Distribution and re-transportation of sodium in three Malus species with different salt tolerance.

To further dissect the mechanism of salt tolerance in Malus, the comparison was made regarding the differences between the salt-tolerant and salt-sensitive species in sodium accumulation and extrusion capability in the roots and stem base as well as the sodium re-transportation from shoot to roots by using 22 Na labeling-based feeding of leaves and roots-split experiments. The results demonstrated that the salt-tolerant Malus species could accumulate more 22 Na in the main roots, lateral roots, stem base phloem and xylem, and extrude more sodium out than the salt-sensitive one. In addition, the salt-tolerant Malus species had the higher sodium re-transportation rate from shoot to roots. Altogether, it is concluded that the stronger sodium accumulation and extrusion in the roots and the stronger sodium re-transportation from shoot to roots in the salt-tolerant species play important roles in salt tolerance of Malus species.