Abstract:Using 2-year old apple dwarf rootstock M9-T337 as test material, pot experiment method was adopted. Watering water (CK) and salt-alkali stress (0.1mol/L NaCl+NaHCO3 solution) were set, followed by spraying five concentrations of H2O2 treatment [0 (T1), 0.2 mmol/L (T2), 0.4 mmol/L (T3), 0.6 mmol/L (T4), 0.8 mmol /L (T5)], chlorophyll content, photosynthetic gas exchange parameters, osmotic adjustment substance content, antioxidant enzyme activity and cell membrane permeability of each treatment were measured, and comprehensive evaluation was conducted by correlation and principal component analysis, so as to explore the physiological mechanism of exogenous hydrogen peroxide (H2O2) enhancing saline-alkali tolerance. The results show that: (1) The contents of chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl t), net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and soluble protein (SP) of M9-T337 seedlings decreased gradually with the extension of saline-alkali stress. Intercellular CO2 concentration (Ci), soluble total sugar (TSS) content, proline (Pro) content, catalase (CAT) activity, ascorbate peroxidase (APX) activity, relative electrical conductivity (REC) and malondialdehyde (MDA) content all showed an increasing trend. The activity of superoxide dismutase (SOD) and peroxidase (POD) increased first and then decreased. Compared with CK, each index of M9-T337 seedling leaves changed in different amplitude after treatment with exogenous H2O2, and there was an obvious concentration effect. The decreases of Chl a, Chl b, Chl t, SP and Gs in leaves treated with T3 (0.4 mmol/L H2O2) were the smallest, and the increases of Ci, REC and MDA were the smallest. TSS, Pro and APX increased the most. (2) Leaf Pn of M9-T337 seedlings was positively correlated with Tr, Gs, Chl a, Chl b, Chl t, SP, SOD and POD, and negatively correlated with Ci, MDA, CAT, APX and REC. (3) Comprehensive evaluation showed that the effect of each treatment on the physiological characteristics of M9-T337 seedling leaves was in the order of CK>T3>T4>T2>T5>T1. The results showed that the suitable concentration of H2O2 could effectively improve the photosynthetic capacity of M9-T337 seedlings under salt-alkali stress, significantly increase the activities of antioxidant enzymes and the content of osmotic regulatory substances, and reduce the membrane permeability, so as to alleviate the salt-alkali stress, and 0.4 mmol/L H2O2 treatment had the best effect.