It has been reported by several researchers that plants selectively take up K+ under saline stress conditions. However, there is a higher foliar accumulation of Na+ and Cl- ion. by what mechanism are these ions (Na and Cl) translocated from roots to the aerial parts of the plants?
According to my Knowledge sodium is highly soluble and its molecular size is also too smaller as compared to to others, that's the reason may be
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The canal of Na+ K+ act as balance in normal condition ,but under salt stress the Na+may be reach to twice compare with K+
No selective uptake is perfect: you will always have an uptake of unwanted ions together with the nutrient ions needed for growth.
The uptake mechanism in the root goes from the apoplast (the cell walls in contact with the soil solution) into the symplast the proplasm of the single cells which are connected via pit membrane pores. Thus, the barrier of the Casparian strip (where cell walls are impermeable in the radial direction) can be circumvented by nurients and all ions not excluded from the symplast for 100%.
These ions are then secreted into the root xylem solution and moved up to the leaves with the transpiration stream. Nutrient ions will be taken up by the the symplast in the leaves, while "unwanted" ones, such as sodium and chloride, will mostly remain in the apoplast and diffuse through the cell walls. Water loss through the leaf stomata will concentrate the apoplastic cell wall solution, until even ions, which had been excluded to a great degree from uptake in in the root, will reach considerable concentration in the leaves.
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