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     

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     

Cyclic Variations in Nitrogen Uptake Rate in Soybean Plants: Uptake During Reproductive Growth

Net uptake of by non-nodulated soybean plants [Glycme max(L ) Merr cv Ransom] growing in flowing hydroponicculture was measured daily during a 63 d period of reproductivedevelopment between the first florally inductive photopenodand late seed growth Removal of from a replenished solution containing 10 mol m^– was determined by ion chromatography Uptake of continued throughout reproductive development The net uptakerate of cycled between maxima and minima with a periodicity of oscillation of 3 to 7 d during the floralstage and about 6 d during the fruiting stage. Coupled withincreasing concentrations of carbon and C:N ratios in tissues,the oscillations in net uptake rates of are evidence that the demand for carbohydrate by reproductiveorgans is contingent on the availability of nitrogen in theshoot pool rather than that the demand for nitrogen followsthe flux of carbohydrate into reproductive tissues. Key words: Nitrate uptake rate, carbon-nitrogen partitioning, Glycme max (L ) Merrill     

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     

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     

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 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     

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     

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 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.     

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.     

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     

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.     

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     

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 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-     

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