Kinetics of caesium and potassium absorptionby roots of three grass pastures and competitiveeffects of potassium on caesium uptake inCynodon sp.

摘要

Caesium uptake by plant roots has been normally associated with the uptake ofpotassium as the potassium transport systems present in plants have also the capacity to transportcaesium. Three grass species (Eragrostis curvula, Cynodon sp and Distichlis spicata) growing inseminatural grassland of central Argentina were selected to study their capability to incorporateCs~+(and K~+)using electrophysiological techniques. Although the ~(137)Cssoil inventory rangedbetween 328- 730 Bq m~(-2)in this region, no ~(137)Csactivity was detected in these plants. However,all the species, submitted previously to K~+starvation, showed the uptake of both Cs~+and K~+when micromolar concentrations of these cations were present in the medium. The uptakeshowed saturation kinetics for both cations that could be fitted to the Michelis-Menten model.K_Mvalues were smaller for K~+than for Cs~+, indicating a higher affinity for the first cation. Thepresence of increasing K~+concentrations in the assay medium inhibited Cs~+uptake inCynodonsp., as expected if both cations are transported by the same transport systems. This effect is dueto the competition of both ions for the union sites of the high affinity potassium transporters. Infield situation, where soil concentration of Cs~+is smaller than K~+concentration, is thenexpectable that caesium activity in plants is not detectable. Nevertheless, the studied plantswould have the capacity to incorporate caesium if its availability in soil solution increases. Inaddition, studies of Cs/K interaction can help us to understand the variability in transfer factors.