Abstract: [Objective]In this study, we investigated the effects of alkali stress on physiological characteristics of Populus alba var. pyramidalis leaves with different maturity, to provide a theoretical basis for indicating the strategy for woody plants to resist alkali stress in arid region.[Method]We chose P.alba var. pyramidalis as the experimental material, and the species is a variant of P.alba and widely distributed in XinJiang area. The young P.alba var. pyramidalis plants were grown in soil pH8.5-9.0 and subjected to 100 mmol·L^-1 Na⁺. The change and distribution of ion content (Na⁺, K⁺, Na⁺/K⁺ ratio, Mg²⁺, Ca²⁺), the response of photosynthetic pigment (Chl a, Chl b, Chl a+Chl b, Car) and the change of antioxidant physiological indices (CAT, SOD, POD, MDA) were investigated in young and old leaves of P.alba var. pyramidalis under alkali stress.[Result]In the control condition, old leaves contained more Na⁺ and less K⁺ than young leaves, and hence a high Na⁺/K⁺ ratio in old leaves; The content of Na⁺ and Na⁺/K⁺ ratio in old leaves increased significantly under alkali stress, but they had no obvious change in young leaves; The content of K⁺ also had no significant change in young and old leaves under alkali stress; On the contrary, Mg²⁺ and Ca²⁺ were mainly accumulated in old leaves in control condition, but the content of them was decreased in old leaves and increased in young leaves under alkali stress. In control condition, the content of photosynthetic pigments (Chl a, Chl b, Car) was much higher in old leaves than in young leaves, but it declined significantly and was lower in old leaves than in young leaves under alkali stress; The content of Chl (Chl a, Chl b, Chl a+Chl b) in young leaves remained unchanged under alkali stress, but the content of Car in young leaves increased significantly under alkali stress. At the same time, the membrane lipid peroxide level (MDA content) in old leaves rose obviously under alkali stress, but it kept at control level in young leaves. The antioxidant enzymes activities (SOD, POD, CAT) in old leaves rose significantly, but they kept at control level in young leaves, except for that the activity of SOD declined.[Conclusion]Under alkali stress, the excessive Na⁺ was mainly accumulated in old leaves of P.alba var. pyramidalis. The ion allocation strategy made old leaves suffer severe oxidative stress and thus protected young leaves that was salt-alkali sensitive from ion injure. Our study indicated that the ion allocation was in favor of young leaves, which was an important strategy for P.alba var. pyramidalis to resist alkali stress and was of great importance for its survival in desert region.

Key words: Populus alba var. pyramidalis, alkali stress, young leaves, old leaves, physiological response