Abstract: Nitrogen metabolism is not only one of the basic processes of plant physiology,but also one of the important parts of global chemical cycle.Plant nitrogen assimilation directly takes part in the synthesis and conversion of amino acid through the reduction of nitrate.During this stage,some key enzymes,e.g,nitrate reductase (NR),glutamine systhetase (GS),glutamate dehydrogenase (GDH),glutamine synthase (GOGAT),aspargine synthetase (AS),and aspartate aminotransferase (AspAT) participate these processes.The protein is assimilated in plant cell through amino acid,and becomes a part of plant organism through modifying,classifying,transporting and storing processes,etc.The nitrogen metabolism is associated with carbonic metabolism through key enzyme regulations and the conversion of products,which consists of basic life process.Among these amino acids in plant cell,glutamic acid (Glu),glutamine (Gln),aspartic acid (Asp) and asparagines (Asn),etc,play a key role,which regulates their conversion each other and their contents in the plant cell through regulating formation and activity of those key enzymes.Environmental factors also affect the conversion and recycle of the key amino acids through regulating gene expression of the key enzymes and their activities.Nitrate and light intensity positively regulate the gene transcription of NR,but ammonium ions and Glu,Gln do the negative way.Water deficit is a very serious constraint on N₂ fixation rate and soybean (Glycine max Merr.) grain yield,in which,ureide accumulation and degradation under water deficit appear to be the key issues of feedback mechanism on nitrogen fixation.Water stress decreases NR activity,but increases proteinase activity,and thus,they regulate plant nitrogen metabolism,although there are some different effects among species and cultivars.Water stress also decreases plant tissue protein content,ratio of protein and amino acid,and reduces the absorption of amino acid by plant.On the contrary,soil flooding decreases the content and accumulation amount of root nitrogen in winter wheat by 11.9% from booting to flowering stages and 39.1% during grain filling stage,and reduces the ratio of carbon and nitrogen by 79.6%.The results misadjust the metabolism between carbon and nitrogen,and result in the end of the root growth.Elevated CO₂ level could decrease plant leaf nitrogen content under well-watered condition,but almost maintain stable under water deficit condition.The radiation of UV-B significantly reduces the partitioning coefficient and synthetic rate of Rubisco,which significantly decreases the photosynthetic rate.This paper reviewed the pathway of plant nitrogen assimilation,characteristics of key enzymes and their regulating mechanisms with picturing the regulating mode of NR,and described the signal sensing and conduct of plant nitrogen metabolism and the formation,transportation,storage and degradation of plant cell protein with picturing the schedule of protein transport of membrane system in plant cell.Seven key tasks are emphasized in this paper in terms of the review on the effects and mechanisms of key ecological factors including water stress on plant nitrogen metabolism.They are:1) the absorption mechanism of plant based on different nitrogen sources and environmental regulations,2) the localization and compartmentalization of the key enzymes of nitrogen mechanism in plant cell,3) the gene and environmental regulating model and their relationships in various key enzymes of nitrogen metabolism,4) the function of main cell organs and their responses to environmental factors in nitrogen metabolism process,5) physiological and chemical mechanism of nitrogen and the relationship between the mechanism and protein formation during crop grain filling,6) improving gene structure of special species or cultivars using gene engineering methods to enhance the resistance to environmental factor stress and the efficiency of absorption and transportation of nitrogen,and 7) the mechanism of natural nitrogen cycle and its response to human activity disturbance.

Key words: Nitrogen metabolism, Environmental regulation, Water stress, Key enzymes, Pathway