Root Physiological, Biochemical and Untargeted Metabolomic Responses of Five Forage Pea (Pisum sativum L.) Varieties at Seedling Stage to Saline-Alkali Stress in Qaidam Salinized Soil

Soil salinization in the Qaidam Basin represents a significant abiotic stressor that restricts the growth, development, and yield of forage pea ( Pisum sativum L.). This study utilized five forage pea genotypes as experimental materials. By integrating morphological phenotyping, physiological and biochemical assays, and untargeted metabolomics techniques, we systematically analyzed the response characteristics and adaptive mechanisms of the roots under salinized soil conditions in the Qaidam Basin. The results indicated significant differences in saline-alkali tolerance among the five genotypes (P < 0.05). Qingjian No. 1 and Dingwan No. 8 were identified as genotypes exhibiting strong saline-alkali tolerance, with Dingwan No. 8 demonstrating optimal performance in terms of plant height (12.50 ± 0.90 cm) and root length (11.80 ± 1.30 cm). Conversely, Grassland No. 7 and Grassland No. 23 were classified as saline-alkali-sensitive genotypes, with Grassland No. 23 exhibiting the shortest plant height (5.40 ± 0.50 cm) and root length (4.00 ± 0.80 cm), thereby indicating significant growth inhibition. At the physiological and biochemical levels, Dingwan No. 8 and Grassland No. 12 exhibited the highest ABTS free radical scavenging activity. Dingwan No. 8 accumulated the highest levels of soluble protein, soluble sugar, and proline (proline: 250 µg·g⁻¹), whereas Qingjian No. 1 and Dingwan No. 8 showed the lowest accumulation of malondialdehyde. Untargeted metabolomic analysis identified a total of 5,193 non-redundant metabolites, with amino acids and their derivatives constituting 31.25% of the core category. A total of 75 common differential metabolites were identified. Root length, lateral root number, catalase activity, soluble sugar content, and secondary metabolite accumulation can serve as key indicators for evaluating saline-alkali tolerance. This study elucidates the differences in saline-alkali tolerance among various forage pea genotypes in the Qaidam Basin and reveals the key physiological and metabolic mechanisms underlying their adaptation to saline-alkali stress.