PLANT ACIDS IN NONSUCCULENTS 267 



All this makes plausible the theory that the dark deacidification of 

 succulents is a dismutation into carbon dioxide and carbohydrates (thus 

 lending indirect support to the "dismutation theory" of photosynthesis). 

 The deacidification in light, too, may be a complex process, involving 

 both photoxidation to carbon dioxide (with the latter becoming available 

 for photosynthesis) and the formation of carbohydrates by reduction or 

 dismutation of the acids without the substrate's passing through the 

 stage of free carbon dioxide. It may be mentioned here that Spoehr 

 (1913) and Volmar (1923) found that some formaldehyde is produced, 

 together with formic acid and carbon dioxide, when oxalic, malic, or 

 succinic acid is photoxidized in ultraviolet light in vitro. 



Wolf (1931) suggested that the mechanism of deacidification in light 

 is the same as in the dark, with light merely accelerating it by assisting 

 in the removal of carbon dioxide. However, the irreversibility of the 

 oxidation process argues against ascribing a retarding effect to the 

 accumulated oxidation products. 



3. Plant Acids in Nonsucculents 



We have seen in the preceding section that a direct participation of 

 malic and citric acid in the photosynthesis of succulents appears possible, 

 but is by no means certain. The existence of a relationship between the 

 metabolism of malic and citric acid and photosynthesis in nonsucculents 

 is even less clear. It was mentioned above, that Ruhland and Wetzel 

 (1929) attributed the acid formation in "oxalate plants" to the de- 

 amination of amino acids (rather than to an oxidation or fermentation 

 of carbohj'drates). In confirmation of this point of view, they mentioned 

 that an equivalent quantity of ammonia is liberated simultaneously with 

 the formation of malic acid. However, Pucher, Wakeman and Vickery 

 (1938) found that rhubarb leaves can produce mahc acid from externally 

 supphed glucose; and Pucher, Clark, and Vickery (1937) denied the 

 existence of a paralleHsm between the liberation of ammonia and the 

 production of malic acid (or any other plant acid). 



Even if we assume that malic and citric acid in nonsucculents are 

 under all circumstances products of the carbohydrate metaboHsm, we do 

 not know whether they are regular respiration intermediates, or by- 

 products. Considerations as to the role of acids in plant respiration have 

 usually been adaptations of the more thoroughly investigated mechanism 

 of glucose oxidation in heterotrophic cells (muscle tissue, yeast cells) 

 which were not supported by direct experimental evidence. 



In the respiration cycle given in scheme 9. II, the conversion of 

 P3^ruvic acid into malic acid is accompanied by an evolution of carbon 

 dioxide. In the acidification of succulents, on the other hand, no carbon 

 dioxide is liberated (c/. Wolf 1931, and Bennet-Clark 1933). This has 



