 

 serted that not only Emerson and Arnold, but Tamiya and Chiba as well, have never 

 even remotely approached flash saturation. The reaction mechanism postulated by 

 Burk, Cornfield and Schwartz (1951, 1952) was in principle similar to that of Franck (or 

 Tamiya): competition between a back reaction and a forward reaction for the primary 

 photoproducts. Burk associated the competing back reaction with the "energy dismu- 

 tation" mechanism. (This concept was first described in chapters 7 and 9, pp. 164 and 

 233, and later used by van der Veen, and by Burk and Warburg; the latter called it the 

 "cyclic reaction," or the "one-quantum mechanism" of photosynthesis.) According to 

 this concept, exothermal back reactions (such as reaction 28.41e on p. 1036, cf. scheme 

 28.11) are used in nature to store chemical energy for subsequent use as supplement to a 

 quantum of light in the reduction of carbon dioxide. This special purpose of the back 



