The impact of NO inf3sup- 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 CO₂ supply. However NO _inf3^sup- concentrations can reach very high levels (100-2000 mmol m^-3 compared with <1-3 in oligotrophic habitats). To investigate the impact of NO _inf3^sup- loading alone, plants were grown under three NO _inf3^sup- regimes (very low, near-natural and high). The interactive effects of NO _inf3^sup- and photon flux density (low and high regimes) on N assimilation and accumulation, CO₂ concentrating mechanisms, C₃ photosynthesis and growth were also examined. The results were unexpected. Increased NO _inf3^sup- supply had very little effect on photosynthetic capacity, crassulacean acid metabolism (CAM) or lacunal CO₂ concentrations ([CO₂]_i), although there was considerable plasticity with respect to light regime. In contrast, increased NO _inf3^sup- supply resulted in a marked accumulation of NO _inf3^sup- , free amino acids and soluble protein in shoots and roots (up to 25 mol m^-3, 30 mol m^-3 and 9 mg g^-1 fresh weight respectively in roots), while fresh weight and relative growth rate were reduced. Total N content even under the very low NO _inf3^sup- regime (1.6-2.3%) was mid-range for aquatic and terrestrial species (and 3.1-4.3% under the high NO _inf3^sup- regime). These findings, together with field data, suggest that L. uniflora is not growth limited by low NO _inf3^sup- 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 NO _inf3^sup- concentrations in eutrophic environments seem likely have detrimental effects on the long-term survival of L. uniflora, possibly as a consequence of N accumulation.

Keywords: Eutrophication; Littorella uniflora; Nitrogen assimilation; Nitrogen use efficiency; Storage.