Uptake of 86Rb and K by Excised Maize Roots

Radioactive isotopes are valuable tools for investigating the uptake and transport of ions by biological material. Their use as tracers provide results that often can be obtained only by such a technique. The reliability of isotopes as tracers, however, is sometimes implicitly accepted. Although the pitfalls are widely known, they are often ignored. The hazards were emphasized for us in the use of 88Rb as a tracer for K and Rb in short-term uptake studies with excised maize roots. We found that analysis for 86Rb did not properly evaluate the uptake of K from KCl solutions. While 86Rb was readily taken up from solutions of 0.1 meq KCl per liter during 4-hr treatment periods, chemical analyses showed a net loss of K from the tissue. The term "uptake" is used here to indicate the net movement of an ion itto the tissue during a given treatment. It is the resultant of both influx and efflux. Seedlings of Zea mays L., (cv. DeKallb 441) were dark grown for 5 days at 250 over aerated solutions of 0.2 mm CaSO4. Six-centimeter end segments were cut from the primary roots and rinsed in distilled-deionized water. After blotting to remove excess water, 2-gram lots were placed in 2 liters of KCI and/or RbCl treatment solutions labeled with approximately 1 ,uc 86Rb per p.eq 'monovalent cation. Specific activities of the solutions were measured at the beginning and end of an experiment. No detectable change occurred. The solutions were aerated continuously and were maintained at 240. The initial pH of 5.5 varied less than 0.05 unit during the absorption period. Treatments were terminated by removing the roots from the solutions and rinsing for 30 sec in 1 liter of distilled-deionized water. After ashing the roots at 4800, the residue was dissolved in a 5 ml mixture of 0.4 N HNO3 and 40 % (v/v) acetic acid. Chemical analyses for K and Rb were made by atomic absorption spectroscopy. The radioanalyses of the anmbient solution and the ashed samples were made by solid scintillation detection.