950 CONCENTRATION FACTORS CHAP, 27 



lower the beginning of the transitional range from 11 to 2 kerg/cm.^ sec. 



In discussing the yield of photosynthesis in relation to the factor [CO2], 

 we repeatedly encountered effects attributable to the exhaustion of the 

 molecular species CO2 in the immediate neighborhood of the plants; this 

 disturbance could be reduced (a) by buffering with bicarbonate, (6) by 

 using unicellular algae, to increase the surface of gas exchange and (c) by 

 stirring. With hydrogen, the solubility of the gas in water is much 

 smaller than that of carbon dioxide, and no buffering is possible; there- 

 fore, exhaustion effects can be expected to occur even more easily, and 

 could perhaps not be fully avoided even with unicellular organisms, such 

 as purple bacteria. Intense stirring is the only help available; the com- 

 paratively rapid diffusion of hydrogen in water ma}^ help to maintain uni- 

 form distribution. Wassink and co-workers (1942) have observed that, 

 in the presence of 15% Ha in the gas phase, the fluorescence of Chromatium 

 in strong light soon rapidly increased by 25-20% (about one minute) 

 after the cessation of shaking (fig. 28.33 would make even a larger increase 

 easily understandable) . 



According to the Dutch authors, the influence of [CO2] on the fluores- 

 cence of bacteria disappears if the supply of reductants is stopped (fig. 

 28.29). Inversely, however, the effect of reductants on fluorescence re- 

 mains considerable even if carbon dioxide is withheld {cf. fig. 28.35). 

 Wassink and co-workers interpreted this difference as confirmation of their 

 general concept that the reductants participate directly as "energy accep- 

 tors" in the photochemical process, while carbon dioxide does not react 

 with the primary photoproduct at all. Franck and Herzfeld, on the other 

 hand, assumed direct participation of carbon dioxide (in the form of a com- 

 plex, ACO2) in the primary photoprocess and quoted the effect of carbon 

 dioxide removal on fluorescence as evidence of this participation (cf. 

 above, page 941), while ascribing the (much stronger) effect of the ab- 

 sence of reductants to the indirect mechanism of "self-narcotization" in 

 consequence of accumulation of unreduced "photoperoxides." Later 

 (cf. page 942), Franck suggested that the effect of CO2 deficiency also is 

 due mainly to narcotization (caused by the products of photoxidation). 



Without using the concept of narcotization, we can explain the effects 

 of reductants on the basis of the picture of a "photocomplex," X- Chi -HZ, 

 which undergoes primary photochemical conversion to HX-Chl-Z, fol- 

 lowed by dark reactions that transfer H from X to CO2, and supply H 

 to Z from H2O (or from one of the "substitute" reductants, such as H2 or 

 H2S2O3) . While CO2 starvation may lead to the accumulation of the photo- 

 complex in the "reduced" form, such as HX-Chl-HZ, the absence of re- 

 ductants may lead to the accumulation of the "oxidized" form, X-Chl-Z; 

 when both CO2 and the reductants are deficient, the tautomeric form, HX • - 



