THE IMPACT OF NO3- LOADING ON THE FRESH-WATER 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 NO3- concentrations can reach very high levels (100-2000 mmol m(-3) compared with <1-3 in oligotrophic habitats). To investigate the impact of NO3- loading alone, plants were grown under three NO3- regimes (very low, near-natural and high). The interactive effects of NO3- and photon flux density (low and high regimes) on N assimilation and accumulation, CO2 concentrating mechanisms, C-3 photosynthesis and growth were also examined. The results were unexpected. Increased NO3- 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 NO3- supply resulted in a marked accumulation of NO3-, 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 NO3- regime (1.6-2.3%) was mid-range for aquatic and terrestrial species (and 3.1-4.3% under the high NO3- regime). These findings, together with field data, suggest that L. uniflora is not growth limited by low NO3- 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 NO3- concentrations in eutrophic environments seem likely have detrimental effects on the long-term survival of L. uniflora, possibly as a consequence of N accumulation.