title: The impact of NOinf3sup-loading on the freshwater macrophyte Littorella uniflora: N utilization strategy in a slow-growing species from oligotrophic habitats.
abstract: The decline and disappearance of Littorella uniflora from oligotrophic waters which have become eutrophic has been associated with shading or reduced CO2supply. However NOinf3sup-concentrations can reach very high levels (100-2000 mmol m-3compared with <1-3 in oligotrophic habitats). To investigate the impact of NOinf3sup-loading alone, plants were grown under three NOinf3sup-regimes (very low, near-natural and high). The interactive effects of NOinf3sup-and photon flux density (low and high regimes) on N assimilation and accumulation, CO2concentrating mechanisms, C3photosynthesis and growth were also examined. The results were unexpected. Increased NOinf3sup-supply had very little effect on photosynthetic capacity, crassulacean acid metabolism (CAM) or lacunal CO2concentrations ([CO2]i), although there was considerable plasticity with respect to light regime. In contrast, increased NOinf3sup-supply resulted in a marked accumulation of NOinf3sup-, free amino acids and soluble protein in shoots and roots (up to 25 mol m-3, 30 mol m-3and 9 mg g-1fresh weight respectively in roots), while fresh weight and relative growth rate were reduced. Total N content even under the very low NOinf3sup-regime (1.6-2.3%) was mid-range for aquatic and terrestrial species (and 3.1-4.3% under the high NOinf3sup-regime). These findings, together with field data, suggest that L. uniflora is not growth limited by low NOinf3sup-supply in natural oligotophic habitats, due not to an efficient photosynthetic nitrogen use but to a slow growth rate, a low N requirement and to the use of storage to avoid N stress. However the increased NOinf3sup-concentrations in eutrophic environments seem likely have detrimental effects on the long-term survival of L. uniflora, possibly as a consequence of N accumulation.
authors.affiliation: Department of Agricultural and Environmental Science, The University, Ridley Building, NE1 7RU, Newcastle upon Tyne, UK.
authors.forename: W E
authors.initials: WE
authors.lastname: Robe
authors.affiliation: Department of Agricultural and Environmental Science, The University, Ridley Building, NE1 7RU, Newcastle upon Tyne, UK.
authors.forename: H
authors.initials: H
authors.lastname: Griffiths
keywords: Eutrophication; Littorella uniflora; Nitrogen assimilation; Nitrogen use efficiency; Storage