The impact of NO3 loading on the freshwater macrophyte Littorella uniflora: N utilization strategy in a slow-growing species from oligotrophic habitats
1994
Robe, W.E. (Newcastle upon Tyne Univ. (United Kingdom). Dept. of Agricultural and Environmental Science) | Griffiths, H.
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 per cubic metre compared with below 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, C3 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)(sub 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 per gram 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.
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