The impact of NO ^sup−_inf3 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 ^sup−_inf3 concentrations can reach very high levels (100–2000 mmol m⁻³ compared with <1–3 in oligotrophic habitats). To investigate the impact of NO ^sup−_inf3 loading alone, plants were grown under three NO ^sup−_inf3 regimes (very low, near-natural and high). The interactive effects of NO ^sup−_inf3 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 ^sup−_inf3 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 ^sup−_inf3 supply resulted in a marked accumulation of NO ^sup−_inf3 , 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 ^sup−_inf3 regime (1.6–2.3%) was mid-range for aquatic and terrestrial species (and 3.1–4.3% under the high NO ^sup−_inf3 regime). These findings, together with field data, suggest that L. uniflora is not growth limited by low NO ^sup−_inf3 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 ^sup−_inf3 concentrations in eutrophic environments seem likely have detrimental effects on the long-term survival of L. uniflora, possibly as a consequence of N accumulation.