Background Tianjin Binhai New district is the first national comprehensive reform and innovation zone in China, but its land is severely salinized. This problem restricts its saline-alkali land reclamation and ecological restoration, resulting in serious soil erosion problems, which is not conducive to its sustainable economic, social and ecological development. Therefore, based on the soil and plant conditions in the coastal saline-alkali area of Tianjin, this study selected 6 representative greening trees in North China to explore their physiological salt tolerance mechanisms and to conduct a comprehensive salt tolerance evaluation. This study aims to add alternative greening trees to the saline-alkali area of Tianjin Binhai New district, and to improve the level of green landscape in the process of ecological restoration in this area.

Methods Six greening tree seedlings (Albizia julibrissin, Hibiscus syriacus, Amorpha fruticosa, Lycium barbarum, Lonicera maackii, and Parthenocissus tricuspidata) commonly used in North China were selected as test materials, and a gradient design of 4 NaCl solution concentrations of 0.2%-0.5% was simulated to calculate the growth and physiological indexes of different tree seedlings under salt stress, and the membership function method was adpated for the comprehensive evaluation of the salt tolerance of 6 tree seedlings.

Results 1) A. fruticosa, H. syriacus and A. julibrissin were all alive after 15 d of salt stress at each concentration. L. barbarum, L. maackii and P. tricuspidata were alive with the increase of concentration and the number of cultivation days, their survival rates decreased, and P. tricuspidata cultivated at 0.5% concentration all died after 15 d. 2) With the increase of salt concentration, the biomass of A. fruticosa increased firstly and then decreased, and the biomass of other trees decreased. Under the 0.2% and 0.3% concentration, the aboveground biomass of A. fruticosa increased, and the belowground biomass of H. syriacus increased. 3) Under salt stress, the relative conductivity, MDA and soluble sugar content of the 6 tree seedling leaves increased. The soluble sugar content of L. maackii changed the most. The soluble sugar content of P. tricuspidata vulgaris reached the peak at 0.3% and then decreased.

Conclusions The growth of L. barbarum is the most sensitive to salt stress, and the growth of A. fruticosa is promoted under 0.2% and 0.3% salt stress. The aboveground part of H. syriacus is more sensitive to salt stress, and the underground part of A. fruticosa is more sensitive to salt stress. A. fruticosa and A. julibrissin showed a more stable cell membrane structure and function under salt stress. P. tricuspidata had a disordered osmotic regulation function of soluble sugar from 0.3% salt stress, and the cell membrane structure was severely damaged.The comprehensive salt tolerance of 6 greening tree seedlings was A. fruticosan > H. syriacus > A. julibrissi > L. barbarum > L. maacki > P. tricuspidata.