Abstract: INTRODUCTION: Thermospermine, as a structural isomer of spermine, is an important substrate involved in plant growth and development, as well as responses to biotic and abiotic stress. 
RATIONALE: At present, there are few reports on the relationships between thermospermine and low-temperature stress, for which a primary analysis of their relationships in wucai was conducted. Firstly, the optimal concentration of exogenous thermospermine alleviating the damage of wucai caused by low temperature was analyzed. Then, thermospermine synthesis inhibitor combined with the selected appropriate concentration of thermospermine was used under low temperature condition in order to further analyze whether thermospermine plays a role in the resistance to low-temperature stress.
RESULTS: At lower concentrations of thermospermine (0.05 and 0.1 mmol·L^–1), electrolyte leakage, hydrogen peroxide, superoxide anion and malondialdehyde contents decreased with different degrees, and the dry and fresh weight increased, while the effects weakened with the increase of thermospermine concentration. The ranking order of the average membership function values obtained decreased gradually with the increase of the concentration, and the score of 0.05 mmol·L^–1 thermospermine was the highest. The addition of thermospermine synthesis inhibitor under low temperature made the two materials with different low-temperature tolerance accumulate more hydrogen peroxide and superoxide anion, electrolyte leakage and malondialdehyde content further increased. In addition, application of thermospermine at the same time can alleviate the damage of low temperature stress aggravated by inhibitor treatment, which was associated with higher activities of SOD and POD.
CONCLUSION: 0.05 mmol·L^–1 thermospermine could effectively alleviate the damage of low temperature on wucai seedlings, and thermospermine synthesis is closely related to the low-temperature stress tolerance of wucai, which may improve low-temperature tolerance through maintaining higher antioxidant enzyme activities, especially SOD and POD.

Phenotype (A), (B), H₂O₂ (C) and •O₂⁻ (D) staining of two wucai materials with different low-temperature stress tolerance (‘W18’, low-temperature tolerant type, and ‘SW-1’, low-temperature sensitive type) under treatment of thermospermine synthesis inhibitor.

Key words: wucai, low-temperature stress, thermospermine