Physiological plastic responses to acute NH4+–N toxicity in Myriophyllum spicatum L. cultured in high and low nutrient conditions

• M. spicatum from eutrophic and oligotrophic lakes cultured in high and low nutrients.
• Treatments exhibit variable growth and chlorophyll fluorescence kinetics.
• Trade-offs between tolerance in high nutrients and photosynthesis in low nutrients.

Eutrophication alters biological and physical characteristics of aquatic freshwater ecosystems, and different macrophytes exhibit variable capacities to tolerate the subsequent stress factors. Myriophyllum spicatum is one of the most eutrophication tolerant submerged macrophyte species. M. spicatum from Fuxian (oligotrophic) and Xingyun (eutrophic) lakes, in South West China, were cultured in water conditions corresponding to their origin and the alternative trophic state (4 treatments) for a year. Each treatment was exposed to NH4+–N (0, 5, 10, 50, 100, and 200 mg/L) for five days (24 treatments) to investigate the effect of the sources and acute NH4+–N enrichment on photosynthetic performance. Both sources and NH4+–N affected photosynthetic performance, and eutrophic source plants in high nutrients and oligotrophic source plants in low nutrients exhibited higher NH4+–N tolerance. High nutrient cultured individuals had higher leaf number, branching, and heights. There were trade-offs between NH4+–N tolerance in turbid high nutrient conditions and photosynthetic performance in low nutrient conditions with high irradiance. Oligotrophic source plants in low nutrients had the highest chlorophyll a levels at ≤10 mg/L, which dropped markedly afterwards. Soluble carbohydrates and starch decreased at ≥50 mg/L and ≥5 mg/L, respectively. Individuals growing in eutrophic conditions could be prone to higher damage in the field owing to potentially weaker stem and leaf biomechanical properties, because of carbohydrate and energy intensive NH4+–N detoxification in higher NH4+–N conditions.