Nitrogen form affects physiological responses and root expansigenous honeycomb aerenchyma in the emergent macrophyte Acorus americanus

Abstract

High nitrogen (N) concentrations and high NH4+:NO3− ratios that are characteristic of heavily eutrophic and constructed wetlands may be detrimental to the growth and establishment of macrophytes in wetlands formed in the oil sands reclamation areas. This study investigates the effects of N form on the physiology, growth, and root expansigenous honeycomb aerenchyma structure of Acorus americanus (Raf.) Raf., an important macrophyte targeted for wetland reclamation in Canada. Three populations of A. americanus were grown in aerated solution culture and provided for up to 3 months with two different concentrations of N (2 mmol·L−1, 8 mmol·L−1) as NH4+, NO3−, or NH4+ + NO3−. Fresh mass and physiological parameters including gas exchange, chlorophyll fluorescence, stomatal traits, nitrogen and chlorophyll concentration, root morphology, and aerenchyma structure were examined. The effects of N form were concentration-dependent. At 2 mmol·L−1 N, NH4+ + NO3− plants had higher fresh mass, photosynthetic rates, number of first-order roots, root diameter, and maximum root length than those provided solely with NH4+ or NO3−. At 8 mmol·L−1 N, both NH4+ and NH4+ + NO3− treatments had a negative impact on growth, net photosynthesis, and chlorophyll concentrations, and they also led to thinner and shorter roots with necrotic tips, a significant reduction in fractional root porosity, and a denser aerenchyma with smaller lacunae. The results indicate that high levels of NH4+-N may negatively affect the establishment of A. americanus plants in constructed wetlands through its impact on growth, net photosynthesis, and root morpho-anatomy.