Submergence Tolerance Strategy Of Completely Submerged Alternanthera Philoxeroides As Affected By Light, Dissolved Oxygen And Water Pressure

The riparian zone is a transitional zone which lies between the terrestrial ecosystem and the river ecosystem. It is an important transitional zone that the material, energy and information exchanged between the terrestrial ecosystem and the river ecosystem. Due to the occurrence of rainfall, flood and the operation of reservoir, the plants perched in the riparian zone are often being flooded, even completely submergence. After the accomplishment of the Three Gorges reservoir, the water level fluctuated in a regular manner each year, ranging from 145 m to 175 m above sea level. The serious flooding environment must threaten the survival of plant, which may cause many ecological problems such as vegetation destruction in the littoral zone and finally affects the ecological function of the riparian zone.Two main survival strategies that allow plants to cope with submergence are: Escape strategy and Quiescence strategy. In the escape strategy, the plant grows or elongates its shoot in an attempt to reach the surface and restore air contact. This strategy is beneficial only in shallow and prolonged floods, when plants are able to reach the surface before the stress becomes lethal. In contrast to the escape strategy, the low-oxygen quiescence strategy is defined by reduced levels of growth, conservation of energy reserves and a delay of the energy crisis. This strategy is particularly beneficial when floods are deep but transient. Plants adjust its strategies depending on the depth of submergence. Escape strategy is beneficial only in shallow and Quiescence strategy is particularly beneficial when floods are deep.Light, dissolved oxygen and water pressure are the main environmental factors under submergence. The three factors are changing with the depth of the submergence. The level of light and dissolved oxygen gradually weakened and the water pressure continuously enhanced with the increasing water level.A study about the effect of light, dissolved oxygen and water pressure on the submergence strategy was done to provide ecological basis and explore the submergence tolerance mechanism of plants.To reveal the effect of light, dissolved oxygen and water pressure on the submergence tolerance strategy, we took Alternanthera philoxeroides which is the common invasive plant specie as the objects. Flooded groups and non-flooded control group were set in the experiment. Every environmental factors divided to two levels: light(L+),dark(L-), high dissolved oxygen(Do+), low dissolved oxygen(Do-), high pressure(Wp+), low water pressure(Wp-). 8 complete submergence treatments were designed: L+Do-Wp-, L+Do-Wp+, L+Do+Wp-, L+Do+Wp+, L-Do-Wp-, L-Do-Wp+, L-Do+Wp-, L-Do+Wp+. During the submergence period, light and oxygen must be controlled at the same level in flooded groups. The following parameters were measured before flooding: stem length, internode number, internode length, stem diameter. At the end of the experiment, we harvested the whole plant, including above-ground parts and below-ground parts, and measured the same parameters above mentioned and some new parameters, such as adventitious root length. The plants were separated into main stem, leaf, and under-ground parts, and cleaned by tap water. All plant parts were oven-dried for 72 h at 60℃. Then the dried parts weighed respectively and measured carbohydrate content. It was found that:(1) The water environmental factors: light, dissolved oxygen, and water pressure have an important impact on the growth and survival of A. philoxeroides. A. philoxeroides can produced carbohydrate to maintain life activities. The lack of light in a short period of time does not lead to the death of the flooded plants. The morphological characteristics of leaf of A. philoxeroides were affected by the factor of light. In contrast, the dissolved oxygen in water was the key environmental factor that limited the survival of A. philoxeroides in water. The low dissolved oxygen content in water would lead the plants death. High pressure in water environment would inhibit the growth of A. philoxeroides, in order to prolong the survival time under water.(2) The light, dissolved oxygen and water pressure in water have an impact on the content of carbohydrate of A. philoxeroides. A. philoxeroides can photosynthesize and produce carbohydrates to ease the crisis with light under water. The concentration of oxygen in water determined the manner of respiration and utilization efficiency of carbohydrate. Under high dissolved oxygen treatment, A. philoxeroides took large amounts of carbohydrates in order to elongate. High water pressure could inhibited the growth the stem and reduce the energy consumption.(3) A. philoxeroides’ s main stem elongation is affected by the effects of light, dissolved oxygen and water pressure. Dissolved oxygen can promote the growth of A. philoxeroides and water pressure can limits its growth and save more carbohydrate. Plants can change the strategy of submergence based on dissolved oxygen and water pressure.