| The Hydro-Fluctuation Belt with area of 300km2 and vertical drop 30m was formed along the two banks of the reservoir because of the natural fluctuation of water level after the finish of the Three Gorges Dam. The original plants could not survive the long time flooding in Hydro-Fluctuation Belt. This consequently leads to the occurrence of serious environmental problems such as soil erosion and reservoir landslide. Many scholars believe that the artificial vegetation reconstruction is one of the important measures to protect the ecological environment of Hydro-Fluctuation Belt. The selection of flooding-tolerant species and understanding their flooding-tolerant mechanism play important roles in the vegetation restoration and rehabilitation of the Hydro-Fluctuation Belt.In this paper, we selected Taxodium hybrids’zhongshansha 405’,’zhongshansha 406’,’zhongshansha 407’,’zhongshansha 502’ as well as their parents Taxodium distichum (Linn.) Rich, and Taxodium mucronatum Tenore. as study materials. We conducted a simulation experiment to study the response mechanisms of the plants’ growth under flooding stress, plants’growth after recovery and plants’photosynthesis after recovery in different submerged environments. Three treatments were installed including no submergence (CK; control), semi-submergence (T1) and fully submergence (T2; top of plants 30cm below water surface). All plants experienced 12 months flooding and a following recovery for 6 months. This research tries to provide a theoretical basis for suitable plant selection in vegetation restoration and reconstruction of Hydro-Fluctuation Belt.The results indicated the following conclusions in the study of the growth characteristics under flooding stress, recovery growth and adaptation of photosynthesis after water flooding of Taxodium hybrids ’zhongshansha’and their parents:(1) Taxodium hybrids ’zhongshansha’ and their parents all survived after semi-submerged for 12 months; Taxodium hybrids ’zhongshansha’and Taxodium distichum (Linn.) Rich.all survived after fully submerged for 12 months; Taxodium mucronatum Tenore all died after fully submerged for 12 months. Taxodium hybrids ’zhongshansha’ and Taxodium distichum (Linn.) Rich., which can survive after fully submerged for more than 30 days, are tree species of strong flooding tolerance according to world standard and can be tentatively applied to the afforestation in Three Gorges Dam.(2)The increase of plant height and ground diameter of Taxodium hybrids ’zhongshansha 405’,’zhongshansha 406’,’zhongshansha 407’,’zhongshansha 502’and their parents were higher than the control under the semi-submergence treatment. That was because the plants adopted an ’Escape Strategy’, in which way the plant height remarkably increased, letting leaves sticking out of the water as many as possible, thus guaranteed its photosynthesis; meanwhile, aerenchyma and lenticels were improved near stem base, which improved gas exchange, thus reduced anaerobic respiration and improved the utilization efficiency of organic matters.(3) The increase of plant height and ground diameter of Taxodium hybrids ’zhongshansha 405’,’zhongshansha 406’,’zhongshansha 407’,’zhongshansha 502’and their parents were inhibited significantly under the full-submergence condition. That was because plants adopted a’Silent Strategy’ when they could survive relying on the nutrient storage. So the author speculate Taxodium mucronatum Tenore’s death after fully submerged was because the nutrient storage was insufficient.(4) After experiencing the 6 month recovery since the remove of flooding stress, the plant height and ground diameter of Taxodium hybrids ’zhongshansha’ and their parents increased more than the control. This is a kind of "Compensation Effect" which makes plants recover fast after flooding. It is a positive self-adjusting function.(5) After the remove of the semi-submergence stress, the increase of the plant height and the ground diameter of Taxodium hybrids ’zhongshansha’ and Taxodium distichum (Linn.) Rich. were bigger than that of those relieved from the full-submergence stress. This is because a plant will utilize its limited nutrient storage and allocate its biomass to grow leaves so that it can generate a large amount of leaves and accumulate photosynthetic product with a faster speed. Therefore its plant height and ground diameter will increase slower. This is probably an optimized recovery growth pattern of deep flooded plants.(6) After experiencing a 6 month recovery since the remove of flooding stress, Taxodium hybrids ’zhongshansha’ were tested for every photosynthesis indexes and compared with the control. It was found that its net photosynthetic rate increased remarkably and its stomatal conductance increased while the intercellular CO2 concentration decreased. It indicated that its photosynthetic potential increased as a result of a combined action of stomatal factor and non-stomatal factor. Moreover, the large change scope of stomatal conductance and the small change scope of intercellular CO2 concentration together indicated that stomatal factors took a larger role. In addition, the light compensation point of Taxodium hybrids ’zhongshansha’ decreased while its light saturation point went up remarkably. This indicated its utilization capacity of both strong light and weak light all strengthened remarkably, which made its net photosynthetic rate increase significantly. At the same time, the increase of its dark respiratory rate indicated that Taxodium hybrids ’zhongshansha’ was more vigorous than the control after waterlog and had a higher demand for materials and energy.(7) The change scopes of every photosynthesis indexes of Taxodium hybrids ’zhongshansha’ after a full-submergence were larger than those after a semi-submergence. This indicated the Taxodium hybrids ’zhongshansha’ after a full-submergence had a stronger photosynthetic recovery ability than that those after a semi-submergence. This is because a deeper waterlog depth leads to a larger porosity of plants. A larger porosity can help improve the aeration condition of the root. A higher oxygen concentration in root can help improve the root vigor and hence promote the plants’absorption of organic nitrogen in soil and ultimately enhance the photosynthesis of plants.(8) After experiencing a 6 month recovery since the remove of a semi-submergence stress, Taxodium mucronatum Tenore were tested for every photosynthesis indexes and compared with the control. It was found that its net photosynthetic rate increased remarkably and its stomatal conductance increased while the intercellular CO2 concentration decreased. It indicated that its photosynthetic potential increased as a result of a combined action of stomatal factor and non-stomatal factor. Moreover, the large change scope of stomatal conductance and the small change scope of intercellular CO2 concentration together indicated that stomatal factors took a larger role. In addition, the light compensation point of Taxodium mucronatum Tenore decreased remarkably while its light saturation point went up remarkably. This indicated its utilization capacity of both strong light and weak light all strengthened remarkably, which in the meantime was an important factor of the increase of its net photosynthetic rate. At the same time, the increase of its dark respiratory rate indicated that the growth and metabolism of Taxodium mucronatum Tenore after flooding were more vigorous than the control.(9) After the remove of the flooding stress, the net photosynthetic rate of Taxodium distichum (Linn.) Rich, decreased with the increase of its stomatal conductance and the intercellular CO2 concentration. The non-stomatal factor tooks the main role for the decrease of the net photosynthetic rate. This indicated that after the remove of flooding stress, the photosynthetic capacity of Taxodium distichum (Linn.) Rich, was still inhibited. Moreover, the net photosynthetic rate of Taxodium distichum (Linn.) Rich, after a full-submergence stress was lower than the statistic of that after a semi-submergence stress. This indicated that a deeper submergence depth results in a stronger inhibiting effect. In addition, after the remove of flooding stress, the utilization capacity of stronger light of Taxodium distichum (Linn.) Rich.decreased remarkably. This was also an important reason why it had a weak photosynthetic capacity. In this research, though Taxodium distichum (Linn.) Rich.also possessed a strong flooding tolerance. But its photosynthetic recovery capacity was not as good as hybrid Taxodium hybrid ’zhongshansha’.(10)The growth under a flooding condition, the recovery capacity after the flooding and the photosynthetic physiologic recovery capacity of Taxodium hybrid ’zhongshansha’ were all stronger than its parent. It performed a favorable heterosis. |
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