Cycling of Amino-Nitrogen and other Nutrients between Shoots and Roots in Cereals--A Possible Mechanism Integrating Shoot and Root in the Regulation of Nutrient Uptake

A split root system was used to investigate the cycling of nitrogenbetween shoots and roots in young wheat and rye plants. ¹⁵N-nitratewas supplied to one part of the root system for various periods,at the end of which these roots were excised. Xylem sap wasthen collected from the other roots which had not been supplieddirectly with ¹⁵N-nitrate. ¹⁵N detected in the xylem sap indicatedcycling of nitrogen between shoots and roots. Calculations showedthat over 60% of the amino-N flux in the xylem was cycling.Thus nitrate assimilation in the root could account for onlya minor part of amino-N in the xylem sap. The specific activity of ¹⁵N in the total N of xylem sap washigher than in the total N of roots and shoots through whichit had cycled. This is because exchange between amino-N in thetransport pools and bulk tissue N is limited. It is proposedthat there is, in effect, a single regulatory pool of amino-N,common to shoots and roots, and that this pool may be a keyelement in the control of N uptake at the level of the wholeplant. The likely energy costs of cycling and implications for thepartitioning of N between shoots and roots are discussed. Infurther investigations the cycling of ⁴²K-potassium and ³²S-sulphurwas demonstrated. Key words: Potassium, sulphur, transport, xylem     

Nitrate Reduction in Solanum tuberosum L.: Development of Nitrate Reductase Activity in Field-grown Plants

Nitrate reductase activity (in vivo method, substrate non-limiting)in unshaded leaves from the top of the canopy has been determinedfor field-grown potato plants over the course of the growingseason. The pattern of change was almost identical for plantsreceiving no added fertilizer and those receiving 24 g N m^–2.Activity increased to a peak at about 90 days after plantingand declined thereafter. On a fresh weight basis activity wasalways higher in fertilized plants. Nitrate reductase activitywas positively and significantly correlated with leaf proteincontent in high N plants (r² = 0.71; P = 0.05), but poorly correlatedwith both the nitrate content of the leaf lamina and the nitrateconcentration in petiole sap. Up until 90 days after planting(mid-July) there appeared to be a positive relationship betweenincreased activity of nitrate reductase and solar radiation.However, results obtained over two seasons showed that the declinein activity after this time was not consistently linked witha fall in the level of solar radiation. Remobilization of reduced-Nand stored nitrate from leaves and stems accompanied this declinein nitrate reductase activity and in the latter part of theseason appeared to account for all of the N gained by growingtubers. In unfertilized plants nitrate-N accounted for 5 per cent orless of total plant N. Fertilized plants contained up to 25per cent nitrate-N. While nitrate availability limited growthin unfertilized plants, sub-optimal rates of nitrate assimilationin fertilized plants, particularly during the early stages ofpost-emergence growth, may contribute to inefficient use ofacquired nitrate. The carbohydrate status of leaf lamina and petiole sap weremodified by N supply. The soluble sugar and starch contentsof low N leaves were higher than in their high N counterparts.By contrast, the concentration of soluble sugars in petiolesap increased to a higher value in high N samples. Althoughsap sugar levels declined in both treatments towards the endof the season, N application delayed this decline for severalweeks. Solanum tuberosum, nitrate reductase, nitrate assimilation, senescence     

Responses of Noogoora Burr (Xanthium occidentale Bertol.) to Nitrogen Supply and Carbon Dioxide Enrichment

We studied the responses of Xanthium occidentale (Bertol.) (cockleburor Noogoora burr), a noxious weed, to atmospheric CO₂ enrichmentand nitrate-N concentrations in the root zone ranging from 0.5to 25 mM. CO₂ enrichment (1500 cm³ m^–3) increased dry-matterproduction to about the same extent (18 per cent) at all levelsof supplied N: most of the increment in dry matter was distributedequally between leaves and roots so that there was little effecton shoot-to-root dry-weight ratios. Growth was stimulated greatlyby N and plateaued at 12 mM supplied N. Shoot-to-root dry-weightand total N ratios increased with increasing N supply. CO₂ enrichmenthad no effect on the total amount of N accumulated by plants,but increased the N-use efficiency of leaves. Enriched plantshad lower concentrations and quantities of N in their leavesthan controls, and therefore lower shoot-to-root total N ratios.Little free NO₃^– accumulated in organs of control or enrichedplants. NO₃^– was the major form of N in xylem sap fromdetopped plants at low supplied NO₃-N, but amino N was equalin importance at high supplied NO₃-N in control and enrichedplants. Concentrations of NO₃^– were lower in the xylemsap of CO₂ enriched plants. It was concluded that the betterN-use efficiency of CO₂ enriched plants could result in increasedgrowth of X. occidentale in regions of marginal soil fertilityas atmospheric levels of CO₂ increase. CO₂ enrichment, nitrogen, Xanthium, Noogoora burr, cocklebur     

Nitrogen Supply Affects the Remobilization of Nitrogen for the Regrowth of Defoliated Lolium perenne L.

Nitrogen remobilization from roots and pseudostems during regrowthof Lolium perenne L. was studied in miniswards grown with contrastinglevels of (NH₄)₂SO₄ in solution culture. Growth with a highN supply (5.0 mol m^–3) increased theweight of leaf laminae recovered at each of five weekly clippings,and decreased the proportion of photosynthate used for rootgrowth. Clipped plants growing in a steady-state were suppliedwith ¹⁵N for 48 h and the recovery of labelled N in laminaemeasured after five weekly cuts. Recovery of labelled N in thelaminae from the second clipping onwards was derived only fromremobilization of N from roots and pseudostem. Miniswards grownwith low N (0.5 mol m^–3) relied moreupon remobilization of N for lamina growth than did high N plants.Thus after 14 d 20% of lamina N was labelled in low N plantsbut only 3% was labelled in the high N treatment. Thereafter,N remobilization declined until at the final clipping after35 d, labelled N represented only 4% and 1 % of the lamina Nin the low and high N plants. When plants were not clipped beforethe labelling period, they took up more ¹⁵N if grown with highN than cut plants. Thereafter, the remobilization of N followeda similar pattern as in the cut plants. Exponential models were used to calculate the rate of N transferfrom roots and pseudostem to laminae. When grown with low N,the half-life of remobilization was 1.56 weeks. High N miniswardshad an initial rapid remobilization with a half-life of 0.66weeks, and a slower phase with a half-life of 2.98 weeks. Key words: Lolium perenne L., nitrogen supply, regrowth, remobilization, internal cycling     

The Effects of Nitrogen Supply and Defoliation on the Seasonal Internal Cycling of Nitrogen in Molinia caerulea

Plants of Molinia caerulea were grown in pots for two seasonsat two levels of nitrogen (N) supply and two levels of defoliation.All N supplied was enriched with ¹⁵N in the first season andwas at natural abundance in the second season. This allowedthe contribution of remobilization from overwintering storesto be discriminated from current root uptake in supplying Nfor new shoot growth in the second season. The effects of Nsupply and defoliation upon the internal cycling of N in M.caerulea were quantified. N was remobilized from both roots and basal internodes to supportnew shoot, especially leaf, growth in spring. Roots suppliedmore N than basal internodes. Since the remobilization mainlyoccurred before the onset of root N uptake, internal cyclingwas important for the earliest period of shoot growth. An increasedN supply increased the amount of N remobilized to new shootgrowth, however, the proportion of N remobilized from overwinteringstores was independent of N supply. Defoliation increased theamount of N remobilized from the roots, and had no effect onthe ¹⁵N content of basal internodes of plants receiving a lowsupply of N. Remobilization of N from leaves of undefoliatedplants occurred later in the season. Remobilization from leavessupplied flowers in plants receiving a low N supply and bothflowers and new basal internodes in plants receiving a higherN supply. Key words: Molinia caerulea, internal cycling, nitrogen, defoliation     

The Effects of Temperature and Form of Nitrogen Supply on the Relative Contribution of Root Uptake and Remobilization in Supplying Nitrogen for Laminae Regrowth of Lolium perenne L.

Plants of Lolium perenne L. cv. S23 were grown in sand culturesupplied with either ammonium (NH₄⁺) or nitrate (NO₃^–)in an otherwise complete nutrient solution at 12°C or 20°C.Three weeks after germination, plants were clipped weekly tosimulate grazing. After 10 weeks growth all nitrogen (N) wassupplied enriched with ¹⁵N to quantify the effects of form ofN supply and temperature on the relative ability of currentroot uptake and remobilization to supply N for laminae regrowth. The form of N supply had no effect on the dry matter partitioning,while at 20°C more dry weight was allocated to laminae regrowthand less to the remaining plant material. The current root uptakeof N, which subsequently appeared in the laminae regrowth, wassimilar for plants supplied with NH₄⁺ or NO₃^–, and bothwere equally reduced at the lower temperature of growth. Remobilizationof N to laminae regrowth was greater for plants receiving NH₄⁺than NO₃^–; remobilization with either form of N supplywas reduced at the lower temperature of growth. Remobilizationwas reduced to a lesser extent at 12°C than current rootuptake. It was concluded that remobilization became relativelymore important in supplying N for regrowth of laminae at lowertemperatures. Key words: Lolium perenne, ammonium, nitrate, temperature, remobilization     

Differences in Nitrogen Metabolism During Growth of Plants from Aged Potato (Solanum tuberosum L.) Meristems

The effect of advanced meristem age on growth and accumulationof plant nitrogen (N) in potato (Solanum tuberosum L.) was studied.Etiolated plantlets, excised from sprouted, single-eye-containingcores from 7 and 19-month-old seed-tubers, were transplantedinto aerated nutrient culture. Rates of shoot and root dry matterand shoot soluble-N (which included nitrate-N) accumulationwere similar for plants from both meristem ages over a 30 dinterval of log-linear growth. The rate at which nitrate-N accumulatedwas consistently 17 per cent higher in shoots from 19-month-oldcompared to those from 7-month-old meristems. However, accumulationof free amino-N and soluble protein-N were 21 and 15 per centlower, respectively in shoots from 19-month-old meristems. Abuild-up of shoot nitrate, along with lower rates of accumulationof amino-N and soluble protein-N, suggests a lower capacityfor nitrate reduction during early growth of plants from oldermeristems. Furthermore, these effects can be attributed to age-inducedchanges in the meristem or bud tissue as the plants were separatedfrom the tuber tissue initially in the study. Long-term ageingof seed-potatoes apparently affects changes within meristemsthat translate into a lower capacity to accumulate reduced formsof nitrogen during early plant growth. Potatoes (Solanum tuberosum L.), meristem age, nitrogen metabolism, plant growth potential     

Evidence for N feedback regulation of N2 fixation in Alnus glutinosa L.

Treatments were applied to vary C and N availability in Alnusglutinosa L. and plant growth, nodule activity (including acetylenereduction) and amino acid composition of the xylem sap weremeasured. Removing the buds, a sink for N, caused a decreasein nodule activity. Flushing root systems daily with 100% O₂destroyed nitrogenase activity and substantially decreased theamount of citrulline in the xylem sap. The amino acid compositionof xylem saps also altered according to the mode of N nutrition.In plants fed , xylem sap composition was similar to N₂-fixing plants, however, when plants were fed, citrulline content increased. The assimilation and subsequent distribution of nitrate wasfollowed in an experiment in which labelled ¹⁵ was added to the base of plant pots. After 12 h7% of root N was from applied ¹⁵ and this increased to 75% at 7 d; substantial enrichment ofN from ¹⁵ also occurred in stems, buds and leaves. After 7 d, 3.5% of nodule N was from¹⁵, consistent with some N being supplied by recycling of shoot N. Xylem saps were alsocollected and after 12 h, glutamate and aspartate were enrichedwith ¹⁵N to 53% and 37% increasing after 7 d to 80% and 49%,respectively. Citrulline content of the xylem sap increasedfrom 3 to 9 µmol cm^–3 following addition of ¹⁵ and at 7 d 80% of the N in the citrullinehad been derived from ¹⁵N. It is hypothesized that the growthand activity of A. glutinosa root nodules is sensitive to theN status of the plant and that the level of citrulline (or otheramino acids) returning to the nodules may feed back to regulatenodule growth and activity. Key words: Alnus glutinosa, citrulline, nitrate, feedback mechanism, N₂-fixation.     

The Effect of Defoliation on the Nitrogen Economy of White Clover: Regrowth and the Remobilization of Plant Organic Nitrogen

Single plants of white clover (Trifolium repens) were establishedfrom stolon cuttings rooted in acid-washed silver sand. Allplants were inoculated with Rhizobium trifolii, and receivednutrient solution containing 0·5 mg ¹⁵N as either ammoniumor nitrate weekly for 12 weeks (i.e. 6 mg ¹⁵N in total). Plantswere then leniently defoliated or left intact, and the labelledN supply was replaced with unlabelled N. Lenient defoliationremoved fully expanded leaves only, leaving immature leaveswhich accounted for 50–55% of the total; growing pointnumbers were not reduced. Nodules, leaves and growing pointswere counted over the following 21 d period, and d. wts, N contents,and ¹⁵N enrichments of individual plant organs were determined. Defoliated plants had fewer nodules, but numbers of growingpoints were unaffected by defoliation. The rates of both leafemergence and expansion were accelerated in defoliated plants;in consequence the number of young leaves remained less thanin intact plants until day 21. Total dry matter (DM) and N accumulationwere less in defoliated plants, and a greater proportion oftotal plant DM was invested in roots. About 97 % of plant totalN was derived from fixed atmospheric N, but there was incompletemixing of fixed and mineral N within the plant. Relatively moremineral N was incorporated into roots, whereas there was relativelymore fixed N in nodules. There was isotopic evidence that Nwas remobilized from root and stolon tissue for leaf regrowthafter defoliation; approximately 2 % of plant N turned overdaily in the 7-d period after defoliation, and this contributedabout 50% of the N increment in leaf tissue. White clover, Trifolium repens L. cv. SI84, lenient defoliation, N economy, regrowth, N remobilization     

Accumulation and Remobilization of Nitrogen in a Vegetative Winter Wheat Crop During or Following Nitrogen Deficiency

Nitrogen (N) deficiency leads to retranslocation of N from shootsto roots in vegetative winter wheat plants grown under controlledconditions. The accumulation and remobilization of nitrogenwere quantified for each individual organ of winter wheat plantsgrown in the field, during a 3-week period of N deficiency (nofertilization) or during the relief of N deficiency (fertilizerapplied), during stem elongation. The rate of accumulation ofN directly from the soil and the rate of remobilization of Nfrom different organs were determined independently, using double-crossed¹⁵Nlabelling. The decrease in soil N availability during the firstweek of the study period reduced the rate of N accumulationby 75%. This low level of N accumulation affected the threeuppermost leaves. At the end of the 3-week study period, nitrogenhad been remobilized from the stems and lower leaves and transportedto the three uppermost leaves of fertilized plants and to thetwo uppermost leaves of the deficient plants. In this case,the third leaf from the top remobilized 40% of total nitrogentranslocated. The roots accumulated 11 to 17% of total nitrogenduring the first week of the study period, and this was thentranslocated to the upper leaves. This reversal of the source-sinkrelationships between organs reflects the ability of the plantto compensate for limited periods of N shortage, using remobilizedN for growth.Copyright 1999 Annals of Botany Company Triticum aestivum, wheat, nitrogen, assimilation, remobilization     

Xylem Transport and Storage of Amino Acids by S.W. Australian Mistletoes and their Hosts

Amino acid composition of xylem (tracheal) sap and ethanolicextracts of shoots of mistletoes (Amyema spp. and Lysiana casuarinae)and their hosts were compared, using material collected in theirnative habitats. Data indicated that certain host xylem soluteswere transferred directly to the parasite xylem, while otherswere either not absorbed or were metabolized prior to transfer.Certain solutes were major constituents of parasite xylem, butundetected or only in trace amount in the host. Shoot aminoacid pools of parasites differed markedly from those of hosts.The mistletoe, Amyema preissii, exhibited differential storageand transport of arginine when parasitizing three differentspecies, but accumulated proline on only two of these hosts.Host- specific amino acids (djenkolic acid in Acacia saligna,and tyramine in Acacia acuminata) were transported and accumulatedin relatively large amounts by the parasite, but were not detectedin other associations. Proline was the major solute of Amyemalinophyllum parasitizing Casuarina obesa, but arginine predominatedin Lysiana casuarinae on the same host. However, when L. csuarinaeparasitized A. linophyllum, in turn parasitic on C. obesa, theLysiana accumulated equal amounts of proline and arginine andmore asparagine than when directly on the Casuarina. Xylem feedingof ¹⁵N-labelled aspartic acid or ¹³N-(amide labelled) asparagineto cut shoots or whole haustoria-bearing plants of the mistletoeA. preissii resulted in 68–73% of the ¹⁵N of aspartateand 24–30% of that of asparagine appearing in ethanol-solubleshoot amino compounds other than the fed solute. ¹⁵N labellingpatterns of detached shoots were not noticeably different fromthat of whole plants suggesting that the haustorium had relativelylittle effect on processing incoming solutes. Alanine, glutamine,and arginine were principal recipients of ¹⁵N from aspartate,alanine and glutamine in the case of fed asparagine. It is estimatedthat 24% of the carbon requirements for dry matter accumulationin Amyema linophyllm were met by intake of xylem sap solutesfrom its host Casuarina obesa. Key words: Amino acids, xylem transport, mistletoes, host: parasite relations, N metabolism     

The Role of the Stem in Phloem Loading of Minerals in Lupinus albus L. cv. Ultra

Collections of phloem sap made over a 40-day period from a varietyof locations on nodulated white lupin plants (Lupinus albusL. cv. ultra) showed considerable enrichment with K⁺ and Mg²⁺in the phloem streams destined for the shoot apices or fruitsrelative to the streams arising from the leaflets (up to 5.5times). Sodium showed enrichment in the streams destined forthe roots (up to 2.5 times) but only when present in the watersupply at a high level (3 mM). The stem, in view of its centrallocation in the transport pathway, is seen as an organ capableof redistributing minerals in the phloem independently of photosynthate. Lupinus albus L., lupin, phloem loading, magnesium, potassium, sodium, mineral elements     

Development of the Xylem Ureide Assay for the Measurement of Nitrogen Fixation by Pigeonpea (Cajanus cajan (L.) Millsp.)

Quantification of N₂ fixation by pigeonpea (Cajanus cajan (L.)Millsp.) in the field has proved difficult using techniquessuch as ¹⁵N isotope dilution, acetylene reduction and N difference.We report experiments to develop the ureide assay of N₂ fixationbased on extraction and analysis of xylem exudate. Plants ofpigeonpea cv. Quantum, inoculated with effective Rhizobium spp.CB756, were grown in a temperature-controlled glasshouse inlarge pots filled with a sand: vermiculite mixture, in waterculture and in a slightly acidic, red-brown earth in replicatedfield plots. Xylem exudate was collected as bleeding sap fromboth nodulated and unnodulated roots, and from detached nodules.Exudate was extracted also from detached shoots and stems ofpigeonpea using a mild vacuum (60–70 kN m^–2). Largedifferences in the composition of N solutes exported from rootsof N₂-dependent and nitrate-dependent plants suggested thatshifts in plant dependence on N₂ fixation may be reflected byconcomitant changes in N solutes. Thus, nodulated plants weresupplied throughout growth with either N-free nutrients or nutrientssupplemented with 1, 2, 5, 5, 10, or 20 mol m^{–3 15}. Plants were harvested at regular intervals fordry matter and vacuum-extracted exudate. The relative abundanceof ureides ([ureide-N/ureide-N + nitrate-N + -amino-N] ? 100)in the exudate was highly correlated with the proportion ofplant N (calculated using a ¹⁵N isotope dilution technique)derived from N₂ fixation. Two distinct phases of plant growthwere recognized and standard curves were prepared for each.The relationship between proportional dependence of plants onN₂ and xylem relative ureides was unaffected by mineral-N source,i.e. nitrate or ammonium. This result is discussed in relationto interpretation of material from field-grown plants. The effectsof plant genotype, strain of rhizobia, section of stem extracted,removal of leaves, time delay between shoot detachment and extraction,and diurnal characteristics were examined in order to identifypotential sources of error and to optimize sampling procedures. Key words: Ureides, allantoin, allantoic acid, N₂ fixation, pigeonpea, Cajanus cajan     

Uptake of 15N by Kiwifruit Vines from Applications of Nitrogen Fertilizer Prior to Budbreak

Mature field-grown kiwifruit vines (Actinidia deliciosa var.deliciosa cv. Hayward) were fertilized with ¹⁵N-labelled fertilizer(ammonium sulphate, 10 atom % ¹⁵N, 50 kgN ha^-1) to investigatethe timing of uptake of fertilizer nitrogen (N) and its availabilityfor new season's growth. Treatments were applied on four occasions,representing 2, 6, 10 and 14 weeks prior to budbreak. Samplesof root, stem, cordon, fruiting cane, vacuum-extracted xylemsap, and new season's growth were collected at fortnightly intervalfrom early winter until 2 months after budbreak. Two weeks after application of each treatment, ¹⁵N equivalentto an average of 7% of the applied label was recovered in rootmaterial. Although label was taken up by roots, there was nomovement of ¹⁵N within the plant until about 1 month prior tobudbreak when it was measured in the stem and cordon. Fertilizernitrogen was not detected at the distal end of fruiting canes,and in new season's growth until 3-4 weeks after budbreak. Beforebudbreak, all nitrogen in the xylem sap was in amino forms.Nitrate appeared 4 weeks after budbreak, and although more enrichedwith ¹⁵N than the amino nitrogen, accounted for only 19% ofthe label. Eight weeks after budbreak, nitrate nitrogen accountedfor 57% of the label. There were no major treatment effects of ¹⁵N on vines in eitherspring or at harvest, although enrichments in fruit and leavesfrom the earliest treatment tended to be less at the end ofthe season than those from the later applications.Copyright1993, 1999 Academic Press Actinidia deliciosa, kiwifruit, nitrogen, ¹⁵N, nutrient uptake     

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     

Partitioning of Nitrogen Derived from N2 Fixation and Reserves in Nodulated Medicago sativa L. During Regrowth

Nodul{macron}ted alfalfa plants were grown hydroponically. Inorder to quantify N₂ fixation and remobilization of N reservesduring regrowth the plants were pulse-chase-labelled with ¹⁵N.Starch and ethanol-soluble sugar contents were analysed to examinechanges associated with those of N compounds. Shoot removalcaused a severe decline in N₂ fixation and starch reserves within6 d after cutting. The tap root was the major storage site formetabolizable carbohydrate compounds used for regrowth; initiallyits starch content decreased and after 14 d started to recoverreaching 50% of the initial value on day 24. Recovery of N₂fixation followed the same pattern as shoot regrowth. Afteran initial decline during the first 10 d following shoot removal,the N₂ fixation, leaf area and shoot dry weight increased sorapidly that their levels on day 24 exceeded initial values.Distribution of ¹⁵N within the plant clearly showed that a significantamount of endogenous nitrogen in the roots was used by regrowingshoots. The greatest use of N reserves (about 80% of N incrementin the regrowing shoot) occurred during the first 10 d and thencompensated for the low N₂ fixation. The distribution of N derivedeither from fixation or from reserves of source organs (taproots and lateral roots) clearly showed that shoots are thestronger sink for nitrogen during regrowth. In non-defoliatedplants, the tap roots and stems were weak sinks for N from reserves.By contrast, relative distribution within the plant of N assimilatedin nodules was unaffected by defoliation treatment. Key words: Medicago sativa L., N₂ fixation, N remobilization, N₂ partitioning, regrowth     

The Effect of the Autumn Senescence of Leaves on the Internal Cycling of Nitrogen for the Spring Growth of Apple Trees

The internal cycling of nitrogen (N) has been studied in applerootstocks grown in sand culture and subjected to a constantN supply, or defoliation, or withholding the N supply in theautumn in order to manipulate the amount of N stored over thewinter. The trees subsequently received either no N or 8–0mol N m^–3 (labelled with ¹⁵N to 498 atom%) with the irrigationthe following spring in order to determine the effect of thecurrent N supply on the remobilization of N for leaf growth. Provision of an autumnal N supply delayed leaf senescence andreduced the amount of N withdrawn from leaves from 156 mg Nplant^–1 to 91 mg N plant^–1. Loss of protein ribulose1,5-bisphosphate carboxylase/oxygenase (RUBISCO) accounted for83–87% of the soluble protein N lost during leaf senescence,there being a preferential loss of RUBISCO compared with othersoluble leaf proteins. Remobilization of N from perennial woody tissues (stems androots) in the spring was used predominantly for leaf growth.The amount of N remobilized depended upon the size of the Nstore, but was unaffected by the current N supply, demonstratingthat fertilization of trees does not alter the efficiency withwhich they cycle N. Degradation of RUBISCO in the autumn accountedfor between 32% and 48% of the N subsequently remobilized forleaf growth the following spring, suggesting that RUBISCO hasa role as a summer store for N. Key words: Malus domestica, Borkh, nitrogen, senescence, ribulose 1, 5-bisphosphate carboxylase, oxygenase, storage, remobilization     

An Isotopic Study of the Fixation of Nitrogen associated with Nodulated Plants of Alnus, Myrica, and Hippophae

Nodulated plants of Alnus glutinosa, Myrica gale, and Hippophaërhamnoides, the root systems of which had been exposed to excessfree ¹⁵N, showed substantial enrichment in fixed ¹⁵N contentin all parts of the plant, but particularly in the root nodules.The data resemble closely those obtained by the present andprevious authors with legumes under comparable conditions, andsupport the conclusion already drawn from experiments of traditionaltype that the nodules of these non-legumes are similar to thoseof legumes in their function and relation to the rest of theplant. Nodulated plants of Alnus and Myrica continued to fixfree nitrogen concurrently with the uptake of combined nitrogenwhen the latter was supplied in the rooting medium in amountsunlikely to be exceeded in the field. Isotopic tests on detachednodules of one of the species (Alnus glutinosa) showed thatfixation continued, and though much reduced as compared withthat shown by attached nodules it considerably exceeded thatexperienced by the present and previous authors with detachedlegume nodules, on the basis of fixation per unit of total nodulenitrogen. It is probable that detached Alnus nodules presentconvenient material for the further study of various aspectsof the fixation process.     

Growth and Nitrogen Status of Soilless Tomato Plants Following Nitrate Withdrawal from the Nutrient Solution

The effects of withdrawing nitrogen (N) from the nutrient solutionof adult tomato plants growing in rockwool in a greenhouse wereinvestigated over a 6 week period during fruit production. Thetreatment reduced total plant growth after a lag period of about2 weeks. The commercial fruit yield after 6 weeks of N deprivationwas 7.7 kg m^-2compared to 9.3 kg m^-2in control plants. Duringthe experiment, growth of the -N plants was fuelled by N reservescontained in both the substrate (rockwool) and in plant organs.The nitrogen budget calculated for -N plants showed that onlya small amount of organic-N was readily available for internalcycling from organs such as stems. It served mainly to feedgrowing fruits which were the main sinks in the plant. The studyalso established that stores of nitrate-N were fully depletedbut it took 45 d for the -N plants to metabolize completelytheir nitrate reserves. This indicates that internal nitrateis not a readily-accessible store of labile N. An estimationof the critical N concentration (%N_c) in the aerial dry matterwas made from the data. Thus, for a crop yielding about 9.9tons DM ha^-1, %N_cwas close to 2.5%. This result is discussedin light of existing models that describe the ontogenic declinein %N_cin dry biomass of C₃plants. The study indicates that thecurrent regime of N fertilization practised in soilless culturesnot only leads to ineffective nitrogen use but also to largelosses of N to the environment; N concentrations should be decreasedin feeding recipes. The use of N-free nutrient solutions priorto the termination of plant culture may also be a means of limitingthe loss of eutrophying elements, such as nitrate, to the environment.Copyright 2001 Annals of Botany Company Lycopersicon esculentum, tomato, organ dry biomass, critical nitrogen concentration, compartment, rockwool, nitrate interruption, distribution, reserves     

The Influence of Nitrogen Supply on the Uptake and Remobilization of Stored N for the Seasonal Growth of Apple Trees

M26 apple rootstocks were grown in sand culture and suppliedwith three rates of nitrogen (N) with the irrigation: none,0·8 mol N m^–2 or 8·0 mol N m^–2. Allthe N supplied to the trees was labelled with ¹⁵N at 5·0atom percent enrichment. The effect of N supply on tree growth,N uptake and the remobilization of N from stems for the annualgrowth of the trees was measured. Increasing the N supply increasedleaf growth, but had no effect upon root mass and so alteredthe root/leaf dry matter ratio Plants receiving no fertilizer N had to rely entirely upon storedreserves of N for their seasonal growth. Initially this N wasused for leaf growth, which stopped after a few weeks. Thereafterthe N-deficient plants retranslocated some of the N from theirleaves to support root growth. Increasing the N supply had littleeffect upon the amount of N remobilized for growth, althoughwell-fertilized plants accumulated N in their leaves and didnot retranslocate any to support root growth. The partitioningof N between roots and shoots was, therefore, altered by increasingthe N supply. Amino acid analysis of stems showed that the majorforms of N remobilized during growth were protein rich in asparagineand arginine The results show the importance of internal N cycling for thegrowth of young apple trees, and are discussed in relation toother studies of N cycling in deciduous trees Malus domestica Borkh., nitrogen, remobilization, growth, partitioning, storage