Soybean Genotypes With Contrasting Root System Size Differ in Saline–Alkaline Tolerance

Soybean (Glycine max L. Merr.) is highly susceptible to saline–alkaline conditions, exhibiting significant genotypic variability in tolerance. The root system plays a pivotal role in saline–alkaline resistance, yet the precise mechanisms, particularly those related to root morphological traits, remain unclear. This study explores genotypic variations in root morphology and saline–alkaline tolerance among diverse soybean genotypes and examines the relationship between root system growth and tolerance mechanisms. Eight soybean genotypes with varying root system sizes were evaluated for saline–alkaline tolerance 26 days after transplantation. Plants were subjected to NaHCO₃ stress (0 and 30 mmol L⁻¹) for 5 days using a semi-hydroponic phenotyping platform in a glasshouse. Saline–alkaline stress caused significant variation in 20 shoot and root traits, as well as 23 physiological and biochemical traits. Transcriptional profiling revealed differential expression of key genes, including GmHKT1;4, GmPLMT, GmERF8 and GmWRKY12. Based on the mean relative shoot dry mass ratio, the eight genotypes were categorised as sensitive, moderately tolerant or tolerant. Under saline–alkaline stress, the tolerant, large-rooted genotype Nannong 26 showed increased Ca²⁺ accumulation and upregulation of GmHKT1;4 and GmPLMT in both shoots and roots. In contrast, the tolerant, smaller-rooted genotype NJP580 exhibited higher K⁺ accumulation and upregulation of GmERF8 and GmWRKY12 in shoots and roots. Root dry mass, fine-root length and the upper-to-lower biomass allocation ratio emerged as potential indicators of saline–alkaline tolerance in soybean. These traits may serve as useful proxies for early-stage screening of tolerant genotypes. The identified saline–alkaline-tolerant genotypes offer promise for cultivation in saline–alkaline soils and for breeding high-yielding, stress-tolerant soybean hybrids.