This study was undertaken primarily to evaluate the role of cation-exchange capacity (CEC) of plant roots in the cation absorption process. Efforts were also made to improve the method to determine the root CEC and to characterize cation absorption by excised barley roots under anaerobic conditions.
Studies on the methodology on the root CEC assay revealed that it is very difficult to determine accurately the root CEC by estimating the root exchange acidity. This is because it is difficult to saturate the roots with H+ and select an arbitrary end point of the titration. It was found that Na-exchange method by Bartlett1) is not a quantitatively accurate method because of the contamination by the native and metabolically absorbed Na, and the short exchange period. The following conditions must be provided for the quantitative measurement of the root CEC by a direct exchange method;
a) A proper exchange period (30 minutes).
b) The use of exchange cations which are not normally contained in the roots.
c) Elimination of metabolic absorption during the exchange period.
Cation absorption under anaerobic conditions was characterized and examined to find whether a drastic reduction of O2 in an exchange solution can completely eliminate metabolic absorption. Lithium absorption by the roots under anaerobic conditions was insensitive to low temperature and ended within 30 minutes. Absorption of Ba, Sr, Rb and Li from 10-3 N single chloride solution under anaerobic conditions showed that the absorption patterns were quite similar and that the amount of cation absorption was in the order of a lyotropic series. Rubidium absorption by N 2-bubbled roots in the presence of Sr was almost negligible. These results indicate that Li, Rb and Sr were absorbed under anaerobic conditions mainly exchange adsorption excluding diffusion and metabolic absorption.
Excised roots kept under anaerobic conditions for 60 minutes, when removed to aerated solutions absorbed Rb at the same rate as the roots initially in aerobic conditions. This indicates that the anaerobic treatments for 60 minutes had no adverse effect on Rh-absorption capacity of the roots.
A new method to determine the root CEC was developed on the basis that Li absorption for 30 minutes under anaerobic conditions was exclusively due to cation exchange adsorption. This method employs a technique using 0.1 N-LiCl as an exchange cation under anerobic conditions during a 30-minute exchange period followed by distilled water rinses to remove the diffusible and occluded portion of Li from the roots.
The CEC value of barley roots determined by the Li-exchange method increased with increasing nitrogen level in the nutrient solution being 10.2, 13.1 and 14.0 meq/100 g dry roots for 0.1 and 2.5 mM of NH4NO3 , respectively.
The relationship among the ratio of mono- to divalent cations adsorbed on the exchange sites in the Donnan Free Space (DFS), the cation concentration of the external solution and the concentration of exchange sites in the DFS was expressed as follows;
where [M++]i and [M+]i are molar concentrations of a divalent cation M++ and a monovalent cation M+ respectively, adsorbed on the exchange sites in the DFS, Ec is the concentration of exchange sites in the DFS in equiv./l. of the DFS, and R is expressed as [M+ ]02/[M++]0 , the ratio of the square of the molar concentration of monovalent cations to the molar concentration of divalent cations in the external solution.
The ratio of Sr to Li absorbed from a mixed solution of SrCl 2 and LiCl under anaerobic conditions increased with decreasing R, indicating that Sr and Li absorption followed the Donnan distribution. On the basis of the Donnan distribution, it is possible to calculate Ec, the concentration of the cation exchange sites in the DFS, from the ratio of Sr to Li absorbed by the roots. The calculated values of Ec for the barley roots which were grown in the nutrient solution without nitrogen source and with 1 mM of NH4NO3 were 760 and 440 meq/l. of the DFS, respectively. These values did not deviate appreciably when 10-4, 10- 3, and 10-2 N were the concentrations of the external solutions.
The experimental values of the ratio of Sr to Li absorbed under anaerobic conditions exactly fitted to the theoretical values derived from the Equation (A). This assumed that the concentration of exchange sites in the DFS was as 760 and 440 meq/l. of the DFS of roots which were grown without nitrogen and with 1 mM of NH4NO3 in the nutrient solution, respectively.
The valence effect which was observed in the absorption of Li and Sr under anaerobic conditions was diminsihed in their absorption under aerobic conditions. Moreover, the rate of Rb absorption from 10-3 N-RbCl after 60-minute absorption period was almost the same regardless of the presence of 10-3 N-SrCl2. These results indicate that exchange adsorption is not a rate-limiting step in the absorption of Li or Rb by the excised barley roots.
On the basis of the information obtained, the possible function of exchange adsorption in differential accumulation of mono- and divalent cations by plants was discussed. It was concluded that the cations adsorbed exchangeably by roots distribute in accordance with the Donnan principle but that exchange adsorption is not necessarily a rate-limiting step in cation absorption resulting in the differential accumulation of mono- and divalent cations by plants.