270 INTERMEDIATES IN REDUCTION OF COj CHAP. 10 



Table lO.III 

 Distribution of Ascorbic Acid in Leaves 



indophenol as a reagent to prove the presence of ascorbic acid in the 

 isolated chloroplastic material from different plants. He also found 

 that Elodea press juice contains 0.008% of dehydroascorhic acid. Neish's 

 figures (from 0.5-2.5% ascorbic acid in the chloroplast matter) are of 

 the same order of magnitude as those given by Giroud (1938) and 

 Moldtmann (1939) for the vitamin C concentration in different leaves. 

 (Giroud's data range from 0.05-3% of the dry weight of the leaves.) 

 The content varies greatly not only from species to species, but also 

 with the age of the leaves, the season of the year, and the time of the day. 

 The proportion of ascorbic acid in the chloroplasts is, by weight, 

 about equal to that of chlorophyll, and consequently several times larger 

 if expressed in moles per liter. 



If the results of Neish can be generalized and the ascorbic acid content of chloro- 

 plasts is, on the average, not higher than that of the surrounding cytoplasm, a new 

 explanation must be sought for the histochemical experiment which has most often 

 been quoted in support of the theory that ascorbic acid is accumulated in the chloro- 

 plasts. This is the blackening of chloroplasts by silver nitrate, a reaction discovered 

 by Mohsch (1918). It was attributed to the presence of different reducing agents: 

 Molisch suspected formaldehyde or hydrogen peroxide, Meyer (1918), hexenaldehyde, 

 and Wieler (1936), an essential oil; but Giroud, Ratsimamanga, and Leblond (1934) 

 found that the reduction of silver nitrate by the chloroplasts can also be observed in 

 an acid medium (e. g., 10% silver nitrate +1% acetic acid). This rules out a number 

 of previously suggested reducing agents, and Giroud suggested that ascorbic acid is 

 responsible for the reaction. He was opposed by MirimanofE (1938, 1939), who stated 

 that the reduction of silver nitrate in acid solution can also be caused by tannins, 

 flavonols, etc. (Fehling reagent is, in Mirimanoff's opinion, a better indicator for 

 ascorbic acid; and it is reduced by the cell sap rather than by the chloroplasts.) 



On the other hand, Dischendorfer (1937) found that silver nitrate in acid solution 

 is not reduced by aldehydes (e. g., chlorophyll b, hexenaldehyde, formaldehyde, furfurol, 

 glucose, and other sugars) which are (or can be) present in the chloroplasts, but only 

 by compounds with two OH- or NH2- groups in para or ortho position (e. g., hydro- 

 quinone, p-phenylenedi amine, or pyrogallol). In his opinion, ascorbic acid is the only 

 substance of this kind which is known to occur in the chloroplasts. 



The black silver deposit in the chloroplasts resulting from Mohsch's test often 

 shows a splotchy structure which is strongly reminiscent of the granular structure of the 

 intact chloroplasts (c/. Chapter 14). Weber (19372) and Pekarek (1938) beUeved that 



