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