Effects Of Water Stress On Physiological And Biochemical Responses Of Metasequoia Glyptostroboides In Water-fluctuation-zone Of The Three Gorges Reservoir

The aim of this study was to investigate the responding characteristics of the M. glyptostroboides seedings to water level change in the Three Gorges Reservoir Area (TGRA), and provide theoretical basis for species selection for revegetation of the riparian zone of the TGRA. Measured were indexes including protective enzymes such as Superoxide dismutase (SOD), Peroxidase (POD), Ascorbate peroxidase (ASP) and Catalase (CAT), osmotic adjustment substances such as soluble protein and free proline, and membrane lipid peroxidation such as Malondialdehyde (MDA), as well as photosynthetic characteristics and biomass accumulation of the two-year old M. glyptostroboides seedings to submergence, upon mimicking water level change in the riparian zone of the TGRA. Based on soil moisture change pattern in the TGRA, water treatments including control (C), half-submersion (HS), and full-submersion (FS) were applied. Field experiment, the same batch of M. glyptostroboides seedings with indoor simulation experiment were planted in zhongxian sites of the TGRA.175 m,170 m and 165 m, three different treatment groups were applied.Results showed that net photosynthetic rate (Pn) of Metasequoia glyptostroboides seedings in the HS group were significantly higher than that in C group after 75-day submersion. Conversely, after a submersion of 150 d, Pn in HS, and root and total biomass both in HS and FS were significantly lower than that in C. However, under drought treatment, Pn and stomatal conductance (Sc) in CD and HSD were significantly decreased as compared to that in C, while there was no significant difference between FSD and C. Similarly, the root and total biomass significantly declined in all of the three drought conditions compared to C. The activities of SOD, POD, ASP, CAT and content of free proline of M. glyptostroboides seedings in HS and FS group were higher than that in CK after 75-day submersion. Under submersion, MDA content in HS and FS group increased as compared to that in CK. Leaves of M. glyptostroboides seedings in FS were leafbud, and survival rates were 100%. After 150-day submersion, contents of membrane lipid peroxidation (MDA), O2, free proline, soluble sugar and soluble protein content in HS were all significantly higher than that in C. Similarly, the contents of superoxide dismutase (SOD), peroxidase (POD) activity, MDA, free proline, and soluble protein content in FS were all significantly increased when compared to C. However, when drought stress was imposed, activities of SOD, POD activity in CD and HSD were significantly higher than that in C, while the activity of catalase (CAT) activity in FSD was significantly higher than C. Moreover, the soluble sugar content significantly increased in all the three drought stress conditions when compared to C; However, no significant difference was found in Pn, Tr, SOD, POD, CAT activity, MDA, O2, free proline, soluble sugar content and root, stem, leaf biomass between CD and HSD, indicating that prior flooding had no detectable effect on subsequent sensitivity of M. glyptostroboides to drought. By the end of the recovery period, Pn, Sc, CAT activity and the contents of free proline of the saplings under different water stresses were comparable as that of C, respectively, except for the Pn in HSD and FS, CAT activity in FSD.Experienced a cycle of seasonal water stress in the TGRA, M. glyptostroboides seedings in 165m group were all death, while in 170m and 175m group were all survive. Similar to indoor short-term (75 d) simulating experimental results, Pn of M. glyptostroboides seedings in 170m group was significantly higher than that in 175m group, while the Ci was significantly lower. The activities of ASP, CAT and content of soluble sugar of M. glyptostroboides seedings in 170m group were significantly higher than that in 175m group. Root and total biomass of of M. glyptostroboides seedings in 170m group was significantly lower than that in 175m group, which was similar to results of indoor long-term (150 d) simulating experiment. Although Sc, Tr, the activities of SOD, POD and content of soluble protein, MDA of M. glyptostroboides seedings in 170m group were increased, no significant difference was found between 170m group and 175m group.The research results showed that prior flooding had no detectable effect on subsequent sensitivity of M. glyptostroboides to drought. Results indicated that antioxidant enzymes and osmotic adjustment substances have a positive response during submersion, M. glyptostroboides saplings showed tolerance to both flooding and drought, and thus can be a candidate for the revegetation of the hydro-fluctuation belt of the TGRA.