Objective  By immersing buds in varied concentrations of sodium azide solutions to induce mutagenesis in vitro on 3 soybean germplasms, optimized induction conditions were determined based on the effect on the low-temp tolerance of the mutants under stress.

  Methods  Young buds of Changshutedawang (CSTDW), Taiwan 292, and Jianhe soybean germplasms were immersed in sodium azide solutions of different concentrations for the in vitro mutagenesis induction. After the optimized induction treatment (immersing buds at 0.8mmol sodium azide/L for 48h), the plants were subjected to low-temp stress at 4 ℃ for 4d prior to the physio-biochemical determinations.

  Results  The mortality rate of the treated plants was only slightly higher than the median lethal dose（LD₅₀）. Under the low-temp stress, the contents of osmosis regulating substance and photosynthetic pigment in the mutant plants were higher than those in control. At room temperature, the increases on the germplasm was in the order of Taiwan 292＞CSTDW＞Jianhe, and the SOD and CAT in Taiwan 292 and Jianhe were significantly increased. Under low temperature, the antioxidant capacities differed among the germplasms. Overall, Jianhe was higher than CSTDW and followed by Taiwan 292. Specifically, SOD and POD activity increased in Jianhe, POD increased in CSTDW, and SOD and POD slightly increased and CAT decreased in Taiwan 292. Compared to control, Taiwan 292 had the greatest antioxidant activities showing an increased POD and slightly decreased SOD activity, CSTDW was next in line with no significant change, and Jianhe had the least with decreased activities on both SOD and POD. The MDA contents of CSTDW and Taiwan 292 increased initially followed by a decline, while Jianhe maintained an increasing trend. The overall increased levels ranked Taiwan 292＞ CSTDW＞ Jianhe.

  Conclusion   The induced mutagenesis increased the osmosis regulating substance and photosynthetic pigment contents in the 3 soybean germplasms. The improvement on low-temp tolerance was higher for CSTDW and Taiwan 292 but lower for the local Jianhe soybean. After mutagenesis and low-temp treatment, Taiwan 292 performed well with a heightened antioxidant capacity. On the other hand, Jianhe changed significantly on the enzymatic activity, the condition was favorable for the plants at room temperature but disadvantageous at low temperatures. In all, the low-temp tolerance of the soybean germplasms generated by sodium azide-induced mutagenesis were found to be Taiwan 292＞CSTDW＞Jianhe.