Effects of exogenous 24-epibrassinolide on chlorophyll content and chlorophyll fluorescence characteristics of camphor seedlings under salt stress

In China, vast areas have faced the problem of soil salinization. To ameliorate this phenomenon, researchers found that it is useful by increasing the salinity tolerance of plants and strengthening the study of plant salt-resistance and the mechanisms of salinity tolerance. Cinnamomum camphora is a king of excellent evergreen tree, street tree and overshadow tree, however, it un-favorites in dry, barren and saline-alkali soil. Besides, soil salinity requires less than 0.2%. In Jiangsu Province, soil pH is on the high side. It is difficult to meet the normal growth needs of camphor plants because of the deficiency of organic matter, which seriously affects the introduction, use and reproduction of C. camphora. Therefore, it is of significance to study the mechanism of salinity tolerance.In order to reveal the effects of EBR on the salt tolerance of C. camphora, with camphor as experiment materials, we set the steadfast salt concentration at 100 mmol/L, applied different concentrations of EBR (0.1, 0.2, 0.3, 0.4, 0.5 mg/L) with the control by only using water, and observed the influence of EBR-stick treatment on camphor root morphological indexes, photosynthetic pigments and chlorophyll fluorescence parameters.The results showed that: compared with the control group, at a specific salt stress, the salt injury index (SI) of camphor seedlings increased, as well as the water saturation deficit (WSD)；meanwhile, the leaf relative water content, the specific leaf area, fresh and dry ratio (m_d/m_f), decreased to some extent and leaf succulence (LSE) also showed a significant decline.EBR treatment with different concentrations could significantly improve the relative moisture content specific leaf areas, fresh and dry ratio and leaf succulence of camphor seedlings；reduce SI, and reached the maximum when the EBR concentration was 0.3 mg/L.After suffering salt stress, the contents of chlorophyll a and chlorophyll b were significantly lower than that of the control groups, reducing 65.33% and 45.87%, respectively. The external application of EBR with different concentrations can significantly increase the content of chlorophyll of C. camphora；Chlb (chlorophyll b), Car and Chla (chlorophyll a) showed a similar variation tendency, and reached the maximum when the EBR concentration was 0.3 mg/L.0.3 mg/L EBR was the best choice that could alleviate the destructive effect of salt stress on PSⅡ reaction centers. It significantly improved the Fm (maximal fluorescence), Fv/Fm (optimal/maximal photochemical efficiency of PSⅡ in the dark), Fv/Fo (potential activity of PSⅡ), ФPSⅡ (potential photochemical efficiency of PSⅡ) of camphor seedling leaves and reduced Fo (minimal fluorescence) and NPQ (non-photochemical quenching) under 100 mmol/L salt stress.There were significant positive correlations among SI, WSD, Chl, specific leaf mass, and the significant negative correlations were observed with LSE, m_d/m_f. Therein, the three groups with larger correlation were: SI and water saturation deficit, water saturation deficit and chlorophyll content, leaf succulence and fresh and dry ratio. Fm, Fv/Fm, Fv/Fo, ΦPSⅡ had positive correlation with each other while had negative correlation with NPQ, Fo, Chl. Therein, the three groups with larger correlation were: Fm and Fv/Fo, Fv/Fo and ΦPSⅡ, NQP and ΦPSⅡ.