Objective To study the effects of continuous flooding on leaf photosynthetic characteristics and physiological and biochemical characteristics of young plant.

Method The Zigui section of the Three Gorges Reservoir riparian zone is affected by the water level operation of winter storage and summer discharge, forming a "drought-flooding-drought" water level fluctuation model, which makes the ecological environment of the riparian zone facing severe challenges. In this study, two-year-old Pinus elliottii seedlings were divided into six groups (including two control groups) with different initial drought and then flooding treatments and to study the effects of a simulated five-month winter flooding stress on photosynthetic characteristics and physiological and biochemical characteristics of seedling leaves after 45 days of drought stress.

Result The results showed that with the increase of flooding time, the net photosynthetic rate of Group 1 (initially under mild drought stress and then root-flooded) and Group 2 (initially under mild drought stress and then full-flooded) showed a "decline-gentle" trend, but the net photosynthetic rate of Group 3 (initially under moderate drought stress and then root-flooded) and Group 4 (initially under moderate drought stress and then full-flooded) showed a downward trend. After 60 days, the net photosynthetic rate was lower than that of the former. After 150 days of flooding, the net photosynthetic rate of P. elliottii decreased by 30.9%, 33.0%, 51.9% and 62.3% respectively compared with the original values. Meanwhile, water flooding significantly reduced the stomatal conductance and transpiration rate. By comparing the relationship between Ci and Ls, it is concluded that the decrease of net photosynthetic rate of P. elliottii was mainly determined by stomatal factors in the early stage, and non-stomatal factors in the later stage. The content of soluble protein in root-flooded groups was mostly not significantly different from that in the control group during the flooding stage, while the content of soluble protein in leaves of P. elliottii in full-flooded group increased significantly at the early stage (60 days before flooding), but decreased gradually at the later stage. At 45 to 60 days of flooding, the activity of SOD in P. elliottii needles were significantly higher than those in the control group. The MDA content of P. elliottii leaves in different flooding conditions was not significantly different from that of the control groups within 30 days before flooding, but after 60 days of flooding, the MDA content of P. elliottii leaves increased significantly and gradually stabilized to a certain level.

Conclusion These results can provide reference for the selection of suitable species and vegetation restoration and reconstruction in Three Gorges Reservoir riparian zone.