Effects of Light on Transport by Azolla pinnata

Effects of light flux density (LFD) during growth and uptakeassay on induction of transport system and kinetics of transport were studied using the Azolla pinnata-Anabaena azollae association (Azolla). Theinduction and uptake kinetics of the transport system were determined using an automated system that measuredthe NO₃ concentration in the growth medium as a function oftime, using an on-line high performance liquid chromatograph(HPLC) with a UV-VIS detector. Full induction of the transport system required about 1.5 to 2.0 h and occurred without any apparent lag phase,regardless of the LFD provided. The level of induction of transport of Azolla grown at 600 µmol m^–2s^–1 LFD was higher than for that grown at 100 µmolm^–2 s^–1. Similarly, 600 µmol m^–1 s^–1LFD during the assay resulted in a higher level of inductionthan did 100 umol m^–2 s^–1. An increase in the LFDeither during the growth or the assay period increased the uptake rate; however, an increase in LFD duringthe latter period had greater effect. Azolla grown and assayedat 600 umol m^–2 s^–1 had the highest uptake rate. The uptake rate at 50 cm³ m^–3ambient CO₂ concentration was initially higher than at 305 cm³m^–3, but the uptake rate decreased rapidly with time andeventually dropped below that at 305 cm³ m^–3 CO₂. Thesedata suggest that the energy required for transport in Azolla may bypass the photosynthetic CO₂ fixationand carbon-cycling. Key words: carbon dioxide, concentration dependence, light flux density, uptake     

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     

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     

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.     

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     

Carbon Metabolism in Seagrasses: I. THE UTILIZATION OF EXOGENOUS INORGANIC CARBON SPECIES IN PHOTOSYNTHESIS

Four species of seagrasses, Halophila stipulacea, Thalassodendronciliatum, Halodule uninervis, and Syringodium isoetifolium,were investigated for their ability to utilize and CO₂ as exogenous carbon sources for photosynthesis. Ratesof photosynthesis were measured as rates of O₂ evolution ina closed system in which the pH was continuously controlled.A computer program was written to calculate the concentrationsof different carbon species as a function of pH and other specifiedexperimental conditions. Bicarbonate as well as CO₂ were readily assimilated by all fourseagrass species. Saturating concentrations of , at saturating light intensities, were 0.5–1.8 mM dependingon the species. Rates of photosynthesis under such conditionswere 0.1–0.55 µmol O₂ min^–1 mg^–1 chlorophyll.At saturating CO₂ concentrations, i.e. 0.5–1.3 mM, ratesof photosynthesis were 0.22–1.4 µmol CO₂ min^–1mg^–1 chlorophyll. Photosynthetic rates in each specieswere considerably higher when CO₂ rather than was supplied at saturating concentrations. The concentration of in natural seawater was found to be saturating, and that of CO₂ insufficient forconsiderable photosynthetic rates in these plants under thegiven conditions It was thus concluded that is the major carbon source for photosynthesis in seagrasses.     

Nitrate Nutrition of Grasses from Steady-State Supplies in Flowing Solution Culture following Nitrate Deprivation and/or Defoliation: II. ASSIMILATION OF AND SHORT-TERM EFFECTS ON UPTAKE

Plants of two genotypes of Lolium perenne L. cv. S23 and a L.perenne ? L. multiflorum Lam. hybrid cv. Augusta were grownin flowing solution culture. N was suppled in one treatmentat 10 mmol m^–3 NO^–3 throughout (HN), and in another(LN) the N supply was terminated after 10 d for 11 d. When was re-supplied both LN and HN plants were leftentire or defoliated. The two genotypes showed similar responsesto all treatments. The concentration of N in shoot dry matterdeclined from 4.4% to 2.0% and in the root from 2.8% to 1.0%over the 11 d of N deprivation, with 95% of the initially present being assimilated during this period. LN plantsassimilated 10% more of their total uptake than did HN plants. The in vitro nitrate reductase activity(NRA) was 10- to 50-fold higher in the youngest fully-expandedleaves than in roots and declined in the leaves during N deprivation.Between 2–6 d after defoliation, there was a large increasein NRA in leaves of HN (but not LN) plants. After defoliationof HN plants, net uptake from 10 mmol m^–3 declined to negligible levels within 15 h, but in defoliatedLN plants it increased to levels similar to those of entireHN plants (15–20 µmol h^–1 g^–1 fr. wt.root) within 8 h. When was re-supplied to entire LN plants, uptake of increased to levels similar to those of entire HN plants within 2.3 h, butdid not markedly exceed that of HN plants for at least 10 h.Net uptake of by LN plants during depletion of stirred static nutrient solutions containing 1.0 mol m^–3 lagged behind that by HN plants by 70–100 min, but the maximum unit absorption rate was similar for LNand HN plants (5–7 µmol h^–1 g^–1 fr.wt. root). The nature of the short-term demand for uptake following recovery from the stresses of defoliation andN starvation is discussed. Key words: Lolium perenne, Lolium multiflorum, N-deficiency, defoliation, nitrate uptake, nitrate reductase, N-assimilation     

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     

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     

Effects of Cl- on and Malate Fluxes and CO2 Fixation in Carrot and Barley Root Cells

Cl^– and ions interact apparently competitively during influx across the plasmalemma of carrot root cells. Cl^–,however, reduces influx much less than predicted from the effect of on Cl^– influx.Cl^– and plasmalemma influxes both increase with time after excision of carrot tissue. Cl^–and may therefore be transported by a common mechanism. The effect of pH changes on the influx of malate across theplasmalemma in barley roots shows that malate crosses the plasmalemmaas the singly charged anion. Stimulations of influx by bothK₂SO₄ and KCl suggest that the malate anion crosses in associationwith. K⁺. If malate entry is passive, P_mal- is about 2?10^–8cm s^–1, but it is thought that malate entry is partlyan active process. A slight, apparently competitive inhibition by Cl^– ofmalate flux into the vacuole of barley root cells suggests thatthe two anions may be transported by a common process at thetonoplast, but this is not thought to be physiologically significant. The accumulation of ¹⁴C from 1 mM is drastically reduced by 10 mM Cl^–. A quantitative analysis of the kineticsof ¹⁴C exchange shows that Cl^– directly inhibits the formationof malate from . The decreased influx of endogenously produced malate to the vacuole in the presenceof Cl^– is probably a secondary consequence of the fallin the cytoplasmic concentration. The nature of the Cl^–inhibition of malate formation is discussed. In KCl-loaded tissue the influx of external malate and the accumulationof ¹⁴C from external are reduced. The location of these effects is not certain, but the effects suggest thatregulation of malate synthesis and accumulation may be relatedto the negative-feedback regulation of Cl^– and transport.     

The Role of Long-distance Transport in Intracellular pH Regulation in Phaseolus vulgaris Grown with Ammonium or Nitrate as Nitrogen Source, or Nodulated

Charge balance and intracellular pH regulation were studiedin Phaseolus vulgaris grown in water culture on or as N source, or nodulated and in N-free medium. Seedlings and 36-d-old plants were analysed for total P, C, organic N and S and ash alkalinity;xylem sap was analysed for mineral ion content and amino acids,amides and dicarboxylates. Both water uptake and H⁺ or baseexcreted during water culture were measured. It was shown that in -grown plants, H⁺ excretedwas related directly to the uptake and assimilation of ; there was no cation uptake associated with netH⁺ exchange. All shoot N was supplied as organic N, mainly glutamine,so that shoot pH perturbation would be limited to uronate productionin situ. This could be regulated by both shoot reduction and the transport of OH^–-generating carboxylatesin the xylem. In plants on , the great majority of reduction occurred in the root and most of the OH^–produced was converted to -COO^–, transported up the xylemand stored, so that the leaves in particular had a high ashalkalinity (20-fold that in the roots). Some OH^–, however,was excreted. Nodulated plants had a low N content, compared with the others,indicating a low rate of N₂-fixation for the earlier periodof water culture. At harvest, however, the xylem sap organicN content was comparable to that of plants. H⁺ excretion was in excess of that required for N assimilation.It was concluded that in these plants some net H⁺/cation exchangehad occurred which may have been a response to low nutrient(i.e. nitrogen) status. Key words: Phaseolus vulgaris, pH regulation, nitrate nutrition, ammonium nutrition, dinitrogen fixation, xylem sap composition.     

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     

Salt-Stimulated Bicarbonate-Dependent Photosynthesis in the Marine Angiosperm Zostera muelleri

Millhouse, J. and Strother, S. 1986. Salt-stimulated bicarbonate-dependentphotosynthesis in the marine angiosperm Zostera muelleri.—J.exp. Bot. 37: 965–976. Photosynthetic oxygen evolution in the seagrass Zostera muelleriIrmisch ex Aschers. was inhibited in iso-osmotic sucrose. Theapparent affinity of the leaves for CO₂ in seawater increasesfrom pH 8?2 to 8?9 indicating that as well as CO₂ may act as a substrate for photosynthesis. Theaffinity for CO₂ was lower in iso-osmotic sucrose and was notaffected by pH. Under these conditions was not a substrate for photosynthesis. The differencebetween the photosynthetic rate in seawater and iso-osmoticsucrose at the same concentration of CO₂ was used to estimate assimilation. The Briggs-Maskell equation, which allows for an unstirred layer around the tissuewas more appropriate than the Michaelis-Menten theory for calculatingthe apparent affinity of the leaf slices for CO₂. The apparentK_m CO₂ was calculated as 116 mmol m^–3 at pH 8?2 by Michaelis-Mentenkinetics but only 8?10 mmol m^–3 by the Briggs-Maskellequation. The stimulation by various ions in Seawater of use was investigated. The cations,in decreasing order of effectiveness were Ca²⁺, Mg²⁺, K⁺ andNa⁺ Anions were ineffective. No single cation at its concentrationin seawater was capable of supporting use at the rate observed in seawater. Acetazolamide,an inhibitor of carbonic anhydrase, inhibited the use of for photosynthesis but had littleeffect on CO₂ photosynthesis. Thus, carbonic anhydrase activityis required for -dependent photosynthesis. Key words: Zostera muelleri, photosynthesis, salinity     

Mechanism of Inorganic Carbon Uptake in Chlorella saccharophila: The Lack of Involvement of Carbonic Anhydrase

The acid-tolerant green alga Chlorella saccharophila maintainedphotosynthesis and accumulated intracellular pools of inorganiccarbon over a a range of external pH from 4.0 to 7.5. This accumulationwas unaffected by treatment of cells with 10 mol m^–3 acetazolamide(AZA). Cells grown at alkaline pH had extracellular carbonicanhydrase (CA), but CA activity was repressed when cells weregrown at pH 5.0. Acid-grown cells retained a high affinity forCO₂, both at acid and alkaline pH, and the ability to accumulateinorganic carbon. Rates of photosynthesis of acid-grown cellsand alkaline-grown AZA-treated cells at pH 8.0 were 2.5-foldhigher than the rate of CO₂ supply from the uncatalysed dehydrationof , indicating that the cells can take up as a source of substrate for photosynthesis. Isotopic disequilibrium experiments with acid-grown cells maintainingsteady-state photosynthesis at pH 7.5 demonstrate that ¹⁴C from¹⁴CO₂ was taken up more rapidly than from H¹⁴. This uptake takes place against a concentration gradient. Theseresults demonstrate that C. saccharophila cells have activetransport systems for the uptake of both CO₂ and and both operate without the mediation of CA. Key words: Bicarbonate transport, carbon dioxide, carbonic anhydrase, Chlorella saccharophila, inorganic carbon accumulation     

The Titrimetric Assay of OH- and HCO3- Excreted by Ricinus Cultivated on NO3--containing Nutrient Media: The Influence of Ionic Strength, End Point pH and CO2 Supersaturation

Allen, S. and Allen, C. R. 1987. The titrimetric assay of OH^–and excreted by Ricinus cultivatedon -containing nutrient media: the influence of ionic strength, end point pH and CO² supersaturation.–J. exp. Bot. 38: 607–617. When spent -containing nutrient media were titrated to the starting pH of 6.5 the titre wasequivalent to 50%orless of the base (i.e. ) excreted. Calculation of the total amount of baseexcreted could only be made from data obtained by titrationto pH 4–5. An accurate calculation of the amount of baserequired the inclusion of activity coefficients, estimated fromionic strength, in the calculations. Spent nutrient media contained from four to ten times the concentrationof CO₂ predicted from equilibrium values. It is probable that is very slow. Key words: Ricinus, nitrate-N nutrient medium, base excretion, ionic activity coefficients, carbon dioxide supersaturation     

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     

White Clover N2-Fixation in Response to Root Temperature and Nitrate: I GROWTH AND UPTAKE OF NITRATE FROM FLOWING NUTRIENT SOLUTIONS

Established, nodulated white clover plants (Trifolium repensL. cv. Blanca) were transferred to eight plant culture unitsof a system of flowing solution culture. Solution temperatureswere 5, 11, 17, and 25 ?C (two units per temperature), withshoot temperature of 25/15 ?C day/night and light regime commonto all plants. After 7 d, was supplied at 10 mmol m^–3 to one of each pair of culture units and thenet uptake of was monitored over 14 d. The remaining four culture units served as controls in which plantswere entirely dependent on N₂ fixation, as estimated by sequentialharvesting and the change in the amount of N in plants. Totalfresh and dry weights increased exponentially with time andwith increasing root temperature, between 5–25 ?C in nitrateplants and 5–17 ?C in control plants, respectively, byfactors of 2.9 and 1.8. Nodule dry weight of nitrate plantsshowed little increase after 6 d compared with control plants.Dry weight shoot: root ratios increased with time in all treatmentsexcept 5 ?C control plants. Total net uptake of over 13 d was 0.41, 4.27, 11.92, and 12.77 mmol plant-1, respectively,at 5, 11, 17, and 25 ?C. At all temperatures except 5 ?C, plantsaccumulated high concentrations of (10–40 mol m^–3) in leaflets and roots, within 2 or 3 days ofsupplying . Daily mean unit absorption rates of increased 12-fold with increasing temperature between 5 and 25 ?C, and showed little change with time at 5,17, and 25 ?C. The total N₂ fixed by nitrate plants over 14d increased 5-fold with temperature between 5 and 17 ?C, butwas always less than the amount fixed by control plants. Thepercentage contributed by N2 fixation to total N acquisitionby nitrate plants over 14 d decreased with increasing temperature,from 77% at 5 ?C to 11 % at 25 ?C. Mean daily rates of N₂ fixationper unit dry weight of nodule were lower in nitrate plants thanin control plants throughout treatment at 5 ?C and 25 ?C, butat 11 ?C and 17 ?C the rates for nitrate plants increased progressivelywith time and exceeded the rates for control plants after 8d. In both nitrate and control plants the effect of temperatureon N₂ fixation per nodule dry weight was proportionately lessthan that on unit absorption rate of . The results are discussed in terms of the overall regulation ofN accumulation by white clover and the adaptive significanceof differences in the sensitivities of uptake and N₂ fixation to root temperature. Key words: Trifolium repens, white clover, root temperature, N₂ fixation, nitrate uptake     

The Uptake of Nitrate by Lolium perenne from Flowing Nutrient Solution: I. EFFECT OF CONCENTRATION

Experiments with simulated swards of perennial ryegrass (Loliumperenne L.) show the relationship between concentration in flowing nutrient solution, nitrate uptake,plant growth, and the chemical composition of roots and shoots.Rates of uptake exceeding 1 g N m^–2 d^–1 were maintainedat concentrations in solution down to 0•02 mg N l^–1. Short-term studies confirmed that at such lowconcentrations the plants were able to maintain rates of uptakeof about 85% of maximum. Between 0•2 and 200 mg N l^–1the concentration of in solution had little effect on rate of uptake or plant growth. With at 1000 and 2000 mg N l^–1 there was a marked reductionin weight of the shoots and, more particularly, in the lengthand tensile strength of the roots. There were several significanttrends in mineral composition of the plants (notably in S, Ca,Mg) which were apparently correlated with increasing concentrationof in solution.     

Intracellular pH Regulation in Ricinus communis Grown with Ammonium or Nitrate as N Source: The Role of Long Distance Transport

Allen, S. and Raven, J. A. 1987. Intracellular pH regulationin Ricinus communis grown with ammonium or nitrate as N source:the role of long distance transport.—J. exp. Bot. 38:580–596. Charge balance and intracellular pH regulation were studiedin Ricinus communis grown in water culture with 1–0 molm^–3 or l-0 mol m ^–3 as sole N source. Seedling and 70-d-old plant parts were analysed for total P, C, organic N and S, and ash alkalinity;xylem and phloem saps were analysed for mineral ion content,and amino acids, amides, and dicarboxylates. Excreted H⁺ andbase were also measured. It was shown that in -N plants, H ⁺ produced and excreted directly bythe roots accounted for all net —COOH produced in theplant, but not for cation uptake by net H⁺ exchange. IntracellularpH perturbation in the shoot was regulated partly by reduction and partly by the transportof OH^- -generating dicarboxylates in the xylem. Phloem sap hadthe capacity to transport organic N and carboxylates excessto shoot requirements back to the root. In -N plants, 60% of total reduction occurred in the root, and 70% of all OH^- produced by root and reduction was excreted directly as base. Very little —COO^–from root reduction was stored in the root: most was transported to the shoot as xylem dicarboxylateand stored in the shoot. Of the OH^- produced from shoot and reduction, 40% was stored as shoot carboxylate: the phloem saphad the capacity to transport the rest back to the roots whereit was excreted as base. Key words: Intracellular pH, ammonium nutrition, nitrate nutrition, xylem dicarboxylates, phloem dicarboxylates, charge balance     

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