PLANT RELATIONS 



A. Uptake 



The uptake by plants of ions from solution has been the subject of many in- 

 vestigations. There have been short-term experiments with adsorption periods of 

 minutes or hours and long-term experiments with adsorption periods of days, weeks, 

 and months. Short-term experiments are useful in studying mechanisms of the 

 initial steps, whereas longer-term experiments elucidate over-all effects and the 

 general distribution within the plant. 



Numerous mechanisms have been proposed for the uptake of ions by plant roots. 

 Most hypotheses state that the process of concentrating the ions within the plant root 

 is a metabolic function. Essential in most of these mechanisms is a biological com- 

 pound that serves as a carrier (20). Evidence has been obtained that calcium, stron- 

 tium, and barium compete for an identical carrier, whereas potassium, rubidium, 

 and cesium compete for a different carrier (20, 21, 28). Hydrogen appears to compete 

 with all ions (27). 



In addition to the accumulation of ions within the root, there is adsorption of 

 ions on the root. The CEC of roots can be increased by nitrogen fertilization (121). 

 A linear correlation was observed between the CEC of different species and the uptake 

 of strontium- 90 (75). The exchange adsorption does not appear to be controlled di- 

 rectly by metabolism (19, 45) and has been considered by some investigators (50) to 

 be independent of active transport. 



The point of maximum uptake of strontium and iodine appears to be within a 

 few mm of the root apex (45), and no enhanced uptake is observed with root hairs. 

 Other work (140) indicates that the tips of barley roots absorb various ions readily 

 but that the greatest translocation occurs from a region 30 mm above the tip. 



It has been suggested that strontium can partially substitute for calcium (133, 

 137) and even that strontium is an essential element (141). Sixty to 70 per cent of the 

 strontium in the plant has been found to be water-soluble, whereas 97 per cent of the 

 cesium and only 16 per cent of the cerium-144 was water-soluble (83). 



A possible error in short-term experiments is the exchange of the radioactive 

 isotope for the stable isotope already in the plant. This is particularly true for ions 

 of slow turnover rate, such as calcium. Some workers (73, 133) consider the first 

 24 hours of calcium uptake to be largely nonmetabolic exchange. 



B. Translocation 



Most investigators believe that translocation of ions is governed less by me- 

 tabolism than by the uptake process (5). The translocation of rubidium from the root 

 to the top has been related to the transpiration stream (29). However, no such rela- 

 tionship was found for calcium (7). 



