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.