Effects Of Flooding And Drought Stress On Photosynthesis And Nutrient Contents Of Hemarthria Altissima And Cynodon Dactylon

In the hydro-fluctuation belt of the Three Gorges Reservoir Area(TGRA), plants are subjected to a relatively long period of annual flooding which can affect their growth and survival. When the flooding recedes, plants may experience drought stress that can also affect their growth subsequently. Thus, screening for suitable species that can survive these fluctuating hydrological conditions is necessary for the successful restoration of vegetation to the hydro-fluctuation belt of the TGRA. Hemarthria altissima and Cynodon dactylon are two common plants in TGRA, and are well adapted to flooding. However, the response of these plants to drought following flooding and the physiological mechanisms behind such a response are unknown. In order to determine whether or not flooding has any effects on plants susceptibility to a subsequent drought, six treatments were imposed, including control(CK, soil water content 60%–63% of soil water field capacity), soil-surface flooding(SF, with water level 5 cm above the soil surface), total flooding(TF, with water level 1 m above the soil surface), control-drought(CD, control group followed by drought treatment), soil-surface flooding-drought(SFD, soil surface flooding followed by drought) and total flooding-drought(TFD, total flooding followed by drought). Three phases were consecutively adopted during the experimentation, including Phase I of flooding, Phase II of drought, as well as Phase III of restoration.The result indicated that flooding and drought stresse had significantly affected growth and biomass allocation of H. altissima and C. dactylon. During flooding stress, plant height, base diameter, branch number, leaf number, root biomass, stem biomass, leaf biomass, and total biomass of two plant specises in the TF groups were significantly lower than that of CK.Compared to CK, plant height, base diameter, branch number, leaf number, root biomass, stem biomass, and total biomass in TFD were significantly lower than in CK, whereas plant height, root biomass, stem biomass, leaf biomass and total biomass in CD and SFD showed no significant difference when compared to that in CK at the end of Phase II, respectively. In addition, no significant differences in plant height and total biomass between CD and SFD were observed. At the conclusion of the recovery period, growth and biomass of t two plant species in all treatments were recovered to some extent.The results also indicated that there were significant treatment effects on photosynthesis of H. altissima and C. dactylon under different water regimes. At the end of flooding stress, net photosynthetic rate(Pn), transpiration rate(Tr), stomatal limitation(Ls), and Water use efficiency(WUE) of H. altissima in the SF and TF groups were significantly lower than that in CK, whereas the intercellular CO2 concentration(Ci) in SF and TF were significantly higher than in CK; Similar to H. altissima, photosynthetic rate(Pn), Ls, and WUE of C. dactylon in SF were significantly reduced as compared to CK. Under drought stress, Pn, Gs, and Tr of H. altissima in groups CD and SFD were significantly lower than in CK, whereas in TFD, they showed no significant differences to the control at the end of drought stress. In contrast to the SF and TF groups, Ci in the CD, SFD, and TFD groups was slightly lower than that in CK, respectively. Compared to controls the drought treatments(CD and SFD) of C. dactylon exhibited lower Pn and Gs, and the changes of these indexes in TFD were similar to H. altissima, which showed no significant difference to the control group. By the end of the recovery period, the Pn, Gs, Ci, Tr, Ls, and WUE of H. altissima and C. dactylon in all treatments showed no significant difference to those of the control, indicating that H. altissima and C. dactylon can adapt to flooding followed by drought stress.The results further indicated that during flooding stress, root nutrient composition(N, P, K, Fe, Mn) of of H. altissima and C. dactylon were higher than that in well water-drained control, whereas the Cu content in all flooding treatments was significantly reduced when compared to the control group. Leaf Fe, Mn concentrations of H. altissima were also higher in the SF and TF groups, but the contents of P and K in leaves were significantly lower in the SF and TF groups, when compared to that of CK, respectively, at the end of Phase I and Phase II. Under drought stress, root, stem and leaf N, K, Ca, Mg, Fe, Cu content of H. altissima and C. dactylon in groups CD and SFD showed no significant differences when compared to the control at the end of phase II. In constrat, in TFD, the root, stem and leaf nutrient contents(N, K, Ca, Fe, Mn) were higher than that of the control group. By the end of recovery period, root N, K, Mn contents of H. altissima and C. dactylon in TF were higher than that in the control group; However, the N, P, Fe contents in TF showed no significant difference to the control group at the conclusion of the recovery period. The above results indicated that previous flooding had no detectable effect on subsequent sensitivity of H. altissima and C. dactylon to drought,and two plant species could be candidate species in re-vegetation of the hydro-fluctuation belt of the Three Gorges Reservoir Area(TGRA) for its adaption to flooding and drought stress.