Growth and membrane permeability of two submersed macrophytes in response to ammonium enrichment

Abstract

Increased ammonium (NH4+) concentrations are reported to be an important cause of decline in submersed macrophytes in eutrophic lakes. In this study, a subacute experiment was conducted to examine the effects of various NH4+ concentrations on growth and membrane permeability of 2 submersed macrophytes, Myriophyllum spicatum and Ceratophyllum demersum. Apical shoots of the plants were incubated in modified Hoagland solution with 0.25, 0.5, 1, 2, 4, 8, 16, 32, 64 and 128 mg l-1 NH4+-N for 4 d. The plants were then collected for examination of cell membrane permeability and the solutions were used to determine the concentrations of NH4+, sodium (Na+), potassium (K+), magnesium (Mg2+) and calcium (Ca2+). The results indicate that high NH4+ concentrations had significant adverse effects (e.g. reduced growth and leaf chlorosis) on the macrophytes and increased membrane permeability, leading to net leakage of NH4+ and the cations. Compared to M. spicatum, C. demersum had higher membrane permeability, which might enhance NH4+ transportation across the membrane and thus increase its tolerance to NH4+ stress. Significant increases in the membrane permeability of M. spicatum and C. demersum were observed in the treatments with external NH4+-N concentrations of ≥2 mg l-1, which fall within the range of NH4+ concentrations in many eutrophic lakes.