| Constructing vegetation to protect the ecological environment of hydro-fluctuation zone in Three Gorges Reservoir is scientific and effective. Due to cyclical fluctuations of water level, the soil of hydro-fluctuation zone repeatedly experienced the process of "flooding-drought-flooding", so selecting plant species that could simultaneously tolerate both flooding and drought environment and identifying the mechanism is very important. Pterocarya stenoptera C. DC. and Pinus elliottii Engelm. seedlings were the research objects, and two experiments that imitated the regular pattern of soil water in fluctuation region were carried out:1) Four water treatments were designed, including control experiment (CK, normal water condition), light drought stress(T1), saturate soil water(T2) and soil submersion(T3, flooding); 2) Another four water treatments were CK (control experiment), DR (normal water supply-light drought-normal water supply), FL (continuous flooding), FD (flooding-light drought-normal water supply). The major research focused on the growth, photosynthesis and chlorophyll fluorescence responses and adaptation mechanisms of Pterocarya stenoptera C. DC. and Pinus elliottii Engelm. seedlings to different soil water conditions. The main results were as following:1) The effects of soil water conditions on Pterocarya stenoptera C. DC. and Pinus elliottii Engelm. seedlingsUnder flooding and wet soil conditions, Pterocarya stenoptera C. DC. both could form hypertrophic lenticels and adve ntitious roots to help plants get more O2, to maintain the normal physiological activity. Light drought, saturate soil water and flooding inhibited the increase of plant height, overground biomass and underground biomass of two species seedlings. In saturate soil conditions, growth of seedlings were least affected. However, ground diameters of the seedlings of two species were significantly increased under saturate soil water and flooding conditions. Seedlings of two species had higher ratio of root to shoot in light drought conditions, which was drought resistance performance.The net photosynthetic rate of Pterocarya stenoptera C. DC. and Pinus elliottii Engelm. seedlings were both decreased by soil water stress, but appeared different tendency. Pn of Pterocarya stenoptera C. DC. seedlings in flooding decreased significantly after 25 days, and after 50 days, the sequence of Pn decreasing proportion was T1(60.3%)>T3(47.8%)>T2(28.1%), and then kept stable. And the flooded Pinus elliottii Engelm. seedlings' Pn decreased by 35.4%, but with time prolonging the net photosynthetic rate increased gradually, and at the end of stress, it was only lower than the control by 20.5%; the Pn of T1 and T2 Pinus elliottii Engelm. seedlings continuously decreased, in the end, T1, T2 and T3 were not significantly different. Stomatal conductance(Gs) and transpiration rate(Tr) of the seedlings of two specices were inhibited significantly by light drought stress, but that of T2 and T3 had no significant difference, and the Ci of T2 and T3 increased while T1 showed down trend; the main factor that causing net photosynthetic rate decreasing of T2 and T3 was non-stomatal limitation, while T1 decreased mainly by stomatal limitation. Water use efficiency (WUE) of T2 and T3 group Pterocarya stenoptera C. DC. and Pinus elliottii Engelm. seedlings decreased by different extents, while seedlings under light drought stress had higher WUE, WUE maximum of Pterocarya stenoptera C. DC. seedlings was 6.94μmol·mmol-1, and maximum WUE of Pinus elliottii Engelm. seedlings was 8.86μmol·mmol-1.To Pterocarya stenoptera C. DC. seedlings in flooding and saturate soil water conditions, the chlorophyll and carotenoid contents declined compared with the control, while they increased under light drought stress. The photosynthetic pigment content of T3 Pterocarya stenoptera C. DC. seedlings were always at the lowest level.The ratio of chlorophyll to carotenoid content ranged 4.475～6.920,while Chla/b ranged 2.518～3.216 among all groups. The content of photosynthetic pigment in T3 Pinus elliottii Engelm. seedlings was the minimum. At the end of stress, T1 had the highest photosynthetic pigments content. The ratio of chlorophyll to carotenoid content fluctuated between 3.604 and 5.042, while chlorophyll a to b between 2.136 and 3.393.2) The effects of alternation change of soil water on Pterocarya stenoptera C. DC. and Pinus elliottii Engelm. seedlingsContinuous flooding significantly reduced height and biomass of the seedlings of two species, but the ground diameter increased. To DR seedlings under light drought conditions, growth parameters and biomass significantly decreased, and Pterocarya stenoptera C. DC. seedlings decreased more; the biomass of FD seedlings did not change significantly after normal water supply. Ratio of free water to bound water of both kinds of seedlings declined under flooding and light drought stress, and it increased rapidly to the control level after normal water supply.To both DR and FD treatments, the net photosynthetic rate of Pterocarya stenoptera C. DC. and Pinus elliottii Engelm. seedlings decreased, but both species seedlings could recover to the normal level after rewatering. FD of two species showed the lowest transpiration rate and stomatal conductance when exposed to light drought stress.Chlorophyll content of DR Pterocarya stenoptera C. DC. seedlings was not significantly different from the control.And that of FD under drought stress was significantly higher than FL, and increased rapidly after rewatering, in the end it was only lower than the control by 18.9%. The carotenoid content of DR under light drought stress decreased, and that of FD was significantly higher than FL.The ratio of chlorophyll to carotenoid content was in the range of 4.798 and 6.729, and Chla/b fluctuated between 2.163 and 2.823.Chlorophyll content of each group of Pinus elliottii Engelm. seedlings decreased first and then increased. The DR carotenoid content significantly decreased in light drought stress, and increased after rewatering. And carotenoid content of FD was significantly higher than the previous drought condition. The ratio of Chlorophyll to carotenoid content was between 4.379 and 6.019, and Chla/b ranged 2.207～2.850.Fv/Fm of FL Pterocarya stenoptera C. DC. seedlings decreased significantly, while that of DR and FD had no significant difference from the control, at the end of experiment, their values were 0.808 and 0.791. The changes in soil water did not significantly affect PS II photochemical efficiency (Fv/Fm) of Pinus elliottii Engelm. seedlings.ETR and Yield of Pterocarya stenoptera C. DC. and Pinus elliottii Engelm. seedlings that were flooded decreased first,and then increased. ETR and Yield of FD Pterocarya stenoptera C. DC. seedlings increased to control levels under drought stress, and that of DR were significantly lower than control but recovered to control level after rewatering. qP of FD were not significantly different in drought condition, but increased significantly after water re-supply. qP and qN of DR was not subject to drought stress. qN of FD was significantly higher than the control after rewatering. ETR and Yield of FD and DR in Pinus elliottii Engelm. seedlings showed the same trend, with the former increasing under drought stress, while the latter declining, but not significantly different from the control. Under drought stress, the qP and qN of DR increased, while FD had no significant difference from the control.In summary,the growth, photosynthetic characteristics and photosynthetic pigment content and chlorophyll fluorescence parameters of Pterocarya stenoptera C. DC. and Pinus elliottii Engelm. seedlings showed tolerances to flooding and drought stress. Therefore, two species both could be considered for vegetative rehabilitation of the Three Gorges reservoir region.
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