SUMMARYThe availability and utilization of nitrate by salt-marsh plants has been studied. All the species examined are able to assimilate nitrate, which appears to be the most important form of nitrogen available on the marsh. Speeies, from the strand line and lower marsh have higher nitrogen contents and nitrate reductase levels, than those from the upper marsh. Competition between species for nitrate is more important in the upper than lower marsh, because of low nitrate supply. Species such as Armeria maritima, Glaux maritima, Limonium vulgare and Triglochin maritima are able to compete more effectively for nitrate in the upper marsh than Puccinellia maritima, Salicornia europaea, Suaeda tnarititna, Spergularia media and Spartinax anglica. The results indicate there is an increase in nitrogen supply down the marsh, which may be associated with the input of nitrate during tidal innundation.
Summary
The characteristics of nitrate reduction exhibited by the tropical leguminous tree Erythrina senegalensis differ in several respects from those of other higher plants. Nitrate reduction in this species can best be characterized as being a high activity‐low affinity system. High levels of nitrate reductase can be demonstrated using in vivo (50 to 278 μmol h−1 g fresh wt−1) or in vitro (360 μmol h−1 g fresh wt−1) assay procedures. The enzyme extracted from leaf tissue can utilize both NADH and NADPH as electron donors. The enzyme has a low affinity for nitrate, Kspp194 8 to 10 mM. The NADH and NADPH activities co‐chromatograph using several chromatographic procedures, suggesting a single molecular form of nitrate reductase is present.
The addition of ammonium to nitrate induced plants of Lemna minor L. brings about a rapid loss in extractable nitrate reductase activity. This inactivation is reversible both in vivo and in vitro. Inhibitors of RNA and protein synthesis do not protect nitrate reductase against ammonium inactivation. It is suggested that factors, in addition to ammonium ions, are components of the inactivating system. Inactivation may involve some form of protein-protein interaction. The physiological significance of rapid ammonium inactivation of nitrate reductase is discussed.
The presence of ammonium is shown to inhibit the nitrate-promoted formation of nitrate reductase in Lemna minor L. The ammonium inhibition does not result from an inhibition of nitrate accumulation. The kinetics of the ammonium inhibition suggest it is not a direct effect of ammonium. The inhibition could result from the build up of a product of ammonium assimilation or an ammonium induced regulatory protein. It is suggested that both the nitrate stimulation and the 'ammonium' inhibition of nitrate reductase formation can be accounted for by a transcriptional control system.
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