1 Department of Bioscience - Aquatic Biology, Department of Bioscience, Science and Technology, Aarhus University2 Department of Bioscience - Plant Biology, Department of Bioscience, Science and Technology, Aarhus University3 Department of Biology, Faculty of Science, Chiang Mai University, Meuang, Chiang Mai 50202, Thailand.4 Department of Bioscience, Science and Technology, Aarhus University5 Department of Bioscience, Science and Technology, Aarhus University
Actinoscirpus grossus, a native species in tropical wetlands of South-East Asia, North Australia and the Pacific islands, has been reported to perform well in experimental scale constructed wetland (CW) systems. However, little is known about how high NH4+ concentrations prevailing in wastewater affect growth and performance of this species. We examined growth, morphological and physiological responses of A. grossus to NH4+ concentrations of 0.5, 2.5, 5, 10 and 15 mM under hydroponic growth conditions. The relative growth rates (RGR) of the plants were highest at 2.5 mM NH4+ but significantly reduced at 10 and 15 mM NH4+. The roots of the plants were stunted and produced subepidermal lignified-cell layers at exposure to 10 and 15 mM NH4+. The photosynthetic rates did not differ between treatments (average An = 21.3 ± 0.4 µmol CO2 m-2 s-1) but the photosynthetic nitrogen and carbon use efficiency (PNUE and PCUE) were significantly depressed at 10 and 15 mM NH4+ treatments. The concentration of NH4+ in the roots, but not in the leaves, reflected the NH4+ concentration in the growth medium suggesting that the species is unable to regulate the NH4+ uptake., and that the NH4+ assimilation occurs primarily in the roots. The high root respiration rates in concert with high tissue NH4+ and declined C/N ratio at 10 and 15 mM NH4+ suggest that the NH4+ assimilation occurs primarily in the roots and the plant has inadequate C-skeletons for NH4+ assimilation and exudation at high NH4+ concentration in the external solution. The concentrations of mineral cations were generally reduced and the root membrane permeability increased at high external NH4+ concentrations. Our study shows that A. grossus tolerates NH4+ concentrations up to 5 mM which is characteristic of most types of wastewater. Hence, A. grossus is a good native candidate species for use in CW systems in tropical and subtropical climates in South-East Asia, North Australia and the Pacific islands.
Ecotoxicology and Environmental Safety, 2014, Vol 107, p. 319-328