Effects of 2,4 epbrassinolide on photosynthetic and physiological charac- teristics of Malusa halliana under saline-alkali stress

Abstract:【Objective】The Loess Plateau is the main producing regions of apple in Northwest China. Due to the abundant sunshine and the great diurnal temperature difference, the apple planting area in this region is constantly expanding. According to statistics, over 7.6 million hectares of China’s cultivat- ed land is salinized, accounting for one fifth of the total cultivated land, and the northwest inland area is more seriously salinized, accounting for more than 13 percent of the total salinized area. The effect of saline-alkali stress on physiological and biochemical characteristics of plants has become a hot topic in recent years. The combined saline-alkali stress can cause osmotic stress and ionic toxicity, change the pH in vacuoles and cytoplasm, increase the content of reactive oxygen species in plant cells, break the balance among ions, decrease osmotic regulation ability, and affect the photosynthetic performance and normal growth of plants. In this study, the effects of exogenous 2, 4-epbrassinolide (EBR) on photosyn- thesis and physiological characteristics of Malus halliana seedlings grown under saline-alkali stress were studied.【Methods】In this experiment, 8 leaf-old M. halliana seedlings were used as the experi- mental material, and 7 treatments were set up, including irrigation with fresh water (CK1), irrigation with 100 mmol · L- 1 (50 mmol · L- 1 NaCl + 50 mmol · L- 1 NaHCO3) sodium salt solution (saline- alkali stress, or CK2), saline-alkali stress + 0.05 mg·L-1 EBR spraying (T1), saline-alkali stress + 0.1 mg·L-1EBR spraying (T2), saline-alkali stress+ 0.2 mg·L-1 EBR spraying (T3), saline-alkali stress + 0.4 mg·L-1EBR spraying (T4), and saline-alkali stress + 0.6 mg·L-1 EBR spraying. The treatments were conducted with 3 biological replicates each consisting of 10 seedlings. The membrane leakage or relative electrici- ty conductivity (REC), relative water content, chlorophyll content, photosynthetic characteristics, fluo- rescence parameters, antioxidant enzyme activity and contents of Na+ and K+ in each treatment were de- termined and principal component analysis was conducted.【Results】Exogenous EBR could effectively reduce the increases in REC, intercellular carbon dioxide concentration (Ci) and Na+ content in leaves of plants under saline-alkali stress. The relative water content, contents of chlorophyll a (Chl a), chloro- phyll b (Chl b), and chlorophyll a+b (Chl a+b), net photosynthetic rate (Pn), stomatal conductance (GS) and transpiration rate (Tr) of leaves were significantly decreased. The initial fluorescence (F0), maxi- mum fluorescence (Fm), photochemical quenching coefficient (qp), regulatory energy dissipation [Y(NPQ)], non-regulatory energy dissipation [Y(NO)], and activities of superoxide dismutase (SOD) and peroxidase (POD) showed first increased and then decreased. The seedlings treated with 0.2 mg·L-1EBR suffered the least damage under saline-alkali stress. Compared with CK2, the REC decreased by 9.90%, 16.96%, 25.05%, 16.78% and 11.12% in T1, T2, T3, T4 and T5, respectively, on the 9th day of stress, and relative water content in the leaves increased by 45.48%, 64.24%, 80.91%, 66.15% and 55.41%, respectively. The results indicated a concentration effect of EBR. The contents of Chl a, Chl b and Chl a+b were increased to different degrees by EBR treatments. Pn, GS, and Tr in all the EBR treat- ments were significantly lower than that of CK1, but significantly higher than in CK2. The results indicated that exogenous EBR could alleviate the photosynthetic damage caused by salt-saline stress. With the increase in EBR concentration, activities of both POD and SOD increased at first and then decreased. The peak values were found in T3 treatment, indicating that T3 had a stronger antioxidant capacity than the other treatments. The content of Na+ firstly decreased and then increased with the in- crease in EBR concentration, while the content of K+ displayed an opposite pattern and was highest in T3 treatment, indicating that the treatment caused the least damage under salt-alkali stress. Principal component analysis showed that the eigenvalues of >1 were 17.242, 2.31 and 1.129, and the variance contribution rates were 82.105%, 10.999% and 5.378%, respectively. The cumulative variance contribu- tion rate reached 98.482%. Based on principal component score sorting, the order from the least to most serious damage among treatments was CK1 > T3 > T2 > T4 > T5 > T1 > CK2【. Conclusion】EBR can alleviate salinity-alkalinity stress in M. halliana seedlings by maintaining the stability of antioxidant en- zyme system and membrane integrity and reducing the absorption of Na+. Application at 0.2 mg·L-1 has the best effect.