Abstract: An experiment was conducted to explore the effects of soaking time and applied level of gibberellic acid (GA₃) on the development of Astragalus membranaceus seedlings under severe drought and salt stresses. There were five application levels (0, 50, 100, 200 and 400 mg·L^-1) of GA₃ and five duration periods (6, 12, 24, 36 and 48 h) for seed soaking, After that, the treated seeds were transferred to -0.6 and -0.9 MPa NaCl solutions respectively. Total germination percentage (TGP), mean germination time (MGT), root/shoot inhibition index (R/SII), shoot length (SL), root length (RL), shoot dry weight (SDW) and root dry weight (RDW) were measured. The results showed that, with the extension of soaking time and the increase of GA₃ application levels, TGPs of A. membranaceus were enhanced under both stresses while MGTs were substantially shortened. SL of soaking seeds with 400 mg·L^-1 GA₃ for more than 36 h under NaCl stress was significantly inhibited, while RL was significantly reduced at the level of 200 mg·L^-1 GA₃ or higher for 36 h and 400 mg·L^-1 GA₃ for 24 h (P<0.05). The significant inhibition of SL and RL under PEG stresses began at 36 h after soaking seeds at 200 mg·L^-1 GA₃ or higher level (P<0.05). The significant inhibition of GA₃ on SDW and RDW under NaCl stress began at 36 h after soaking seeds at 200 mg·L^-1 or higher, while the significant inhibition of GA₃ on SDW and RDW under PEG stress began at 48 h after soaking seeds at 200 mg·L^-1 GA₃ or higher (P<0.05). There were significant correlations among TGP, MGT, SL, RL, SDW and RDW (P<0.05). The optimum treatment combinations of 103 mg·L^-1 GA₃ for 40 h and 51 mg·L^-1 GA₃ for 48 h could contribute to the germination and seedling development under NaCl and PEG stresses, respectively.

Key words: vegetation restoration, growth stage, soil microbial biomass carbon, soil physicalchemical property.