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 demer- sum. 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 (Mg 2+ ) and calcium (Ca 2+ ). 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.