Response of Root System to Sequential Salinity Conditions in Salinity-Tolerant/Sensitive Soybean Varieties

Abstract The root is one of the vital organs of soybean for combating salt stress. Previous studies have indicated that the salt-tolerant gene Ncl reduces the chloride and sodium ion content in stems and leaves, and it can significantly increase yield under saline conditions. The expression of theNcl gene is the highest in soybean roots. To further clarify the response of salt-tolerant soybean roots to salt stress, this study conducted hydroponic experiments on three groups of near-isogenic lines (NILs) with different salt tolerance levels, observing the effect of the gene Ncl on soybean root growth under ten salt concentrations. The results indicate that under salt stress, the Ncl gene not only maintains the biomass of aboveground parts but also sustains root biomass. However, the effect of the Ncl gene on root length varies among different NILs. Additionally, under salt stress, the Ncl gene improves morphological characteristics of the root, especially lateral roots number and root surface area. Concerning xylem sap, no significant difference was observed between salt-tolerant and salt-sensitive varieties, indicating that the Ncl gene does not affect osmotic water absorption under salt stress. However, the hydraulic resistance of salt-tolerant varieties is significantly higher than that of salt-sensitive varieties. Finally, interactions between salt concentration and salt tolerance were observed in root dry weight and root volume, suggesting the Ncl gene makes a trade-off between root growth and soybean salt tolerance.