IMASL   20939
INSTITUTO DE MATEMATICA APLICADA DE SAN LUIS "PROF. EZIO MARCHI"
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Kinetics of caesium and potassium absorption by roots of three grass pastures and competitive effects of potassium on caesium uptake in Cynodon
Autor/es:
JURI AYUB, J.; RUBIO VALVERDE, L.; GARCÍA-SÁNCHEZ, M.J.; FERNÁNDEZ, J.A.; VELASCO, R. H.
Lugar:
Buzios, Brasil
Reunión:
Simposio; 8th International Symposium on the Natural Radiation Environment; 2007
Resumen:
Caesium uptake by plant roots has been normally associated with the uptake of potassium. It is widely accepted that caesium influx into plant cells mainly occurs through potassium transporters. Classically two discrete uptake systems for potassium have been defined: system 1 and 2. These systems operate at low and high substrate concentration, respectively. System 1 obeys Michaelis-Menten kinetics and it has been related to the operation of high affinity carriers that are expressed in radical cells of plants growing in K+ deficiency. We have selected three vegetal species (Eragrostis curvula, Cynodon sp and Distichlis spicata) growing in seminatural grassland of central Argentina, where 137Cs soil inventory ranged from 328-730Bqm-2 but 137Cs activity was not detected in these plants. The three species were submitted to K starvation to analyse caesium and potassium root uptake. Cynodon sp and D. spicata plants were extracted from field and cultivated in K+ deficiency for at least 4 days. In the case of E. curvula, seeds were germinated in distilled water and later transferred to K+ deficiency (4 days). Michaelis-Menten kinetic parameter (Km) of K+ and Cs+ uptake by root cells were determined by electrophysiology techniques. Km values for K+ were 142mM, 97.4±16.2mM and 73.3±34.1mM whereas Km values for Cs+ were 607mM, 163.3±51.9mM and 120.2±52.7mM for E. curvula, D. spicata and Cynodon, respectively. In all cases Km was smaller for potassium than for caesium, indicating higher affinity of the carriers for the first than for the second cation. In addition, the effect of increasing K+ concentrations on Cs+ uptake was evaluated in Cynodon plants submitted to K+ deficiency. In these plants, the addition of 500mM Cs+ (saturating concentrations) causes a membrane depolarization of 14mV. However, in presence of 500mM and 1000mM K+ in assay medium, the sequential addition of Cs+ (500mM) causes a depolarization 3- and 7-fold lower, respectively, than in absence of K+. An increase in K+ concentration causes then a diminution in the absorption of Cs+. This effect is due to the competition of both ions for the union sites of the high affinity potassium carriers. In field situation, where soil concentration of Cs+ is smaller than K+ concentration, is then expectable that caesium activity in plants is not detectable. Nevertheless, the studied plants would have the capacity to incorporate caesium if its availability in soil solution increases. In addition, kinetic studies of Cs/K interaction can help to understand the variability in transfer factors.