Seasonal Dynamics of Nitrogenous Compounds in a Nitrophilic Weed I. Changes in Inorganic and Organic Nitrogen Fractions of the Different Plant Parts of Urtica dioica

The seasonal dynamics of nitrogenous compounds in the differentplant parts of Urtica diocia, a perennial nitrophilic weed,were investigated under natural conditions during the courseof a year. In the below ground plant parts (roots and rhizomes)a nitrogen store was built up during summer and autumn, consistingmainly of free amino acids. In the seeds, however, proteinswere of major importance as nitrogen reserve. In early springthe below ground nitrogen store was quickly mobilized, translocatedto the upper plant parts and used for the protein synthesisof the shoots, enabling them a rapid growth prior to the developmentof their own capacity for nitrogen assimilation. (Received September 3, 1984; Accepted November 2, 1984)     

The Influence of Nitrogen Nutrition on the Accumulation of Free Amino Acids in Root Tissue of Urtica dioica and their Apical Transport in Xylem Sap

Urtica dioica plants were grown on a nitrogen supply of 3, 15and 22 mM with nitrate and ammonium as nitrogen source. In contrastto nitrate reductions amino acid synthesis occurred in roottissue. At 3 mM ammonium obviously the amino acids were rathertransported via xylem upwards to the shoots than stored in theroots. Particularly increased ammonium supply led to stimulatedstorage of free amino acids in the roots, mainly as asparagineand arginine. In xylem asparagine was the dominant nitrogentransporting compound, while arginine was hardly translocated.With the enhancement of nitrogen supply, the second amide, glutamine,became more and more important with respect to the transportof nitrogen. (Received September 3, 1984; Accepted November 2, 1984)     

Organic nitrogen reserves and their mobilization during sprouting of purple nutsedge (Cyperus rotundus L.) tubers

Purple nutsedge is a perennial weed propagating vegetativelyby an extended network of rhizomes and tubers. Large quantitiesof starch are stored in the tubers, but the properties of nitrogenreserves and their importance for early plant growth have receivedlittle attention in the past. Organic nitrogen compounds werestudied in mature tubers by protein determination, SDS-PAGEand amino acid quantification and separation using reversed-phaseHPLC. Changes in these compounds were followed in sproutingtuber pieces fed with a complete nutrient solution (containingnitrate, control) or with a nutrient solution without nitrogensource. As judged by gel electrophoresis or direct protein quantification,some net protein degradation occurred after 2–4 weeksof sprouting in the presence or absence of exogenous N. Aminoacids decreased much faster, especially during the first 2 weeks.The major amino acids were arginine and asparagine, which togetheraccounted for 70% of total amino acids at day 0, and which hadalmost disappeared after 4 weeks of sprouting. Sprout growthdepended strongly on the availability of exogenous nitrogenduring the second week. The results indicate that amino acidsare the main N storage form of purple nutsedge tubers and, essentially,that no storage proteins are present. Future investigationson the N metabolism of sprouting nutsedge should focus on arginineand asparagine metabolism and eventually on nitrogen assimilation,which becomes important in an early phase of plant growth. Key words: Cyperus rotundus L., arginine, asparagine, nitrogen assimilation, storage proteins     

Seasonal Changes in Nitrogen Fractions of Coptis japonica, a Temperate Forest Evergreen Chamaephyte, and their Ecological Implications

Seasonal changes in several forms of nitrogen were investigatedin Coptis japonica, an evergreen rosette hemicryptophyte intemperate deciduous forest. The concentration of total nitrogenin rhizomes and roots decreased during the period of new shootgrowth from winter to spring. In the rhizomes, total solubleprotein stored by early summer decreased gradually until winter,coupled with an increase in free amino acids. Nitrogen was largelystored in free amino acids in the roots, especially during summer.The total soluble protein in current-year leaves decreased fromspring to summer and then increased during winter. The seasonalchanges in nitrogen components were coincident with the changein light-saturated photosynthetic rates recorded in a previousstudy. The ratio of total soluble protein to total nitrogendecreased from spring to summer and then increased from latesummer to winter in the current-year leaves. In contrast, chlorophyllcontent and the ratio of chlorophyll to total nitrogen werehigher in summer than in other seasons. The results indicatethat nitrogen was used in a manner that better utilizes thevery weak light in summer and the higher light intensities inother seasons. The major component of the free amino acid poolwas asparagine, in every organ throughout the season, exceptfor the senescent leaves. Since asparagine has a high N:C ratio,we suppose that the asparagine-dominated amino acid pool isadvantageous in the carbon-limited environment of the forestfloor.Copyright 1994, 1999 Academic Press Free amino acid composition, total nitrogen, total soluble protein, photosynthesis, evergreen hemicryptophyte     

Changes in Total Nitrogen and Free Amino Acids in Stem Cuttings of Mulberry (Morus alba L.)

Changes in total nitrogen and free amino acid contents in stemcuttings of Morus alba have been studied. The fresh and dryweights and total nitrogen amounts of the parent stems of cuttingsdecreased initially after cutting. Their increase follows theformation of main roots in cuttings, suggesting that, like carbohydrates,sugars and starch, stored nitrogenous substances are used forsprouting and rooting of cuttings. Amino acids found in stems,roots and shoots are those common in other higher plants withthe exception of pipecolic acid and 5-hydroxypipecolic acid.Significant changes in the levels of asparagine, proline, arginine,-aminobutyric acid and alanine in roots, bark and wood of parentstems were observed during cutting growth, whereas those ofother amino acids remained comparatively constant; the mostpredominant amino acid in the starting materials was proline.while that in the cuttings during growth was asparagine. Theresults suggest that, among free amino acids, asparagine, prolineand arginine play the major part in storage of nitrogen in mulberry.The importance of glut-amine and asparagine in nitrogen metabolismin mulberry has been discussed.     

Origin of Foliar Nitrogen and Changes in Free Amino-Acid Composition and Content of Leaves, Stubble, and Roots of Perennial Ryegrass During Re-growth after Defoliation

Nitrogen re-mobilization and changes in free amino acids werestudied as a function of time in leaves, stubble, and rootsduring ryegrass (Lolium perenne L.) re-growth. Experiments with¹⁵N labelling clearly showed that during the first days nearlyall the nitrogen in new leaves came from organic nitrogen re-mobilizedfrom roots and stubble. On the days of defoliation, stubblehad the highest content of free amino acids with 23 mg per gdry weight against 15 mg and 14 mg in leaves and roots, respectively.The major amino acids in leaves were asparagine (23% of totalcontent in free amino acids), aminobutyrate, serine, glutamine,and glutamate (between 7% and 15%) whereas in roots and stubblethe contribution of amides was high, especially asparagine (about50%). Re-growth after cutting was associated with a rapid increaseof the free amino acid content in leaves, with a progressivedecrease in roots while stubble content remained virtually unchanged.In leaves, asparagine increased from the first day of re-growth,while the aspartate level remained unchanged and glutamine increasedstrongly on the first day but decreased steadily during thenext few days of re-growth. Asparagine in stubble and rootschanged in opposite directions: in stubble it tended to increasewhereas in roots it clearly decreased. In contrast, stubbleand roots showed a similar decrease in glutamine. In these twoplant parts, as in leaves, aspartate remained at a low level.Results concerning free amino acids are discussed with referenceto nitrogen re-mobilization from source organs (stubble androots) to the sink organ (regrowing leaves). Key words: Lolium perenne L, re-growth, nitrogen, free amino acids, glutamine, asparagine     

Seasonal Changes of Nitrogen Storage Compounds in a Rhizomatous Grass Calamagrostis epigeios

The seasonal dynamics in content and distribution of N-rich compounds between overwintering organs of Calamagrostis epigeios were examined. Samples were taken both from plants grown in natural conditions and in containers with controlled nutrient supply. There were significant changes in content of nitrate, free amino acids and soluble protein in all investigated plant parts during the course of a year. Amino acids showed both the highest maximum and seasonal fluctuation among the all N compounds observed and, therefore, appear to have a central role in N storage. Their content rises in the autumn, remains stable during winter and declines quickly at the beginning of spring. The most abundant amino acids in the end of winter storage period - asparagine, arginine and glutamine - constituted about 90 % of N in fraction of free amino acids. The portion of N stored in soluble proteins, however, was considerably smaller compare to both amino acids and nitrate. The amount of N stored in rhizomes of C. epigeios was smaller than in roots and stubble base before the onset of spring re-growth. This indicates that roots and stubble base are particularly important for winter N storage in this species.     

Seasonal Variation of Nitrogenous Compounds in Components of the Kiwifruit Vine

Free amino acids in 6-year-old kiwifruit vines [Actinidia deliciosa(A. Chev.) C. F. Liang et A. R. Ferguson] were measured overthe course of 1 year using components obtained from whole-vineharvests. Tissues examined from the perennial structure consistedof the wood and cortex of structural roots, wood and bark ofstem, leader and 1-year-old fruiting canes. Free acids in theannual growth (fine roots, flowers, fruit, leaves and non-fruitingshoots) were also measured. The range of amino acids extracted indicated that kiwifruitconforms to a conventional pattern of nitrogen metabolism. Acidspresent in greatest concentration depended on tissue type andsampling time. In perennial components and fine roots, arginineand glutamine were the predominant species, followed by gamma-aminobutyrate,aspartate, glutamate, alanine and valine. Generally, maximumconcentrations of all free acids were measured in a 10-weekperiod around budbreak. These same acids, plus asparagine, serineand threonine, were also prevalent in annual growth. In leaves,flowers and non-fruiting shoots, concentrations were greatestin the young tissue and declined with age. By contrast, concentrationsof arginine, asparagine and glutamine in fruit peaked approximately10 weeks after anthesis, subsequent to the cell division phaseof growth. During the year, free arginine averaged 44, 48 and 58 % of thetotal N in the fine roots, and the cortex and wood of structuralroots, respectively (the quantity of total N and amino-N inother components of the structural framework was much less thanthat in root tissue). Arginine was the principal N-containingspecies measured in xylem sap vacuum-extracted from 1-year-oldcanes over winter. During the period of vegetative growth, however,glutamine and nitrate were the principal N-transport forms present.The study highlights the importance of the fine root systemas the primary location of nitrogenous reserves in this plantand identifies arginine as the dominant N-storage form. Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson, amino acid composition, kiwifruit, nitrogen, whole-plant harvesting     

Phloem Translocation of Storage Nitrogen in Apple

Mobilization of nitrogenous compounds during the spring was studied in ringed isolated shoot sections (bearing one intact bud each) from Golden Delicious apple trees and in intact stem-ringed apple rootstocks M VII. The changes in total, protein and soluble nitrogen and soluble amino acids and amides were followed in the bark of the shoot sections for 3 weeks during leafing-out and in the shoot and stem bark of the rootstocks for 6 weeks starting at bud-break. Ringing prevented nitrogen movement from below the ring both in the shoot sections and in the rootstocks almost completely, thus demonstrating the importance of the phloem as translocation pathway for stored nitrogenous compounds, even over longer distances. Asparagine and arginine were the major soluble amino compounds throughout. The values of the asparagine/arginine quotient in the various tissues suggest that when the distance between points of nitrogen supply and demand is short asparagine is translocated preferentially, but that at increasing distance this preference shifts to arginine.     

Distribution of non-structural nitrogen and carbohydrate compounds in mature oak trees in a temperate forest at four key phenological stages

The distribution of nitrogen (N) and carbon (C) pools in 100-year-old sessile oaks was investigated in situ at four key periods of tree phenology. Leaves, twigs, trunk and roots were sampled, and total non-structural nitrogen compounds (TNNC) and total non-structural carbohydrates (TNC) were quantified. TNC concentrations decreased more than 50 % between February and May, especially in the above-ground parts. During the same period, TNNC concentrations drastically decreased especially in twigs (more than 55 % below the winter baseline). This indicates high sink strength of new organs at bud break. TNC concentrations increased in summer in all tree compartments highlighting the TNC storage. TNNC deposition for storage constitution began in autumn during leaf senescence. The first organs acting as a N store were coarse roots followed by twigs; whereas all oak tree compartments were potential C store organs except phloem tissue. Arginine and asparagine were the two prevailing amino acids involved in both storage and transport. Besides identifying the compounds involved in the storage and transportation of N, our results highlight that oak seasonal cycles of C and N are not in phase: after intensive use of C and N stores in spring, the replenishment of N stores lags behind that of C stores.     

Free amino acids and total nitrogen during shoot development in Scots pine seedlings

The concentration of free amino acids and total nitrogen was studied in needles, stems and roots of seedlings of Pinus sylvestris L. for five weeks during the second growth period ("summer"). In one group of seedlings the source/sink relation was disturbed through removal of the terminal buds. The seedlings were cultivated in artificial year-cycles in a climate chamber.
Total nitrogen increased in needles and sterns of intact seedlings in the beginning of the "summer" and decreased during shoot growth. In seedlings, from which the buds had been removed, nitrogen remained at high levels in the primary needles and accumulated in steins and roots. The results are consistent with utilization of nitrogen in older needles and in the stem during shoot elongation.
The pool of free amino acids increased in the beginning of the "summer" and decreased after bud break in primary needles, stems and roots. Arginine and glutamine, in the roots also asparagine, were the dominating amino acids (amides included). Together, these compounds (plus glutamate and aspartate) contributed about 90% of the nitrogen in the amino acid pool in all organs. In primary needles and in the stem, arginine predominated at the end of hardening (75–85% of the amino acid nitrogen). Free amino acids contributed at most ca 10% of the total nitrogen in primary needles, where the ratio of free amino acid nitrogen: total nitrogen was highest at the end of dormancy and in the early "summer". Free amino acids accumulated after bud removal in primary needles and especially in stems and roots. Glutamine became relatively more dominant than arginine in the different organs.
The observations are consistent with the role of arginine and glutamine for storage and transport of nitrogen in conifers. Because of the low concentrations of amino acid nitrogen in the primary needles, arginine is not considered a major nitrogen reserve in needles of Scots pine seedlings.     

Seasonal changes in nitrogen fractions of Pachysandra terminalis, a forest evergreen chamaephyte

Seasonal changes in several forms of nitrogen were investigated in the evergreen chamaephyte Pachysandra terminalis Sieb. et Zucc. growing in temperate evergreen coniferous forest. After plants sprouted new shoots, nitrogen accumulated largely as proteins in the leaves from summer to late autumn and, additionally, during a short spring period in the following year. Proteins accumulated in the overwintered leaves decreased markedly in summer, indicating that they were used for new shoot growth. A similar change was found in Fraction 1 protein. This is consistent with the seasonal changes in photosynthetic capacity and carboxylation efficiency observed in previous studies. The allocation of nitrogen to Fraction 1 protein increased in the spring, presumably to utilize better the relatively high light intensity at the forest floor before leaf expansion of the understory deciduous plants. In contrast to protein nitrogen, soluble nitrogen was present largely in stems and rhizomes. Aspargine was the major component of the amino acid pool in all organs throughout the year, especially in stems in summer. Since asparagine has the highest N:C ratio (2N:4C), an amino acid pool dominated by asparagine is economic in the use of carbon and advantageous for the carbon-limited environment of the forest understory.     

Alterations of Free Amide and Amino Acid Contents During the Development of Maize Plants, Zea mays L.

Concentrations of free amino acids and amides were measuredin organs of maize plants, Zea mays L. in the period from 14d before pollen liberation until complete seed maturation. Inthis time anthesis took place and only the ovaries of ear 9(numbered from below) developed into seeds. In mature leaf bladesNH₄ ion assimilation had ceased and asparagine and glutaminewere not found there. N redistribution induced the occurrenceof large amounts of aspartate, glutamate and alanine. The amountsof amides in leaf sheaths and stem parts depended on the neighbourhoodof generative parts. The generative plant parts can be distinguishedfrom adjacent vegetative plant parts in concentrations of freeproline or asparagine. Proline occurred in pollen but not inthe empty anthers. Ears had a small, early peak amount of prolinemostly before pollination. Only the ninth ear had a first maximumproline amount after the fifth day of pollen liberation. A secondproline peak in the ears coincided with the period of maximumincrease in d. wt. The occurrence of proline in the generativeorgans relative to metabolic processes inducing fertility orseed maturation is discussed. Zea mays L., amides, amino acids, amino-transferring components, asparagine, glutamine, proline     

Nitrogen and carbohydrate pools of two rhizome types of Phragmites australis (Cav.) Trin. ex Steudel

Carbohydrate and total nitrogen contents as well as free amino acid (FAA) concentrations were determined in distinct types of rhizomes of two genetically homogeneous stands of reed (Phragmites australis (Cav.) Trin. ex Steudel) differing in morphology, productivity and nutrient supply in order to evaluate the storage capacity of vertical rhizomes and expansion rhizomes. The expansion rhizomes possess significantly higher amounts of FAA and of total nitrogen but similar carbohydrate concentrations in comparison to the vertical rhizomes. However, no significant differences were found for total nitrogen, FAA and total carbohydrates between both investigated stands indicating a comparable storage capacity of rhizomes independent of nutrient availability. Only the composition of the FAA pool varied in the alanine/asparagine ratio probably influenced by the oxygen supply of the rhizome/root system.     

Changes in the Content of Amino acids, Carbohydrates and Adenine Nucleotides following Budding in the Hardwood Stems of Mulberry Plants (Morus alba)

Quantitative changes in the free amino acids, sugars and adeninenucleotides in the hardwood stems of mulberry plants (Morusalba L.) were followed during bud opening. Following this, thestarch and ATP content decreased rapidly. The value of the adenylateenergy charge, ATP+; ADP/ATP + ADP+AMP, or ATP/ADP ratio becamelower. Asparagine, proline and arginine existed prominentlyat the onset of budding. After budding, the content of prolineand asparagine decreased consecutively, but that of arginineincreased initially and then decreased. Thus, the fluctuationsin the carbohydrate and amino acids metabolism in the hardwoodstems caused by budding was characterized by a rapid decreasein energy and carbohydrates reserves, and a concurrent decreasein proline and asparagine. The conversion of asparagine to arginineis also presumed to have taken place at the same time. Morus alba, mulberry, amino acids, carbohydrates, adenine nucleotides     

Effect of root temperature on the amino-nitrogen composition of leaves in successive segments of apple shoots

Young apple trees ( Malus pumila Mill. cv. Cox's Orange Pippin) given nitrogen either at or 40 days after bud-break were kept at a root temperature of 6, 18 or 30°C under otherwise constant conditions. Twelve weeks after the start of the experiment leaves from successive shoot segments and roots were collected and in most cases analysed to assess total nitrogen, protein nitrogen, and the main amides and amino acids. The percentage composition of the amino-nitrogen fraction of the roots was not or was hardly at all affected by the treatments; asparagine predominated, followed by arginine. In contrast, in the leaves the share of arginine dropped from about 90% at 6°C to about 30% at 30°C in favour of especially asparagine. This pattern was mainly attributable to the situation in the basal sections of the shoot. In the middle and top segments the temperature effects were small. In general, a high level of amino nitrogen corresponded to a high contribution of arginine. Soluble nitrogen was higher after the late than after the early application of nitrogen. Shoot growth was reduced at 6°C root temperature, but little difference was seen between 18 and 30°C. It was concluded that with respect to nitrogen metabolism roots and shoots function more or less independently of each other. The hypothesis that the roots affect leaf nitrogen metabolism via the supply of growth substances produced in the roots, presumably cytokinins, is discussed.     

Nitrogen nutrition in the cyanobacterium Nostoc ANTH, a symbiotic isolate from Anthoceros: uptake and assimilation of inorganic-n and amino acids

Amino acid uptake and utilization of various nitrogen sources (amino acids, nitrite, nitrate and ammonia) were studied in Nostoc ANTH and i ts mu tant (Het(-)Nif(-)) isolate defective in heterocyst formation and N2-fixation. Both parent and its mutant grew at the expense of glutamine, asparagine and arginine as a source of fixed-nitrogen. Growth was better in glutamine-and asparagine-media as compared to that in arginine media. Glutamine and asparagine repressed heterocyst formation, N2-fixation and nitrate reduction in Nostoc ANTH, but arginine did so only partially. The poor growth in arginine-medium was not due to poor uptake rates, since the uptake rates were not significantly different from those for glutamine or asparagine. The glutamine synthetase activity remained unaffected during cultivation in media containing any one of the three amino acids tested. The uptake of amino acids was substrate-inducible, energy-dependent and required de novo protein synthesis. Nitrate and ammonium repressed ammonium uptake, but did not repress uptake of amino acids. In N2-medium (BG-11(0)), the uptake of ammonium and amino acids in the mutant was significantly higher than its parent strain. This was apparently due to nitrogen limitation since the mutant was unable to fix N2 and the growth medium lacked combined-N.     

Composition of Amino Compounds Transported in Xylem of Casuarina sp.

Seedlings (180-d-old) of Casuarina cunninghamianaM L., C. equisetifoliaMiq. and C. glauca Sieber inoculated with each of two differentsources of Frankia, were analysed for translocated nitrogenouscompounds in xylem sap. Analyses were also made on sap fromnodulated and non-nodulated plants of C. glauca grown with orwithout a range of levels of combined nitrogen. Xylem exudateswere collected from stems, roots, and individual nodules ofnodulated plants and from stems and roots of non-nodulated plants.While the proportional composition of solutes varied, the samerange of amino compounds was found in xylem sap from the threedifferent symbioses. In C. glauca asparagine was the major aminoacid in the root sap followed by proline, while in symbioticC. cunninghamiana arginine accounted for more than 25% of theamino compounds. Citrulline was the major translocated productfound in the stem exudate of symbiotic C. equisetifolia. Increasingconcentrations of ammonium nitrate in the nutrient solutionresulted in increasing levels of free ammonia and glutaminein xylem sap from stems of nodulated and non-nodulated C. glauca,but there was relatively little change in the prominent solutes,e.g. citrulline, proline, and arginine. The composition of nitrogenoussolutes in stem or root exudates of C. glauca was similar tothat of exudate collected from individual nodules and on thisbasis it was not possible to distinguish specific products ofcurrent N₂ fixation in xylem. The main differences in N solutecomposition between the symbioses were apparently due to hostplant effects rather than nodulation or the levels of combinedN. Also, the data indicate that the use of the proportion ofN in sap as citrulline (or indeed any other organic N solute)could not be used as an index of nitrogen fixation.     

Metabolism of Phloem-borne Amino Acids in Maternal Tissues2Pisum sativum L.)

The amino acid composition of the EDTA-induced phloem exudatereaching the fruit and the seed, and of the solutes releasedby the seed coat during fruit development were determined inglasshouse-grown pea (Pisum sativum L. cv. Finale) suppliedeither with nitrate-free nutrients (nodulated plants) or withcomplete medium (non-nodulated plants). The EDTA-promoted exudationtechnique was used supposedly to collect phloem sap and theempty seed technique supposedly to collect the solutes secretedby the seed coat to the embryo sac cavity. In young seeds embryosac liquid was sampled directly from the embryo sac. The maincarbohydrate transported and secreted was sucrose. The mainamino acids reaching the fruit were asparagine, glutamine, andhomoserine. Their proportions were steady during a day-nightcycle and throughout fruit development. Amino acid compositionchanges occurred first in the pathway from fruit stalk to seedfunicle, due to the formation of threonine (probably from homoserine)and in the seed coat due to production of glutamine, alanineand valine which, together with threonine were the main secretedamino acids. The temporary nitrogen reserves of the pod walland seed coat were remobilized as asparagine during senescence.Phloem exudate of nodulated plants showed a higher (about twice)proportion of asparagine but lower proportions of homoserineand glutamine than in EDTA-induced phloem exudate of nitrate-fedplants. The two types of nitrogen nutrition also produced somechanges in relative proportions of threonine and homoserinesecreted by the seed coat. Key words: Pisum sativum, phloem, amino acids, pod wall, seed coat     

Phloem Loading and Metabolism of Xylem-Borne Amino Compounds in Fruiting Shoots of a Legume

Cut, fruiting shoots of Lupinus albus L. supplied with ¹⁴C-and ¹⁵N-labelled L-asparagine, L-glutamine, L-aspartic acid,or L-glutamic acid through the transpiration stream readilytransferred the labelled carbon and nitrogen of each compoundto pods and seeds of fruits. A time course of labelling of phloemsap collected from petioles and fruit tips following feedingof labelled asparagine indicated that xylem to phloem exchangein leaflets was an immediate and effective route of transferof the amide to fruits and that this and the loading onto phloemof additional asparagine from unlabelled pools of the amidein stems furnished a major source of the nitrogen for fruitfilling. Xylem to phloem exchange of nitrogen was accomplishedin different ways for each amino acid. The amide nitrogen ofasparagine was transferred mainly in the unmetabolized compound,the nitrogen of aspartate and glutamate largely in a wide rangeof amino acids synthesized in the leaf, and the amide nitrogenof glutamine was transferred in a manner intermediate betweenthese extremes. Glutamine and asparagine were the principalphloem solutes labelled with nitrogen from any of the suppliedcompounds, but the photosynthetically produced amino acids,glutamate, aspartate, serine, alanine, and valine also became¹⁵N-labelled in phloem. The main pathway for glutamine synthesisin vegetative parts of the shoot appeared to be by amidationof glutamate, but asparagine was not considered to be derivedsimilarly from aspartate. Leaflets metabolized glutamine morereadily than asparagine, but in each case the amide nitrogenwas used for synthesis of a variety of amino acids and the carbonwas recovered largely in non-amino compounds.