Nitrogen/carbon metabolism in response to NH4+ pulse for two submersed macrophytes

Abstract

Occasional and sharp increase in NH4+ availability due to massive input of sewage and agricultural runoff is not uncommon in inland aquatic systems; and its aftermath effects on the system should be evaluated as high NH4+ concentration is toxic to many animals and plants. In this study, an experiment mimic of natural NH4+ pulse was used to examine its effects on carbon (C) and nitrogen (N) metabolism of two submersed macrophytes, Myriophyllum spicatum L. and Ceratophyllum demersum L., by determination of NH4N, free amino acid (FAA), starch, and soluble carbohydrate (SC) contents. Results showed that FAA contents were significantly higher even on the 10th day after stopping of NH4+ pulse. Both species accumulated large amounts of NH4N and FAA mainly in the leaves in response to the NH4+ pulse. M. spicatum accumulated more FAA than C. demersum despite both species accumulated similar amounts of NH4N. The higher starch and SC reserves in M. spicatum benefited its higher NH4+ capture and assimilation compared to C. demersum. The C source for N assimilation came mainly from SC in the roots of M. spicatum. The species-specific strategy in CN metabolism and the long-lasting effects of NH4+ pulse on the CN metabolism would affect growth, distribution, and interaction of the plants in lakes.