## 1 Introduction  
Nitrogen, especially ammonium, is one of the most indispensable nutrients for growth and reproduction of macrophytes in aquatic ecosystems [1; 2]. However, dramatically increased ammonium input and accumulation in aquatic ecosystems severely deteriorated the water quality and ecological community as a whole [3; 4; 5]. Different advanced treatment approaches have been developed and deployed for wastewater treatment. Constructed wetland (CW) is one that has been gaining greater attention in recent years [6]. Due to advantages including easy maintenance, on-site treatment, and good self-purification capacity [6], CWs are widely used to treat wastewater from industrial,agricultural, and livestock breeding activities in many small rural areas where large-scale wastewater treatment plants are scarce [7]. In CWs, macrophytes play important roles in nitrogen removal since macrophytes can absorb and assimilate the nitrogen nutrients to constitute amino acids and proteins in tissues [8; 9]. Meanwhile, as economic plant species, many wetlands plant species show rapid growth, produce a large amount of aboveground biomass, and own the potential to be used as a source of feed and biomaterials as well as bioenergy [4; 10]. However, ammonium of high concentrations in wastewaters were usually toxic to most macrophytes, which influenced the treatment effects of CWs [4; 6]. Generally, plants including macrophytes prefer to uptake ammonium nitrogen since it is less energy-consuming [2; 11], and all inorganic nitrogen is first reduced to ammonium, which will be assimilated by plants into nitrogen-carrying amino acids [12; 13]. Ammonium in plants such as _Arabidopsis_ can originate from nitrate reduction, direct absorption, photorespiration, or deamination of nitrogenous compounds, such as asparagine [2; 11]. Afterward, ammonium enters the biosynthetic pathways of plant cells in order to produce different amino acids [2], which are used to synthesize proteins and other nitrogenous compounds. Typically, ammonium is assimilated into glutamine and glutamate in plant cells, which are catalyzed by glutamine synthetase (GS) and glutamine oxoglutarametotransferase (GOGAT), respectively [13; 14]. Besides the main pathway of GS/GOGAT, plants also possess alternative routes, such as reversible amination of 2-oxoglutarate (2-OG) to produce glutamate through glutamate dehydrogenase (GDH) [15; 16]. GDH can release amino nitrogen from amino acids yielding a keto acid and NH\({}_{3}\) that can be separately recycled in respiration and amide formation, respectively [17]. Meanwhile GDH may catalyse in the deamination direction in tissues that converts amino acids to transport compounds at a low C:N ratio [2; 17]. Although high concentrations of ammonium are toxic to plants, since it can induce proton extrusion, which is associated with ammonium uptake, changes in cytosolic pH and uncoupling of photophosphorylation in plants [14; 16; 18], plants have developed metabolic processes, such as nutrient remobilization, to alleviate ammonium toxicity. Toxic ammonium will be rapidly converted into organic form (amino acids) to avoid negative effects and provide nitrogenous compounds suitable for source-sink transport [5; 16]. Meanwhile, pathways of second metabolites such as phenolics, flavonoids, and anthocyanins can also alleviate ammonium toxicity [14; 16; 19]. Therefore, based on the requirement of wastewater treatment, macrophytes that can grow and tolerate ammonium at high concentrations are preferred in CWs [4]. Recent studies showed that a C4 plant, _M. aquaticum_, can better tolerate ammonium exceeding 14 mmol/L, which indicated that this macrophyte is more adaptive to high ammonium loading wastewater, when compared with widely used macrophytes, such as _Typha latifolia_, _Juncus effusus_, and _Cyperus alternifolia_s [4; 20].