Diurnal K+ and Anion Transport in Phaseolus Pulvinus

Diurnal movement of Phaseolus leaf is caused by deformationof the laminar pulvinus located at the joint of the leaf bladeand the petiole. The plants were cultured in solutions withvarious ion compositions, and changes of K⁺, Na⁺, Ca²⁺, Mg²⁺,Cl^–, NO₃– and P₁ concentrations both in the upperand lower parts of the laminar pulvinus were measured. Culturein 10 mM KCl solution caused an increase in K⁺ and Cl^–concentrations both in the upper and lower parts without anysignificant change in the concentration of NO₃^–; culturein 10 mM KNO₃ solution caused an increase in K⁺ and NO₃^–concentration without any significant change in the concentrationof Cl^–; and culture in 10 mM KH₂PO₄ solution caused anincrease in K⁺ and P₁ concentrations without any significantchange in the concentrations of NO₃- and Cl^–. K⁺ moved from the upper to lower parts or from the lower toupper parts diurnally in all plants cultured in any solutionmentioned above. The main inorganic anion that accompanied thisK⁺ movement was Cl^– in KCl solution, and NO₃^– inKNO₃ solution. When the seedlings were cultured in distilledwater or in KH₂PO₄ solution, neither Cl^– NO₃^– norP₁ accompanied this K⁺ movement. In these cases, mainly H⁺ and/ororganic anions are supposed to move in exchange for and/or incombination with K⁺ movement. (Received November 8, 1982; Accepted June 13, 1983)     

Growth of Actinorhizal Plants as Influenced by the Form of N with Special Reference to Myrica gale and Alnus incana

Growth of two actinorhizal species was studied in relation tothe form of N supply in water culture. Non-nodulated bog myrtle(Myrica gale) and grey alder (Alnus incana) were grown withNH₄⁺, NH₄NO₃^– or NO₃^– (4 mol m^–3 N). A nodulatedseries of bog myrtle was also cultivated in N-free medium. Relative growth rate (RGR), utilization rate of N, and shoot/rootratio were highest for the two species with the N completelysupplied as NH₄⁺. In both species, nitrate was largely reducedin the roots and the presence of NO₃^– in combined-N supplyalways affected the RGR and N utilization rate negatively. BothN₂ fixation and complete NO₃^– nutrition represented conditionsof relative N-deficiency resulting in relatively low tissue-Nconcentrations and a greater allocation of dry mass to the roots.The physiological N status of nodulated M. gale plants was comparativelygood, as indicated by a normal nodule weight ratio and a relativelyhigh N₂-fixing rate per unit nodule mass. However, whole-plantN₂-fixing capacity remained relatively low in comparison withacquisition rates of N in combined-N plants. The anion charge from the nitrate reduction was largely directlyexcreted as an OH^– efflux. H ⁺ /N ratios generally agreedwith the theory. In comparison with NH₄⁺ nutrition, carboxylateconcentrations were higher in N₂-fixing M. gale plants and theH ⁺ /N ratio in nodulated plants was less than unity below thevalue for ammonium plants as previously found for other actinorhizalspecies. Therefore, NH₄⁺ should be an energetically more attractiveN source for actinorhizal plants than N₂. The results agree with commonly accepted views on energeticsof N uptake and assimilation in higher plants and support theconcept of a basically similar physiological behaviour betweennon-legumes and legumes. Key words: Actinorhizal symbioses, ammonium, H⁺/OH^– efflux, nitrate, N₂ fixation, NRA     

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

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

Examination of analytical methods for ions and saccharides in the extract of Phaseolus pulvini

With slight modifications, conventional assay procedures forK⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^–, NO₃^–, H₂PO₄^–, fructoseand fructose-yielding saccharides, and glucose were applicableto the extract of Phaseolus pulvini. About 10 ml of a hot-waterextract from about 30 mg fresh weight of the pulvini was sufficientfor separate measurement of the ions and saccharides named above. (Received August 7, 1979; )     

Concurrent Rates of N2 Fixation, Nitrate and Ammonium Uptake by White Clover in Response to Growth at Different Root Temperatures

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown for 71 d in flowing nutrient solutions containingN as 10 mmol m_–3 NH₄NO₃, under artificial illumination,with shoots at 20/15°C day/night temperatures and root temperaturereduced decrementally from 20 to 5°C. Root temperatureswere then changed to 3, 7, 9, 11, 13, 17 or 25°C, and theacquisition of N by N₂ fixation, NH₄+ and NO₃^– uptakewas measured over 14 d. Shoot specific growth rates (d. wt)doubled with increasing temperature between 7 and 17°C,whilst root specific growth rates showed little response; shoot:root ratios increased with root temperature, and over time at11°C. Net uptake of total N per plant (N₂ fixation + NH₄++ NO₃^–) over 14 d increased three-fold between 3 and 17°C.The proportion contributed by N₂ fixation decreased with increasingtemperature from 51% at 5°C to 18% at 25°C. Uptake ofNH₄+ as a proportion of NH₄+ + NO₃^– uptake over 14 d variedlittle (55–62%) with root temperature between 3 and 25°C,although it increased with time at most temperatures. Mean ratesof total N uptake per unit shoot f. wt over 14 d changed littlebetween 9 and 25°C, but decreased progressively with temperaturebelow 9°C, due to the decline in the rates of NH₄+ and NO₃^–uptake, even though N₂ fixation increased. The results suggestthat N₂ fixation in the presence of sustained low concentrationsof NH₄+ and NO₄^– is less sensitive to low root temperaturethan are either NH₄+ or NO₃^– uptake systems. White clover, Trifolium repens L. cv. Huia, root temperature, nitrogen fixation, ammonium, nitrate     

Characterization of Two Proton Transport Systems in the Tonoplast of Plasmalemma-Permeabilized Nitella Cells

ATP-dependent and PP_i-dependent H⁺-transport systems of thetonoplast were characterized in plasmalemma-permeabilized Nitellacells, where direct access to the protoplasmic surface of thetonoplast was possible. Since H⁺ transport across the tonoplastcan be measured in situ, the identity of the membrane responsiblefor H⁺ pumping is unequivocal. H⁺ transport was evaluated bythe accumulation of neutral red. While both transport systemswere obligately dependent on Mg²⁺, the two transport systemsshowed completely different sensitivity to NO₃^– and K⁺,suggesting the presence of two types of H⁺-pumps in Nitellatonoplast. NO₃^– applied to the protoplasmic surface, completelyand reversibly inhibited ATP-dependent transport but had noeffect on PP_i-dependent transport. By contrast, NO₃^– appliedinto the vacuole by the vacuolar perfusion technique did notinhibit ATP-dependent or PP_i-dependent H⁺ transport. Replacementof K⁺ with the organic cation, BTP, inhibited PP_i-dependenttransport but not the ATP-dependent one, indicating that PP_i-dependenttransport is K⁺ dependent. The sensitivities of the H⁺ transportsystems found in the tonoplast of Nitella are quite similarto those of higher plant tonoplasts. ¹ Present address: Department of Botany, Faculty of Science,University of Tokyo, Hongo, Tokyo 113, Japan. (Received February 21, 1987; Accepted May 27, 1987)     

Unequal distribution of potassium and anions within the Phaseolus pulvinus during circadian leaf movement

Ion and saccharide concentrations in the upper and lower partsof the laminar pulvinus of the primary leaf of Phaseolus vulgariswere measured in relation to the circadian movement. Concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^–, organic acid,NO₃^–, H₂PO₄^–, fructose and fructose-yielding saccharidesin the pulvinus were 75–120, 0.3–0.7, 5–8,6–12, 40–60, 60–73, 19–35, 2–9and 1–5 mM, respectively, and the osmotic pressure ofthe pulvinus was considered to be due to these ions. The cell volume in the expanding part was larger than that inthe contracting part. The change of the cell volume alteredthe molar concentration in the cell sap and therefore the amountof solutes actually transported from the upper to the lowerpart and vice versa was estimated from the concentration expressedin moles per gram of dry weight. Results showed that K⁺, Cl^–, organic acid (or H⁺) andNO₃^– moved from the upper to lower parts or vice versain the pulvinus in relation to its deformation, keeping theelectroneutrality among those ions, whereas C_a2⁺ and Mg²⁺ didnot move. The difference in the K⁺ concentration between theupper and lower parts when the leaf was up or down amountedto 30% of the whole osmotic pressure. This lead to the conclusionthat the endogenous clock-controlled unequal distribution ofK⁺, Cl^–, organic acid (or H⁺) and NO₃^– in the pulvinuscould be the force for the circadian leaf movement. (Received August 7, 1979; )     

Ion Composition of the Chara Internode

Ion compositions of the cytoplasm and the vacuole of Chara australiswere analyzed according to Kishimoto and Tazawa (1964) and Kiyosawa(1979a). The ions in the cytoplasm and the vacuole analyzedwere K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^–, NO₃^– and H₂PO₄^–.Assuming that the volume of the cytoplasm V_p is 10% of thatof the whole cell V, the concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺,Cl^–, NO₃^– and H₂PO₄^– in the cytoplasm averaged70, 15, 13, 4.6, 31, 2.2 and 16 mM, respectively. If the volumeof the cytoplasm was assumed to be 5% of that of the whole cell,their averaged concentrations were 139, 31, 25, 9.2, 62, 4.4and 33 mM, respectively. The averaged ion compositions of thecell sap were K⁺, 111; Na⁺, 47; Ca²⁺, 4.4; Mg²⁺, 8.9; Cl^–,91; NO^–₃, 3.3 and H₂PO₄^–, 6.0 mM. These values,taking the concentrations and the charges of the protein (Kiyosawa1979b) and amino acids (Sakano and Tazawa 1984) into accountand assuming the presence of some uni- or oligovalent anionsand/or small nonelectrolyte molecules, could explain fairlywell both the electroneutrality and the osmotic pressure ofthe cell, except when V_p/V = 5%. (Received May 18, 1987; Accepted September 29, 1987)     

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

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

Plasma Membrane H+-ATPase in Guard-Cell Protoplasts from Vicia faba L.

The activity of plasma membrane H⁺-ATPase was measured withmembrane fragments of guard-cell protoplasts isolated from Viciafaba L. ATP hydrolytic activity was slightly inhibited by oligomycinand ammonium molybdate, and markedly inhibited by NO₃^–and vanadate. In the presence of oligomycin, ammonium molybdateand NO₃^–, the ATP-hydrolyzing activity was strongly inhibitedby vanadate. It was also inhibited by diethylstilbestrol (DES),p-chloromercuribenzoic acid (PCMB) and Ca²⁺, but slightly stimulatedby carbonyl cyanide m-chlorophenylhydrazone (CCCP). The acitivityhad higher specificity for ATP as a substrate than other phosphoricesters such as ADP, AMP, GTP and p-nitrophenylphosphate; theK_m was 0.5 mM for ATP. The activity required Mg²⁺ but was notaffected by K⁺, and it was maximal around pH 6.8. When guard-cellprotoplasts were used instead of membrane fragments, the ATPaseactivity reached up to 800µmol Pi.(mg Chl)^–1.h^–1in the presence of lysolecithin. These results indicate thatthe guard cell has a high plasma membrane H⁺-ATPase activity. (Received December 23, 1986; Accepted April 28, 1987)     

Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

Nitrogen utilization by plant species from acid heathland soils: II. Growth and shoot/root partitioning of NO3- assimilation at constant low pH and varying NO3-/NH4+ ratio

The growth of four heathland species, two grasses (D. flexuosa,M. caerulea) and two dwarf shrubs (C. vulgaris, E. tetralix),was tested in solution culture at pH 4.0 with 2 mol m^–3N, varying the N0₃^–/NH₄⁺ ratio up to 40% nitrate. In addition,measurements of NRA, plant chemical composition, and biomassallocation were carried out on a complete N0₃^–/NH₄⁺ replacementseries up to 100% nitrate. With the exception of M. caerulea, the partial replacement ofNH₄⁺ by NO₃^– tended to enhance the plant's growth ratewhen compared to NH₄⁺ only. In contrast to the other species,D. flexuosa showed a very flexible response in biomass allocation:a gradual increase in the root weight ratio (RWR) with NO₃^–increasing from 0 to 100%. In the presence of NH₄⁺, grassesreduced nitrate in the shoot only; roots did not become involvedin the reduction of nitrate until zero ambient NH₄⁺. The dwarfshrubs, being species that assimilate N exclusively in theirroots, displayed an enhanced root NRA in the presence of nitrate;in contrast to the steady increase with increasing NO₃^–in Calluna roots, enzyme activity in Erica roots followed arather irregular pattern. Free nitrate accumulated in the tissuesof grasses only, and particularly in D. flexuosa. The relative uptake ratio for NO₃^– [(proportion of nitratein N uptake)/(proportion of nitrate in N supply)] was lowestin M. caerulea and highest in D. flexuosa. Whereas M. caeruleaand the dwarf shrubs always absorbed ammonium highly preferentially(relative uptake ratio for NO₃^– <0.20), D. flexuosashowed a strong preference for NO₃^– at low external nitrate(the relative uptake ratio for N0₃^– reaching a value of2.0 at 10% NO₃^–). The ecological significance of thisprominent high preference for NO₃^– at low NO₃^–/NH₄⁺ratio by D. flexuosa and its consequences for soil acidificationare briefly discussed. Key words: Ammonium, heathland lants, N0₃^–/NH₄⁺ ratio, nitrate, nitrate reductase activity, soil acidification, specific absorption rate     

Quantitative Analysis of ATP-Dependent H+ Efflux and Pump Current Driven by an Electrogenic Pump in Nitellopsis obtusa

Internodal cells of Nitellopsis were made tonoplast-free byperfusion with a medium containing EGTA. Cytoplasmic concentrationsof solutes were controlled by a second perfusion with mediaof known composition. The electrogenic pump current (I_p), whichwas calculated from electrical data obtained from cells withand without ATP, was compared with the current carried by H⁺(I_H⁺) across the plasma membrane. A close correlation betweenI_p and I_H⁺ was found under various internal and external conditions.(1) I_p and I_H⁺ depended on the internal ATP and showed Michaelis-Mententype saturation curves. For I_p, K_m was 120 µM and themaximum current V_max was 15.1 mA m^–2, while for I_H⁺, K_mwas 160 µM and V_max was 16.6 mA m^–2. (2) I_p andI_H⁺ showed almost the same I_H²⁺ dependence. The Mg²⁺-dependentI_p was 19.5 mA m^–2, while the Mg²⁺-dependent I_H²⁺ was17.7 mA m^–2. (3) I_H²⁺ was maximal at an external pH of8 and decreased both in acidic and alkaline pH ranges. I_p wasnearly equal to I_H⁺ in the pH range between 8 and 5. (4) I_H⁺became maximal at an internal pH of 7.3, which is nearly thesame as the pH for maximal electrogenecity found by Mimura andTazawa (1984). All these facts support the idea proposed in our previous paper(Takeshige et al. 1985) that the electrogenic ion pump locatedin the plasma membrane of Nitellopsis is the H⁺ pump. ¹ Dedicated to Professor Dr. Erwin Bünning on the occasionof his 80th birthday. (Received June 21, 1985; Accepted December 20, 1985)     

N2 Fixation and Nitrate Uptake by White Clover Swards in Response to Root Temperature in Flowing Solution Culture

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown as simulated swards for 71 d in flowing nutrientsolutions with roots at 11 C and shoots at 20/15 C, day/night,under natural illumination. Root temperatures were then changedto 3, 5, 7, 11, 13, 17 or 25 C and the total N₂, fixation over21 d was measured in the absence of a supply mineral N. Alltreatments were subsequently supplied with 10 mmol m^–2NO₂ ^– in the flowing solutions for 14 d, and the relativeuptake of N by N₂, fixation and NO₃ ^– uptake was compared.Net uptake of K⁺ was measured on a daily basis. Root temperature had little effect on root d. wt over the 35-dexperimental period, but shoot d. wt increased by a factor of3.5 between 3 and 25 C, with the sharpest increase occurringat 7–11 C. Shoot: root d. wt ratios increased from 25to 68 with increasing temperature at 7–25 C. N₂-fixationper plant (in the absence of NO₂ ^–) increased with roottemperature at 3–13C, but showed little change above13 C. The ratios of N₂ fixation: NO₂ ^– uptake over 14d (mol N: mol N) were 0.47–0.77 at 3–7 C, 092–154at 11–17 C, and 046 at 25 C, reflecting the dominanceof NO₃ ^– uptake over N₂ fixation at extremes of high andlow root temperature. The total uptake of N varied only slightlyat 11–25 –C (095–110 mmol N plant^–1),the decline in N₂ fixation as root temperature increased above11 C was compensated for by the increase in NO^– ₃ uptake.The % N in shoot dry matter declined with decreasing root temperature,from 32% at 13 C to 15% at 3 C. In contrast, concentrationsof N expressed on a shoot water content basis showed a modestdecrease with increasing temperature, from 345 mol m^–3at 3 C to 290 mol m^–3 at 25 C. Trifolium repens L, white clover, root temperature, N₂ fixation, potassium uptake, nitrate uptake, flowing solution culture     

Acclimation of NO-3 fluxes to low root temperature by Brassica napus in relation to NO-3 supply

Acclimation of NO^–₃ transport fluxes (influx, efflux)in roots of oilseed rape (Brassica napus L. cv. Bien venu) andtheir sensitivity to growth at low root temperature was studiedin relation to external NO^–₃ supply, defined by constantconcentrations ranging from sub- to supra-optimal with respectto plant growth rate. Plants were grown from seed in flowingnutrient solutions containing 250 mmol m^–3 NO^–₃at 17°C for 20d, and solution temperature in half the cultureunits was then lowered decrementally over 3 d to 7°C. Threedays later plants were supplied with NO^–₃ at 1, 10, 100or 1000 mmol m^–3 maintained for 18 d. Dry matter productionwas decreased more by low root zone temperature than low [NO^–₃]_e. Root specific growth rates were inversely related to [NO^–₃]_eand shoot:root ratios increased with time at [NO^–₃]_e between10–1000 mmol m^–3. Net uptake of NO^–₃ at 17°Cwas twice that at 7°C, and at both temperatures it doubledwith increasing [NO^–₃]_e between 1–10 mmol m^–3with further small increases at higher [NO^–₃]_e. Mean unitabsorption rates of NO^–₃ between 0–6 d and 6–14d were linearly related (r² of 0.79–0.99) to log₁₀[NO].Steady-state Q₁₀ (7–17°C) for uptake between 0–6d were 0.91, 1.62, 1.27, and 1.10, respectively, at [NO^–₃]_eof 1, 10, 100, and 1000 mmol m^–3, compared with correspondingvalues of 0.98, 1.38, 1.68, and 1.89 between 6–14 d. Thedata indicated that net uptake rates at 7 and 17°C divergedover time at high [NO^–₃]_e. Short-term uptake rates from1 mol m^–3 NO^–₃ measured at 17°C were higherin plants grown with roots at 7°C than at 17°C; for7°C plants there was a strong inverse linear relationship(r²=0.94) between uptake rate and treatment log₁₀ [NO^–₃]_ewhilst rates in 17°C plants were independent of prior [NO^–₃]_e. Rates of NO^–₃ influx and efflux under different steady-stateconditions of NO^–₃ supply and root temperature were calculatedfrom dilution of ¹⁵N added to culture solutions. Efflux wassubstantial relative to net uptake in all treatments, and wasinversely related to [NO^–₃]_e at 17°C but not at 7°C.Ratios of influx: efflux ranged from 1.6–2.9 at 17°Cand 1.3–1.8 at 7°C, indicating the proportionatelygreater impact of efflux at low root temperature. Ratios ofefflux: net uptake were 0.53–1.56 at 17°C and 1.21–3.58at 7°C. The apparent sensitivities of influx and effluxto steady-state root temperature varied with [NO^–₃]_e.Both fluxes were higher at 17°C than 7°C in the presenceof 100–1000 mmol m^–3 NO^–₃ but the trend wasreversed at 1–10 mmol m^–3 NO. Concentrations oftotal N measured in xylem exudate were at least 2-fold higherat 7°C compared with 17°C, attributable mainly to higherconcentrations of NO^–₃ glutamine and proline. The resultsare discussed in terms of acclimatory and other responses shownby the NO^–₃ transport system under conditions of limitingNO^–₃ supply and low root temperature. Key words: Brassica napus, nitrate supply, efflux, influx, root temperature, xylem exudate     

Osmotic and Ionic Regulation in Chara L-cell Fragments

Ion absorption from rather complicated artificial pond water(APW) by cell fragments having a lower osmotic pressure thanthe intact internodal cell (L-cell fragments) was studied. L-cellfragments were prepared by taking advantage of trans cellularosmosis and ligating the cell with thread. The results wereas follows: (1) L-cell fragments absorbed more K⁺ than Na⁺ fromaKCL + NaCl mixture in the presence of Ca²⁺, Mg²⁺ and SO₂₄ inthe light; (2) the influx of KCI was larger than that of KNO₃;(3) the amount of positive charge carried by K⁺, Na⁺ and Mg²⁺across the cell membrane balanced well with the amount of negativecharge carried by Cl^– in Cl^–-containing and NO₃^–-free APW; (4) no conclusion could be made as to whether ornot the rule of electro neutrality held for the K⁺, Na⁺, Ca²⁺and NO₃^– fluxes across the cell membrane, because dilutedKNO₃ is unstable; (5) L-cell fragments in KCl-containing APWsurvived longer than those in KNO₃-containing and Cl^–-free APW; (6) after incubation in KNO₃-containing and Cl^–-freeAPW, L-cell fragments absorbed a great amount of KCI immediatelyafter being transferred to KCl-containing and NO₃^– -freeAPW; and (7) lowering the turgor pressure of the intact cellby raising the external osmotic pressure did not induce ionflux into the cell. Thus, we concluded that the L-cell fragmentsabsorbed ions from the external solution not because of theirlower turgor pressure, but because of the diluted ion concentrationof the cytoplasm and the vacuole. The electroneutrality ruleheld, at least, for K⁺, Na⁺, Mg²⁺ and Cl^– influxes acrossthe cell membrane inthe KCl-containing and NO₃^–-free APW.These results were analyzed on the basis of an extended poremodel which presumed the existence of ATP-dependent processesin the membrane, and suggested that K⁺, Na⁺ and Mg₂₊ inflowsinto an L-cell fragment are likely to be induced by active Cl^–inflow. (Received May 18, 1987; Accepted September 29, 1987)     

Nodulation and Nitrogen Fixation of Faba Bean (Vicia faba L.) as Affected by Removal of the Cotyledons and Nitrate Supply

Cotyledons of faba bean (^{Vicia faba} L. cv. Fiord) were removedto determine whether an apparent delay in nodulation of thiscultivar could be attributed to an inhibitor from these organs.Cotyledons were left intact or excised from seedling plants14 and 18 d after sowing and plants grown with or without 2·5mm NO₃^–. Seedling growth was depressed when cotyledons were removed onday 14 but not when removed on day 18. Removal of the cotyledonsat day 14 reduced nodule number and nodule weight in the absenceof NO₃^–, but in the presence of NO₂^–, nodule numberwas unaffected and only nodule weight was reduced. Cotyledonremoval at day 18 increased both nodule number and nodule weightwith +NO₃^– but not with –NO₂^–. Acetylene reduction(AR) was markedly depressed by NO₃^–. Cotyledon removalat day 14 decreased AR but removal at day 18 resulted in anincrease in AR. We suggest from these results that faba beancotyledons have an inhibitory effect on nodule activity andon nodulation and this interacts with NO₃^–. This can beexplained through a ‘feed-back’ regulation of N₂fixation by soluble N in the seedling. Vicia faba, faba bean, nodule number, nodulation, nodule activity, acetylene reduction, N₂ fixation, cotyledon removal, nitrate     

Ammonium release by nitrogen sufficient diatoms in response to rapid increases in irradiance

It has been hypothesized that nitrogen-replete diatoms, butnot flagellates, may release NO₂^–, NH₄⁺ or dissolved organicnitrogen (DON) following rapid increases in irradiance (andconsequently an increase in cellular electron energy), as mightbe expected to occur in a vertically well mixed estuarine system.Just as the increase in irradiance leads to an increase in cellularenergy, so too would a decrease in temperature, due to the temperaturedependency of biosynthetic enzymes. This hypothesis was testedby comparing the response of nitrogen-replete diatoms (Skeletomenacostatum, Thalassiosira weissflogii and Chaetoceros sp.) andflagellates (Dunaliella tertiolecta, Pavlova lutheri and Prorocentrumminimum) to rapid increases in irradiance and decreases in temperature.Short-term (<3 h) changes in extracellular NO₂^– andNH₄⁺ concentrations were measured in cultures following theseexperimental shifts, as well as in cultures retained at thegrowth irradiance. Net rates of NO₂^– and NH₄⁺ releasewere calculated from the time course of extracellular nitrogenconcentrations. As a fraction of NO₃^– uptake, NO₂^–release rates under the increased irradiance increased marginallyrelative to NO₂^– release rates under the growth irradiance.Release rates of NH₄⁺ under the increased irradiance increasednearly fivefold over release rates at the growth irradiance,and accounted for 84% of the NO₃^– uptake rate. In directcontrast to the diatom species, the flagellate species releasedNO₂^– under the higher experimental irradiance at ratesone half those of the release rates under the growth irradiance,and continued to take up NH₄⁺ under both irradiance conditions.Within the experimental boundaries, these findings have importantphysiological and ecological implications. The magnitude ofthe observed nitrogen release represents a significant physiologicalsink for electrons and, in fact, calculations suggest that upto 62% of the total electrons harvested could be consumed. Froman ecological perspective, these findings add to the body ofliterature which suggests that a significant fraction of thenitrogen that is taken up is ultimately released in dissolvedform. More importantly, these data suggest that DON is not theonly compound that phytoplankton may release in the aquaticenvironment.     

The influence of salinity on the utilization of root anaplerotic carbon and nitrogen metabolism in tomato seedlings

In hydroponically grown Lycopersicon esculentum (L.) Mill. cv.F144 the site of NO₃^– reduction and assimilation withinthe plant was shifted from the shoot to the root by salinity.Uptake of NO₃^– from the root solution was strongly inhibitedby salinization. Consequently, NO₃^– concentrations inthe leaf, stem and root tissues as well as the nitrate reductaseactivities of the leaves were lower in salinized than in controlplants. Lower NO₃^–, but higher reduced-N, concentrationswere observed in the xylem sap as a result of the enhanced participationof the root in NO₃^– reduction in salinized plants. Lowerstem K⁺ concentrations and leaf malate concentrations were foundin salinized compared to control plants which indicates reducedfunctioning of the K⁺–shuttle in the salinized plants. Incorporation of inorganic carbon by the root was determinedby supplying a pulse of NaH¹⁴CO₃ followed by extraction andseparation of the labelled products on ion exchange resins.The rate of H¹⁴CO₃^– incorporation was c. 2-fold higherin control than in salinized plants. In salinized plants theproducts of H¹⁴CO₃^– incorporation within the roots werediverted into amino acids, while the control plants divertedrelatively more ¹⁴C into organic acids. Products of inorganiccarbon incorporation in the roots of salinized plants providean anaplerotic source of carbon for assimilation of reducedNO₃^– into amino acids, while in control plants the productswere predominantly organic acids as part of mechanisms to maintainionic balance in the cells and in the xylem sap. Key words: Tomato, nitrate, PEPc, respiration, salinity     

Effect of NaCI Salinity on Flows and Partitioning of C, N, and Mineral Ions in Whole Plants of White Lupin, Lupinus albusL.

Plants of Lupinus albus L., cv. Ultra, were grown hydroponicallywith NO₃^–-nutrition for 51 d under control (0.05 mol m^–3Na⁺ and 10 mol m^–3 Cl^–) and saline (40 mol m^–3NaCI) conditions. Plants were harvested 41 and 51 d after germinationand analysed for content and net increment of C, N and the mineralcations K⁺, Na⁺, Mg²⁺, and Ca²⁺ and the anions Cl^–, NOJ,malate, phosphate, and SO₄^2–. Roots, stem interaodes,petioles and leaflets were analysed separately. During the studyperiod net photosynthesis, respiratory losses of CO₂ from shootand root and the composition of the spontaneously bleeding phloemsap and the root pressure xylem exudate were also determined.Using molar ratios of C over N in the transport fluids, incrementsof C and N, and photosynthetic gains as well as respiratorylosses of C, the net flows of C and N in the xylem and phloemwere then calculated as in earlier studies (Pate, Layzell andMcNeill, 1979a). Knowing the carbon flows, the ratios of ionto carbon in the phloem sap, and ion increments in individualorgans, net flows of K⁺, Na⁺, and Cl^– over the study periodwere also calculated. Salt stress led to a general decrease of all partial componentsof C and N partitioning indicating that inhibitions were notdue to specific effects of NaCI salinity on photosynthesis oron NO₃ uptake. However, there were differences between variouslyaged organs, and net phloem export of nitrogenous compoundsfrom ageing leaves was substantially enhanced under saline conditions.In addition, NO₃^–reduction in the roots was specificallyinhibited. Uptake and xylem transport of K⁺ was more severelyinhibited than photosynthetic carbon gain or NO₃^– uptakeby the root. K⁺ transport in the phloem was even more severelyrestricted under saline conditions. Na⁺ and Cl^– flowsand uptake, on the other hand, were substantially increasedin the presence of salt and, in particular, there were thenmassive flows of Na^– in the phloem. The results are discussedin relation to the causes of salt sensitivity of Lupinus albus.The data suggest that both a restriction of K⁺ supply and astrongly increased phloem translocation of Na⁺ contribute tothe adverse effects of salt in this species. Restriction ofK⁺ supply occurs by diminished K⁺ uptake and even more by reducedK⁺ cycling within the plant. Key words: Lupinus albus, salt stress, phloem transport, xylem transport, partitioning, carbon, nitrogen, K⁺, Na⁺, CI^–