Carbon and nitrogen metabolism of an eutrophication tolerative macrophyte, Potamogeton crispus, under NH4+ stress and low light availability

Submersed macrophytes in eutrophic lakes often experience high NH4+ concentration and low light availability in the water column. This study found that an NH4+–N concentration of 1 mg L−1 in the water column apparently caused physiological stress on the macrophyte Potamogeton crispus L. The plants accumulated free amino acids (FAA) and lost soluble carbohydrates (SC) under NH4+ stress. These stressful effects of NH4+ were exacerbated under low light availability. Shading significantly increased NH4+ and FAA contents and dramatically decreased SC and starch contents in the plant shoots. At an NH4+–N concentration of 1 mg L−1 in the water column, neither growth inhibition nor NH4+ accumulation was observed in the plant tissues of P. crispus under normal light availability. The results showed that 1 mg L−1 NH4+–N in the water column was not toxic to P. crispus in a short term. To avoid NH4+ toxicity, active NH4+ transportation out of the cell may cost energy and thus result in a decline of carbohydrate. When NH4+ inescapably accumulates in the plant cell, i.e. under NH4+ stress and shading, NH4+ is scavenged by FAA synthesis.