Effect of Nitrogen Form and Counterion on Establishment of the Rhizobium trifolii-Trifolium repens Symbiosis

The effect of level of different counterion forms of or or both, on establishment of the Rhizobium trifolii-Trifolium repens symbiosis was evaluatedin plants cultured under bacteriologically controlled conditions. Ammonium had little effect on nodule formation, and even after exhaustion there was little compensatory nodule formation. Plant growth, nodule mass and acetylene reductionactivity all declined with increasing levels of . Except for , different counterion forms had little effect on plant growth, nodule numberand mass, and acetylene reduction activity. Ammonium markedlydecreased pH, the extent being dependent upon level and counterion present. Nitrate inhibited nodulation while levels remained in excessof plant uptake. Compensatory nodulation followed exhaustionof or at lower or levels. Nodule mass and acetylene reduction activity decreased, but plant growth increased withincreasing or levels. Nitrate raised the pH of the nutrient solution by one unit orless. Different or counterion forms had little effect on the symbiosis or plant growth. Ammonium nitrate severely inhibited nodulation but on exhaustionmarked compensatory nodulation occurred. The patterns of nodulemass, acetylene reduction activity and plant growth with increasingNH₄NO₃ levels were similar to or alone. Plants provided with NH₄NO₃ or no nitrogenwere similar in only slightly decreasing pH. Key words: Ammonium, Nitrate, Ammonium nitrate, Nitrogen fixation, Rhizobium trifolii, Trifolium repens, Symbiosis establishment, Nodulation     

Translocation of Some Anions, Cations, and Acids in Turnip (Brassica napus L.) Mitochondria

The translocation of various anions, cations, and acids acrossthe membrane system of turnip mitchondria has been investigatedby light scattering techniques. The results imply that the membranesof turnip mitochondria are not permeable to choline, K⁺, Na⁺,, , , and . It has been established that Cl^–,, and SCN– passively permeate the mitochondrial membrane. It is probable that permeates as NH₃ and that and cross the membrane either as the corresponding acid or on ananion/ hydroxide antiporter. The alkali metals Na⁺ and K⁺ equilibrateacross the membrane via specific H⁺/Na⁺ and H⁺/K⁺ antiporters,having similar activity at neutral pH.     

Interactions Between the Uptake and Assimilation of Inorganic Nitrogen and Carbon in Amphidinium spp. (Dinophyceae)

The rate of exponential growth of Amphidinium carterae Hulburtwas the same (0.025 h^–1) with either or as sole N-source. Nevertheless, in short-term experiments, cells growing exponentially with as N-source took up added 5–6 times faster than . accumulated in the cells. Addition of inhibited, reversibly, disappearance of from the medium; prior N-deprivation of the cells did not affect this inhibition. N-repIetecells, grown with as N-source, took up for several hours both in light and darkness, butthe uptake by such cells soon ceased in darkness although it continued in light. When uptake ceased, the cells could still take up rapidly in darkness. Ammonium taken up was assimilated rapidly into organic-N includingglutamine, other amino acids and protein. Ammonium uptake in darkness was accompanied by the utilizationof cellular polysaccharide, mainly glucose polysaccharide. Mostof this carbon was unavailable for the dark assimilation of. Addition of the analogue, methylammonium, did not initiate polysacchande catabolism. Itis suggested that a control mechanism is in operation, througha product of assimilation, which operates on one or more of the enzymes concerned with polysaccharidebreakdown, for example, -amylase or phosphorylase. Uptake of was accompanied by a high rate of dark ¹⁴CO₂ fixation and with both Amphidintum klebsii Kof.et Swezy and A. carterae N-deprivation led to a marked increasein this rate following addition of ; addition of had much less effect. The possible implications of these findings for the eco-physiology of marine dinoflagellatesare discussed. Key words: Ammonium, nitrate, CO₂ fixation     

Differential rates of inhibition of N2 fixation by sustained low concentrations of NH4+ and NO3- in northern ecotypes of white clover (Trifolium repens L.)

An experimental study in flowing solution culture compared threeNorwegian ecotypes (from Saerheim, Pasvik and Bod) and a commercialcultivar (Ac51) of white clover (Trifolium repens L.) and demonstrateddifferences with respect to the rate and intensity with whichN₂ fixation was affected when or were supplied at 20 mmol m^–3 over 14 d. Plants were nodulated and N₂ fixation wasestablished over 17 d prior to supplying mineral N, with shoottemperature 25/15 C day/night and root temperature adjustedprogressively to 12C. Control plants received no mineral Nthroughout, and did not grow as rapidly as those supplied with or . Mineral N generally depressed the total N₂ fixation per plantrelative to control plants, with two exceptions. The effectof on N₂ fixation was moresevere than that of and, over the 14 d, -fed plants fixed more N₂ than the comparable -fed plants. Interpolated daily rates of N₂ fixation per plant andnodule dry wts were used to calculate specific rates of N₂ fixation.These showed that (1) addition of either or initially stimulated fixation relative to control plants and to mineral N uptake;(2) this stimulation was greatest and was delayed by 1-2 d in-fed plants; and (3) nutrition sustained higher residualrates of N₂ fixation after 8 d compared with nutrition, under which fixation all but ceased after10 d in three of the genotypes. Ecotype Bode showed by far themost severe induced depression of N₂ fixation. Key words: N₂ fixation, nitrate, ammonium, white clover, northern ecotypes     

Enhanced Superoxide Radical Production in Roots of Zinc-Deficient Plants

The production of superoxide radical () was studied in roots of cotton (Gossypium hirsutum L. cv. Deltapine15/21), bean (Phaseolus vulgaris L. var. Pr?lude) and tomato(Lycopersicon esculentum L. cv. Super marmande) plants grownin nutrient solution with different Zn concentrations. UsingTiron as a spin-probe, electron spin resonance (ESR) was employedfor the measurements of levels. In the 48 000 g and 140 000 g supernatants of cotton root extracts theamplitude of the Tiron ESR signals reflecting production steeply increased with the appearance of visual Zndeficiency symptoms in the shoots. The changes in the amplitudeof the Tiron ESR signals were closely correlated with an NADPH-dependent generating oxidase activity with a high pH optimum. Increases in NADPH-dependent generation were also found in root extracts of Zn-deficientbean and tomato plants. In all experiments re-supply of Zn todeficient plants for 12 h or 24 h markedly decreased generation. Further, with advancing Zn deficiencyrates of NADPH oxidation increased and the activities of superoxidedismutase (SOD) and catalase decreased. The results suggest that cotton, bean and tomato roots possessan NADPH-dependent generating activity which is affected by the Zn nutritional status of the plants. UnderZn deficiency, enhanced generation and impaired detoxification of Of and H₂O₂ could lead to elevated levelsof and -derived oxidizing O₂ species and thus to increased peroxidation of membrane lipids. Key words: NADPH oxidase, superoxide radical, zinc deficiency     

Nitrate Uptake and Assimilation following Nitrate Deprivation

Upon first exposure to , the uptake and reduction capacities of dark-grown corn (Zea maysL.) roots are initially low, but increase markedly within 6h. The development of the accelerated uptake rate appears to be substrate ‘induced’ as is reductase (NR), the first enzyme in the assimilatory pathway. However, the ‘induction’of uptake is independent of NR induction. The effect of deprivation was studied to determine the role of endogenous on subsequent uptake and reduction. Corn roots were ‘induced’ for 24 h in 0–5 mol m^–3 nutrient solution and then exposed for 0 to 32 h to -free nutrient solution. Uptake and reduction of were determined periodically by exposing sets of roots to a1 h pulse of 0.5 mol m^–3 . Neither uptake (4.57 µmol root^–1 h^–1)nor the percentage of absorbed reduced (27%) was changed significantly (P 0.05) by exogenous deprivation. However, the estimated ‘induced’ componentof uptake decreased significantly (50% after 32 h). Concurrently, the ‘non-induced’ basal componentof uptake increased. Previously accumulated decreased from 23 to 4.5 µmol root^–1 after 32 h of exogenous deprivation. Nearly equivalent quantities of endogenous were used for translocation and reduction during deprivation. During each 1 h pulse, the amounts of translocation and net efflux of to the uptake solution were similar. Net efflux of was strongly correlated (r = 0.991) to the amount of endogenous . The remaining endogenous and its rate of utilization were apparently sufficient to minimize a rapid declineor complete loss in both the ‘induced’ uptake state and the rate of in vivo assimilation. Key words: reduction, translocation, efflux, root, Zea mays L     

The Effect of and Ions on the Reactions of Ribulose Bisphosphate Carboxylase

The effects of sulphite ion () and sulphate ion () on both the activation and the catalytic activities of ribulose- 1, 5-bisphosphate carboxylase (EC 4.1.1.39 [EC] )were studied and compared to those of other effectors of theenzyme, particularly inorganic phosphate (P₁). The activationby CO₂ and Mg²⁺ of a slow activating form of the carboxylasein the presence of the two anions produced high specific activitieswith significant lower concentrations of CO₂ than normally required.This was due to stabilization of the ternary complex betweenthe enzyme, CO₂ and Mg²⁺. With a rapidly activating speciesof enzyme, and caused only a small increase in activation with subsaturating CO₂. , and P₁, with saturatingconcentrations of CO₂ also enhanced the catalytic activity abovethat achieved with CO₂ and Mg²⁺ alone; P₁ was the most effectiveof the anions, producing a 50% increase in the specific activity,both with the slow and rapidly activating species. and were potent inhibitors of the carboxylase and oxygenase reactions of the enzyme. was a non-competitive inhibitor with respect to CO₂, and competitive/mixedwith respect to ribulose-1, 5-bisphosphate. The time courseof the carboxylase and oxygenase reactions in the presence of were biphasic with inhibition apparent only in the second phase. Key words: Ribulose bisphosphate carboxylase, Activation, SO₃^2-, SO₄^2-     

Expression of Characteristics of Ammonium Nutrition as Affected by pH of the Root Medium

To study the effect of root-zone pH on characteristic responsesof -fed plants, soybeans (Glycine max {L.}Merr. cv. Ransom) were grown in flowing solution culture for21 d on four sources of N (1.0 mol m^–3 , 0.67 mol m^–3 plus 0.33 mol m^–3, 0.33 mol m^–3 plus 0.67 mol m^–3 , and 1.0 mol m^–3) with nutrient solutions maintained at pH 6.0, 5.5, 5.0, and 4.5. Amino acid concentration increased inplants grown with as the sole source of N at all pH levels. Total amino acid concentration in the rootsof -fed plants was 8 to 10 times higher than in -fed plants, with asparagine accounting for more than 70% of the total in the roots of these plants.The concentration of soluble carbohydrates in the leaves of-fed plants was greater than that of -fed plants, but was lower in roots of -fed plants, regardless of pH. Starch concentration was only slightlyaffected by N source or root-zone pH. At all levels of pH tested,organic acid concentration in leaves was much lower when was the sole N source than when all or part of theN was supplied as . Plants grown with mixed plus N sources were generally intermediate between - and -fed plants. Thus, changes in tissue compositioncharacteristic of nutrition when root-zone pH was maintained at 4.5 and growth was reduced, still occurredwhen pH was maintained at 5.0 or above, where growth was notaffected. The changes were slightly greater at pH 4.5 than athigher pH levels. Key words: Ammonium, nitrogen nutrition, root-zone pH, soybean, tissue composition     

Comparison of the Effects of Root Temperature on Nitrate and Ammonium Nutrition of Oilseed Rape (Brassica napus L.) in Flowing Solution Culture: I. GROWTH UPTAKE OF NITROGEN

Oilseed rape plants (Brassica napus L. cv. Bien venu) were grownin flowing nutrient solution and acclimatized over 49 d to lowroot temperature (5 ?C) prior to treatment at root temperaturesof 3, 7, 11 or 17?C for 14 d, with common air temperature of20/15?C day/night and solution pH 6.0. The effects of root temperatureon growth, uptake and assimilation of N were compared with Nsupplied either as or at 10 mmol m^–3. Plant fresh weight, dry weight and leafarea increased with root temperature. Q₁₀ values for specificgrowth rates between 7?C and 17?C were 1.10 and 1.33, respectively,under and nutrition. Fresh weight and leaf area were favoured by nutrition, particularly at 11 ?C and 17 ?C. At each temperaturethe net uptake of over 14 d was similar to that of , increasing 4-fold between 3?C and 17 ?C. Unit absorption rates of increased over the first 3 or 4 d, to an extent that increased with roottemperature. Thereafter, near constant values were maintainedat 3, 7 and 11 ?C. Unit absorption rates of at 3 ?C and 7 ?C did not increase over the first 3 or 4 d. Plantsat 17?C responded differently from those at other temperatures,with steep increases in unit absorption rates of and after day 8. The tendency for plants to adjust unit absorption rates of N over 14 d to become independentof root temperature was limited to the range 7–11 ?C.Q¹⁰ values between 7 ?C and 17 ?C for unit absorption rateswere 1.7—1.8 and 1.4—1.5, respectively, for and . Plants at 7, 11 and 17?C absorbed more than was subsequently assimilated, but at 3 ?C the absorption of was insufficient to meet the assimilatory demand. The results are discussed comparativelyfor and and in terms of the factors that determine unit absorption rates of N inresponse to a change in root temperature. Key words: Brassica napus, oilseed rape, root temperature, nitrate, ammonium, ion uptake rate     

Interactions between Root Temperature and Nitrogen Deficiency Influence Preferential Uptake of NH+4 and NO-3 by Oilseed Rape

Oilseed rape (Brassica napus L. cv. Bien venu) plants were grownin a system of flowing solution culture and pre-treated at roottemperatures of 3 ?C or 13 ?C for 7 d with or without N suppliedas 10 mmol m^–3 NH₄NO₃. Subsequently, N was re-suppliedand root temperatures were reversed for 7 d. Shoot temperatureswere common at 20/15 ?C day/night. Net uptake of , and K⁺, leaf area, root length, and transpirationwere measured and compared with control plants having root temperaturesof 3 ?C and 13 ?C throughout. Plants that were continuouslysupplied with N and pre-treated at 3 ?C showed a 50% increasein total uptake of , and measured at 13 ?C over 7 d compared with control plants at 3?C,but uptake of was 28% lower and uptake of was 43% higher than that shown by control plants at 13 ?C. Pre-treatment at 3 ?C did not enhance the subsequentuptake of total N or of K⁺ at 13 ?C relative to the 13 ?C control.Transpiration rates at 3 ?C were on average 70% of those at13 ?C. The concentration of total N in plants was halved after7 d without a supply of N, but total dry matter production wasnot significantly affected. N starvation also increased thetemperature sensitivity of subsequent uptake relative to that of uptake. After N starvation at 13 ?C the uptake of and measured at 13 ?C was 50% higher over 7 d than that measuredunder continuous N supply. In contrast, after N starvation at3 ?C the uptake of at 3 ?C was 70% less, whilst uptake was 50% more than the respective totals absorbed by plants that were continuously supplied withN at 3 ?C. After N starvation the uptake of was generally 40–60% of the daily total N uptake, comparedwith 60–80% in plants continuously supplied with N. Key words: Brassica napus, oilseed rape, root temperature, nitrate, ammonium, potassium, N-deficiency, ion uptake rate, transpiration     

Relative Uptake Rates of Inorganic Nutrients by NO3- and NH4+ Grown Ricinus communis and by two Plantago Species

Allen, S., Thomas, G. E. and Raven, J. A. 1986. Relative uptakerates of inorganic nutrients by and grown Ricinus communis and by two Plantago species.—J. exp. Bot. 37: 419–428. The relative rates of uptake and assimilation of C, N, P, S,Cl^–, K⁺ , Na⁺ Ca²⁺ and Mg²⁺ by and grown Ricinus conimunisand by NH₄NO₃- grown Plantago lanceolata and P. major were calculatedfrom data presented elsewhere. Results showed that for grown Ricinus the short term relativeuptake rates, for each nutrient X did not change significantly over the steady-state periodof exponential growth. The average gave , the mean relative uptake rate during exponential growth, for each nutrient. The amountof each nutrient taken up from a nutrient solution over a periodof time could, therefore, be calculated. For and -grown R. communis,the relative uptake rate of each nutrient was a constant fractionof the relative rate of carbon assimilation. It is suggestedthat this is typical of plants of cauline habit. For both Plantago spp., the relative rates of nitrogen uptakeand assimilation fell significantly during the exponential growthphase It is suggested that this could be a characteristic ofthe growth habit of the rosette plant. Key words: Relative uptake rates, Ricinus, Plantago, ammonium, nitrate, cauline, rosette     

Nitrate Metabolism in Alfalfa Root Nodules under Water Stress

Three-month-old plants (vegetative stage) of alfalfa (Medicagosaliva L cv. Aragon) were supplied for one week with 1.0dm³(uniformly distributed) nutrient solutions containing 0 or 20mol m^–3 . One week after initiation of treatment, the plants were subjected to drought by withholding water. Bacteroidsand cytosol of nodules were obtained at different stages ofstress, and used for enzyme assays and for determination of, and . Proteins of bacteroids were more stable than cytosolic proteinswith respect to the detrimental effects of water stress and. Protein contents of bacteroids and cytosol were inversely related to proteolytic activitiesagainst azocasein in both nodule fractions. Specific nitrate reductase activity (NRA) and nitrite reductaseactivity (NiRA) of bacteroids from -treated plants were inhibited by c. 70% and 45%, respectively, as leafwater potential (_w) declined from –0.5 MPa (control) to–1.8 MPa. At still lower _w both activities began to increase:NRA was doubled, whereas NiRA only returned to its control level.Cytosolic NRA was strongly inhibited by drought, but the correspondingNiRA remained constant. Ammonia concentration in bacteroids and nodule cytosol keptbasically constant, whereas accumulated in the cytosol at severe stress, due to the activationof bacteroid nitrate reductase. Results indicate that nitrate and nitrite reductases of thebacteroids and the nodule cytosol act in different form: assimilatory,the cytosolic enzymes; and dissimilatory, the enzymes of bacteroidsat low _w The possibility that assimilation of also occurs in bacteroids at control or mild waterstress conditions is suggested. Key words: Assimilatory and dissimilatory reduction, bacteroids, Medicago saliva, nodule cytosol, water stress     

Cyclic Variations in Nitrogen Uptake Rate of Soybean Plants: Effects of External Nitrate Concentration

Net uptake of by non-nodulated soybean plants [Glycine max (L.) Merr.cv. Ransom] growing in flowing hydroponiccultures containing 0–5, 1.0 and 10-0 mol m^–3 was measured daily during a 24-d period of vegetativedevelopment to determine if amplitude of maximum and minimumrates of net uptake are responsive to external concentrations of . Removal of from the replenished solutions during each 24-h period was determinedby ion chromatography. Neither dry matter accumulation nor theperiodicity of oscillations in net uptakerate was altered bythe external concentrations. The maxima of the oscillations in net uptake rate, however, increased nearly3-fold in response to external concentrations. The maxima and minima, respectively, changed from 4.0 and 0–6mmol per gram root dry weight per day at an external solution level of 05 mol m^–3 to 15–2and -2–7 mmol per gram root dry weight per day at an external solution level of 10–0mol m^–3 . The negative values for minimum net uptake rate from10–0 mol m^–3 solutions show that net efflux was occurring andindicate thatthe magnitude of the efflux component of net uptake was responsiveto external concentration of . Key words: Nitrate uptake rate, carbon-nitrogen partitioning, flowing solution culture     

Calcification in the Green Alga Halimeda: III. THE SOURCES OF INORGANIC CARBON FOR PHOTOSYNTHESIS AND CALCIFICATION AND A MODEL OF THE MECHANISM OF CALCIFICATION

Calcification and photosynthetic rates in Halimeda tuna weremeasured by the ¹⁴C method under conditions of differing pHand total inorganic carbon (CO₂) concentrations. The effectsof pH and CO₂ on photosynthesis and respiration were also monitoredwith a polarographic O₂ electrode. The results obtained indicatethat the intercellular pH and CO₂ differ from those of the externalmedium. Experiments carried out over a range of pH values show thatHalimeda can use for photosynthesis. Photosynthesis appears to stimulate calcification by removing CO₂ from theintercellular spaces. As these spaces are isolated from theexternal sea water by the layer of cell wall of the adpressedperipheral utricles, the removal of CO₂ results in a rise in[] and a rise in pH. This results in an increased rate of CaCO₃ precipitation. Respiratory CO₂ evolution has aninhibitory effect on calcification by decreasing the pH and[]. A model for calcification in Halimeda is proposed based on theresults of this and previous papers. Calcification in Halimedais seen to be a result of the anatomy of the thallus in whichthe sites of calcification are within a semi-isolated chamberwhere removal or addition of CO₂ due to photosynthesis or respirationcan effectively change [CO] thereby resulting in precipitation of CaCO₃. In the Appendix to this paper theoreticalcalculations illustrate the effects of CO₂, , and removal or addition in a closed system on the relative concentrations of the other inorganic carbonspecies.     

Influence of Inorganic Phosphate on Photosynthesis of Wheat Chloroplasts: I. PHOTOSYNTHESIS AND ASSIMILATE EXPORT AT 5 ?C AND 25 ?C

Chloroplasts were isolated from 10 d old wheat seedlings andilluminated at 5 ?C or 25 ?C in various concentrations of . Photosynthetic oxygen evolution, ATP content, andexport of triose phosphates and 3-phosphoglycerate were measured.Incorporation of ¹⁴C from NaH¹⁴CO₃ into pentose monophosphates,fructose monophosphate, and glucose monophosphates was determined. The ATP content in illuminated chloroplasts decreased when the concentration in the medium was low. The ATP content increased when the concentration was increased. A higher . concentration in the medium was needed to increase the ATP at 5 ?C than at 25?C. This would suggest that deficiency occurs more readily at low than at high temperatures. More ¹⁴C wasincorporated into photosynthetic metabolites within the chloroplastsat 5 ?C than at 25 ?C, indicating decreased assimilate exportwhen the temperature was low. Dihydroxyacetone phosphate waspreferentially exported when the concentration enabled a high rate of photosynthesis at 25 ?C. However, underconditions of deficiency, either due to low concentration in the medium or due to low temperature, 3-phosphoglycerate was preferred for export. The results suggest that the relatively high photosyntheticrates at low temperature are due to increased concentrationsof photosynthetic metabolites. The assimilate export at lowtemperature seems to be decreased due to decreased concentrationsof dihydroxyacetone phosphate in the stroma. Preferential exportof 3-phosphoglycerate at low temperature or at low concentration in the medium may be a consequence of high stromalconcentrations of this metabolite. On the other hand, it couldalso be due to decreased stromal pH. Key words: Chloroplast, Photosynthesis, Phosphate, Temperature, Translocation, ATP, Wheat     

Nitrate Transport into Chara corallina Cells using as an Analogue for Nitrate: III. INTERACTIONS BETWEEN CHLORIDE AND NITRATE TRANSPORT PROCESSES

The effects of Cl^– and pretreatment on ³⁶Cl^–influx and influx into Characorallina cells were examined. Both treatments reduced ³⁶Cl^–influx into C. corallina cells in the acid pH range (4.5–7.0). pretreatment stimulated influx into C. corallina cells, but Cl^– pretreatment hadno effect. There was a direct inhibitory effect of CI on influx into C. corallina cells, but no apparenteffect on net NO uptake. The time course of ³⁶Cl accumulation into cytoplasmic and vacuolarcompartments during incubation of the cells with showed that significant radioactivity appeared in the vacuolarsap after 30 min. There was a linear increase in the percentageof total ³⁶CI which crossed the tonoplast (_c-v). There was noeffect of Cl^– or pretreatment on accumulation of radioactivity in the vacuole. Thin layer chromatography ofthe vacuolar sap showed that after 2 h only one component waspresent which had an R_F which was similar to ³⁶CI^–. Therate of accumulation of ³⁶C1^– in the vacuole could beused to estimate rates of reduction. Key words: Chloride, Chlorate, Chara, Nitrate     

Nitrate Transport Characteristics in Hordeum vulgare L. Seedlings using Three Different Tracer Techniques

Deane-Drummond, C. E. and Thayer, J.R. 1986. Nitrate transportcharacteristics in Hordeum vulgare L. seedlings using threedifferent tracer techniques.—J. exp. Bot. 37: 429–439. and have been used to investigate various properties of nitrate uptake and translocation intoHordeum vulgare L seedlings. Short term / influx into seed lings grown in CaSO⁴ was stimulated by after a lag of 2 h. The apparent kinetics of shortterm / influx over the concentration range 0?0–0?7mol m fitted Michaelis-Menten equations The apparent V_max didnot change when seedlings were used that had been pretreatedin 10 or 100 mmol m^–3 and V^max=3.77 and 3?56µmol g^–1 fr. wt. h^–1respectively. The apparent Michaelis constants were also similarand K_m=0?47 and 0?45 mol m^–3 respectively. Longer term pulse chase experiments with the heavy isotope ¹⁵Nhave shown that feeding roots with resulted in the preferential appearance of ¹⁵N labelled aminoacids in the xylem sap. Pulse chase experiments with the radioisotope¹³N have shown that feeding shoots with resulted in a radial pattern of distnbution of labelin the leaf veins, which can be detected using autoradiography. The limitations and advantages of all three techniques are comparedby reference to other known experimental data. Key words: ³⁶Chlorate, ¹³nitrate, ¹⁵nitrate, Hordeum vulgare     

Uptake of Methylammonium Ions by Hydrodictyon africanum

Methylamine influx into Hydrodictyon has been measured with[^l4C]methylamine. The influx increases with rising externalpH up to about pH 8. Between pH 8 and 9 influx remains quiteconstant, with a further increase above pH 9. Influx is light-dependent,temperature-sensitive, and is decreased by . During short-term influx (less than 4 h) metabolism of methylamineappears negligible. Prolonged influx results in CH₃ accumulation, and efflux of K⁺, Na⁺, and H⁺. There is no effecton Cl^– influx. Methylamine decreases the membrane electricp.d. by 60–120 mV at external concentrations of 0?2–1?0mM. The results indicate that, below pH 9, methylamine enters thecell almost entirely as CH₃. It is suggested that a passive electrogenic uniporter is involved, and thatby analogy uniport of may also be expected in Hydrodictyon. The results are discussed in relation to theevidence for uptake of CH₃ and by other plant cells.     

Compensatory Changes in Ion Fluxes into Barley (Hordeum vulgare L. cv. Betzes) Seedlings in Response to Differential Root/Shoot Growth Temperature

The effect of growth temperature on the short term influx of⁸⁶Rb⁺/K⁺, ³⁶Cl^–/Cl^–, , ⁴⁵Ca²⁺/Ca²⁺and into barley plants (Hordeum vulgareL. cv. Betzes) has been examined. When barley plants were grownwith a differential root: shoot temperature (15 ?C/25 ?C) therewas a marked stimulation of influx of all ions except Ca²⁺.Q₁₀ measurements were close to 2.0 for all ions except Ca²⁺,where Q₁₀ 1.0 was found. Kinetics of ion influx showed thatthere had been almost complete compensation for the lower growingtemperature over a range of concentrations. The regulation of uptake was affected by growth temperature. On exposure to 15?C efflux/influx fell initially but was restored after 5 d. Sensitivity of net uptake to was increased by growth at a lower temperature. The importance of these observations in relationto application to field experiments and the proposed mechanismfor uptake is discussed. Key words: Growth temperature, Nitrate, Ammonium     

Nitrate Nutrition of Grasses from Steady-State Supplies in Flowing Solution Culture following Nitrate Deprivation and/or Defoliation: I RECOVERY OF UPTAKE AND GROWTH AND THEIR INTERACTIONS

Two Lolium genotypes, i.e. Lolium perenne cv. S23 and a L. perenne? L. multiflorum hybrid cv. Augusta were, in one treatment,grown in flowing solution culture with a maintained supply of at 10 mmol m–3 throughout. In another treatment, the plants were deprived of for 11 d, was then re-supplied and both control and N-deprived plants were either defoliatedor left entire. Effects of the stresses imposed by N deprivationor defoliation on subsequent net uptake of , shoot and root growth and their interactions were examined.There were no major differences in the behaviour of the twogenotypes throughout. The removal of NO3 supply did not altertotal dry matter production during the 11 d period of deprivationbut the rate of root growth accelerated markedly. There waslittle effect of N deprivation on the specific growth ratesof the recovering defoliated shoots. Cumulative net uptake of closely paralleled the pattern of growth and there were obvious and marked effects of both N deprivationand defoliation. Effects on uptake were more distinctly demonstrated when uptake was expressed on aper unit root or shoot basis, i.e. specific unit absorptionrates (S_NO3). In the control plants with sustained supply, there was a progressive decline with time in S_NO3, inspite of an increasing shoot: root ratio. After both N deprivationand defoliation treatments there was a rapid initial increasein S_NO3, followed by an oscillatory cycle of increasing anddecreasing rates with a 10–11 d period. The relationshipsbetween growth and uptake were complex and did not conform to a previously predicted linear relationshipbetween S_NO3, and the ratio total fresh plant weight: root freshweight. The results are discussed in relation to the possiblemechanisms involved. It is suggested that the oscillations inunit absorption rate indicated that the roots of N-deprivedor defoliated plants retained a high capacity for absorptionimmediately after re-supply or defoliation and that the stressedplants were unable to assimilate or utilize taken up; net uptake then decreased through a shift in favourof efflux over influx until the influx mechanism was able todominate and the pattern was then repeated. Key words: Lolium, genotypes, N deprivation, nitrate uptake, defoliation, unit absorption, growth