INFLUENCE OF DIFFERENT FACTORS ON PIGMENT FORMATION 429 



unable to confirm these conclusions. Hill and Lehmann (1941) found 

 that changes in the chlorophyll content of leaves with the season of the 

 year are preceded by changes in their content of iron. Boresch (1913, 

 1921) observed that iron deficiency also has a chlorotic effect on Cyano- 

 phyceae, affecting both the chlorophyll and the phycocyanin contents. 



Other heavy metals. — The importance of manganese for the forma- 

 tion of chlorophyll was asserted by McHargue (1922), Bishop (1928), 

 and Ulvin (1934). Pirson (1937), on the other hand, observed no chloro- 

 sis in Chlorella cells grown in the absence of manganese, even though the 

 photosynthesis of these cells was inhibited (cf. page 338). Hoffman 

 (1942) found a stimulating effect of uranium on the greening of seeds. 



(b) Oxygen, Water, and Sugars 



The role of oxygen in greening has been investigated by Gortikova 

 and Lubimenko (1934) and Gortikova and Sapozhnikov (1939), who 

 based their studies on Lubimenko's theory of greening as an oxidation 

 process (cf. page 431). They found that the greening of etiolated wheat 

 plantules does not occur in absence of an oxidant, but that molecular 

 oxygen can be replaced, for example, by 2,6-dichlorophenol-indophenol. 

 Gortikova and Sapozhnikov (1940^) have studied the influence of water 

 on this process. 



With the oxidation theory of Lubimenko as a guide, Gortikova and 



Sapozhnikov (19400 also have investigated the effect of sugars on greening. 



They observed a retardation of greening by some of these compounds 



and attributed it to their reducing power. However, this hypothesis 



does not explain why glucose and fructose had almost no effect, while 



sugars not usually present in the plant, e. g., galactose, arabinose, and 



particularly mannose, produced a strong inhibition, as illustrated by 



table 15.x. 



Table 15.X 



Effect of Sugars on Greening 

 (milligrams chlorophyll synthesized within a given period of time) 



Sugar concentration, % 0.1 0.5 3 



Fructose 100 98 99 98 



Mannose 168 100 62 12 



(c) Light and Heat 



Seedlings of the higher plants, sprouted in darkness, remain colorless, 

 "etiolated," but begin to turn green immediately upon being brought 

 into hght. This phenomenon has been much studied, and there is no 

 doubt that the formation of chlorophyll in etiolated plants is a photo- 

 chemical reaction. However, as early as 1885, Schimper discovered 



