Effect Of Different Flooding Time On Photosynthetic And Physiological Characteristics Of Phoebe Zhennan Saplings

Flooding is a form of waterlogging which is a common stress for plants.Water flooding leads to soil to be in a state of water saturation for a long time,resulting in anoxic soil,which inhibits the energy metabolism and ion absorption of plants and then affects the physiological metabolism and morphological characteristics.Phoebe zhennan is a rare and endangered species that is unique to China and considered as a protection species.It is not only a valuable timber,but also an excellent ornamental tree species.Due to its important economic value and wide range of uses,in the ecological restoration and landscaping construction of recent years,large areas of P.zhennan plantations have been developed in the low hilly areas of western Sichuan Basin?Rainy Area of Western China?.In the construction of urban gardens and rural“four sides”greenery,a lot of P.zhennan was also planted.However,the heavy rainfall in summer and continuous autumn rain in this area often cause water flooding or accumulation in the forest land,which poses a serious threat to the growth of P.zhennan saplings.Due to the cyclical waterlogging every year,P.zhennan saplings planted in low-lying areas in the western Sichuan Basin grow badly and even die.Therefore,the correct choice of site conditions and scientific water management are the necessary conditions to develop P.zhennan plantation.In this study,a submerged method taking 7 d as a flooding gradient was used to study the effects of waterlogging on photosynthesis,resistance physiology and growth dynamic of 2.5-year-old P.zhennan saplings.The correlation of each indicator with the duration of flooding was also analyzed.The results are as following.The net growth of height and root activity of P.zhennan saplings significantly?P<0.05?decreased with the increase of duration of flooding.The net growth and root activity reduced by 80.8%and 89.44%respectively compared with CK after 28days of flooding?D28?.Chloroplast pigments content,leaf net photosynthetic rate?P_n?,stomatal conductance?G_s?,and transpiration rate?T_r?decreased significantly with the increase of flooding stress time?P<0.05?,while intercellular carbon dioxide?C_i?concentration increased significantly with the extension of stress time?P<0.05?.Under the same light intensity,P_n,showed a significant downward trend with the increase of waterlogging stress time.The decrease of light saturation point?Lsp?and the increase of light compensation point?Lcp?indicate that the effective use of photosynthetic active radiation by P.zhennan saplings was reduced.P_n increases with the increase of CO₂ concentration,but the magnitude of the rise decreased with the increase of waterlogging stress time,suggesting that the assimilation ability of CO₂weakened.During the flooding stress,the active oxygen accumulated in roots of P.zhennan saplings,expressing as the contents of hydrogen peroxide?H₂O₂?and malondialdehyde?MDA?increased slowly,indicating that the degree of cell membrane lipid peroxidation increased.However,H₂O₂ and MDA in the leaves decreased during the flooding,and the activity of superoxide dismutase?SOD?and peroxidase?POD?increased significantly.It may be because t the water-flooding stress stimulated the anti-oxidation protection system in the P.zhennan saplings,leading to an enhancement inability of active oxygen removal and reduction in contents of H₂O₂ and MDA.The change trend of soluble sugar content in leaves was different from that in roots,while the change trend of soluble protein was consistent in both organs,indicating that the sensitivity of different osmotic substances in different organs of plants to flooding stress is different.In summary,under waterlogging stress,stomata of leaves of P.zhennan saplings closed and the photosynthetic pigment content reduced.The photosynthetic organs were destroyed to a certain extent,resulting in a decline in the utilization of light and CO₂.Although P.zhennan saplings showed a certain degree of tolerance to short-term flooding stress,long-term waterlogging stress could decrease the osmotic adjustment ability,and damage the antioxidant system,the cell structure and function,and eventually leaded to plant death.