Introduction

During the last five decades, human activities have substantially altered the global nitrogen cycle, increasing both the availability and the mobility of nitrogen, especially inorganic nitrogen, over large regions of aquatic ecosystems (Vitousek et al., 1997; Carpenter et al., 1998). As a consequence, increased input of inorganic nitrogen has led to eutrophication of many water bodies, and has been attributed to change of ecological process in fresh water ecosystems. One of the significant changes of fresh water ecosystems is biological invasion of exotic aquatic weeds, which bring many serious problems, such as disruption of ecosystems, competition with native species and economic losses (Pimentel et al., 2000). In many cases, it has been shown that invasions in aquatic ecosystems are associated with elevated or fluctuating resource levels, and invasion success is thought to be the combined outcome of nutrient conditions and plant traits in aquatic habitats (Davis and Pelsor, 2001; Zedler and Kercher, 2004; Hastwell et al., 2008). Nitrate and ammonium, two inorganic nitrogen forms, are main components of eutrophic water bodies, and serve as available nitrogen resources for aquatic plant growth. Invasive species often have higher growth rates than that of non-invasive species, especially at high nitrogrn levels. Therefore, invasion of aquatic plant probably attributed to its strong capability to assimilate nitrogen at a fast rate and hence have greater ability to take advantage of high nitrogen availability.

Nitrate is the major nitrogen form for higher plant, and most plant species tolerate high nitrate concentrations without any sign of toxicity. However, ammonium as the sole nitrogen source leads to physiological disorders and reduced growth when compared with nitrate or mixed nitrogen nutrition. Most vascular plants acquire nitrogen as nitrate or ammonium, and the physiological response to different nitrogen forms were reported in many previous studies (Majerowicz and Kerbauy, 2002; Munzarova et al. 2006; Fernandes and Gelvan, 2007). Preference for nitrate or ammonium varies according to the plant species, which is probably related to the physiological adaptations of plants to natural ecosystems (Adams and Attiwill, 1982). In inorganic nitrogen assimilation process, nitrate reducrase and glutamine synthetase are two key enzymes, which regulate nitrogen utilization in plants. It is well established that nitrogen forms displayed seasonal variation, and physiological response to nitrogen forms may be related to ecological performance of plant species (Smirnoff and Stewart, 1986; Paulissen et al, 2004).

Eichhornia crassipes, a fast clonal growth plant, is considered as one of the worst invasive weeds in water body. After establishment, the invader rapidly spreads by generation of the ramets and occupies broad range of habitats in a short time (Howard and Harley 1998). The growth rate of E. crassipes seems to exceed that of any other aquatic plants, and this high propagation rate may depend on the peculiar physiological characteristics and nutrient absorption efficiency of water hyacinth. Many studies have dealt with the influence of nutrient supply on phenology, growth, nutrient absorption and storage of E. crassipes (Center and Spencer, 1981; Reddy et al., 1989; Greenway, 1997). Although large numbers of data, including biomass production, population dynamics, and phenotypic plasticity showed that advantage of nitrogen utilize in E. crassipes, only few investigation focused on its performances of nitrogen assimilation. E. crassipes displays more aggressive in adequate either nitrate or ammonium environments, which probably attributed to its flexible capability to assimilate different nitrogen forms effectively. Therefore, the study of the activity of key nitrogen assimilating enzymes may give valuable information on the physiological adaptive mechanism of invasive plant species under different nitrogen sources conditions.

The objectives of the present study are to examine the effects of nitrogen form on: (i) the relative growth rate and clonal growth; (ii) concentration of nitrate and free ammonium in tissues; (iii) the activity of nitrate reductase (NR), glutamine synthetase (GS) in roots and leaves of invasive plant species E. crassipes.

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