Zygophyllum xanthoxylum, a C₃ woody species, is a succulent xerophyte that is well adapted to arid environments. Our previous investigations showed that Na⁺ has a positive effect on the growth of Z. xanthoxylum under drought conditions, which was closely related to high Na⁺ accumulation in leaves. To reveal the physiological mechanisms underlying how Na⁺ accumulation improves the drought resistance of Z. xanthoxylum, 3-week-old seedlings were treated with a series of additional external NaCl concentrations (5–150 mM) in sand culture experiments. Seedlings were also subjected to water deficit (30% of field water capacity) in the presence or absence of additional NaCl (50 mM) in pot experiments. The results indicated that 50 mM NaCl could mitigate deleterious impacts of water deficit on the growth of Z. xanthoxylum, by improving the relative water content, inducing a significant drop in leaf water potential and, concomitantly, increasing leaf turgor pressure and chlorophyll concentrations resulting in an enhancement of overall plant photosynthetic activity (i.e., photosynthetic rate and water use efficiency). Furthermore, NaCl (50 mM) could alleviate the inhibitory effect of water deficit on the activity of photosystem II in Z. xanthoxylum. The contribution of Na⁺ to the total osmotic potential varied from 8% in the control to 13% in plants subjected to water deficit and, surprisingly, to 28% in plants grown in the presence of 50 mM NaCl under water deficit; however, the contribution of K⁺ significantly decreased from 13 to 8%. These findings suggest that, under arid environments, Z. xanthoxylum is able to accumulate a high concentration of Na⁺ in its leaves and use it directly for osmotic adjustment, which was coupled with an improvement in leaf hydration and photosynthetic activity.

Na+ uptake by plant roots has largely been explored using species that accumulate little Na+ into their shoots. By way of contrast, the halophyte Suaeda maritima accumulates, without injury, concentrations of the order of 400 mm NaCl in its leaves. Here we report that cAMP and Ca2+ (blockers of nonselective cation channels) and Li+ (a competitive inhibitor of Na+ uptake) did not have any significant effect on the uptake of Na+ by the halophyte S. maritima when plants were in 25 or 150 mm NaCl (150 mm NaCl is near optimal for growth). However, the inhibitors of K+ channels, TEA+ (10 mm), Cs+ (3 mm), and Ba2+ (5 mm), significantly reduced the net uptake of Na+ from 150 mm NaCl over 48 h, by 54%, 24%, and 29%, respectively. TEA+ (10 mm), Cs+ (3 mm), and Ba2+ (1 mm) also significantly reduced 22Na+ influx (measured over 2 min in 150 mm external NaCl) by 47%, 30%, and 31%, respectively. In contrast to the situation in 150 mm NaCl, neither TEA+ (1–10 mm) nor Cs+ (0.5–10 mm) significantly reduced net Na+ uptake or 22Na+ influx in 25 mm NaCl. Ba2+ (at 5 mm) did significantly decrease net Na+ uptake (by 47%) and 22Na+ influx (by 36% with 1 mm Ba2+) in 25 mm NaCl. K+ (10 or 50 mm) had no effect on 22Na+ influx at concentrations below 75 mm NaCl, but the influx of 22Na+ was inhibited by 50 mm K+ when the external concentration of NaCl was above 75 mm. The data suggest that neither nonselective cation channels nor a low-affinity cation transporter are major pathways for Na+ entry into root cells. We propose that two distinct low-affinity Na+ uptake pathways exist in S. maritima: Pathway 1 is insensitive to TEA+ or Cs+, but sensitive to Ba2+ and mediates Na+ uptake under low salinities (25 mm NaCl); pathway 2 is sensitive to TEA+, Cs+, and Ba2+ and mediates Na+ uptake under higher external salt concentrations (150 mm NaCl). Pathway 1 might be mediated by a high-affinity K transporter-type transporter and pathway 2 by an AKT1-type channel.

Soil salinity is an increasingly serious problem worldwide that reduces agricultural output potential. Selected beneficial soil bacteria can promote plant growth and augment tolerance to biotic and abiotic stresses. Bacillus subtilis strain GB03 has been shown to confer growth promotion and abiotic stress tolerance in the model plant Arabidopsis thaliana. Here we examined the effect of this beneficial soil bacterium on salt tolerance in the legume forage crop, white clover. Plants of white clover (Trifolium repens L. cultivar Huia) were grown from seeds with or without soil inoculation of the beneficial soil bacterium Bacillus subtilis GB03 supplemented with 0, 50, 100 or 150 mM NaCl water into soil. Growth parameters, chlorophyll content, malondialdehyde (MDA) content and osmotic potential were monitored during the growth cycle. Endogenous Na+ and K+ contents were determined at the time of harvest. White clover plants grown in GB03-inoculated soil were significantly larger than non-inoculated controls with respect to shoot height, root length, plant biomass, leaf area and chlorophyll content; leaf MDA content under saline condition and leaf osmotic potential under severe salinity condition (150 mM NaCl) were significantly decreased. Furthermore, GB03 significantly decreased shoot and root Na+ accumulation and thereby improved K+/Na+ ratio when GB03-inoculated plants were grown under elevated salt conditions. The results indicate that soil inoculation with GB03 promotes white clover growth under both non-saline and saline conditions by directly or indirectly regulating plant chlorophyll content, leaf osmotic potential, cell membrane integrity and ion accumulation.