三峡库区消落带不同水淹强度下池杉与落羽杉的光合生理特性

Photosynthetic characteristics of Taxodium ascendens and Taxodium distichum under different submergence in the hydro-fluctuation belt of the Three Gorges Reservoir

为探究三峡库区消落带原位人工种植的池杉(Taxodium ascendens)和落羽杉(Taxodium distichum)在该特殊生境下的适应机制,设置了浅淹(SS,海拔175m,对照)、中度水淹(MS,海拔170m)和深度水淹(DS,海拔165m)3个水淹处理组,测定了两树种在连续4个周期性水淹处理后的光合响应过程,并采用直角双曲线模型、非直角双曲线模型、直角双曲线修正模型和指数模型进行拟合,比较各模型拟合结果差异并选出最优模型,通过最优模型来分析水淹后落干期两树种的光合生理变化。结果表明:(1)不同模型间的光响应曲线拟合结果存在差异(P<0.05)。综合分析可知,4种模型中,指数模型是拟合两树种光响应曲线的最优模型,更符合植物的生理学意义。(2)两树种的光响应曲线有相似的变化规律,光合速率(P_n)的表现均为DS组>MS组>SS组。(3)中度水淹和深度水淹两树种的最大净光合速率(P_(nmax))、表观量子效率(α)、光饱和点(LSP)均高于浅淹,而光补偿点(LCP)则显著降低。以上结果说明落干生长期两树种对强光和弱光的利用能力增强,水淹对两树种的光合潜力有一定的促进作用,这可能与其遭受水淹胁迫后的自我调节能力和光合补偿机制有关。由此表明,当生境发生不良变化时,植物的适应性变化往往表现为沿着有利于光合作用最大化的方向发展。

After the completion of the Three Gorges Dam Project, the water level of the Three Gorges Reservoir Area （TGRA） fluctuated between 145m a.s.1, in summer and 175m a.s.1, in winter. These extreme water fluctuations along with an annual 30m water-level drawdown have led to the formation of the hydro-fluctuation zone around the reservoir, turning the terrestrial ecosystem into a wetland experiencing winter flooding every year. Under these water fluctuation conditions, the richness and diversity of the vegetation decreased and soil erosion was exacerbated, which further damaged the ecological structure and function of the riparian ecosystem of the reservoir area. The effective way to solve this problem is to restore the riparian vegetation artificially. Understanding the photosynthetic adaptation mechanism of water-tolerant species in the hydro-fluctuation zone is of great importance to undertake vegetation restoration in the TGRA. In order to research the adaptation mechanism of the species suitable to the hydro-fluctuation belt of the TGRA, we designed the following three water regimes： periodic shallow submergence （SS, 175m） , moderate submergence （MS, 170m） , and deep submergence （DS, 165m）. Li-6400 portable photosynthetic measurement system was used to test the light response curves of photosynthesis of Taxodium ascendens and Taxodium distichum after four cycles of submergence, in situ, in the TGRA. The light response curves were fitted and analyzed using the rectangular hyperbola model, the non-rectangular hyperbola model, the rectangular hyperbola modified model, and the exponential equation model. The optimal model was screened out from the four models by analyzing proximity between the simulated and measured values of photosynthetic parameters, and then it was used to analyze the photosynthetic characteristics of the two species under different submergence conditions. Results showed that ： （ 1 ） The fit of different models was significantly different. The exponential equation model was the best model for the light response curve in accordance with the physiology of the two species ; （2） The light response curves of the two species showed similar trends in variation, and along with an increase in the intensity of submergence, the photosynthetic rate of the two species also increased ; （3） The maximum photosynthetic rate, apparent quantum yield, and light saturation point of the two species growing under both deep and moderate submergence conditions were higher than that in shallow submergence, while the light compensation point of the two species decreased significantly in deep submergence conditions. These results suggest that the utilization capacity of both low light and high light of the two species were improved after submergence. Submergence also stimulated the photosynthetic potential of the two species, which may be related to the positive self-adjustment ability and the photosynthetic compensation of the two species after submergence stress. This also indicates that when the habitat is stressed, plants often adapt themselves to maximize photosynthesis. Thus, it is appropriate to use these two species for reconstructing the vegetation in the hydro-fluctuation belt of the TGRA.