684 PLANT GROWTH AND PLANT COMMUNITIES 



Contact decomposition of Fe^Os and Fe-diffusion 



The clear-cut relationship between iron uptake and the number of 

 action sites on the root surface indicates acquisition of iron in a con- 

 tact-microregion. Two questions immediately present themselves: By 

 what chemical mechanism does the root surface acquire iron? How is 

 the iron subsequently transported to the inner regions of the root? 



Although chelate excretion and round-trip diffusion of chelate 

 compounds is not a plausible cause in the present study, chelating 

 molecules might reside on the root surface, as postulated for algae by 

 Hutner (1947-48) and for yeast by Rothstein and Hayes (1956). After 

 acquisition of iron by contact with Fe-sand, the chelate molecule could 

 move inward as a mobile constituent of the cytoplasm, but in that case 

 the cytoplasm would have to penetrate the cell wall, reach the root 

 surface, and touch the Fe-sand, which is rather improbable. As an al- 

 ternative, the chelating compound at the root surface might be firmly 

 anchored in the non-living matrix of cellulose fibrils and pectic sub- 

 stances. If so, a specific reaction, presumably enzymatic, would have 

 to break the bond between lodged chelate and Fe, releasing the iron 

 ion into pore-space solution, from where it would have to reach the 

 cytoplasm without being reprecipitated. 



The CO2 mechanism, though ruled out for niobium (Vlamis and 

 Pearson, 1950), remains a possibility for iron solubility at localized 

 spots in the microregion of contact. 



A third alternative, and a very effective one, is provided by the 

 carboxyl groups of the interfibrillar pectic matrix that decompose 

 FeaOs by contact. Grunes ( Grunes and Jenny, 1960 ) has made a thor- 

 ough study of this decomposition process, using iron oxide and amber- 

 lites, and Charley (Charley and Jenny, 1960) has tested it with H- 

 roots. The acquired iron may migrate to the cytoplasm by exchange 

 diffusion ( Lopez-Gonzales and Jenny, 1959 ) . 



The two processes— decomposition of Fe203 and gel diffusion of 

 Fe— may be examined simultaneously. 



Plugs of H-roots. Twenty grams of fresh alfalfa roots were killed 

 with ether, leached with dilute HCl, and washed thoroughly with dis- 

 tilled water. In this process they lost, on a dry-weight basis (60° C), 

 0.545 gram of material, including 0.0159 g. of nitrogen. The remaining 

 H-root material ( oven dried ) weighed 0.747 g. and contained 3.84 per 

 cent nitrogen. Its exchange capacity, as determined by the Ca-acetate 

 method (Keller and Deuel, 1957), was 66.8 m. eq. per 100 g., oven 

 dried. In a hydraulic press at 42 kg/cm^, the diy roots (2.5 g. ) 

 were compacted into a dense plug or disc, having a diameter of 1.91 

 cm. and a height of 0.90 cm. A Incite cell was constructed (Figure 10) 

 which permitted study of iron transfer to roots from radioactive iron 



