Photosynthetic characteristics of Taxodium ascendens and Taxodium distichum under different submergence in the hydro-fluctuation belt of the Three Gorges Reservoir
After the completion of the Three Gorges Dam Project,the water level of the Three Gorges Reservoir Area (TGRA) fluctuated between 145m a.s.l.in summer and 175m a.s.l.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 spccies 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.
Three Gorges Reservoir Areahydro-fluctuation beltsubmergenceTaxodium ascendensTaxodium distichumphotosynthesislight response curves
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