The impact of NO inf3sup- loading on the freshwater macrophyte Littorella uniflora: N utilization strategy in a slow-growing species from oligotrophic habitats.

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 inf3sup- concentrations can reach very high levels (100-2000 mmol m-3 compared with <1-3 in oligotrophic habitats). To investigate the impact of NO inf3sup- loading alone, plants were grown under three NO inf3sup- regimes (very low, near-natural and high). The interactive effects of NO inf3sup- 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 inf3sup- 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 inf3sup- supply resulted in a marked accumulation of NO inf3sup- , 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 inf3sup- regime (1.6-2.3%) was mid-range for aquatic and terrestrial species (and 3.1-4.3% under the high NO inf3sup- regime). These findings, together with field data, suggest that L. uniflora is not growth limited by low NO inf3sup- 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 inf3sup- concentrations in eutrophic environments seem likely have detrimental effects on the long-term survival of L. uniflora, possibly as a consequence of N accumulation.