378 CHLOROPLASTS AND CHROMOPLASTS CHAP. 14 



be increased by treatment with catalyst poisons (hydrocyanic acid or 

 sulfur dioxide) ; the simultaneous increase in the intensity of the hema- 

 toxylin staining of the chloroplasts indicated that these agents act on 

 chloroplastic iron. (However, Noack's suggestion that only chloroplastic 

 iron is affected by sulfur dioxide was proved to be incorrect by Wieler's 

 experiments on variegated leaves.) The mechanism of this "unveiling" 

 (a term used by Wieler) of iron in the leaves is unknown, but it must 

 consist in the decomposition of complex organic iron compounds. Ac- 

 cording to Wieler, insoluble "organic" sulfur is also transformed into a 

 soluble "inorganic" form simultaneously with the "unveiling" of iron. 

 Mineral acids and — more slowly — organic acids (e. g., glacial acetic acid 

 or concentrated oxalic acid) also bring organic iron in the leaves into a 

 soluble form. 



According to Wieler, the inorganic iron hberated by acids is in the 

 divalent state when sulfur dioxide, nitric acid, or tartaric acid is used, 

 and in the trivalent state in the case of other acids (sulfuric, hydrochloric, 

 phosphoric, and acetic). In certain leaves, hydrochloric acid liberates 

 both ferric and ferrous iron. 



According to Noack (1930) and Griessmeyer (1930), the proportion 

 of noncomplex, soluble iron in barley leaves can be increased from 6 to 

 12% by sulfur dioxide and to 10% by cyanide. The same Hmit (12%) 

 can be reached also by boiling the leaf powder. 



Hill and Lehmann (1941) found four times more iron in the chloro- 

 plasts of Claytonia than in the leaf as a whole. The molecular ratio of 

 iron : chlorophyll was between 1 : 4 and 1 : 10 in most plants. A large 

 part of chloroplast iron reacts immediately with 2,2'-bipyridine (it is 

 thus present in the form of free ferrous ions) ; another part reacts only 

 after boiling with acids, and still another only after ashing. Noack and 

 Liebich (1941) have determined iron in separated chloroplasts, and in the 

 chloroplastic matter (coagulated by ammonium sulfate or precipitated by 

 centrifugation from a suspension of ground spinach leaves). The 

 chloroplasts were found to contain as much as 82% of all leaf iron (0.05% 

 of dry weight, in agreement with Neish's data in table 14. VI); the 

 cytoplasm 5%; and the water-soluble fraction 13%. The nuclear matter 

 and the cell walls were iron-free. About 18% of iron in spinach chloro- 

 plasts was water-soluble, 32% could be extracted by 0.01 molar hydro- 

 chloric acid (this was probably bound to phosphorus-free proteins), and 

 the remaining 60%, which was not extractable with 0.01 molar hydro- 

 chloric acid, was probably bound to nucleic acid or to phosphorylated 

 proteins. The latter fraction, which must include also the hemin iron 

 of different enzymes, proved to be more resistant to iron starvation than 

 the more loosely bound 40%. The soluble iron is divalent and the in- 

 soluble trivalent; the lipide fraction contains no iron. These results do 



