Comparison of Effects of Ammonia-nitrogen Concentrations on Growth of Four Kinds of Submerged Macrophytes

Four local submerged macrophytes of Hydrilla verticillata, Vallisneria natans, Ceratophyllum demersum and Najas minor were chosen to investigate the effects of ammonia-nitrogen(NH4+-N) on their growth by measuring their biomass, chlorophyll contents, soluble sugar contents and peroxidase(POD) activities. The results indicated that the maximum optimum concentrations(MOCs) of ammonia-nitrogen for Hydrilla verticillata, Vallisneria natans, Ceratophyllum demersum and Najas minor were 2, 2, 1 mg·L-1 and 0.5 mg·L-1 respectively, while the maximum tolerant concentrations(MTCs) were 6, 6, 4 mg·L-1 and 2 mg·L-1. The relative growth rate of different submerged macrophytes showed the trend of increasing first followed by decreasing with the elevated ammonia-nitrogen levels. The maximum relative growth rates of Hydrilla verticillata, Vallisneria natans, Ceratophyllum demersum and Najas minor occurred at the MOCs of NH4+-N, with the values of 1.21, 0.94, 0.52 and 0.28, respectively. Chlorophyll contents and soluble sugar contents in different submerged macrophytes had an increasing trend at the range of MOCs of NH4+-N, while there was no significant changes for POD activities. Meanwhile, chlorophyll contents and soluble sugar contents changed not significantly, and POD activities were significantly higher than CK, while aquatic ammonia-nitrogen concentration was between MOC and MTC. When aquatic NH4+-N was beyond MOT, chlorophyll conents and soluble sugar contents decreased, and POD activities reached the maximum values at 7~21 d and then decreased since then. It can be concluded as follows:aquatic NH4+-N is one of the limiting factors on affecting growth of submerged macrophytes and different types have their corresponding MOC and MTC; Low NH4+-N concentrations can promote growth of submerged macrophytes due to their resistance ability, while high levels may inhibite their growth and even leads to death.