266 INTERMEDIATES IN REDUCTION OF COj CHAP. 10 



liberation of this gas. Since the carbon dioxide liberation is larger than 

 ordinary "dark" respiration, an additional photoxidation of accumulated 

 acids is clearly indicated by these observations (cf. Chapter 19). Another 

 argument in favor of deacidification in light being an oxidative process 

 is the observation of Kraus (1873) and Richards (1915) that it requires 

 the presence of oxygen. 



On the whole, evidence favors a primary oxidation or photoxidation 

 of accumulated acids in succulents, rather than a direct photochemical 

 reduction of these acids to carbohydrates; but the issue is not settled. 



Another complication arises from the fact that deacidification is not 

 necessarily a photochemical effect. De Fries (1884) found that, after 

 eight or ten hours of acid accumulation, deacidification begins even if 

 the plants remain in darkness. The decrease in acidity in artificially 

 prolonged darkness was confirmed by Bennet-Clark (1933, 1934) and 

 Thoday and Jones (1939). We do not know whether the "dark" 

 deacidification also leads to a resynthesis of carbohydrates, or whether 

 it is a purely oxidative process. From the point of view of the theories 

 which assume that all reduction steps in photosynthesis between {CO2} 

 and {CO2O} must be photochemical (cf. Franck and Herzf eld's scheme, 

 7.VA), a "dark" conversion of mafic or citric acid into carbohydrates 

 appears impossible. The reduction levels of these acids are less than 

 unity, i. e., they cannot be converted into carbohydrates without a supply 

 of energy. However, we have also discussed, in chapter 7, reaction 

 schemes in which only the first step in the reduction of carbon dioxide 

 utilized light energy, while the energy required for the subsequent 

 reduction steps was supplied by dismutations. Thus, malic and citric 

 acid could be reduced to carbohydrates without the help of light, if one 

 part of them were simultaneously oxidized. Such an enzymatic dismu- 

 tation was deemed probable by Bennet-Clark (1933), and is supported 

 by the fact that the respiratory quotient of succulents during dark 

 deacidification is often much higher than 1.33, the value corresponding 

 to the combustion of malic acid (Wolf 1939). (For pure dismutation, 

 this coefficient should be infinity.) Other experimental facts can be 

 quoted in connection with this discussion. It was mentioned on page 

 262 that in experiments on starch production by algae in the dark, the 

 rule that only substances with L > 1 can be utiHzed for this purpose 

 was found to allow of some exceptions. Bokorny (1897) listed succinic, 

 citric, and tartaric acid (L = 0.875, 0.75, and 0.625, respectively) as 

 acceptable foods. Treboux (1903) found that, while succinates, malates, 

 and tartrates are ineffective, citric acid (L = 0.75) is utihzed by the 

 algae; this was confirmed by Zumstein (1899). Similarly, Lwoff (1932) 

 and Lwoff and Dusi (1935) found pyruvate (L = 0.833) to be a satisfac- 

 tory source of carbon for the dark growth of some species of Flagellata. 



