Compensatory root proliferation and physiological adjustments enable Velvet Ash (Fraxinus velutina) to tolerate high nonuniform salinity.

Partial desalination offering a promising approach in the saline soil reducing the costs of vegetation construction. However, the quantitative salinity thresholds for tree species in such zones remain unclear, and understanding plant responses to nonuniform salinity is critical for designing effective afforestation strategies via partial zone desalination. In this study, a split-root system was used to examine the growth, physiological responses, and salinity tolerance thresholds of velvet ash (Fraxinus velutina) seedlings exposed to uniform and nonuniform salinity. In the nonuniform treatments, one root half was kept salt free and the other was subjected to progressively NaCl concentrations (100-500 mM), the average salinity across the entire root zone was set to matched that of the uniform treatments (50-250 mM). At equivalent mean salinities, seedlings under nonuniform salinity showed significantly greater total biomass, shoot height, leaf water potential, and water consumption than those under uniform salinity. In the salt free zone, fine root biomass was higher than in control, and total root biomass remained above that of the uniform 50 mM treatment even when the saline side reached 400 mM NaCl. Leaf Na+ accumulation was lower under nonuniform than uniform salinity at the same mean salinity, while the K+/Na+ ratio remained higher. Leaf water potential of the nonuniform salinity was governed by the salt free zone, indicating that roots in the salt free zone supplied enough water to maintain shoot hydration. Osmotic adjustment shifted from proline dominated regulation under uniform salinity to soluble sugar dominated regulation under nonuniform salinity, and the lower leaf malondialdehyde (MDA) content indicated reduced oxidative stress. The salinity required for a 50% reduction in biomass was 163 mM under uniform conditions, compared with an estimated threshold of 436 mM under nonuniform salinity. Nevertheless, achieving the same shoot growth under nonuniform salinity demanded a larger allocation to fine roots than under uniform salinity, implying lower operational efficiency of roots in the saline zone. Collectively, these results demonstrate that compensatory fine root proliferation in the salt free zone, together with preferential water uptake and restricted Na+ translocation, enables velvet ash to tolerate far higher partial salinity than uniform salinity. This work provides a theoretical foundation for partial root zone desalinization as a cost effective approach to afforestation in coastal saline areas.