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Báo cáo lâm nghiệp: "Partitioning of assimilated nitrogen in beech (Fagus sylvatica)"

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Partitioning of assimilated nitrogen in beech (Fagus sylvatica) M. Ben Driss Amraoui F. Martin Microbiologie Foresti6re, INRA, Centre de Recherches Forestieres de Nancy, Laboratoire de 54280 Seichamps, France Champenoux, introduction [in roots and patterns of with N]NH§ 15 transport, accumulation and utilization of assimilated nitrogen. The transport and incorporation of nitro- gen in trees involve many complex pro- cesses. Nitrogen ions taken up by the root and its symbiotic associates may be Materials and Methods retained in root cells for the synthesis of organic compounds or may be trans- located directly to the aerial parts. The Growth and labeling of plants types of compounds transported, the Seeds of beech (F. sylvatica L.) were germinat- channels selected for transport and the ed in the dark on peat moistened with water at characteristics of the transport mechan- 20°C and 98% relative humidity. After 3 wk, uni- form seedlings were transferred to a modified isms, themselves are dependent upon the Ever’s solution containing 1 mM NH4 in sand nitrogen sources available, the charac- culture in a naturally lighted glasshouse. Label- teristics of the plant species, its stage of ing studies were performed on 100 d old plants. growth and the environment in which it is 5 d prior to the !5N-labeling experiment, the plants were transferred to a culture holder grown. consisting of a polystyrene sheet punched with Nitrogen assimilation and translocation holes to accept the roots. The cultures were have been extensively studied in orchard placed in aerated, fresh modified Evers’s solu- trees including apple (Hill-Cottingham and tion and returned to the glasshouse. At the start of an 1 experiment, the pump was 5N-labeling Lloyd-Jones, 1977; Tromp and Ovaa, stopped, the container rapidly drained and 1979) and Citrus (Kato, 1980). Data are refilled with culture solution containing also available on conifers (Martin et al., [(50% 1 atom excess). Two cultures 5N 4 + 5N]NH 1 1981 a; Scheromm et al., 1988) but, so far, (4 plants each) were harvested at regular inter- no detailed information exists dealing with vals over a 4 d uptake period. The tissues of one culture were divided into roots, stem and hardwood species. This investigation ex- leaves, blotted, weighed, and frozen immediat- amines the importance of the roots in the ely. After being lyophilized, samples were nitrogen economy of beech (Fagus sylva- weighed separately and then combined prior to tica L.) plants, describing labeling studies grinding. Xylem exudate was collected from the
Labeling of soluble compounds in 2nd culture, using a Scholander chamber, and roots frozen for later analysis. Plants were rinsed with and xylem sap distilled water, weighed and frozen. 1 shows the pattern of 15 labeling of N Fig. Analyses intracellular NH+, soluble N in root tissues Soluble and insoluble nitrogen compounds and xylem sap N. There are some indic- extracted in a methanol-water mixture as were ations that multiple NHIpools are present described by Martin et al. (1981b). Amino acid in the root. Thus, the labeling of ammonia contents in the xylem exudate and tissues were appears to become saturated at around determined according to the method of Genetet 30% 15 abundance (vs 50% 15 abun- N N et al. (1984). Ammonium in xylem exudates, tis- sues and digests was measured colorimetrically dance of extracellular NHI), indicating that (Martin et al., 1981b). The % 15 in soluble and N ’storage pools’ of this ion exist in the roots. insoluble compounds, and xylem exudates was Absorbed ammonium-N was rapidly as- measured by emission spectrometry after diffu- similated into amides and amino acids in sion of ammonia from the digest (Martin et al., 1981b). root cells. Glutamine, glutamate and as- paragine were the most highly labeled components over the time course of the experiment (Table I). Bleeding sap was Results collected from the stems of decapitated plants after the addition of 15 label. The N major nitrogenous constituents of the Distribution of !5N in plant organs xylem sap of NHI-grown beech plants were arginine and asparagine, with very little ammonia being transported. These At the end of the first day of [ N]NH+ 15 4 amino acids accounted for 80% of the N of feeding, 73% of the absorbed N remained root xylem sap and more than half of its in the root tissues (data not shown). The nitrogenous solutes on a molar basis. The corresponding value on day 4 was 55%, amino acid level of the xylem sap aver- indicating a low translocation rate of aged 19 pmol/ml. Feeding beech roots assimilated N to the shoots. On day 4, with p5)BJjNH!resulted in a simultaneous 35% of the total absorbed N had been labeling of xylem-borne amino acids and incorporated into insoluble root N. root soluble N. Xylem translocation of assimilated 15 occurred during the first N xii j j
hours of exposure, with the isotope abun- thesis and 2) storage in the vacuole. The dance gradually increasing to 30%. The physical basis of the compartmentation isotope abundance of sap amino N was has not been elucidated. At the tissue higher compared to that of amino N in the level, it is unlikely that the N assimilation root. This suggests that amino acids trans- product partitioning can be described located to shoots were not mixed with the among all cell types as a uniform 3-way large root amino acid pool. Thus, translo- branched pathway to accumulation, trans- cation of products of 15 assimilation N location and incorporation. We envisage occurred within a few hours in spite of a that, depending upon on the location of a large source of potentially available root cell within the root, its early N assimilation N-amino 14 acids for translocation. metabolites may be more or less available for translocation to shoots than for root maintenance. Discussion References It has been confirmed by many investiga- tions using an 15 tracer method or by N Genetet L, Martin F. & Stewart G. (1984) Nitro- direct analysis of xylem sap that aspara- gen assimilation in mycorrhizas. Ammonium gine and arginine are the major forms of assimilation in the N-starved ectomycorrhizal nitrogen translocated from assimilating fungus Cenococcum graniforme. Plant PhysioL roots to sink organs, such as young leaves 76, 395-399 and buds in trees (Hill-Cottingham and Hill-Cottingham D.G. & Lloyd-Jones C.P. (1977) translocation of nitrogenous compounds in Lloyd-Jones, 1977; Tromp and Ovaa, plants. In: fVitrogen Assimilation of Plants. 1979). The results presented here are also (Hewitt E.J. & Cutting C.V., eds.), Academic consistent with the view that asparagine Press, London, p!p. 397-406 and arginine are the primary source mate- Kato T. (1980) Nitrogen assimilation in Citrus rials in the translocation flux of nitrogen- trees. 1. Ammonium and nitrate assimilation by ous compounds to shoots in beech. The intact roots, leaves and fruits. Physiol. Plant. 48, 416-420 percentages of assimilated 15 in amino N acids in the xylem sap were considerably Martin F., Chem!ardin M. & Gadal P. (1981a) Nitrate assimilation and nitrogen circulation in higher than in the roots. This suggests the Austrian pine. Physio/. Plant. 53, 105-110 0 existence of a least two compartmented Martin F., Chemardin M. & Gadal P. (1981b) pools of amino N in the roots. The lower D6termination isotopique du 15 par spectro- N pool has a rapid turnover rate and is dedi- m6trie d’dmission dans les tissus v6g6taux. cated to translocation, whereas the other PhysioL V6g. 19, 513-521 is a larger pool with a relatively low turno- Scheromm P., Plassard C. & Salsac L. (1988) ver rate. The ’translocation pool’ is closely Nitrogen nutrition of non-mycorrhized maritime pine (Pinus pini5ister) grown on nitrate or am- connected to the stream of currently assi- monium. PIantPhysioG Biochem. 26, 261-269 milated nitrogen. Early products of NH4 Tromp J. & Ovaa J.C. (1979) Uptake and distri- assimilation (e.g., asparagine, glutamine, bution of nitrogen in young apple trees after etc.) remaining in the root cells may sub- application of nitrate or ammonium, with special sequently undergo: 1) incorporation into reference to asparagine and arginine. Physiol. other amino acids for root protein syn- Plant 45, 23-28

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