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1.
Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m–3NO3 or 1 mol m–3 NH+4 as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH+4. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH+4 wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K+, Na+ and Ca2+ but not of Mg2+ were transported to theshoot. As NH+4 was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride  相似文献   

2.
Ammonium influx into roots and N translocation to the shootswere measured in 3-week-old hydroponically grown rice seedlings(Oryza sativa L., cv. IR72) under conditions of N deprivationand NH4+ resupply, using 13NH4+as a tracer. Root NH4+ influxwas repressed in plants continuously supplied with NH4+ (at0.1 mM), but a high proportion of absorbed N (20 to 30%) wastranslocated to the shoot in the form of N assimilates duringthe 13-min loading and desorption periods. Interruption of exogenousNH4+ supply for periods of 1 to 3 d caused NH4+ influx to bede-repressed. This same treatment caused N translocation tothe shoot to decline rapidly, until, by 24 h, less than 5% ofthe absorbed 13N was translocated to the shoot, illustratinga clear priority of root over shoot N demand under conditionsof N deprivation. Upon resupplying 1 mM NH4+, root NH4+ influxresponded in a distinct four-phase pattern, exhibiting periodsin which NH4+ influx was first enhanced and subsequently reduced.Notably, a 25 to 40% increase in root influx, peaking at {smalltilde}2 h following re-exposure was correlated with a 4- to5-fold enhancement in shoot translocation and a repression ofroot GS activity. The transient increase of NH4+ influx wasalso observed in seedlings continuously supplied with NO3and subsequently transferred to NH4+. Extended exposure to NH4+caused root NH4+ influx to decrease progressively, while shoottranslocation was restored to {small tilde}30% of incoming NH4+.The nature of the feedback control of NH4+ influx as well asthe question of its inducibility are discussed. (Received August 7, 1998; Accepted September 21, 1998)  相似文献   

3.
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.  相似文献   

4.
Seven heathland species, four herbaceous plants and three dwarfshrubs, were tested for their capacity to utilize NH4+ or NO3. When cultured in solution at pH 4.0 with 2mol m–3 N,all species showed similar growth responses with respect toN source. Nitrate was assimilated almost equally well as ammonium,with relative growth rate generally averaging 5–8% lowerfor NO3 grown plants, albeit not always significantly.However, N source was significantly and consistently correlatedwith biomass partitioning, as NH4+-fed plants allocated moredry matter to shoots and less to roots when compared to NO3-fed plants. The strong difference in biomass partitioning mayrelate to the relative surplus of carbon per unit plant N (or,alternatively, the relatively suboptimal rate of N assimilationper unit plantC) in NO3-fed plants Inherently slow-growing dwarf shrubs accumulated virtually nofree nitrate in their tissues and reduction of nitrate was strictlyroot-based. Faster-growing herbaceous plants, however, partitionedthe assimilation of nitrate over both shoots and roots, therebyaccumulating relatively high tissue NO3 levels. Ion uptakerates depended clearly on the ‘relative shoot demand’.At similar shoot demands, especially in the herbaceous species,specific uptake rates for N and total inorganic (non-N) anionswere higher in NH4+ -fed plants, whereas the uptake rate fortotal (non-N) cations was higher in NO3-fed plants. Rateof P uptake was enhanced with increasing plant demand, but wasindependent of the N source. Net H+ extrusions ranged from 1.00to 1.34 H+ per NH4+, and from –0.48 to –0.77 H+per NO3 taken up. Key words: Ammonium, biomass partitioning, heathland plants, low pH, nitrate, nitrate reductase activity, relative shoot demand, specific absorption rate  相似文献   

5.
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  相似文献   

6.
Allen, S. and Smith, J A. C. 1986. Ammonium nutrition in Ricinuscommunis: its effect on plantgrowth and the chemical compositionof the whole plant, xylem and phloem saps.—J. exp. Bot.37: 1599–1610. The growth and chemical composition of Ricinus communis cultivatedhydroponically on 12 mol m – 3 NO3-N were comparedwith plants raised on a range of NH4+-N concentrations. At NH4+-Nconcentrations between 0·5 and 4·0 mol m–3,fresh- and dry-weight yields of 62-d-old plants were not significantlydifferent from those of the NO3-N controls. Growth wasreduced at 0·2 mol m–3 NH4+-N and was associatedwith increased root. shoot and C: organic N ratios, suggestingthat the plants were N-limited. At 8·0 mol m–3NH4+-N, growth was greatly restricted and the plants exhibitedsymptoms of severe ‘NH4+ toxicity’. Plants growingon NH4+-N showed marked acidification of the rooting medium,this effect being greatest on media supporting the highest growthrates. Shoot carboxylate content per unit dry weight was lower at mostNH4+-N concentrations than in the NO3-N controls, althoughit increased at the lowest NH4+-N levels. Root carboxylate contentwas comparable on the two N sources, but also increased substantiallyat the lowest NH4+-N levels. N source had little effect on inorganic-cationcontent at the whole-plant level, while NO3 and carboxylatewere replaced by Cl as the dominant anion in the NH4+-N plants.This was reflected in the ionic composition of the xylem andleaf-cell saps, the latter containing about 100 mol m–3Cl in plants on 8·0 mol m–3 NH4+. Xylem-saporganic-N concentration increased more than threefold with NH4+-N(with glutamine being the dominant compound irrespective ofN source) while in leaf-cell sap it increased more than 12-foldon NH4+-N media (with arginine becoming the dominant species).In the phloem, N source had little or no effect on inorganic-cation,sucrose or organic-N concentrations or sap pH, but sap fromNH4+-N plants contained high levels of Cl and serine. Collectively, the results suggested that the toxic effects ofhigh NH4+ concentrations were not the result of medium acidification,reduced inorganic-cation or carboxylate levels, or restrictedcarbohydrate availability, as is commonly supposed. Rather,NH4+ toxicity in R. communis is probably the result of changesin protein N turnover and impairment of the photorespiratoryN cycle. Key words: Ricinus, ammonium nutrition, nitrate, whole-plant composition, xylem, Phloem, amino acids, carboxylate  相似文献   

7.
The effects of NO-3 and NH+4 nutrition on hydroponically grownwheat (Triticum aestivum L.) and maize (Zea mays L.) were assessedfrom measurements of growth, gas exchange and xylem sap nitrogencontents. Biomass accumulation and shoot moisture contents ofwheat and maize were lower with NH+4 than with NO-3 nutrition.The shoot:root ratios of wheat plants were increased with NH+4compared to NO-3 nutrition, while those of maize were unaffectedby the nitrogen source. Differences between NO-3 and NH+4-fedplant biomasses were apparent soon after introduction of thenitrogen into the root medium of both wheat and maize, and thesedifferences were compounded during growth. Photosynthetic rates of 4 mM N-fed wheat were unaffected bythe form of nitrogen supplied whereas those of 12 mM NH+4-fedwheat plants were reduced to 85% of those 12 mM NO-3-fed wheatplants. In maize supplied with 4 and 12 mM NH+4 the photosyntheticrates were 87 and 82% respectively of those of NO-3-fed plants.Reduced photosynthetic rates of NH+4 compared to NO-3-fed wheatand maize plants may thus partially explain reduced biomassaccumulation in plants supplied with NH+4 compared to NO-3 nutrition.Differences in the partitioning of biomass between the shootsand roots of NO-3-and NH+4-fed plants may also, however, arisefrom xylem translocation of carbon from the root to the shootin the form of amino compounds. The organic nitrogen contentof xylem sap was found to be considerably higher in NH+4- thanin NO-3-fed plants. This may result in depletion of root carbohydrateresources through translocation of amino compounds to the shootin NH+4-fed wheat plants. The concentration of carbon associatedwith organic nitrogen in the xylem sap of maize was considerablyhigher than that in wheat. This may indicate that the shootand root components of maize share a common carbon pool andthus differences induced by different forms of inorganic nitrogenare manifested as altered overall growth rather than changesin the shoot:root ratios.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize, nitrogen, growth, photosynthesis, amino acids, xylem  相似文献   

8.
Nitrate assimilation was examined in two cultivars (Banner Winterand Herz Freya) of Vicia faba L. supplied with a range of nitrateconcentrations. The distribution between root and shoot wasassessed. The cultivars showed responses to increased applied nitrateconcentration. Total plant dry weight and carbon content remainedconstant while shoot: root dry weight ratio, total plant nitrogen,total plant leaf area and specific leaf area (SLA) all increased.The proportion of total plant nitrate and nitrate reductase(NR) activity found in the shoot of both cultivars increasedwith applied nitrate concentrations as did NO3: Kjeldahl-Nratios of xylem sap. The cultivars differed in that a greaterproportion of total plant NR activity occurred in the shootof cv. Herz Freya at all applied nitrate concentrations, andits xylem sap NO3: Kjeldahl-N ratio and SLA were consistentlygreater. It is concluded that the distribution of nitrate assimilationbetween root and shoot of V. faba varies both with cultivarand with external nitrate concentration. Vicia faba L., field bean, nitrate assimilation, nitrate reductase, xylem sap analysis  相似文献   

9.
An experimentally-based modelling technique was developed todescribe quantitatively the uptake, flow, storage and utilizationof NO3-N over a 9 d period in mid-vegetative growth of sandcultured castor bean (Ricinus communis L.) fed 12 mol m–3nitrate and exposed to a mean salinity stress of 128 mol m–3NaCl. Model construction used information on increments or lossesof NO3-N or total reduced N in plant parts over the study periodand concentration data for NO3-N and reduced (amino acid) Nin phloem sap and pressure-induced xylem exudates obtained fromstem, petiole and leaf lamina tissue at various levels up ashoot. The resulting models indicated that the bulk (87%) of incomingnitrate was reduced, 51% of this in the root, the remainderprincipally in the laminae of leaves. The shoot was 60% autotrophicfor N through its own nitrate assimilation, but was oversuppliedwith surplus reduced N generated by the root and fed to theshoot through the xylem. The equivalent of over half (53%) ofthis N returned to the root as phloem translocate and, mostly,then cycled back to the shoot via xylem. Nitrate comprised almosthalf of the N of most xylem samples, but less than 1% of phloemsap N. Laminae of leaves of different age varied greatly inN balance. The fully grown lower three leaves generated a surplusof reduced N by nitrate assimilation and this, accompanied byreduced N cycling by xylem to phloem exchange, was exportedfrom the leaf. Leaf 4 was gauged to be just self-sufficientin terms of nitrate reduction, while also cycling reduced N.The three upper leaves (5–7) met their N balance to varyingextents by xylem import, phloem import (leaves 6 and 7 only)and assimilation of nitrate. Petioles and stem tissue generallyshowed low reductase activities, but obtained most of theirN by abstraction from xylem and phloem streams. The models predictedthat nodal tissue of lower parts of the stem abstracted reducedN from the departing leaf traces and transferred this, but notnitrate, to xylem streams passing further up the shoot. As aresult, xylem sap was predicted to become more concentratedin N as it passed up the shoot, and to decrease the ratio ofNO3-N to reduced N from 0·45 to 0·21 from thebase to the top of the shoot. These changes were reflected inthe measured N values for pressure-induced xylem exudates fromdifferent sites on the shoot. Transfer cells, observed in thexylem of leaf traces exiting from nodal tissue, were suggestedto be involved in the abstraction process. Key words: Ricinus communis, nitrogen, nitrate, nitrate reduction, partitioning, phloem, xylem, flow models  相似文献   

10.
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  相似文献   

11.
When radish plants were grown in nutrient solutions that containedammonium ions (NH4+) as the sole source of nitrogen, they grewpoorly and accumulated high levels of NH4+ in their leaves.However, radish plants cultured in 5 mM NH4+ plus 1 mM NO3(a ratio of 5 : 1 in forms of nitrogen; referred to as 5:lmix-N)grew well and accumulated very low levels of NH4+ in their leaves.After radish plants were cultured in solutions that containedNO3, or NH4+, or 5: lmix-N for a week, they were thensupplied with the same nitrogen source labeled with 15N forone day. The uptake of 15N from labeled NH4+ into total nitrogenwas the highest in plants supplied with 5:1mix-N. These plantsconverted far fewer labeled NH4+ into free NH4+ than did NH4+-fedplants, but converted many more labeled NH4+ into the insolublefraction than did NH4+- or NO3-fed plants. The presence of a small amount of nitrate was shown to stimulatethe assimilation of ammonium ions and the synthesis of proteins. (Received October 26, 1988; Accepted January 24, 1989)  相似文献   

12.
Concentrations of inorganic cations are often lower in plantssupplied with NH4+ as compared with NO3. To examine whetherthis is attributable to impaired root uptake of cations or lowerinternal demand, the rates of uptake and translocation of K,Mg, and Ca were compared in maize plants (Zea mays L.) withdifferent growth-related nutrient demands. Plants were grownin nutrient solution with either 1·0 mol m–3 NO3or NH4+ and the shoot growth rate per unit weight of roots wasmodified by varying the temperature of the shoot base (SBT)including the apical shoot meristem. The shoot growth rate per unit weight of roots, which was takenas the parameter for the nutrient demand imposed on the rootsystem, was markedly lower at 12°C than at 24°C SBT.As a consequence of the lower nutrient demand at 12°C SBT,uptake rates of NO3 and NH4+ declined by more than 50%Compared with NO3 supply, NH4+ nutrition depressed theconcentrations of K and particularly of Ca in the shoot, bothin plants with high and with low nutrient demand. This indicatesa control of cation concentration by internal demand ratherthan by uptake capacity of the roots. Translocation rates of K, Mg and Ca in the xylem exudate werelower in NH4+- than in NO3-fed plants. Net accumulationrates of Ca in the shoot were also decreased, whereas net accumulationrates of K in the shoot were even higher in NH4+-fed plants.It is concluded that reduced cation concentrations in the xylemsap of plants supplied with NH4+ are due to the lower demandof cations for charge balance. The lower K translocation tothe shoot is compensated by reduced retranslocation to the roots.For Ca, in contrast, decreased translocation rates in NH4+-fedplants result in lower shoot concentration. Key words: Nitrogen form, cation nutrition, charge balance, xylem exudate, recirculation  相似文献   

13.
Peoples, M. B., Sudin, M. N. and Herridge, D. F. 1987. Translocationof nitrogenous compounds insymbiotic and nitrate-fed amide-exportinglegumes.–J. exp. Bot. 38: 567–579. The transport of nitrogen from the roots and nodules of chickpea(Cicer anetinum L.), lentil (Lens culinaris Medic), faba bean(Vicia faba L.) and pea (Pisum sativum L.) was examined in glasshouse-grownplants supplied either with nitrate-free nutrients or with nutrientssupplemented with 1,2,4 or 8 mol m-3153N-nitrate. A sixth treatmentcomprised uninoculated plants supplied with 8–0 mol m-31513N-nitrate. For each species, more than 75% of the nitrogenwas exported from the nodules as the amides, asparagine andglutamine. In fully symbiotic plants, the amides also dominatednitrogen transport to the shoot When N2 fixation activity wasdecreased by the addition of nitrate to the rooting medium,the N-composition of xylem exudate and stem solutes changedconsiderably. The relative concentrations of asparagine tendedto increase in the xylem whilst those of glutamine were reduced;the levels of nitrate increased in both xylem exudate and thesoluble nitrogen pool of the stem with a rise in nitrate supply.The changes in relative nitrate contents reflected generallythe contributions of root and shoot to overall nitrate reductaseactivity at the different levels of nitrate used. The relationshipsbetween the relative contents of xylary or stem nitrate andamino nitrogen and the plants' reliance on N2 fixation (determinedby the 15N isotope dilution procedure) were examined. Data suggestthat compositional relationships based on nitrate may be reasonableindicators of symbiotic dependence for all species under studyexcept faba bean when greater than 25% of plant nitrogen wasderived from N2 fixation. Key words: Nitrogen, translocation, legumes  相似文献   

14.
Growth of two actinorhizal species was studied in relation tothe form of N supply in water culture. Non-nodulated bog myrtle(Myrica gale) and grey alder (Alnus incana) were grown withNH4+, NH4NO3 or NO3 (4 mol m–3 N). A nodulatedseries of bog myrtle was also cultivated in N-free medium. Relative growth rate (RGR), utilization rate of N, and shoot/rootratio were highest for the two species with the N completelysupplied as NH4+. In both species, nitrate was largely reducedin the roots and the presence of NO3 in combined-N supplyalways affected the RGR and N utilization rate negatively. BothN2 fixation and complete NO3 nutrition represented conditionsof relative N-deficiency resulting in relatively low tissue-Nconcentrations and a greater allocation of dry mass to the roots.The physiological N status of nodulated M. gale plants was comparativelygood, as indicated by a normal nodule weight ratio and a relativelyhigh N2-fixing rate per unit nodule mass. However, whole-plantN2-fixing capacity remained relatively low in comparison withacquisition rates of N in combined-N plants. The anion charge from the nitrate reduction was largely directlyexcreted as an OH efflux. H + /N ratios generally agreedwith the theory. In comparison with NH4+ nutrition, carboxylateconcentrations were higher in N2-fixing M. gale plants and theH + /N ratio in nodulated plants was less than unity below thevalue for ammonium plants as previously found for other actinorhizalspecies. Therefore, NH4+ should be an energetically more attractiveN source for actinorhizal plants than N2. The results agree with commonly accepted views on energeticsof N uptake and assimilation in higher plants and support theconcept of a basically similar physiological behaviour betweennon-legumes and legumes. Key words: Actinorhizal symbioses, ammonium, H+/OH efflux, nitrate, N2 fixation, NRA  相似文献   

15.
Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown for 71 d in flowing nutrient solutions containingN as 10 mmol m–3 NH4NO3, under artificial illumination,with shoots at 20/15°C day/night temperatures and root temperaturereduced decrementally from 20 to 5°C. Root temperatureswere then changed to 3, 7, 9, 11, 13, 17 or 25°C, and theacquisition of N by N2 fixation, NH4+ and NO3 uptakewas measured over 14 d. Shoot specific growth rates (d. wt)doubled with increasing temperature between 7 and 17°C,whilst root specific growth rates showed little response; shoot:root ratios increased with root temperature, and over time at11°C. Net uptake of total N per plant (N2 fixation + NH4++ NO3) over 14 d increased three-fold between 3 and 17°C.The proportion contributed by N2 fixation decreased with increasingtemperature from 51% at 5°C to 18% at 25°C. Uptake ofNH4+ as a proportion of NH4+ + NO3 uptake over 14 d variedlittle (55–62%) with root temperature between 3 and 25°C,although it increased with time at most temperatures. Mean ratesof total N uptake per unit shoot f. wt over 14 d changed littlebetween 9 and 25°C, but decreased progressively with temperaturebelow 9°C, due to the decline in the rates of NH4+ and NO3uptake, even though N2 fixation increased. The results suggestthat N2 fixation in the presence of sustained low concentrationsof NH4+ and NO4 is less sensitive to low root temperaturethan are either NH4+ or NO3 uptake systems. White clover, Trifolium repens L. cv. Huia, root temperature, nitrogen fixation, ammonium, nitrate  相似文献   

16.
The growth of four heathland species, two grasses (D. flexuosa,M. caerulea) and two dwarf shrubs (C. vulgaris, E. tetralix),was tested in solution culture at pH 4.0 with 2 mol m–3N, varying the N03/NH4+ ratio up to 40% nitrate. In addition,measurements of NRA, plant chemical composition, and biomassallocation were carried out on a complete N03/NH4+ replacementseries up to 100% nitrate. With the exception of M. caerulea, the partial replacement ofNH4+ by NO3 tended to enhance the plant's growth ratewhen compared to NH4+ only. In contrast to the other species,D. flexuosa showed a very flexible response in biomass allocation:a gradual increase in the root weight ratio (RWR) with NO3increasing from 0 to 100%. In the presence of NH4+, grassesreduced nitrate in the shoot only; roots did not become involvedin the reduction of nitrate until zero ambient NH4+. The dwarfshrubs, being species that assimilate N exclusively in theirroots, displayed an enhanced root NRA in the presence of nitrate;in contrast to the steady increase with increasing NO3in Calluna roots, enzyme activity in Erica roots followed arather irregular pattern. Free nitrate accumulated in the tissuesof grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO3 [(proportion of nitratein N uptake)/(proportion of nitrate in N supply)] was lowestin M. caerulea and highest in D. flexuosa. Whereas M. caeruleaand the dwarf shrubs always absorbed ammonium highly preferentially(relative uptake ratio for NO3 <0.20), D. flexuosashowed a strong preference for NO3 at low external nitrate(the relative uptake ratio for N03 reaching a value of2.0 at 10% NO3). The ecological significance of thisprominent high preference for NO3 at low NO3/NH4+ratio by D. flexuosa and its consequences for soil acidificationare briefly discussed. Key words: Ammonium, heathland lants, N03/NH4+ ratio, nitrate, nitrate reductase activity, soil acidification, specific absorption rate  相似文献   

17.
Nitrate reductase activity (NRA, in vivo assay) and nitrate(NO-3) content of root and shoot and NO-3 and reduced nitrogencontent of xylem sap were measured in five temperate cerealssupplied with a range of NO-3 concentrations (0·1–20mol m–3) and three temperate pasture grasses suppliedwith 0·5 or 5 0 mol m–3 NO-3 For one cereal (Hordeumvulgare L ), in vitro NRA was also determined The effect ofexternal NO-3 concentration on the partitioning of NO-3 assimilationbetween root and shoot was assessed All measurements indicatedthat the root was the major site of NO3 assimilation in Avenasatwa L, Hordeum vulgare L, Secale cereale L, Tnticum aestivumL and x Triticosecale Wittm supplied with 0·1 to 1·0mol m–3 NO-3 and that for all cereals, shoot assimilationincreased in importance as applied NO-3 concentration increasedfrom 1.0 to 20 mol m–3 At 5.0–20 mol m–3 NO3,the data indicated that the shoot played an important if notmajor role in NO-3 assimilation in all cereals studied Measurementson Lolium multiflorum Lam and L perenne L indicated that theroot was the main site of NO-3 assimilation at 0.5 mol m–3NO-3 but shoot assimilation was predominant at 5.0 mol m–3NO-3 Both NRA distribution data and xylem sap analysis indicatedthat shoot assimilation was predominant in Dactylis glomerataL supplied with 0.5 or 5.0 mol m–3 NO-3 Avena sativa L., oats, Hordeum vulgare L., barley, Secale cereale L., rye, x Triticosecale Wittm., triticale, Triticum aestivum L., wheat, Dactylis glomerata L., cocksfoot, Lolium multiflorum Lam., Italian ryegrass, Lolium perenne L., perennial ryegrass, nitrate, nitrate assimilation, nitrate reductase activity, xylem sap  相似文献   

18.
Ruan  Jianyun  Zhang  Fusuo  Wong  Ming H. 《Plant and Soil》2000,223(1-2):65-73
The effects of nitrogen form and phosphorus source on the growth, nutrient uptake and rhizosphere soil property of tea (Camellia sinensis L.) were investigated in a pot experiment. The experiment was performed with a compartmental cropping device, which enables the collection of rhizosphere soil at defined distances from the root of tea plant. Nitrogen was supplied as nitrate or ammonium in combination with soluble phosphorus as Ca(H2PO4)2 or insoluble P as rock phosphate. The leaf dry matter production of tea was significantly greater in the treatments with NH4 + than NO3 -, whereas dry matter production of root and stem was not significantly affected. Addition of phosphorus as either source did not influence the dry matter production. The concentrations of K in root, Mg and Ca in both the shoot and root supplied with NO3 - were significantly higher than in NH4 + and influence of P sources was minor. On the contrary, Al and Mn concentrations were significantly larger in NH4 --fed plants which could be attributed to remarkably increased availability of Al and Mn caused by acidification of the rhizosphere soil (the first 1-mm soil section from the root surface) with NH4–N nutrition. The concentration of N in shoot was also significantly higher in NH4- than in NO3-fed plants, indicating higher use efficiency of NH4–N. Whatever the phosphate source, rhizosphere pH declined in ammonium compared to in nitrate treatment. The pH decrease was much larger when no P or soluble P were applied and reached 0.85–1.30 units which extended to 3–5 mm away from the root surface. Exchangeable acidity, content of exchangeable Al and Mn were also considerably higher in the rhizosphere soils of NH4 + fed tea plants. Significant amounts of P dissolved from rock phosphate accumulated in rhizosphere of NH4 +, not NO3 -, suggesting that the dissolution of rock phosphate was induced by the proton excreted by tea root fed with ammonium. With soluble P addition, shoot and root P concentrations were greater in NH4 + than in NO3 - treatment and it appeared that this difference could not be sufficiently explained by the available P content in soil which was only slightly higher in NH4 + treatment. With rock phosphate addition, the shoot and root P concentrations were hardly affected by nitrogen form, although the available P content was much higher and accumulated in the rhizosphere soil supplied with ammonium. The reason for this was discussed with regard to the inter-relationship of Al with P uptake. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   

19.
The xylem exudation of detopped 7-d-old seedlings of Zea maysL. doubled when KCI was present in the root medium comparedto seedlings maintained on water. It was further enhanced whenKCI was replaced by nitrogen compounds such as nitrate, ammoniumand glutamine. The role of the nitrate assimilation pathwayon the enhancement of xylem exudation rate was investigatedusing tungstate, an inhibitor of nitrate reductase (NR) activity,and phosphinothricin or methionine sulphoximine, inhibitorsof glutamine synthetase (GS) activity. The sap levels of NO3,NH4+, glutamine, and asparagine was used to ascertain the invivo inhibition of both enzymes. The tungstate effects werealso checked by measuring leaf in vitro NA activity and NR proteincontent. Xylem exudation rate of detopped seedlings fed withKNO3 decreased when the nitrate assimilation pathway was blockedeither at the NR or at GS sites. This decrease was preventedwhen urea (acting as NH4+ supply) was given simultaneously withtungstate. KNO3 does not act directly on exudation, but throughthe involvement of NH4+. The involvement of glutamine was alsoshown since GS inhibition resulted in a cancellation of theenhancing effect of KNO3 on exudation. As change of exudationrate was not linked to change in sap osmolarity, it is assumedthat the assimilation chain could modify root water conductance.The role of glutamine was discussed. Key words: Exudation, maize, nitrate, conductance, NR, GS  相似文献   

20.
Relationships between nitrate (NO-3) supply, uptake and assimilation,water uptake and the rate of mobilization of seed reserves wereexamined for the five main temperate cereals prior to emergencefrom the substrate. For all species, 21 d after sowing (DAS),residual seed dry weight (d.wt) decreased while shoot plus rootd.wt increased (15–30%) with increased applied NO-3concentrationfrom 0 to 5–20 mM . Nitrogen (N) uptake and assimilationwere as great with addition of 5 mM ammonium (NH+4) or 5 mMNO-3but NH+4did not affect the rate of mobilization of seedreserves. Chloride (Cl-) was similar to NO-3in its effect onmobilization of seed reserves of barley (Hordeum vulgare L.).Increased rate of mobilization of seed reserves with additionalNO-3or Cl-was associated with increases in shoot, root and residualseed anion content, total seedling water and residual seed watercontent (% water) 21 DAS. Addition of NH+4did not affect totalseedling water or residual seed water content. For barley suppliedwith different concentrations of NO-3or mannitol, the rate ofmobilization of seed reserves was positively correlated (r >0.95)with total seedling water and residual seed water content. Therate of mobilization of seed reserves of barley was greaterfor high N content seed than for low N content seed. Seed watercontent was greater for high N seed than for low N seed, 2 DAS.Additional NO-3did not affect total seedling water or residualseed water content until 10–14 DAS. The effects of seedN and NO-3on mobilization of seed reserves were detected 10and 14 DAS, respectively. It is proposed that the increasedrate of mobilization of seed reserves of temperate cereals withadditional NO-3is due to increased water uptake by the seedlingwhile the seed N effect is due to increased water uptake bythe seed directly. Avena sativa L.; oat; Hordeum vulgare L.; barley; Secale cereale L.; rye; xTriticosecale Wittm.; triticale; Triticum aestivum L.; wheat; nitrate; seed; germination; seed reserve mobilization  相似文献   

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