Effects of proline on the growth and physiological response of American maple seedlings under complex saline-alkali stress

The tissue culture seedlings of ‘freeman maple’ were used as experimental materials, The effects of saline-alkali stress and exogenous proline on physiological characteristics of American maple seedlings were investigated by exogenous proline treatment (0, 5, 10, 20, 40mmol/L) with compound saline-alkali (NaCl, Na₂SO₄, NaHCO₃, Na₂CO₃) as stress conditions. The results showed that under the conditions of compound saline-alkali stress, the leaves of American maple seedlings curl and turn yellow. With the increase of the concentration of compound saline-alkali stress, the chlorophyll and carotenoids in the leaves decreased significantly, and the salt damage index increased significantly. Compared to the 0 mmol/L proline-treated group (F₀), the 10 mmol/L proline-treated group (F₂) could effectively promote the growth of ‘freeman maple’ and increase the content of photosynthetic pigments in leaves, which significantly increased the growth rate of plant height by 36.41%, and the total content of chlorophyll and carotenoids increased by 176.12% and 39.58%. It is conducive to the growth of ‘freeman maple’ seedlings and the accumulation of plant biomass. By enhancing the contents of soluble sugar, soluble protein, proline and the activities of antioxidant enzymes POD, SOD and CAT, the contents of H₂O₂, O₂⁻ and MDA in the leaves were reduced, and the content of osmoregulatory substances was increased, so as to slow down the toxic effect of saline-alkali stress on American maple seedlings.