Ethylene-bis-Nitrourethane: Investigation of the Plant Growth Stimulant Action of Analogous Compounds

The results of a systematic attempt to relate the growth-stimulatingfunction of compounds analogous to ethylene-bis-nitrourethaneto their chemical structure and properties showed that, withthe exception of a series of nitraminocarboxylic acids, NO₂NH(CH₂)COOH,activity was confined to compounds of the form R—N(NO₂)—X—N(NO₂)—R,where X is an alkylene or substituted alkylene chain. It issuggested that active compounds are converted to dinitramines(R = H) in the plant, and that the acidic nature of the —NHNO₂groups is of importance. The oil/water partition coefficientsand the geometrical structure of the molecules are also shownto influence their growth-stimulating activity.     

Ethylene Biosynthesis and Xylogenesis in Lactuca Pith Explants Cultured In Vitro in the Presence of Auxin and Cytokinin: The Effect of Ethylene Precursors and Inhibitors

The possible involvement of ethylene in the induction of xylemdifferentiation was studied in lettuce (Lactuca saliva L. cv.Romaine) pith parenchyma explants. The addition of the ethyleneprecursors L-methionine (0.25 µM), S-adenosylmethionine(25 µM) and 1-aminocyclopropane-l-carboxylic acid (0.01µM), or the ethylene-releasing agent 2-chloroethylphosphonicacid (1.0 µM), to a standard IAA-kinetin-containing mediumenhanced xylogenesis compared to control explants cultured inthe absence of these compounds. In the presence of the ethyleneinhibitors aminoethoxyvinylglycine, Co(NO₃)₂ and AgNO₃, xylogenesiswas inhibited. Inhibition of xylogenesis by aminoethoxyvinylglycine(75 µM), Co(NO₃)₂ (50 µM) and AgNO₃ (6.0 µM)was reversed by exogenous 1-aminocyclopropane-l-carboxylic acid(0.01 µM), 2-chloroethylphosphonic acid (5.0 µM)and L-methionine (0.25 µM), respectively. Ethylene productionby explants cultured on media containing L-methionine or 1-aminocyclopropane-l-carboxylicacid was greater than the biosynthesis of ethylene by explantscultured in the absence of these compounds. The incorporationof 2-chloroethylphosphonic acid into the culture medium resultedin higher rates of ethylene production compared to explantscultured on the IAA-kinetin medium. The presence of either aminoethoxyvinylglycineor Co(NO₃)₂ inhibited ethylene production by explants culturedon the IAA-kinetin medium. The data support the hypothesis thatethylene plays a positive role in the initiation of xylem differentiation. Key words: Xylogenesis, Differentiation, Ethylene, IAA, Kinetin, Lactuca sativa     

Changes in the growth rates of saline-lake diatoms in response to variation in salinity, brine type and nitrogen form

The growth rates of four saline-lake diatom taxa were measuredunder varying conditions of salinity (5, 8 and 11), brine type(sulfate- versus bicarbonate-dominated) and nitrogen form (NH₄⁺versus NO₃^–), using a full factorial design. With NO₃^–as the nitrogen source, Cyclotella quillensis, Cymbella pusillaand Anomoeoneis costata exhibited lower growth rates in thesulfate versus bicarbonate media. The strain of Chaetoceroselmorei used in these experiments, isolated from a sulfate-dominatedlake, was unable to grow on NO₃^– alone. In the NH₄⁺ treatments,neither salinity nor brine type affected the growth rates ofC.quillensis or C.elmorei. When supplied with NH₄⁺, C.pusillaand A.costata had higher growth rates in the bicarbonate versussulfate media, although for C.pusilla the difference on NH₄⁺was not as great as on NO₃^–. The impact of brine typeon NO₃^– use is consistent with the theory that sulfateinhibits molybdate uptake, as molybdenum is required for NO₃^–use but not NH₄⁺. Cymbella pusilla was the only taxon affectedby changes in salinity. The four taxa used in these experimentsare frequently found in saline lakes and saline-lake sediments,hence they are used in paleoclimate reconstructions; the resultspresented here provide additional information that may enhancethese diatom-based reconstructions.     

Plant Growth in Relation to the Supply and Uptake of NO3-: A Comparison Between Relative Addition Rate and External Concentration as Driving Variables

Two approaches to quantifying relationships between nutrientsupply and plant growth were compared with respect to growth,partitioning, uptake and assimilation of NO₃^– by non-nodulatedpea (Pisum sativum L. cv. Marma). Plants grown in flowing solutionculture were supplied with NO₃^– at relative addition rates(RAR) of 0·03, 0·06, 0·12, and 0·18d^–1, or constant external concentrations ([NO₃^–)of 3, 10, 20, and 100 mmol m^–3 over 19 d. Following acclimation,relative growth rates (RGR)approached the corresponding RARbetween 0·03–0.12 d^-1, although growth was notlimited by N supply at RAR =0.18 d^-1. Growth rates showed littlechange with [NO₃–] between 10–100 mmol m^–3(RGR=0·15 –0·16 d^-1). The absence of growthlimitation over this range was suggested by high unit absorptionrates of NO₃^–, accumulation of NO₃^– in tissues andprogressive increases in shoot: root ratio. Rates of net uptakeof NO₃^– from 1 mol m^–3 solutions were assessed relativeto the growth-related requirement for NO₃^–, showing thatthe relative uptake capacity increased with RGR between 0·03–0·06d^–1 , but decreased thereafter to a theoretical minimumvalue at RGR     

Perfusion of charophyte cells: a critical analysis of the method

The data obtained by different types of intracellular perfusionwere compared. As the ligated cells cannot be space-clamped,the efficiency of compartment-clamping was evaluated, showingthat the difference I/V (current-voltage) profile between space-clampedand compartment-clamped data could be approximated by a straightline. The time-dependence of the clamp currents was not affectedby the clamp technique. The comparison of different sets of data was quantified by fittingthe I/V curves with a mathematical model (Beilby and Walker,1996). The I/V curves of ligated cells perfused with 1 mM ATPshowed the closest similarity to intact cells with resting potentialsof –22010 mV (7 cells) and similar model parameter values.The cells under open-end perfusion with ATP showed less hyperpolarizedresting p.d.s (potential differences): –17512 mV (4 cells).For both preparations the —ATP data were similar withresting p.d.s at –80 12 mV (5 ligated cells, 7 open-endcells). The excited state was more pronounced in open-end cells(resting level: –5912 mV, 5 cells) than in ligated cells(resting level: –6512 mV, 7 cells). In open-end cellsthe pump responded faster to changes of ATP concentration thanthe excitation channels. The cells stabilized with Pb(NO₃)₂were strongly depolarized both with ATP: –8010 mV (6cells) and without: 010 mV (6 cells). Most model parametersdiffered from those in the intact cells. The excited state wasabolished. Key words: Intracellular perfusion, current-voltage characteristics, Chara, stabilization with Pb(NO₃)₂, ATP effects, voltage clamp techniques     

The mineral nutrition of cultured chlorophyllous cells of tobacco I. Effects of {pi}salts, {pi}sucrose, Ca, Cl and B in the medium on the yield, friability, chlorophyll contents and mineral absorption of cells

1. The growth rate of cultured free-cells and their contents ofchlorophyll, Mg, K and N reached peaks on media containing M_n+= 60 and 80 meq per liter, or in which _salts=2.7 and 3.6 atm.These concentration levels in the media were considerably higherthan those (_Salts=1.5–2.0 atm) which produced good growthof intact plants. This discrepancy in media concentrations maybe due to differences in the culture conditions and in the morphologyof the materials.
2. Very low Ca²⁺/(Mⁿ⁺–Ca²⁺) equivalentratios of 0.1 to 0.01in a medium were sufficient for good growthof free cells, whilegenerally about unity of the ratio is requiredin a medium forgood growth of intact plants. The differencein the medium compositionmay also be due to morphological differencesin the materials.
3. Addition of Cl^– to the medium furtherpromoted the growthrate and friability of cell clumps.
4. Alow concentration of 0.01 mM BO₃^3– in the medium alsopromoted the friability of cell clumps.

(Received August 15, 1972; )     

Culture of chlorophyllous tobacco-cells not requiring any organic additives except sucrose in the medium I. Effects of light and temperature on the growth of the cells

1. In the dark, HNG (habituated Nicotiana glutinosa) and NG cellsscarcely grew at 15?C, and the difference between the growthrates of HNG and NG was small at both 15?C and 25?C.
2. The stimulatoryeffect of light (4000 lux) on the cell growthrate was higherat 25?C than at 15?C for both HNG and NG.
3. Light exerted muchmore effect on the growth rate of HNG thanof NG.
4. The thermaleffect was higher in the light than in the darkfor both HNGand NG, and was somewhat greater on NG than onHNG.
5. The synergisticeffect of light and temperature on cell growthwas greater onHNG than on NG.
6. HNG contained more chlorophyll than NG.
7. Inaddition, there was little difference between the friabilitiesof cell groups of HNG and NG.

(Received December 3, 1976; )     

Influence of nutrients on the development of gibbosity in fronds of the duckweed Lemna gibba L.

The duckweeds Lemna gibba L. and Lemna minor L. only grew wellin undisturbed culture under axenic conditions in low lightintensity when provided with a suitable energy source such asglucose. In media containing N0₃-N gibbosity (a convex ventralsurface) was induced in the presence of the chelating agentethylene-diamine-di-o-hydroxyphenylacetic acid (EDDHA). In nutrientsolutions containing NO₃-N as the only N source, but withoutEDDHA, L. gibba occasionally exhibited gibbosity in culturesolutions of 40 cm³ volumes. More fronds were induced to exhibitgibbosity when the volume of the culture medium was increasedfrom 40 cm³ to 200 cm³. Gibbosity was never induced in L. minor,neither was it induced in L. gibba in media containing NH₄-N,even in the presence of NO₃-N. There was no direct correlationbetween the occurrence of gibbosity and frond growth rate, butgibbosity occurred only when there was good frond growth. In the absence of a sugar, frond growth was enhanced by bubblingair through the culture solution in the light. Increasing theCO₂ concentration in the air up to 1% enhanced growth and inducedgibbosity. Carbon dioxide did not induce gibbosity in mediacontaining NH₄-N. Key words: Ammonium-N, carbon dioxide, gibbosity, Lemna, nitrate-N     

The In Vitro Flowering of Kalanchoe blossfeldiana Poellniz: I. ROLE OF CULTURE CONDITIONS AND NUTRIENTS

Dickens, C. W. S. and Van Staden, J. 1988. The in vitro floweringof Kalanchoe blossfeldiana Poellniz. 1. Role of culture conditions.—J.exp. Bot. 39: 461—471. Nodal explants of Kalanchöe blossfeldiana Poellniz. werecultured in vitro on a low nutrient hormone-free medium. Floweringwas achieved in response to short-day inductive cycles. Thissystem was used to test the influence, on the flowering response,of a variety of culture conditions and media. Reduced vesseland medium volume both inhibited flowering, as did renderingthe vessel impervious to gasses. Nitrogen in the form of NH₄NO₃and KNO₃ promoted flowering and vegetative growth in differentways. Increasing sucrose content in the medium caused some increasein the flowering response and in leaf anthocyanin production,but inhibited most aspects of vegetative growth. All of theseaspects are discussed in relation to the induction and evocationof flowering. Key words: Kalanche, flowering, in vitro     

Culture of chlorophyllous tobacco-cells not requiring any organic additives except sucrose in the medium II. Mineral accumulation by the cells

1. There appeared to be a high degree of constancy in the nutrientcontents in NG (Nicotiana glutinosa) cells where the mediumcomposition remained stable, while the nutrient contents inHNG (habituated NG) cells varied with the changes in the compositionof the medium during the culture period of 30 days.
2. The respectivenutrient-content ratios of HNG to NG cells were:N (1.06)Ca(l.08)<Mg (1.26)<K (2.06)<P (3.57) in theorder ofmagnitude on the 5th day from the start of culture,when bothmedia compositions of NG and HNG cells were stillunchanged.

During this culture period, contents of Mg, K and P were markedlyhigher in HNG than in NG cells, and nutrient contents were generallyhigher in HNG than in NG cells. (Received January 17, 1978; )     

Effects of Sulphur Dioxide and Nitrogen Dioxide on Growth and Translocation in Winter Wheat

Gould, R. P. and Mansfield, T. A. 1988. Effects of sulphur dioxideand nitrogen dioxide on growth and translocation in winter wheat.—J. exp. Bot 39: 389–99 Winter wheat (Triticum aestivum L. cv. Avalon) was grown undersimulated autumn conditions for 4 weeks and exposed to a mixtureof SO₂ and NO₂. Biomass was measured after 2, 3 and 4 weeksand the flag leaves of sample plants were labelled with ¹⁴CO₂.Biomass yields revealed an increase in shoot-to-root ratiosunder polluted conditions. The labelling experiments showedthat less assimilate was transported to the roots, whilst morewas allocated to the younger components of the plant. It appearedthat NO₂ and SO₂ also caused labelled photosynthate to be retainedin the labelled leaf. Reducing the photon flux exacerbated theeffects of SO₂ and NO₂ as indicated by changes in biomass andby the distribution of ¹⁴C. Key words: Wheat, SO₂, NO₂, growth, translocation     

The Effect of Root Temperature on Growth and Uptake of Ammonium and Nitrate by Brassica napus L. cv. Bien venu in Flowing Solution Culture: II. UPTAKE FROM SOLUTIONS CONTAINING NH4NO3

Macduff, J. H., Hopper, M. J. and Wild, A. 1987. The effectof root temperature on growth and uptake of ammonium and nitrateby Brassica napus L. CV. Bien venu in flowing solution culture.II. Uptake from solutions containing NH₄NO₃.—J. exp. Bot.38: 53–66 The effects of root temperature on uptake and assimilation ofNH₄⁺ and NO₃^– by oilseed rape (Brassica napus L. CV. Bienvenu) were examined. Plants were grown for 49 d in flowing nutrientsolution at pH 6?0 with root temperature decrementally reducedfrom 20?C to 5?C; and then exposed to different root temperatures(3, 5, 7, 9, 11, 13, 17 or 25?C) held constant for 14 d. Theair temperature was 20/15?C day/night and nitrogen was suppliedautomatically to maintain 10 mmol m^–3 NH₄NO₃ in solution.Total uptake of nitrogen over 14 d increased threefold between3–13?C but was constant above 13?C. Net uptake of NH₄⁺exceeded that of NO₃^– at all temperatures except 17?C,and represented 47–65% of the total uptake of nitrogen.Unit absorption rates of NH₄⁺ and of 1?5–2?7 for NO₃^–suggested that NO₃^– absorption was more sensitive thanNH₄⁺ absorption to temperature. Rates of absorption were relativelystable at 3?C and 5?C compared with those at 17?C and 25?C whichincreased sharply after 10 d. Tissue concentration of N in theshoot, expressed on a fresh weight basis, was independent ofroot temperature throughout, but doubled between 3–25?Cwhen expressed on a dry weight basis. The apparent proportionof net uptake of NO₃^– that was assimilated was inverselyrelated to root temperature. The results are used to examinethe relation between unit absorption rate adn shoot:root ratioin the context of short and long term responses to change ofroot temperature Key words: Brassica napus, oilseed rape, root temperature, nitrogen uptake     

Nitrate Effects on Growth of the First Four Main Stem Leaves of a Range of Temperate Cereals and Pasture Grasses

The effects of different applied NO₃^– concentrations onextension growth and final length and area of leaves 1–4of five cereals and six pasture grasses of temperate originwere examined. Increased applied NO₃^– in the range 0.1–0.5.0mol m^–3 caused decreased duration of growth but increasedgrowth rate and final length of leaves 2–4 of the cerealsAvena saliva, Hordeum vulgare, Secale cereale, x Triticosecaleand Triticum aestivum. For all cereals, increased NO₃^–resulted in increased area of leaves 1–4. Pasture grasseswere supplied either 0.5 or 50 mol m^–3 NO₃^–. Increasedapplied NO₃^– (0.5–5.0 mol m^–3) resulted indecreased duration of growth and increased growth rate and finalarea of leaves 1–4 of Bromus wiltdenowii, leaves 2–4ofFestuca arundinaceae and leaves 3 and 4 of Lolium muitiflorum.In addition, length of leaves 3 and 4 of B. witidenowii increasedwith increased NO₃^–. Increased NO₃^– resulted inincreased area of leaves 2–4 of Dactylis gtomerata andLolium perenne and leaves 3 and 4 of Phalaris aquaiica but hadno effect on extension growth of all three species. Avena sativa L, oat, Hordeum vulgare L, barley, Secale cereale L, rye, x Triticosecale Wittm, triticale, Triticum aestivum L, wheat, Bromus willdenowii Kunth, prairie grass, Dactylis gtomerata L, cocksfoot, Festuca arundinaceae Shreb, tall fescue, Lolium multijlorum Lam, Italian ryegrass, Lolium perenne L, perennial ryegrass, Phalaris aquatica L, nitrate, leaf extension, leaf expansion     

Plant Growth-promoting Rhizobacteria and Soybean [ Glycine max (L.) Merr.] Growth and Physiology at Suboptimal Root Zone Temperatures

Application of plant growth-promoting rhizobacteria (PGPR) hasbeen shown to increase legume growth and development under optimaltemperature conditions, and specifically to increase nodulationand nitrogen fixation of soybean [Glycine max (L.) Merr.] overa range of root zone temperatures (RZTs). Nine rhizobacteriaapplied into soybean rooting media were tested for their abilityto reduce the negative effects of low RZT on soybean growthand development by improving the physiological status of theplant. Three RZTs were tested: 25, 17.5, and 15 °C. At eachtemperature some PGPR strains increased plant growth and development,but the stimulatory strains varied with temperature. The strainsthat were most stimulatory at each temperatures were as follows:15 °C—Serratia proteamaculans 1–102; 17.5 °C—Aeromonashydrophila P73, and 25 °C—Serratia liquefaciens 2–68.Because enhancement of plant physiological activities were detectedbefore the onset of nitrogen fixation, these stimulatory effectscan be attributed to direct stimulation of the plant by thePGPR rather than stimulation of plant growth via improvementof the nitrogen fixation symbiosis. Legume; nitrogen fixation; nodulation; root zone temperature; PGPR     

Photosynthetic Nitrogen Metabolism in High and Low CO2-adapted Scenedesmus: II. EFFECT OF AMMONIUM AND METHIONINE SULPHOXIMINE ON NITRATE UTILIZATION

Larsson, C.-M., Larsson, M. and Guerrero, M. G. 1985. Photosyntheticnitrogen metabolism in high and low CO²-adapted Scenedesmus.II. Effect of ammonium and methionine sulphoximine on nitrateutilization.—J. exp. Bot. 36: 1387–1395 In 3% CO²-grown Scenedesmus obtusiusculus Chod. utilizing NO₃^–J as the N source, NH₄⁺ addition caused a prompt inhibitionof NO₃^– utilization. Nitrate reductase (NR) activity declinedrapidly in response to the presence of NO₄⁺, but the cessationof NO₃^– utilization was too rapid to be accounted forby the loss in NR activity. The first site of NO₄⁺ inhibitionin these cells seems to be the entrance of NO₃^– into thecells. Upon exhaustion of NO₄⁺ from the medium, NO₃^– utilizationwas rapidly restored and NR activity increased. Air-grown cellswere much less sensitive to the effect of NO₄⁺, more than 30min being required for added NO₄⁺ to cause complete inhibitionof NO₃^– utilization. In these cells, NO₃^– uptakeand NR activity decreased in parallel in response to NO₄⁺ addition.In 3% CO²-grown cells simultaneously subjected to NO₄⁺ and air-levelof CO², NO₄⁺ initially inhibited NO₃^– utilization completely,but a slight recovery took place after approximately 20 min The glutamine synthetase (GS) inhibitor L-methionine D, L-sulphoximine(MSO) behaved as a potent inhibitor of NO₃^– uptake in3% CO²-grown cells, but had considerably less effect in air-growncells, although the time-course of the MSO-induced inhibitionof GS was the same in both cases Key words: Ammonium, nitrate utilization, Scenedesmus     

Nitrate Effects on Growth of the First Four Main Stem Leaves of a Range of Temperate Cereals and Pasture Grasses

The effects of different applied NO₃^– concentrations onextension growth and final length and area of leaves 1–4of five cereals and six pasture grasses of temperate originwere examined. Increased applied NO₃^– in the range 0.1–50mol m^–3; caused decreased duration of growth but increasedgrowth rate and final length of leaves 2–4 of the cerealsAvena saliva, Hordeum vulgare, Secale cereale x Triticosecaleand Triticum aestivum. For all cereals, increased NO₃^–resulted in increased area of leaves 1-4. Pasture grasses weresupplied either 0.5 or 50 mol m^–3; NO₃^–. Increasedapplied NO₃^– (0.5–50 mol m^–3) resulted indecreased duration of growth and increased growth rate and finalarea of leaves 1–4 of Bromus willdenowii leaves 2–4of Festuca arundinaceae and leaves 3 and 4 of Lolium multiflorum.In addition, length of leaves 3 and 4 of B. willdenowii increasedwith increased NO₃. Increased NO₃ resulted in increased areaof leaves 2–4 of Daciylis glomerata and Lolium perenneand leaves 3 and 4 of Phalaris aquatica but had no effect onextension growth of all three species. Avena saliva L., oat, Hordeum vulgare L., barley, Secale cereaie L., rye, x Triticosecale Wittm, triticale, Triticum aestivum L., wheat, Bromus willdenowii Kunth, prairie grass, Dactylis glomerata L., cocksfoot, Festuca arundinaceae Shreb, tall fescue, Lolium multiflorum Lam, Italian ryegrass, Lolium perenne L, perennial ryegrass, Phalaris aquatica L, nitrate,, leaf extension, leaf expansion     

Resting membrane potential and action potential of Nitella expansa protoplasts

Inside negative membrane potentials were observed for protoplastsobtained from Nitella expansa leaf internodal cells in mediacontaining 1 to 100 mM CaCl₂ using the microelectrode technique.The potential values were less negative than the membrane potentialof intact N. expansa leaf internodal cells. In addition, anaction potential consisting of two components—a fast componentand a slow component—was induced by electrical stimulationfor the protoplasts as well as the intact cells. (Received December 18, 1979; )     

Orthophosphate Relations of Root: NO-3 Effects on Orthophosphate Influx, Accumulation and Secretion into the Xylem

Lamaze, T., Sentenac, H. and Grignon, C. 1987. Orthophosphaterelations of root: NO^–₃effects on orthophosphate influx,accumulation and secretion into the xylem.—J. exp. Bot.38: 923–934. Orthophosphate (Pi) accumulation by barley (Hordeum vulgareL.) roots was specifically inhibited by NO^–₃ as comparedto Cl^– and SO₄^{2 –}, and Pi secretion into the xylemwas stimulated. The inhibition of Pi accumulation by NO^–₃was also observed in roots of intact photosynthesizing horsebean(Vicia faba L.), rice (Oryza sativa L.) and soybean (Glycinemax L.) plants. NO^–₃ effects on Pi transport by rootswere more thoroughly investigated with corn (Zea mays L.). Theywere due to intracellular NO^–₃. Pi secretion was stillstimulated by NO^–₃ after Pi withdrawal from the absorptionsolution. ³²Pi influx decreased during Pi accumulation, supportingthe hypothesis that this ion allosterically regulated its owntransport system by feedback control. This control was modulatedby other anions: the decrease was more pronounced in the presenceof nitrate. Chronologically, the depressive effect of NO^–₃on ³²Pi influx appeared after the inhibition of Pi accumulation.Furthermore, under conditions where Pi accumulation was notaffected by NO^–₃, ³²Pi influx and Pi secretion into thexylem became insensitive to the presence of nitrate. Our hypothesisis that the stimulative effect of NO^–₃ on Pi secretionand the depressive one on ³²Pi influx are the repercussionsof an increase in the Pi cytosolic concentration due to an NO^–₃-induced decrease in Pi uptake by the vacuoles. Key words: Root, orthophosphate fluxes, orthophosphate accumulation, nitrate, ionic interaction     

Effect of Denitrification on Nitrogen Fixation in Light in a Photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans

The effect of denitrification on N₂ fixation was studied ina denitrifying photosynthetic bacterium, Rhodopseudomonas sphaeroidesforma sp. denitrificans. KNO₃ inhibited diazotrophic growthin light, but NC₃–-dependent diazotrophic growth in darknesswas found. NO₃– inhibited C₂H₂ reduction activity in lightin cells grown with NO₃–. NO₃–-dependent C₂H₂ reductionactivity in darkness also was present in cells grown with N₂plus NO₃–, but not in cells grown on glutamate with NO₃–.NO₃– repressed the synthesis of nitrogenase in light.This repression was not overcome by the addition of methioninesulfoximine. The inhibitory and repressive effect of NO₃– was not mediatedby the NO₂– produced from NC₃– nor by the NH₄+ excretedinto the medium. But, as N₂ fixation is controlled by O₂ (redoxcontrol) it seems to be mediated through the denitrificationprocesses. Much of the glutamine synthetase was adenylylatedin cells grown with NO₃– and its adenylylation state closelyparallelled nitrogenase activity in the cells. (Received September 4, 1982; Accepted December 11, 1982)     

Ammonium Nutrition in Ricinus communis: Its Effect on Plant Growth and the Chemical Composition of the Whole Plant, Xylem and Phloem Saps

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