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     

Evidence for N feedback regulation of N2 fixation in Alnus glutinosa L.

Treatments were applied to vary C and N availability in Alnusglutinosa L. and plant growth, nodule activity (including acetylenereduction) and amino acid composition of the xylem sap weremeasured. Removing the buds, a sink for N, caused a decreasein nodule activity. Flushing root systems daily with 100% O₂destroyed nitrogenase activity and substantially decreased theamount of citrulline in the xylem sap. The amino acid compositionof xylem saps also altered according to the mode of N nutrition.In plants fed , xylem sap composition was similar to N₂-fixing plants, however, when plants were fed, citrulline content increased. The assimilation and subsequent distribution of nitrate wasfollowed in an experiment in which labelled ¹⁵ was added to the base of plant pots. After 12 h7% of root N was from applied ¹⁵ and this increased to 75% at 7 d; substantial enrichment ofN from ¹⁵ also occurred in stems, buds and leaves. After 7 d, 3.5% of nodule N was from¹⁵, consistent with some N being supplied by recycling of shoot N. Xylem saps were alsocollected and after 12 h, glutamate and aspartate were enrichedwith ¹⁵N to 53% and 37% increasing after 7 d to 80% and 49%,respectively. Citrulline content of the xylem sap increasedfrom 3 to 9 µmol cm^–3 following addition of ¹⁵ and at 7 d 80% of the N in the citrullinehad been derived from ¹⁵N. It is hypothesized that the growthand activity of A. glutinosa root nodules is sensitive to theN status of the plant and that the level of citrulline (or otheramino acids) returning to the nodules may feed back to regulatenodule growth and activity. Key words: Alnus glutinosa, citrulline, nitrate, feedback mechanism, N₂-fixation.     

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     

Stage of development is an important determinant in the effect of nitrate on photoassimilate (13C) partitioning in chicory (Cichorium intybus)

The effect of nitrogen supply to chicory plants on carbon partitioningbetween shoot, root and tuberized root was studied at differentstages of vegetative growth, using long-term ¹³CO₂ labelling-chaseexperiments. This approach was complemented by measurement ofstorage carbohydrates and activities of enzymes involved inroot sucrose metabolism (sucrose-sucrose fructosyl transferase(SST), sucrose synthase, invertase). In both young and matureplants, low resulted in a 30–35% decrease in ¹³C assimilation. However, the partitioningof ¹³C between shoot and root was affected differently at differentstages of development. In young plants, in which carbohydrateswere being used for structural root and shoot growth, neither¹³C shoot/¹³C root ratio nor root activities of the above enzymeswere modified by supply. In contrast, in mature plants storing large amounts of carbohydratesas fructan in the tuberized root, low caused the ratio to decrease from 0.6 to 0.2, despiteunchanged net flux of ¹³C from shoot to root. The extractableactivity of SST was elevated in mature plants, compared to youngplants, at both low and high , consistent with its role in fructan synthesis. However, matureplants grown at low exhibited SST activity double that of plants grown at high . From these results, it is concluded that the observeddecrease in shoot/root dry weight ratio at low supply is caused by increased utilization of carbohydratesfor storage due to elevated root SST activity. Key words: Chicory, nitrate, ¹³C, shoot/root ratio, fructans, SST     

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.     

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     

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     

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-     

Respiratory Utilization of 13C-Labelled Photosynthate in Nodulated Root Systems of Soybean Plants

Kouchi, H., Akao, S. and Yoneyama, T. 1986. Respiratory utilizationof ¹³C-labelled photosynthate in nodulated root systems of soybeanplants.—J. exp. Bot. 37: 985–993. An improved method for the measurement of respiratory utilizationof current photosynthate in the nodulated root system of water-culturedsoybean (Glycine max L.) plants was developed using a steady-state¹³CO₂ labelling technique. Well-nodulated plants at the latevegetative stage were allowed to assimilate ¹³CO₂ for 10 h incontinuous light at a constant CO₂ concentration with a constant¹³C abundance. The respiratory evolution of ¹³CO₂ from rootsand nodules was measured continuously throughout the periodof ¹³CO₂ assimilation and during a subsequent 36 h chase periodby using a differential infrared ¹³CO₂ analyser. The plantswere grown with nitrogen-free or (15 mmol dm^–3)-containing culture solution for 3 d before¹³CO₂ assimilation. In plants grown without , nodule respiration averaged 69% of the total respiration of the undergroundparts over the full experimental period and the CO₂ respiredreached an apparent isotopic equilibrium at 80–85% labellingafter initiating ¹³CO₂ assimilation. By contrast, the CO₂ respiredfrom the roots did not reach an isotopic equilibrium and labellingwas only 56% at the end of exposure to ¹³CO₂ These findingsdemonstrated that nodule respiration is strongly dependent onrecently assimilated carbon compared with root respiration. Plants supplied with in the culture solution showed a decreased rate of nodule respirationand a slightly increased rate of root respiration. The extentsand time courses of labelling of respired CO₂ from both theroots and nodules were similar in the presence and absence of except that the maximum level of labelling of CO₂ derived from nodule respiration in plantswith was significantly higher (about 91%) than for plants growing without . Key words: Soybean (Glycine max L.), nodule respiration, ¹³CO₂, assimilation, carbon partitioning     

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     

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.     

Proliferation of Maize (Zea mays L.) Roots in Response to Localized Supply of Nitrate

Maize (Zea mays L.) plants with two primary nodal root axeswere grown for 8 d in flowing nutrient culture with each axisindependently supplied with . Dry matter accumulation by roots was similar whether 1.0 mol m^–3 was supplied to on( or both axes. When was supplied to only one axis, however, accumulationof dry matter within the root system was significantly greaterin the axis supplied with . The increased dry matter accumulation by the +N-treated axis was attributableentirely to increased density and growth of lateral branchesand not to a difference in growth of the primary axis. Proliferation of lateral branches for the + N axis was associatedwith the capacity for in situ reduction and utilization of aportion of the absorbed , especially in the apical region where lateral primordia are initiated. Althoughreduced nitrogen was translocated to the –N axis, concentrationsin the –N axis remained significantly lower than in the+N axis. The concentratio of reduced nitrogen, as well as invitro reductase activity, was greater in apical than in more basal regions of the +N axis. The enhancedproliferation of lateral branches in the + N axis was accompaniedby an increase in total respiration rate of the axis. Part ofthe increased respiration was attributable to increased massof roots. The specific respiration rate (umol CO₂ exolved perhour per gram root dry weight) was also greater for the +N thanfor the –N axis. If respiration rate is taken as representativeof sink demand, stimulation of initiation and growth of lateralsby in situ utilization of a localized exogenous supply of establishes an increased sink demand through enhancedmetabolic activity and the increased partitioning of assimilatesto the + N axis responds to the difference in sink demand between+N and –N axes. Key words: NO₃^- reduction, NO₃^- uptake nitrogen partitioning, root respiration, sink demand     

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     

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     

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.     

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     

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     

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     

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