Abstract: Soil alkalization is one of the major abiotic stresses restricting soybean growth, development, and yield. Developing efficient evaluation methods for soybean alkali tolerance and screening alkali-tolerant germplasm resources is crucial for improving soybean productivity on sodic soils. In this study, vermiculite was used as the seedling substrate, and gradient treatments were conducted with mixed alkali solutions (0, 30, 35, and 40 mmol L?¹ NaHCO₃:Na₂CO₃ = 9:1, molar ratio). By comparing aboveground and belowground phenotypic traits including plant height, leaf area, root length, and root volume, we aimed to identify suitable evaluation indices and establish an efficient evaluation method for alkali tolerance at the soybean emergence stage. The results showed that treatments with 35 mmol L?¹ and 40 mmol L?¹ mixed alkali solutions significantly inhibited the aboveground growth and root development of soybean seedlings, with significant phenotypic differences observed among the tested germplasm accessions. Correlation analysis indicated that relative leaf area was significantly correlated with multiple root traits and could reliably reflect the alkali tolerance of different accessions. Ultimately, 35 mmol L?¹ mixed alkali solution was selected as the treatment concentration. The relative leaf area measured 10 days after alkali treatment was defined as the alkali tolerance coefficient (ATC), which served as the evaluation index for soybean alkali tolerance at the emergence stage. This method was used to evaluate the alkali tolerance of 52 soybean germplasm accessions, which were classified into four grades according to ATC values. Twelve grade 1 highly alkali-tolerant accessions and ten grade 2 alkali-tolerant accessions were identified. Meanwhile, salt tolerance identification at both emergence and seedling stages was carried out on all 52 accessions. Three elite soybean accessions, namely SZH101, DND254, and SN33, were finally identified. These accessions exhibited both alkali and salt tolerance at the emergence stage and stable salt tolerance at the seedling stage. This study presents a simple, non-destructive, and high-throughput method for evaluating alkali tolerance in soybean at the emergence stage, which is highly valuable for efficient identification of alkali-tolerant soybean germplasm, mining of key alkali-tolerance genes, and accelerated breeding of alkali-tolerant varieties.

Key words: soybean, emergence stage, alkali tolerance, relative leaf area, phenotyping method