Physiological Regulation of Photosynthetic-Related Indices, Antioxidant Defense, and Proline Anabolism on Drought Tolerance of Wild Soybean ( L.)

Wild soybean ( L.), drought-tolerant cultivar ( L.), and drought-sensitive cultivar ( L.) were used as materials to investigate the drought tolerance mechanism after 72 h 2.5 M PEG 8000 (osmotic potential -0.54 MPa)-simulated drought stress at the seedling stage. The results indicated that the leaves of the did not wilt under drought stress. However, both the drought-tolerant and drought-sensitive cultivated soybean cultivars experienced varying degrees of leaf wilt. Notably, the drought-sensitive cultivated soybean cultivars exhibited severe leaf wilt after the drought stress. Drought stress was determined to have a significant impact on the dry matter of the above-ground part of the drought-sensitive cultivar , followed by the drought-tolerant cultivar , with the lowest reduction observed in . Furthermore, the presence of drought stress resulted in the closure of leaf stomata. exhibited the highest proportion of stomatal opening per unit area, followed by the drought-tolerant cultivar , while the drought-sensitive cultivar displayed the lowest percentage. Photosynthesis-related indexes, including photosynthetic rate, intercellular CO, transpiration rate, and stomatal conductance, decreased in and after drought stress, but increased in . In terms of the antioxidant scavenging system, lower accumulation of malondialdehyde (MDA) was observed in and , along with higher activities of superoxide dismutase (SOD, EC 1.15.1.1) and catalase (CAT, EC 1.11.1.6) to counteract excess reactive oxygen species and maintain cell membrane integrity. In contrast, the drought-sensitive cultivar had higher MDA content and higher activities of ascorbate peroxidase (APX, EC 1.11.1.11) and peroxidase (POD, 1.11.1.7). and also exhibited less accumulation of osmolytes, including soluble sugar, soluble protein, and free proline content. The activities of δ-OAT, ProDH, and P5CS, key enzymes in proline anabolism, showed an initial increase under drought stress, followed by a decrease, and then an increase again at the end of drought stress in . Before drought stress, had higher activities of ProDH and P5CS, which decreased with prolonged drought stress. experienced an increase in the activities of δ-OAT, ProDH, and P5CS under drought stress. The gene expression levels were up-regulated in all three germplasms. The expression levels of the gene in and were down-regulated, while showed no significant change. The expression of the gene and gene was down-regulated in and , but up-regulated in . This indicates that proline content is regulated at both the transcription and translation levels.