Assimilation and transport of nitrogen in rice II. 15N-labelled nitrate nitrogen

An investigation was made to study the assimilation and transportof ¹⁵N-labelled nitrate nitrogen in rice plant (Oryza sativaL.). Nitrogen from labelled nitrate at the end of plant feedingwas found mainly in nitrate form, and was more prevalent inroots, stem and leaf sheaths. The nitrite fraction had the nextlargest ¹⁵N enrichment. The ¹⁵NO₃ assimilation in the newlyemerged panicle was mainly in amide and amino acid. The ¹⁵N-incorporation at day 0 was greatest in amino acid andnitrate of roots and decreased towards the stem and leaves.Incorporation in these fractions considerably decreased fromday 0 to day 10. Probably most of the nitrogen from the nitratesource was transported from the roots to the shoot in nitrateand amino acid forms. A decrease of ¹⁵N-incorporation in the soluble N fraction andincrease in the insoluble N fraction from day 0 to day 10 inplant parts, particularly the blades, suggested that proteinsynthesis occurred mostly in young parts of the shoot duringthis period. The marked variation in ¹⁵N distribution in differentparts of the plant during the 10 days indicated that the nitrogenin roots and tillers was probably remobilized and transportedto other parts, particularly the upper leaf blades. Ammonium and nitrate nitrogen transport in rice plant are compared. (Received May 11, 1974; )     

Response of mycorrhizal seedlings of SW Australian sandplain Epacridaceae to added nitrogen and phosphorus

The responses of seedlings of four species of southwestern Australiansandplain Epacridaceae to added phosphate (P), added NH₄N0₃(N) or Complete nutrients were studied in glasshouse pot cultureusing cores of habitat soil as rooting substrate. Positive responsesto N and Complete nutrients were evident for three species interms of shoot height and shoot dry weight in comparison withControl plants supplied only with deionized water, but no speciesresponded significantly in its shoot growth to P. Root dry weightwas generally less in Complete and N treatments compared toP and Control, leading to considerably higher shoot: root ratiosin the former two treatments. There was no effect of treatmenton infection intensity of hair roots. Root xylem sap compositionshowed greatly elevated levels of nitrogen in the Complete andN treatments and of phosphate in the P treatment. Ammonium comprisedthe major nitrogenous solute of xylem in Control and P treatmentswhile nitrate levels exceeded ammonium in Complete and N treatments.Glutamine levels were particularly high in the P treatments.Labelling of the Complete or N treatments with ¹⁵NH₃ or ¹⁵NO₃(supplied as single labelled ammonium nitrate) indicated thatboth forms of N were taken up and incorporated into plant insolubleN. Key words: Epacridaceae, nitrogen, phosphorus, mycorrhiza, south-west Western Australia     

Evidence of ammonium assimilation via the glutamine synthetase-glutamate synthase system in rice seedling roots

When rice seedling roots were fed ¹⁵N-ammonium for 1 hr, theamide nitrogen of glutamine showed the highest ¹⁵N abundance.Moreover, glutamine amino, glutamic acid, aspartic acid andalanine showed higher ¹⁵N abundance than ammonium did. In roots whose GS activity was inhibited with MS, both the amountof ammonium and its ¹⁵N abundance were increased. In contrast,both the amount of all examined amino acids containing glutamicacid and their ¹⁵N abundance decreased in roots whose GS activitywas inhibited. From these results, it could be concluded thatthe first step of ammonium assimilation in rice seedling rootswas mainly glutamine synthesis by GS and the second was glutamicacid formation by the GOGAT system. The results of an experiment using ¹⁵N glutamine also supportedthis conclusion. (Received February 23, 1977; )     

Waterlogging and fire impacts on nitrogen availability and utilization in a subtropical wet heathland (wallum)

Protein, amino acids and ammonium were the main forms of soluble soil nitrogen in the soil solution of a subtropical heathland (wallum). After fire, soil ammonium and nitrate increased 90- and 60-fold, respectively. Despite this increase in nitrate availability after fire, wallum species exhibited uniformly low nitrate reductase activities and low leaf and xylem nitrate. During waterlogging soil amino acids increased, particularly γ-aminobutyric acid (GABA) which accounted for over 50% of amino nitrogen. Non-mycorrhizal wallum species were significantly (P < 0.05) ¹⁵N-enriched (0.3–4.3‰) compared to species with mycorrhizal associations (ericoid-type, ecto-, va-mycorrhizal) which were strongly depleted in ¹⁵N (-6.3 to -1.8‰). Lignotubers and roots had δ¹⁵N signatures similar to that of the leaves of respective species. The exceptions were fine roots of ecto-, ecto/va-, and ericoid type mycorrhizal species which were enriched in ¹⁵N (0.1–2.4‰). The 5¹⁵N signatures of δ¹⁵N_{total soil N} and δ¹⁵N_{soil NH4+} were in the range 3.7–4.5‰, whereas δ¹⁵N_{soil NO3?} was significantly (P < 0.05) more enriched in ¹⁵N (9.2–9.8‰). It is proposed that there is discrimination against ¹⁵N during transfer of nitrogen from fungal to plant partner. Roots of selected species incorporated nitrogen sources in the order of preference: ammonium > glycine > nitrate. The exception were proteoid roots of Hakea (Proteaceae) which incorporated equal amounts of glycine and ammonium.     

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     

Variations in the Natural Abundance of 15N in Nitrogenous Fractions of Komatsuna Plants Supplied with Nitrate

Komatsuna (Brassica campestris L. var. rapa) plants were grownhydroponically under various conditions with respect to thesupply of nitrate, and the variations in levels of natural ¹⁵Nabundance (¹⁵N) in nitrogenous fractions of leaf blades, petiolesplus midribs, and roots in these plants were analyzed. The fractionation of nitrogen isotopes during uptake of nitratewas null irrespective of the concentrations of nitrate in theculture medium. The roots had lower ¹⁵N values than that inthe nitrate applied to plants. The nitrate in the three tissuesexamined had higher ¹⁵N values than that in the nitrate applied:the values were highest in the leaf blades which were presumedto have highest activities in terms of reduction of nitrate.In contrast, the amino acids and residual fractions had lower¹⁵N values than those in the nitrate applied. These resultssuggest that reduction of nitrate is a critical step in thefractionation of nitrogen isotopes in plant tissues in vivo. ¹Permanent address: Fukuoka Agricultural Experiment Station,Yoshiki 587, Chikushino, 818 Japan. (Received March 10, 1989; Accepted July 10, 1989)     

Seasonal nitrogen speciation in temperate seagrass Posidonia oceanica (L.) Delile

To better understand some basic aspects of the nitrogen economy in Posidonia oceanica and, specifically, the seasonality of the processes of storage, translocation and assimilation, we examined nitrogen speciation into soluble compounds, both inorganic (nitrates, nitrites and ammonium) and organic (free amino acids, FAA, and total soluble protein, TSP), and the nitrogen assimilation potential (through the glutamine synthetase activity measurement) in the leaves, rhizomes and roots of P. oceanica over a 1-year cycle. Only a limited amount of inorganic nitrogen was found, accounting for less than 3.3% of the total nitrogen content, and it was mostly in the form of ammonium. Nitrate and nitrite concentrations were very low, always below 7.2 μmol g⁻¹ dw in annual average. Among the organic soluble fractions, FAAs were the most abundant, accounting for up to 50% of N pools. Rhizomes were the organs in which FAA concentrations reached their maximum value. The leaves showed higher nitrogen assimilation potential than the roots and this assimilation potential was highest during and after the period of maximum leaf growth, probably corresponding to the assimilation of both new and recycled nitrogen. Our results suggest that 5% of the total nitrogen assimilation occurs in roots and 79% in leaves on an annual average. In addition, rhizomes contributed to the total shoot nitrogen assimilation by 32-54% between autumn and spring. Rhizomes appear as key organs in the nitrogen economy of the plant, not only as a major site for nitrogen assimilation but also as an organ for nitrogen storage. This storage, mostly in the form of FAA, occurs during periods of high availability and low demand (winter). This stored nitrogen can supply up to 33% of plant demands during the moment of maximum leaf growth (i.e. late spring).     

A kinetic study of the assimilation of 15N-labelled ammonium in rice seedling roots

The pathway of ammonium nitrogen assimilation, its incorporationinto amino acids and synthesis of protein was studied with theaid of nitrogen-15. The analysis of ¹⁵N involves the use ofoptical emission spectrometry. Kinetic analysis of nitrogen assimilation by the roots indicatesthat glutamine and glutamic acid were the primary products ofammonium assimilation. Possibly some of the amino acids, suchas aspartic acid and alanine received their amino nitrogen directlyfrom free ammonia in the roots. Amino groups were transformedinto other amino acids from these primary products, especiallyfrom glutamic acid through transamination. (Received April 1, 1974; )     

Mobilization of nitrogen reserves during regrowth of defoliated Trifolium repens L. and identification of potential vegetative storage proteins

Although it is well established that carbon reserves contributeto shoot regrowth of leguminous forage species, little informationis available on nitrogen reserves except in Medicaqo sativaL. and Trifolium subterraneum L. In this study, reserves werelabelled with ¹⁵N to demonstrate the mobilization of endogenousnitrogen from roots and stolons to regrowing leaves and newstolons during 24 d of regrowth in white clover (Thfolium repensL.). About 55% and 70%, respectively, of the nitrogen contentsof these organs were mobilized to support the regrowth of leaves.During the first 6 d, nitrogen in regrowing leaves came mainlyfrom N reserves of organs remaining after defoliation. Afterthese first 6 d of regrowth, most of the shoot nitrogen wasderived from exogenous nitrogen taken up while the contributionof nitrogen reserves decreased. After defoliation, the buffer-solubleprotein content of roots and stolons decreased by 32% duringthe first 6 d of regrowth. To identify putative vegetative storageproteins, soluble proteins were separated using SDS-PAGE ortwo-dimensional electrophoresis. One protein of 17.3 kDa instolons and two proteins of 15 kDa in roots seemed to behaveas vegetative storage proteins. These three polypeptides, initiallyfound at high concentrations, decreased in relative abundanceto a large extent during early regrowth and then were accumulatedagain in roots and stolons once normal growth was re-established. Key words: White clover, regrowth, ¹⁵N-labelled, vegetative storage proteins, electrophoresis     

Phosphate Regulation of Nitrate Assimilation in Soybean

It is known that phosphorus deficiency results in alterationsin the assimilation of nitrogen. An experiment was conductedto investigate mechanisms involved in altered ¹⁵NO^–₃ uptake,endogenous ¹⁵N translocation, and amino acid accumulation insoybean (Glycine max L. Merrill, cv. Ransom) plants deprivedof an external phosphorus supply for 20 d in solution culture.Phosphorus deprivation led to decreased rates of ¹⁵NO^–₃uptake and increased accumulation of absorbed ¹⁵N in the root.Both effects became more pronounced with time. Asparagine, theprimary transport amino acid in soybean, accumulated in largeexcess in roots and stems. In roots of phosphorus-deprived plants,concentrations of ATP and inorganic phosphate declined rapidly,but dry weight accumulation was similar to or above that ofthe control even after 20 d of treatment. Arginine accumulationin leaves was greatly enhanced, even though ¹⁵N partitioninginto the insoluble reduced-N fraction of leaves was unaffected.The results suggest that decreases in NO^–₃ uptake in lowphosphorus plants could be caused by feedback control factorsand by limited ATP availability. The decline in endogenous Ntransport from the root to the shoot may be associated withchanges in membrane properties, which also result in paralleleffects on hydraulic conductance and the upward flow of waterthrough the plant. Key words: Phosphorus stress, nitrate uptake, nitrate translocation, arginine     

Metabolism of Inorganic Carbon Taken Up by Roots in Salix Plants

The metabolic products of inorganic carbon taken up throughthe roots from nutrient solution were studied in willow plants.Willow cuttings (Salix cv. Aquatica gigantea) were suppliedwith unlabelled or ¹⁴C-labelled NaHC0₃ for 1, 5, 10, and 24h in light or in darkness. After feeding, the plants were dividedinto six samples (upper and lower leaves and corresponding stems,cuttings and roots), which were frozen in liquid N₂. Freeze-driedground samples were extracted into water-soluble, chloroform-solubleand insoluble fractions. The water-soluble fraction was furtherseparated into basic, acidic, and neutral fractions by ion-exchangechromatography. In the light experiment pronase treatment wasused to separate the insoluble fraction into proteins and insolublecarbohydrates. After I h feeding time, most of the ¹⁴C was fixed into organicacids and amino acids both in light and in darkness in all partsof the plants. In the roots a large part of the ^l4C-carbon wasincorporated into the protein and insoluble fractions alreadyduring short feeding times, and the amounts incorporated increasedwith time. In the leaves, after 1 and 5 h the main labelledcompounds were the organic acids and amino acids, but after10 h about half of the total ¹⁴C was in protein and in the insolublefraction. A further analysis of amino acids and organic acidswith HPLC showed that C-4 acids were labelled initially andthat over time the proportion of different acids changed. These results indicate that the metabolism of carbon in rootsmight take place via ß-carboxylation of PEP. Partof the fixed ¹⁴C is transported from the roots, probably asamino acids and organic acids, to the shoot. In roots the C-4acids are metabolized further into structural compounds (proteinsand insoluble carbohydrates). Key words: DIC, Salix, roots, metabolism, HPLC     

Nitrogen and Phosphorus Nutrition in Mycorrhizal Epacridaceae of South-west Australia

Xylem transport of nitrogen and phosphorus was examined in maturemycorrhizal plants of 41 species in 15 genera of Epacridaceaein native habitat in south-west Australia. Glutamine was theprincipal nitrogenous solute of xylem of all but four species.In the latter species, arginine or asparagine predominated.Nitrate and ammonium comprised minor fractions of xylem (tracheal)sap N, except in two species in which nitrate contributed overhalf of the N. Ratios of total-N:phosphate-P in xylem sap variedwidely (mean 67±18, range 0.2–495) between speciesand habitats. Plants ofCroninia kingiana (syn.Leucopogon kingeanus)from the one habitat showed higher levels of N and P in xylemearly than late in the mycorrhizal season, but there was noconsistent evidence of higher N and P levels from upper thandeeper parts of their root systems. Study of juvenile populationsof four species of epacrids indicated that substantial fractionsof the yearly increment of N, P and dry matter was accumulatedduring the three summer months when infected mycorrhizal hairroots were absent. Glasshouse culture of mycorrhizal plantsof Epacridaceae in habitat soil enriched with decomposed andleached double (¹³C,¹⁵N)-labelled dry matter of wheat showedsubstantial labelling of shoots with¹⁵N but not with¹³C. Plantsfed similarly treated¹⁵N-labelled root residues of maize acquired¹⁵Nbut failed to generate ¹³C values different from those of controlplants. Possible avenues of mycorrhizal and non-mycorrhizalnutrition of Epacridaceae are discussed. amino acids; mycorrhizal nutrition; xylem transport; south-west Australia; Epacridaceae; nitrogen; phosphorus     

Post-uptake metabolism affects quantification of amino acid uptake

? The quantitative significance of amino acids to plant nutrition remains controversial. This experiment determined whether post-uptake metabolism and root to shoot export differ between glycine and glutamine, and examined implications for estimation of amino acid uptake. ? Field soil containing a Eucalyptus pauciflora seedling was injected with uniformly (13)C- and (15)N-labelled glycine or glutamine. I quantified (15)N and (13)C excess in leaves and roots and intact labelled amino acids in leaves, roots and stem xylem sap. A tunable diode laser quantified fluxes of (12)CO(2) and (13)CO(2) from leaves and soil. ? 60-360 min after addition of amino acid, intact molecules of U-(13)C,(15)N glutamine were < 5% of (15)N excess in roots, whereas U-(13)C,(15)N glycine was 30-100% of (15)N excess in roots. Intact molecules of glutamine, but not glycine, were exported from roots to shoots. ? Post-uptake metabolism and transport complicate interpretation of isotope labelling such that root and shoot contents of intact amino acid, (13)C and (15)N may not reflect rates of uptake. Future experiments should focus on reconciling discrepancies between intact amino acid, (13)C and (15)N by determining the turnover of amino acids within roots. Alternatively, post-uptake metabolism and transport could be minimized by harvesting plants within minutes of isotope addition.     

Partitioning of inorganic nitrogen assimilation between the roots and shoots of cerrado and forest trees of contrasting plant communities of South East Brasil

Summary Woody plants growing in cerrado and forest communities of south-east Brasil were found to have low levels of nitrate reductase activity in their leaves suggesting that nitrate ions are not an important nitrogen source in these communities. Only in the leaves of species growing in areas of disturbance, such as gaps and forest margins, were high levels of nitrate reductase present. When pot-grown plants were supplied with nitrate, leaves and roots of almost all species responded by inducing increased levels of nitrate reductase. Pioneer or colonizing species exhibited highest levels of nitrate reductase and high shoot: root nitrate reductase activities. Glutamine synthetase, glutamate synthase and glutamate dehydrogenase were present in leaves and roots of the species examined.¹⁵N-labelled nitrate and ammonium were used to compare the assimilatory characteristics of two species:Enterolobium contortisiliquum, with a high capacity to reduce nitrate, andCalophyllum brasiliense, of low capacity. The rate of nitrate assimilation in the former was five times that of the latter. Both species had similar rates of ammonium assimilation. Results for eight species of contrasting habitats showed that leaf nitrogen content increased in parallel with xylem sap nitrogen concentrations, suggesting that the ability of the root system to acquire, assimilate or export nitrate determines shoot nitrogen status. These results emphasise the importance of nitrogen transport and metabolism in roots as determinants of whole plant nitrogen status.     

Nitrate Reduction and Partitioning of Nitrogen in Komatsuna (Brassica campestris L. var. rapa) Plants: Compartmental Analysis in Combination with 15N Tracer Experiments

Komatsuna (Brassica campestris L. var. rapa cv. Misugi) is aleafy vegetable that readily accumulates nitrate in its tissues.Plants grown hydroponically with 2 mM nitrate in a greenhousewere fed ¹⁵N-labeled nitrate for 2 h, followed with nonlabelednitrate for 8 h. At intervals of 2 h, the plants were sampledand analyzed for the distribution of 15N in the nitrate, aminoacids, and proteins in the tissues of roots, petioles plus midribs,and leaves. Nitrate reduction and nitrogen fluxes were examinedusing a compartmental analysis with 19 compartments and 28 transferrate constants. Nitrate existed in the three types of tissues as a large storagepool and a small metabolic pool. Nitrogen reduction was observedin these tissues, but mainly in the leaf tissue. Nitrate uptakeand reduction rates were smaller in the dark than in light,and particularly nitrate reduction in the shoot was less inthe dark. The rate of protein synthesis was much greater inthe light. The simulation, using compartment models and ¹⁵Ndistribution data, may be useful for estimating actual ratesof nitrogen transport and metabolism in the whole plant system. (Received October 15, 1986; Accepted March 26, 1987)     

Partitioning of carbon and nitrogen and the nutrition of root and shoot apex in a nodulated legume

Empirically based models depicting exchanges of C, N, and H₂O in phloem and xylem among organs of nodulated white lupin (Lupinus albus cv Ultra) were constructed for the interval 51 to 58 days after sowing. Information was incorporated on the economy of C, N, and H₂O in plant parts, the solute composition of transport fluids collected at selected sites on the plant, and the photosynthetic inputs, transpirational losses, and translocatory activities of different age groups of leaflets and stem + petiole segments of the shoot. Partitioning of C and N showed preferential transfer of N to the shoot apex, which imported 13 milligrams C per milligram N, compared with 54 milligrams C per milligram N for the nodulated root. Leaves translocated assimilates at a C:N weight ratio of 43 to 59, and older leaves serving the roots produced the translocate most rich in N relative to C. The shoot apex was enriched with N, additional to its intake from leaves, by direct uptake of xylem fluid (C:N ratio, 2.4) and receipt of nitrogenous solutes transferred from xylem to upward-moving phloem streams in upper regions of the stem. The models for flow of N and H₂O indicated that xylem streams passing to leaves were substantially less rich in N than the adjacent stream moving through the body of the stem and that a progressive increase in concentration of N occurred within stem xylem elements from base to top of the shoot. This apparently resulted from an abstraction of N from xylem of departing leaf traces, possibly by xylem transfer cells, and a subsequent feedback of this N to xylem streams passing on up the shoot. Upper leaves and shoot apex, therefore, acquired more N from xylem per unit of H₂O transpired than lower parts of the shoot.     

Chemical Changes In Young Tea Plant (Camellia sinensis L.) Tissues Following Application of Fertilizer Nitrogen

The uptake of ammonium sulphate by 14-month-old potted tea plantsgrown in a glasshouse was studied over the 11-week period followingapplication. Concurrent changes in the starch of root-wood,amino acids of xylem sap, and total nitrogen of leaves, stems,root-wood, and feeder roots were determined. Depletion of nitrogenfrom the soil at different depths and transformation of NH₄⁺to NO₃^– was also followed. The results show that the uptake of nitrogen commences within2 days of application as indicated by a marked increase in theamino-acid content of the xylem sap. Glutamine and, to a lesserextent, theanine were quantitatively the most important aminoacids in the sap. The amino-acid content of the sap was a maximumat about the time rapid depletion of the ammonium of the soiltook place. An interesting feature of the work is the reciprocalrelationship between the changes in the starch of root-woodand amino acids in the sap a few days after fertilizer application.Studies on the ammonium and nitrate levels of the soil at differentdepths showed that transformation of NH₄⁺ to NO₃^– occurredin the soil. The response of the various tissues to applied fertilizer nitrogenand increase in the fresh weight of the shoot system showedsimilar trends and may be correlated with the depletion of ammonium-nitrogenfrom the soil.     

Environmental factors affecting basic nitrogen metabolism and seasonal levels of various nitrogen fractions in tissues of bilberry, Vaccinium myrtillus

Bilberry tissues accumulated nitrogen for the winter in the form of reduced low-molecular weight amino compounds. The storage organ was principally the underground stem and the oldest parts of the aerial shoot. Most of the nitrogen was stored in arginine and ammonium compounds, and less in glutamine and other amino acids. Proteins did not accumulate during the winter. The soluble nitrogenous compounds were discharged from storage in May, when nitrogen was translocated from the lower parts of the stem to the growing leaves and buds. Amino acid compositions and concentrations in winter were almost identical under the snow and in snowless areas, only the concentration of glutamine being lower and that of glutathione higher in the snowless area. The level of total protein, particularly in the leaves and buds was much higher in a nitrogen-polluted industrial area than in unpolluted urban forests. The same difference was observed in total amino compounds, but among individual substances it only appeared in ammonium compounds. Certain species differences in the amino acid pool were recorded between V. myrtillus and V. vitis-idaea.     

Nitrate Assimilation in Higher Plants with Special Reference to the Cocklebur (Xanthium pennsylvanicum Wallr.)

A soluble NADH-dependent nitrate reductase is described forthe shoot system of Xanthium. Young leaves and immature stemtissues contain high levels of the enzyme. They are relativelyrich in free amino acids and amides but store little free nitrate.The specific activity of the enzyme is lower in fully expandedleaves, although these leaves exhibit higher rates of fixationof carbon in photosynthesis than do younger leaves. Neithernitrate nor free amino acids accumulate in the mesophyll ofthe leaf. Older parts of the stem axis accumulate large amountsof soluble nitrogen, almost entirely as free nitrate. Reservesof nitrate in the shoot and root are rapidly depleted if nitrateis removed from the external medium. Nitrate reductase is apparently absent from roots of Xanthium.This finding is supported by analyses of bleeding sap from nitrate-fedplants which show that 95 per cent of the nitrogen exportedfrom roots is present as free nitrate. However, roots are capableof synthesizing and exporting large amounts of amino nitrogenif supplied with reduced nitrogen such as urea or ammonium. A scheme is presented summarizing the main features of the metabolismof nitrate in Xanthium and this is compared with the situationin nitrate-fed plants of the field pea (Pisum arvense L.), aspecies previously shown to be capable of reducing nitrate inits root system.