Fine root compensation in the non-saline zone increases the velvet ash (Fraxinus velutina) growth salt threshold under nonuniform salinity

Soil salinity is often heterogeneous in natural environments, yet most studies on plant salt tolerance have focused on uniform salinity conditions. Understanding how trees respond to nonuniform salinity is critical for developing effective afforestation strategies in saline lands. In this study, we investigated the growth, physiological responses, and salinity threshold of velvet ash (Fraxinus velutina) seedlings under uniform salinity and nonuniform salinity using a split-root experiment. The nonuniform treatments consisted of one salt-free side and one side subjected to increasing NaCl concentrations (100–500 mM), with mean salinities equivalent to the uniform salinity treatments (50–250 mM). The results showed that at equal mean salinities, plants under nonuniform exhibited significantly higher biomass, shoot height, leaf water potential, and whole-plant water consumption compared to those under uniform salinity. Compensatory fine root proliferation in the salt-free zone was significantly higher than control roots, maintaining total root biomass higher than uniform 50mM, even when partial salinity reached 400 mM. Leaf Na+ accumulation was substantially lower under nonuniform salinity than under uniform salinity at equivalent mean salinities, while K+/Na+ ratios remained higher. Leaf water potential in nonuniform salinity plants was primarily determined by the salt-free root zone, indicating that roots in the non-saline compartment supplied sufficient water to sustain shoot hydration. Osmotic adjustment shifted from proline-dominated regulation under uniform salinity to soluble sugar-dominated regulation under nonuniform salinity, with lower MDA content suggesting reduced oxidative stress. Under uniform salinity, the threshold (mean salinity) for 50% biomass reduction was 163 mM. Under nonuniform salinity, when one side was saltfree and the other side received increasing NaCl, the threshold on the saline side for 50% biomass reduction was estimated as 436 mM . However, more fine root biomass was required to support the same shoot growth under nonuniform compared to uniform salinity, indicating reduced efficiency of roots in the saline zone. These findings demonstrate that compensatory fine root proliferation in the non-saline zone, coupled with preferential water uptake and restricted Na+ transport, enables velvet ash to tolerate much higher partial salinity than uniform salinity. This study provides a theoretical basis for partial root-zone desalinization as a cost-effective approach for afforestation in coastal saline lands.