As a salt-tolerant shrub/tree species, Elaeagnus angustifolia L. is widely planted for afforestation in many marginal lands or environmentally harsh conditions in northern China. Although E. angustifolia is well known for its strong adaptation to harsh conditions, the underpinning physiological mechanisms associated with ion transport and homeostasis under high-salt conditions have not been revealed. Has it developed some physiological mechanisms to avoid the high Na⁺ in soil or sequester the Na⁺ in some specific tissues or organs? The use of E. angustifolia to answer these questions can greatly enhance our understanding of the general physiological mechanisms that plants deploy to combat the environmental challenges.
To unravel the underlying physiological mechanisms responsible for the extra-ordinary adaptation to high salt in E. angustifolia, we used the well-controlled water culture experiment in greenhouse to investigate the biomass accumulation, and the absorption, transportation and allocation of multiple ions including K⁺, Na⁺, Ca²⁺ and Mg²⁺ in different plant tissues (roots, stems and leaves) of E. angustifolia seedlings upon being challenged by different NaCl concentrations (0, 100 and 200 mmol/L) for 30 days. Interestingly, the root growth was stimulated to a different extent by salt stress. The biomass accumulation of E. angustifolia seedlings was not obviously affected by 100 mmol/L NaCl stress, whereas it was significantly inhibited by 200 mmol/L NaCl stress. Compared with non-salt control, the K⁺-Na⁺ selective transportation coefficients (S_{K, Na}) and Ca²⁺-Na⁺ selective transportation coefficients (S_{Ca, Na}) of different plant tissues (roots, stems and leaves) under two salt concentrations were all significantly elevated, while the contents of K⁺, Ca²⁺ and Mg²⁺, and the ratios of K⁺/Na⁺, Ca²⁺/Na⁺ and Mg²⁺/Na⁺ in the three plant tissues were all significantly decreased. The Na⁺ concentration and net Na⁺ accumulation in 200 mmol/L NaCl-stressed seedlings' roots were 22.15 mg/g DW and 1.87 mg/plant, respectively, which were 16.20 and 20.06 times higher than that in the control roots, respectively. The concentration and the accumulating amplitude of Na⁺ in roots were more conspicuous than any of other two tissues, implicating that roots may contribute vitally to the observed salt-tolerance of E. angustifolia. The Na⁺ concentration in stems and leaves of 200 mmol/L NaCl-stressed seedlings increased to 5.15 and 7.71 mg/g DW, which were 7.22 and 9.58 times the content in corresponding control, respectively, and net Na⁺ accumulation in 200 mmol/L NaCl-stressed seedlings' shoots was 3.29 mg/plant (5.45 times as much as in control shoot). However, all seedlings stressed by two salt concentrations exhibited a normal growth, no typical salt-damaged symptoms like succulent shoot and abscised leaves in treated seedlings were observed, indicating that shoots (including stems and leaves) can tolerate high concentration's Na⁺ stress. In conclusion, our findings suggested that the salt-adaptation mechanisms of E. angustifolia are root salt-rejection and shoot salt-tolerance, which are primarily implemented by root growth stimulation, root Na⁺ accumulation and restriction, and shoot Na⁺ endurance, and are also correlated with a remarkably increased ability of K⁺ and Ca²⁺ selective transportation in roots, stems and leaves.