ASCORBIC ACID IN THE CHLOROPLASTS 269 



converted in the dark into citric acid (rather than vice versa), as if the 

 Krebs-Martius-Knoop cycle were running in reverse. Thus, Mikhlin 

 and Bakh (1938) found that tobacco leaves convert externally supplied 

 pyruvic, malic, and oxalacetic acids into citric acid. Pucher, Wakeman, 

 and Vickery (1937) added the observation that, in tobacco leaves, mahc 

 acid disappears in darkness and is replaced by an equivalent quantity of 

 citric acid. 



It is by no means certain that all glucose respiration in plants proceeds through a 

 triose stage. To the contrary, evidence has been obtained of glucose oxidation processes 

 which begin by direct oxidation to glucuronic acids (c/., for example, Mliller 1928, 

 Boysen-Jensen 1931, and Harrison 1933). Recently, this mechanism was also discussed 

 by Emerson, Stauffer and Umbreit (1944). It is possible that the formation of citric 

 acid in plants occurs by a similar mechanism; at least this assumption has been found 

 useful in the discussion of the way in which citric acids is produced from sugars by 

 certain moulds, Aspergillus niger, for example (c/. Butkevich 1925; Butkevich and 

 Gaevskaja 1935; Barinova and Butkevich 1936; Wells, Moyer and May 1936). AUsop 

 (1937) suggested that oxaUc acid, too, is formed by moulds by the way of uronic acids, 

 and not by the intermediary of Cj compounds. 



E. Ascorbic Acid in Green Plants* 

 1. Ascorbic Acid in the Chloroplasts 



The function of ascorbic acid (vitamin C) in plants is unknown, but 

 its formula, which shows it to be a dehydrogenation product of hexoses, 

 indicates the possibility of its being an intermediate product of either 

 photosynthesis or respiration. On the other hand, the capacity of 

 ascorbic acid for reversible oxidation-reduction may assign to it the 

 part of an oxidation-reduction catalyst, rather than that of an intermediate. 



Ascorbic acid was the subject of very extensive research, and only a 

 few results can be communicated here. (For more ample information, 

 we may refer to the monograph by Giroud 1938.) The compound is 

 present in the green parts of all plants, and in many colorless plant 

 tissues as well. Giroud, Ratsimamanga and Leblond (1934) and Giroud, 

 Leblond and Ratsimamanga (1934) found a significant parallehsm be- 

 tween the concentration of ascorbic acid and that of chlorophyll in 

 different plants; this conclusion was later contested by Mirimanoff (1938, 

 1939), who suggested an association of ascorbic acid with flavonols in 

 the cell sap; but Giroud's statistical relationships were confirmed by 

 other investigators, e. g., Reid (1938) and Moldtmann (1939). However, 

 Neish's (1939) direct determinations of the ascorbic acid content of 

 separated chloroplast matter (c/. Chapter 14) did not show much differ- 

 ence between its concentration in the chloroplasts and in the leaves as a 

 whole, as shown by table 10. III. Bukatsch (1940) used dichlorophenol- 



* Bibliography, p. 279. 



