Objectives Germination and seedling stages of crops are most sensitive to salt stress by restricting seed germination and seedlings growth. In this study, a fertilizer synergist containing compound amino acids (hereinafter referred to as synergist) was produced from the waste liquids of glutamate production, and the effects of the synergist on pakchoi (Brassica chinensis L.) seed germination, seedling growth and physiological indexes of leaves under salt (NaCl) stress were investigated. The results may provide guidance for uses of glutamic acid tail liquids in saline soils.

Methods A pakchoi (Brassica chinensis L.) seed germination experiment and a hydroponic experiment were conducted, respectively. The first one was for the seed germination using standard method of germination test. Seeds of pakchoi were presoaked in 0, 0.05, 0.1, 0.2, 0.4, 0.8 g/L synergist solution, respectively. The presoaked seeds were moved one by one to 0, 25, 50, 75 mmol/L NaCl solution in germination dish. The germination potential, germination percentage, length of radicle and plumule were evaluated in this experiment. The second experiment was the hydroponics experiment. After recovering the seedlings, the uniform seedlings were cultivated in nutrient solution containing the same concentration of both synergist and NaCl solution with the first experiment. When salt stress showed up, plant samples were taken, and the fresh biomass weight, SPAD value, root length, plant height, contents of SOD, POD, CAT, \rmO_\small 2^\overline \,\cdot\, producing rate, Pro and MDA in leaves were measured.

Results Seed germination and seedling growth of pakchoi were both significantly inhibited with the increasing concentrations of NaCl. Without salt stress, lower concentrations (≤0.2 g/L) of synergist had a slight inhibitory effect on the seed germination, while 0.4 g/L and 0.8 g/L synergist concentrations significantly inhibited seed germination. Under the same NaCl concentration, increase of synergist concentrations led to early increasing but later decreasing trend in germination potential, percentage germination, length of radicle and plumule of pakchoi seeds during the incubation. Furthermore, it was found that pakchoi grew best under 0.2 g/L synergist concentration while was inhibited under 0.4 g/L and 0.8 g/L synergist concentrations. Without salt stress, increase of synergist concentrations led to increasing first and then decreasing trend in pakchoi growth, and the best concentration of synergist was 0.1 g/L while pakchoi growth was inhibited under 0.4 g/L and 0.8 g/L synergist concentrations. At different NaCl concentrations, addition of 0.05 g/L of synergist improved the fresh biomass weight and SPAD value of pakchoi seedlings, promoted root elongation and stem extension, improved SOD, POD and CAT activity and Pro content, and reduced \rmO_\small 2^\overline \,\cdot\, producing rate and MDA content in leaves. Further increasing concentrations of synergist to higher than 0.05 g/L led to decrease in the fresh biomass weight, SPAD value, plant height and root length of pakchoi seedling, while activity of SOD, POD, CAT and proline content in leaf maintained for some time before falling down, and \rmO_\small 2^\overline \,\cdot\, producing rate and MDA content showed the trend of rising for a while and then stablizined. When the synergist concentration reached 0.4 g/L and 0.8 g/L, seedling growth of pakchoi was seriously suppressed.

Conclusions At 25–75 mmol/L NaCl stress, lower concentrations (≤ 0.2 g/L) of the synergist can promote seed germination of pakchoi, although it has a slight inhibitory effect on the seed germination without NaCl stress. Lower concentration of synergist can also improve antioxidant enzyme activity in leaves, maintain Pro content, enhance leaf photosynthesis, promote the seedling growth of pakchoi, and show higher resistance to salt stress (0.05 g/L synergist is optimal).