Effects of NH4+ and K+ enrichments on carbon and nitrogen metabolism, life history and asexual reproduction of Vallisneria natans L. in aquarium experiments

Ammonium (NH ₄⁺) is an important nitrogen nutrition for plant growth; however, it is toxic to many plants at high concentration. Eutrophication induces NH ₄⁺enrichment in shallow lakes and increases risk of NH ₄⁺ toxicity to submersed macrophytes. Potassium (K ⁺) was known as an alleviant to the toxic syndromes of plant under NH ₄⁺ stress. In the present study, we examined the single effects and co-effects of NH ₄⁺ and K ⁺ enrichments (∼0.65 mg L ⁻¹ NH ₄–N, ∼45 mg L ⁻¹ K ⁺) in water column on submersed macrophyte Vallisneria natans L. with focus on carbon (C) and nitrogen (N) metabolism, life history and asexual reproduction of the plant in outdoor aquariums lasting for about five months. We found that the NH ₄⁺ enrichment alone increased growth and decreased death rate of the plant, leading to higher biomass, longer survivorship and more tuber production and ramets of the plants, indicating that the NH ₄⁺ enrichment was not stressful to the plant. The combination of NH ₄⁺ and K ⁺ enrichments enhanced the plant growth to less extent as did the NH ₄⁺ enrichment alone, and reduced the plant death rate. The NH ₄⁺ enrichment increased the concentrations of free amino acids and nitrogen, and decreased the concentrations of soluble carbohydrate in leaves and tubers of the plant irrespective of the K ⁺ enrichments. The K ⁺ enrichments alone did not affect growth, death rates and C–N metabolism of the plant. The K ⁺ alone or together with NH ₄⁺ enrichments had negligible effect on tuber production and tuber C–N metabolism of the plant. The present results indicated that the K ⁺ enrichment had negligible effects on the plant; the NH ₄⁺ enrichment (0.65 mg L ⁻¹ NH ₄–N) benefited the plant growth, while it might negatively affect the asexual reproduction due to low carbohydrate contents of the tubers particularly under low light stress in eutrophic lake.