| Submerged macrophytes Potamogeton maackianus A.Benn, Potamogeton crispus L. and Vallisneria spinulosa Yan. are the dominant species in middle to lower reaches of the Yangtze River. In this thesis, large-scale population surveys and experimental studies are carried out to reveal the response of those species to environmental factors. The life-history strategies were used to estimate the response of submerged macrophytes to different water depth, sediment nutrient and the stress of light. The main conclusions are as follows:1. The life-history strategies of submerged macrophytes were influenced by water depth gradient. It shows that the above-ground and below-ground biomass of V. spinulosa gradually increased with the increasing water depth when it was less than 2 meters; when the water depth was more than 2 meters, the biomass of V. spinulosa decreased significantly, as well as the sexual reproduction was limited. From 1.4 meters to 2.2 meters water depth, the increase of water depth caused the decrease of biomass, harvest index and number of sexual propagules of V. spinulosa. Winter bud formation in V. spinulosa was significantly affected by water-level fluctuations in the growing season and propagating season. The disturbance would affect winter bud formation in V. spinulosa in the growing season more than in the propagating season. V. spinulosa subjected to changes in water depth had refuging life-history strategy for responding to this change. For submerged macrophytes P. maackianus, this species tended to produce less number of shoots, and allocate more biomass to leaves with the increase of water depth. P. maackianus adapted the change of water depth by means of adaptive strategy.2. The life-history strategies of submerged macrophytes were influenced by sediment nutrient. For V. spinulosa, submerged macrophyte V. spinulosa tended to show a high degree of morphological plasticity through the changes of its total stolon length, maximal leaf area and root length under low sediment nutrient, and the very high morphological variability of V. spinulosa raised the issue of the capacity of the species to colonise the more width range of sediment nutrient. Seed biomass of V. spinulosa inceased significantly with gradually increasing sediment nutrient level. V. spinulosa subjected to changes in sediment nutrient level had tolerant life-history strategy for responding to this change. For P. crispus, the growth of submerged macrophyte P. crispus showed a temporary physiological response to different sediment nutrients. The ratios between below-ground and above-ground biomass exhibited a dependent change on any one sediment nutrient. Allometric growth relationships between below-biomass and above-biomass of P. crispus were significantly affected by the sediment nutrient. With biomass allocation regulation, P. crispus firstly allocated more biomass to the formation of turions in the stress of sediment nutrients. The allocation mode of P. crispus was benefit for the mainetance of the populations. P. crispus adopted refuging life-history strategy in stress of sediment nutrients. For submerged macrophytes P. maackianus, the plants tended to decrease the speed of clonal growth, and produce shorter shoots with shorter internodes, less number of nodes as well as the plants produced longer roots at lower levels of substrate nutrient.3. The life-history strategies of submerged macrophytes were influenced by the stress of light. Light quantity adjusted by shaded treatments had profound influences on the associated morphological traits of V. spinulosa. The longest leaf length and leaf area significantly decreased with the degree of shaded treatments. The more degree shaded treatments were, the more biomass was allocated to the below-ground parts, resulting in the less ratios between above-ground and below-ground biomass. Shaded treatments limited sexual reproductive ability of V. spinulosa, which has reproduced less of ramets and tubers under shaded conditions versus natural conditions. V. spinulosa adapted the change of light by means of refuging life-history strategy.
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