Influence of intracellular Na+, K+ and Cl- on the salt tolerance in suspension cell cultures of Medicago media
In the process of selection for salt tolerance, it is important to understand the physiological basis of ion management executed by the cells through the exclusion, accumulation or maintenance of ratios of specific ions. Intracellular accumulation of Na+, K+ and Cl- ions in the cells in vitro was studied as a factor in salt tolerance in suspension cultures of Medicago media cv. Rambler. Cells selected for NaCl tolerance (R151-S) and the non-selected R151 (normal) cells were grown in MS medium containing 0, 100, 250, 400 and 550 mM NaCl, Na+, Cl- or K+ for 5 weeks and examined for relative growth rate per week (RGR/week) and intracellular concentration of Na+, Cl- and K+ ions. R151-S cells were found to be significantly more tolerant to the elevated Na+ and Cl- levels than R151. When the effects of Na+ and Cl- ions were compared, R151-S appeared to be more tolerant of higher concentrations of Cl- as compared to the equimolar concentrations of Na+. As the concentration of NaCl in the medium increased, the intracellular concentrations of Na+ increased, though not at a proportional rate. Also, when the external Cl- concentration increased, there was no significant change in the intracellular accumulation of Na+. On the contrary, intracellular Na+ decreased when the external K+ was increased. In the cells adapted to high concentration of NaCl, the concentration of intracellular K+ was lower than the non-adapted cells. The adequate intracellular K+ concentration for optimum growth in M. media suspension cultures was found to be 3000 µM/g dry weight. Excess or deficiency of intracellular K+ caused severe reduction in the growth rates in all cell lines.
Key words: Salt tolerance, Medicago media, suspension cell culture, intracellular ions, organic relative growth rate.