Objectives Exogenous amino acids have been proved of alleviating the stress of NaCl on crops, but not yet about their effects on that of alkaline salt (Na₂CO₃). In this study, we investigated the effects of a preparation made of gourmet powder tail liquid, aiming at providing more support for the recycle of the tail liquid in agriculture, particularly in salt-alkaline soil.

Methods The tested tail liquid contained total crude protein 47.62%, total free amino acid 15.4%, Na⁺ 1.2% and the pH was 3.9 (solid: water ratio of 1∶250). Seed germination experiment showed that seeds of pakchoi were presoaked in solutions containing 0, 0.05, 0.1, 0.2, 0.4, 0.8 g/L of tail liquid for 12 h, then they were loaded inside germination dishes containing 5 mL of 0, 10, 20, 30 mmol/L Na₂CO₃ solution. The germination potential, percentage of germination and lengths of radicle and plumules were measured. Hydroponic experiment showed that pakchoi seedlings were cultivated in nutrient solutions containing 0, 0.05, 0.1, 0.2, 0.4, 0.8 g/L of the tail liquid solution, and 0, 2.5, 5, 10 mmol/L of Na₂CO₃, respectively. The fresh weight, SPAD value, root length, shoot height, content of SOD, POD, CAT, \rmO_2 ^\bar \dot\rm producing rate, proline and MDA content in pakchoi leaves were measured when Na₂CO₃ stress was observed.

Results 1) Under the same Na₂CO₃ concentration, germination potential, percentage of germination, lengths of radicle and plumule of pakchoi showed a trend of increasing first and then decreasing with the increase of tail liquid concentration. Pakchoi grew best at 0.1 g/L preparation concentration under all the tested Na₂CO₃ stress condition. 2) Under the same Na₂CO₃ concentration, seedling fresh weight, SPAD value, SOD, POD, CAT activities increased, and the \rmO_2 ^\bar \dot\rm producing rate and MDA content decreased within low concentrations of tail liquids (0.05–0.1 g/L). When the concentration of tail liquid was in range of 0.4–0.8 g/L, pakchoi growth was severely inhibited, and fresh weight, SPAD value, root length, shoot height decreased greatly, while \rmO_2 ^\bar \dot\rm producing rate and MDA content in leaves increased. 3) Under 2.5 mmol/L of Na₂CO₃, 0.05 g/L of tail liquid was the best option, while 0.1 g/L of tail liquid was the best concentration under 5 mmol/L and 10 mmol/L of Na₂CO₃ stress conditions.

Conclusions The desalted tail liquid of gourmet powder could alleviate the inhibitory of Na₂CO₃ stress on seed germination and seedling growth of pakchoi. Within stresses range of Na₂CO₃10–30 mmol/L, adding 0.1 g/L of the tail liquid into the hydroponic solution is satisfactory in promoting pakchoi seed germination. Under 2.5–10 mmol/L of Na₂CO₃ stress, addition of 0.05–0.1 g/L tail liquid is effective in improving antioxidant enzyme activity of leaves, reducing cells membrane permeability, enhancing leaf photosynthesis, which is the key of higher resistance to Na₂CO₃ stress and healthy growth of pakchoi.