Absorption of Ions by Plant Roots 



EMANUEL EPSTEIN 



U. S. Departfnent of Agriculture, Bdtsville, Maryland 



HIGHER PLANTS are the main gateway through which the carbon and 

 mineral nutrients of the inorganic environment are made available to 

 the multitude of terrestrial life. Two principal organs are involved: the leaf, 

 by whose activities carbon dioxide is fixed from the air, and the root, which 

 absorbs water and mineral nutrients from the soil. Roots are, therefore, the 

 ion-transporting system par excellence, differing from such organs as the kidney 

 or the erythrocyte in that they may normally be exposed to and absorb ions 

 from an environment essentially or wholly inorganic. 



Two principal problems have engaged plant physiologists interested in the 

 phenomenon of ion absorption by roots: the energetics of the process, and the 

 chemical selectivity which characterizes it. A large body of evidence indicates 

 that in the absence of aerobic respiration there is little or no absorption of K, 

 NO3 and other ions normally absorbed at relatively high rates. Energy supplied 

 by aerobic respiration operates the ion absorption 'pump'. Under anaerobic 

 conditions, not only will the roots fail to absorb ions, even ions accumulated 

 prior to the period of anaerobiosis may be lost again; not merely absorption 

 but even retention of ions already absorbed requires a continuous e.xpenditure 

 of metabolites during respiration. 



The second major problem is that of selectivity. The cation and anion of a 

 salt may be absorbed at very unequal rates, and the same is true of ions of the 

 same sign, present in equal concentration. As in many other biological systems, 

 the difference between K and Na is conspicuous: most plant species absorb K 

 at a rapid rate, while Na may be largely excluded — hence the necessity for salt 

 blocks in pastures. 



EXPERIMENTAL APPROACHES 



Plant roots normally grow in soil — itself a highly complex system involving 

 rock and its weathering products, among which clay minerals play a prominent 

 role, as well as water, salts of diverse solubilities and their ions, and varying 

 amounts of organic matter and microbial life. Even this sketchy indication 

 of the complexity of the soil system will suffice to explain why simpler sub- 

 strates more amenable to accurate control are preferred in investigations of the 

 physiology of absorption. Solution cultures are the usual medium. They are 



