264 INTERMEDIATES IN REDUCTION OF CO2 CHAP. 10 



The role of malic and citric acid in the metabolism of plants is certainly 

 a more active one, since even in plants which accumulate large quantities 

 of these acids, their concentrations are subject to rapid fluctuations. They 

 are to be considered as intermediary metabolites, and not as excretions. 



Their precise metabolic function has not yet been definitely established, despite 

 the extensive studies of Ruhland, Wetzel, and coworkers in Germany (Ruhland and 

 Wetzel 1926; UUrich 1926; Ruhland and Wetzel 1927; Wetzel 1927; Ruhland and 

 Wetzel 1929; Bendrat 1929; Wetzel and Ruhland 1931; Wolf 1931; Schwartze 1932; 

 Ruhland and Wolf 1934, 1936; Wolf 1937, 19391'''), as well as of Vickery, Pucher, and 

 coworkers in America (tobacco leaves: Vickery and Pucher 1931, 1933'' ^ 1935; Pucher, 

 Wakeman, and Vickery 1937; Pucher, Vickery, and Wakeman 1938; Vickery, Pucher, 

 Wakeman, and Leavenworth 1937, 1938, 1939; Vickery and Pucher 1939; rhubarb 

 leaves: Pucher, Clark, and Vickery 1937i'^; Pucher, Wakeman, and Vickery 1938; 

 Vickery and Pucher 1939; buckwheat leaves: Pucher, Wakeman, and Vickery 1939) 

 and of Bennet-Clark in England (Bennet-Clark 1933, 1934; Bennet-Clark and Woodruff 

 1935). Cf. reviews by Ruhland and Wolf (1934, 1936), Bennet-Clark (1937), and 

 Vickery and Pucher (1940). 



There is considerable disagreement between these authors as to the interpretation 

 of many results. Ruhland and Wetzel suggested that, in the plants of the so-called 

 "acid type" (e. g., rhubarb and sorrel), both oxalic and mahc acid are formed by de- 

 amination of aminoacids rather than by oxidation of carbohydrates; but Vickery and 

 Pucher, as well as Bennet-Clark, opposed this view. They agreed, however, that funda- 

 mental differences exist between the acid metabohsm of succulents, that of "acid- type 

 plants," and that of other nonsucculents (of the latter, only tobacco and buckwheat 

 have been investigated in some detail) . 



2. Acidification of Succulents 



The question of the role of plant acids in photosynthesis arises most 

 acutely in the interpretation of the acid metabolism of succulents. Its 

 most striking characteristic is a diurnal rhythm. The accumulation of 

 acids in succulents during the night and their disappearance during the 

 day has attracted much attention since its discovery by Heyne in 1819. 

 A table compiled by Bennet-Clark (1933) shows that in some plants the 

 titratable acidity increases from evening to morning by as much as a 

 factor of 12, whereas in others the increase is only of a few per cent. 

 In some cases a nightly decrease in acidity was observed. (However, 

 titratable acidity is not an entirely adequate measure of the production 

 of plant acids, since other factors also may affect the pH of the sap.) 

 The daily fluctuation of acidity in most succulents is due mainly to the 

 formation and disappearance of malic acid, but in some species, citric 

 acid accounts for the largest part of the effect. However, even acids 

 present in a relatively small concentration, participate in the fluctuations 

 together with malic and citric acid (Wolf 1939). 



An explanation of the acidification cycle was suggested by Meyer in 

 1887. He pointed out that succulents, because of their relatively small 

 surface, may have difficulty in obtaining from the outside an adequate 



