268 Mineral Nutrition of Plants 



proximately 1:1. Figure 4 shows that as iron and, simultaneously, phos- 

 phorus increase on the roots, movement of phosphorus into the top is 

 impaired. This, with the foregoing experiment, furnishes substantial 

 evidence that precipitation reactions may occur on roots which may in- 

 fluence the ready entrance of iron and phosphorus into the root, and, 

 therefore, the plant as a whole. 



4 th. leof 

 3rd. leaf 

 2 nd. leaf 



st. leaves 



: e/g.EM. 

 Roots 



1.0 1.0 1.0 0.5 mg.Fe/g. D.M. 



Iron concentration of nutrient media below: 

 I.Op.p.m.Fe O.lp.p.m.Fe O.OIp.p.m.Fe 0.002 p. p.m. Fe 



Figure 3. The amount of radioiron moving into the various leaves on 

 four bean plants each grown at different iron concentrations and having dif- 

 ferent amounts of iron associated with the roots. 



The most direct evidence that mineral nutrients may concentrate in 

 veins and become immobilized has been attained for iron. Fe 5 ° auto- 

 radiographs on no-screen X-ray film furnish a particularly good medium 

 with which to show this phenomenon. Autoradiography were prepared 

 of bean plants grown at (a) pH 4.0 and medium phosphorus (0.0001M), 

 (b) pH 7.0 and medium phosphorus (0.0001M), and (c) pH 7.0 and 

 high phosphorus (0.001M). Condition a results in healthy green plants. 

 Condition b yields plants with chlorotic mesophyll, later becoming 

 generally chlorotic. (The autoradiographs were made at the time that 

 the tissue adjacent to the veins developed chlorosis). Condition c results 

 in general chlorosis throughout the leaf (Figure 5). 



The distribution of Fe r ' 5 in leaves under the above conditions shows: 

 rapid entry and equal distribution throughout at pH 4.0 and medium 



