IV. BIOCHEMICAL SYSTEMS 213 



has been attributed to ascorbic acid. Improved tests have shown, however, 

 that the reduction of the silver reagent may occur in the colorless stroma 

 of the plastid, but it is extremely doul^tful if the reaction can l)e taken as 

 being specific for the presence of ascorbic acid. As an alternative explana- 

 tion of the reaction it has been claimed that the reduction of AgNOs by 

 green tissue is in effect a photolysis of AgNOs in which chlorophyll acts as 

 an optical sensitizer, irrespective of the presence of ascorbic acid.'® 



It also seems clear that chlorophyll is not necessary for the synthesis of 

 ascorbic acid, and any apparent relationship between the formation of ascor- 

 bic acid and of chlorophyll is prol)ably the result of conditions which favor 

 the development of both substances. Ascorbic acid may accumulate in 

 plant tissue independently of the formation of chlorophyll; e.g., the corolla 

 lacks chlorophyll but contains more ascorbic acid than the calyx." The 

 best e\adence, however, that ascorbic acid may be synthesized in the ab- 

 sence of chlorophyll is the fact that seeds germinated in complete darkness 

 produce ascorbic acid but not chlorophyll. 



2. Formation from Sugars 



Interpretation of studies on the synthesis of ascorbic acid is complicated 

 by the fact that an increase in the concentration of the vitamin in the cell 

 may, as far as our present knowledge indicates, arise either (a) by an in- 

 crease in the rate at which it is synthesized or (b) by a decrease either in the 

 rate at which it is oxidized to dehydroascorbic acid or in the rate at which 

 the latter is irreversibly converted into 2,3-diketogulonic acid. It is usually 

 extremely difficult to decide between the two possibilities. In other words, 

 the presence of ascorbic acid in higher concentration in some tissues than 

 in others may be related to conditions favoring stability rather than to 

 those favoring synthesis; in fact its presence in a cell does not necessarily 

 mean that it was synthesized there at all. 



The mechanism of the synthesis of ascorbic acid, whether in plants or in 

 animals, is not understood. Studies on this subject have been concerned 

 with supplying possil)le precursors to the plant or animal and to ascertain 

 the effect of these substances on the synthesis. Since seeds are devoid of 

 ascorbic acid, but produce it during germination, this phase of plant life 

 has been examined by many workers as offering the best material for study. 

 Seedlings grown in either water or inorganic mineral cultures form ascorbic 

 acid in conditions in which photosynthesis cannot occur. Under such condi- 

 tions the synthesis must take place at the expense of a carbon soinx'c de- 

 rived from the reserves of the seed. Excised pea embryos grown on sterile 

 synthetic media form ascorbic acid if supplied with certain hexose sugars. 

 Of these, fructose, mannose, glucose, and galactose have been found to l)e 



'« R. J. Goutheret, ThSse Sciences, Paris (1935). 

 " M. E. Reid, Am. J. Bot. 24, 445 (1937). 



