1034 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



(as was assumed above), but also to the oxidation of certain metabolites 

 (sugars?) which produces a "narcotic" (a fatty acid?). The latter settles 

 on the chlorophyll complex, prevents any further acceleration of the pri- 

 mary photochemical reaction, and causes a strong increase in fluorescence. 



In the case of purple bacteria the following two alternative descriptions 

 of the rate-limiting and fluorescence-enhancing effect of a limited supply 

 of the reductants (H2, H2S2O3 . . . ) are possible : Either one considers this 

 supply as a preparatory reaction, whose slowness causes the photosensitive 

 complex to accumulate in a changed (more strongly fluorescent) form, 

 such as X-Chl-Z; alternatively, (using Franck's theory) one can treat 

 this supply as part of a finishing reaction ("disposal of photoperoxides"), 

 and explain its effect on fluorescence as a consequence of the production of 

 the "narcotic" by accumulated peroxides. In both cases, the light curve 

 of photosynthesis will approach, with increasing light intensity, a limit de- 

 termined by the maximum rate of supply of the reductants (either by dif- 

 fusion or by a preliminary enzymic transformation). 



Another argument against general attribution of the assimilation num- 

 bers of green plants to the rate-limiting influence of preparatory reactions 

 — specifically, those on the "carbon dioxide side" — was mentioned in Chap- 

 ter 12 (Vol. I) in connection with the influence of cyanide on photosynthesis. 

 It was stated there that the difference in the amounts of cyanide required 

 to reduce by a certain factor the rate of photosynthesis in different plant 

 species is most easily understood if one assumes that the cyanide-sensitive 

 catalyst (which, in all probability, is the "carboxylase" Ea) is not rate- 

 limiting in strong light in the absence of the poison, and becomes limiting 

 only when a considerable part of it is inactivated. If a different ratio 

 Ea/Eb prevails in various species and strains, the fraction of Ea that has 

 to be inactivated in order to make this catalyst "limiting" also must change 

 from case to case. 



What kind of back reactions can compete with finishing catalytic re- 

 actions in photosynthesis? In section d of this chapter, we considered the 

 "primary" back reaction, HX-Chl-Z -^ X- Chi -HZ, which can be called 

 "detautomerization" of the chlorophyll complex. In equations (28.20) and 

 (28.21), this reaction competes with the secondary photochemical forward 

 reaction, e. g., (28.20b) or (28.21b). We noted on page 1024 that this 

 competition can cause carbon dioxide saturation, but not light saturation, 

 because the proportion of quanta lost by this kind of back reaction is inde- 

 pendent of light intensity. If one would treat (28.20b) or (28.21b) as a 

 catalytic reaction, assigning to it a catalyst with the concentration and 

 working speed attributed above to "catalyst B," this would produce light 

 saturation, but the latter will again be associated with the accumulation of 

 the form X • Chi • Z or HX • Chi • HZ and thus with a change in the intensity 



