Mycorrhizal symbiosis enhances tolerance to NaCl stress through selective absorption but not selective transport of K+ over Na+ in trifoliate orange

• Mycorrhizas increased plant growth and LRWC under no-NaCl or NaCl stress.
• Leaf and root Na+ decreased while K+ increased by mycorrhizas under NaCl stress.
• AMF increased SAK+/Na+SAK+/Na+ and K+/Na+ ratio but decreased STK+/Na+STK+/Na+ under NaCl stress.
• SAK+/Na+SAK+/Na+ was significantly correlated with LRWC and almost all growth traits.
• STK+/Na+STK+/Na+ was significantly correlated with leaf number and root dry weight only.

Selectivity of potassium ion (K+) over sodium ion (Na+) is essential to understand plant's tolerance to salt stress, whereas information is limited whether arbuscular mycorrhizal fungi (AMF) increase selective absorption or transport of K+ over Na+(SAK+/Na+   or STK+/Na+)(SAK+/Na+   or STK+/Na+) in host plants. The 61-d-old trifoliate orange (Poncirus trifoliata) inoculated with or without an AM fungus Funneliformis mosseae was subjected to 45-day 100 mM NaCl stress. The AMF inoculation significantly increased plant growth (height, leaf number, stem diameter and biomass production), leaf relative water content (LRWC), and tissue K+ absorption but decreased Na+ absorption under no-NaCl or NaCl stress. Mycorrhization also significantly increased ratio of K+/Na+ in leaf, root and total plant under no-NaCl and NaCl stress. Meanwhile mycorrhizal seedlings showed higher SAK+/Na+SAK+/Na+ under no-NaCl and NaCl stress, and higher STK+/Na+STK+/Na+ under no-NaCl stress but lower STK+/Na+STK+/Na+ under NaCl stress. In addition, SAK+/Na+SAK+/Na+ significantly positively correlated with LRWC and almost all tested growth traits, whilst STK+/Na+STK+/Na+ only with leaf number and root biomass. These results suggested that it was the mycorrhizal-mediated increase of SAK+/Na+SAK+/Na+, rather than STK+/Na+STK+/Na+, under NaCl stress, that could enhance the plant's tolerance to NaCl stress, thus conferring a greater LRWC and plant growth in mycorrhizal citrus seedlings.