Oxygen Diffusion in Lupin Nodules: II. MECHANISMS OF DIFFUSION BARRIER OPERATION

The oxygen diffusion resistance of Lupinus albus (L.) cv. Multoluparoot nodules was increased by subjection to short-term stresses;lowering rhizosphere temperature from 25 to 16 °C (2 h),detopping plants (3 h), darkening plants (21 h) or exposingroots to 20 mol m^–3 KN03 for 2, 4 or 6 d. Microscopicobservations and measurements showed that this resulted in thearea of open intercellular spaces within the inner cortex beingreduced due to both cell expansion and increased productionof an occluding glycoprotein. Electrophoretic and Western Blotanalysis using the monoclonal antibodies MAC236 and MAC265 showedtwo distinct glycoprotein antigens with molecular weights of240 and 135 kDa, respectively. Both antigens are localized withinintercellular spaces of the inner cortex. The amount of glycoproteinwas determined using either ELISA, with MAC265, or quantificationof immunolabelling with MAC236. This immunolabelling also localizedthe glycoprotein within globules adhering to the inside of theinner cortical cell walls. Key words: Oxygen diffusion resistance, glycoprotein, nodules, nitrogen fixation, Lupinus albus     

Time-course of changes involved in the operation of the oxygen diffusion barrier in white lupin nodules

Nodulated white lupins (Lupinus albus L. cv. Multolupa) weresubject to either darkening for 12 h, followed by 24 h recoveryin light, or to 50% O₂ for 30 min. For each treatment, noduleswere harvested at intervals for analysis by light and electronmicroscopy and determination of glycoprotein content using EnzymeLinked Immunosorbent Assays (ELISA). This allowed for an analysisof the sequence of events causing an increase in intercellularspace occlusion within the inner cortex. The temporal sequencein response to darkening appears to be: (1) an initial rapidincrease in the detectable levels of intracellular glycoprotein,due to either a state change or de novo synthesis, (2) a concomitantincrease in the volume of thickened cell walls, causing a reductionof intercellular space volume and (3) after 1–3 h a releaseof glycoprotein into the intercellular space network of theinner cortex, accompanied (and possibly spread) by the continuedconstriction of the spaces due to cell wall and cell contentexpansion. The results for exposure to 50% O₂ showed a similar,but much more rapid, sequence of events, operating within 15–30min. The main difference between the two sequences was the lackof expansion of thickened cell walls with increased pO₂. Also,it was possible to detect glycoprotein within cell walls followingexposure to 50% O₂ but not following darkening. These observationsare discussed in relation to proposed mechanisms for the operationof a variable oxygen diffusion barrier in legume nodules. Key words: Oxygen diffusion resistance, glycoprotein, nodules, Lupinus albus     

Possible reasons for relative salt stress tolerance in nodules of white lupin cv. Multolupa

The effects of different NaCl concentrations on the growth andnitrogen fixation activity of white lupin (Lupinus albus [L.])was studied over a 6 d period. Plant growth parameters, photosynthesisand shoot respiration were unaffected by NaCl concentrationsup to 150 mol m^–3. However, nitrogenase activity decreasedwith increased NaCl concentration up to 100 mol m_–3, whilstthe O₂ diffusion resistance increased with 100 mol m^–3NaCl, but showed no further change when 150 mol m^–3 NaClwas applied for 6 d. Increases in NaCl concentration decreasednodular starch content while increasing sucrose content, suggestingan osmotic regulation. These changes were associated with a77% decrease in sucrose synthase activity. The effect on theO₂ diffusion resistance was paralleled by changes in glycoproteincontent of the nodules, as determined by immunogold localizationand ELISA. X-ray microanalysis studies of nodules showed that,following a 6 d exposure to 150 mol m^–3 NaCl, Na⁺ ionswere largely excluded from the infected zone, whilst only lowlevels of Cl^- ions penetrated into this region. Na⁺ entry intoroots and leaves was also at a low level. Leghaemoglobin contentdecreased with saline stress, as did superoxide dismutase; whichdecreased by 36% following exposure to 100 mol m^–3 saltfor 6 d. These results are discussed in relation to the relativesalt tolerance of the Multolupa/ISLU-16 symbiosis. Key words: Salt stress, nodules, nitrogen fixation, oxygen diffusion, carbohydrates, Lupinus albus     

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^–     

Leaf-like Structure in the Photosynthetic, Succulent Stems of Cacti

This research examined the hypothesis that as cacti evolve tothe leafless condition, the stem epidermis and cortex becomemore leaflike and more compatible with a photosynthetic role.All cacti in the relict genus Pereskia have non-succulent stemsand broad, thin leaves. All members of the derived subfamilyCactoideae are ‘leafless’, having an expanded cortexthat is the plant's only photosynthetic tissue. In Pereskia,leaves have a high stomatal density (mean: 50.7 stomata mm^–2in the lower epidermis, 38.1 mm^–2 in the upper epidermis),but stems have low stomatal densities (mean: 11.3 mm ₂, threeof the species have none). Stems of Cactoideae have a high stomataldensity (mean: 31.1 mm^–2, all species have stomata). Theouter cortex cells of stems of Cactoideae occur in columns,forming a palisade cortex similar to a leaf palisade parenchyma.In this palisade cortex, the fraction of tissue volume availablefor gas diffusion has a mean volume of 12.9%, which is identicalto that of Pereskia leaf palisade parenchyma. Pereskia stemcortex is much less aerenchymatous (mean: 5.3% of cortex volume).Cactoideae palisade cortex has a high internal surface density(0.0207 cm₂ cm^–2 which is higher than in Pereskia stemcortex (0.0150 cm₂ cm^–3) but not as high as Pereskia leafpalisade parenchyma (0.0396 cm₂ cm^–3). Pereskia stem cortexhas no cortical bundles, but Cactoideae cortexes have extensivenetworks of collateral vascular bundles that resemble leaf veins. Cactaceae, cactus, intercellular space, stomatal density, internal surface/volume, evolution     

Translocation Profiles of [11C] and [13N]-Labelled Metabolites after Assimilation of 11CO2 and [13N]-Labelled Ammonia Gas by Leaves of Helianthus annuus L. and Lupinus albus L.

Tracer amounts of atmospheric [¹³N]-Iabelled ammonia gas, wereabsorbed by leaves of Lupinus albus and Helianthus annuus inboth the light and the dark. Exogenous [¹³N]-ammonia was onlyabsorbed in the dark when the feeding occurred shortly aftera period of illumination and the tissue was not depleted ofits carbohydrate reserves (e.g. starch). Incorporation of the[¹³N]-ammonia appeared to occur via the leaf glutamine synthetase/glutamatesynthase (GS/GOGAT) cycle since 2.0 mol m^–3 MSX, an inhibitorof the GS reduced uptake in both the light and dark. Photosyntheticincorporation of ¹¹CO₂ was not affected by this treatment Therate of movement of [¹³N]-assimilates in the petiole of attachedleaves of Helianthus and Lupinus was similar to that of the¹¹Cl-photo assimilates. Export of both [¹³N] and [¹¹C]-Iabelledassimilates from the leaf and movement in the petiole in boththe light and the dark was inhibited by source leaf anoxia (i.e.nitrogen gas). Translocation was re-established at the samerate when the feed leaf was exposed to gas containing more than2% O₂ which permitted dark respiration to proceed. After aninitial feeding of either ¹¹CO₂ or [¹³N]-ammonia at ambient(21%) O₂ exposure of the source leaf to 2% O2, or 50% O² didnot alter the rates of translocation, indicating that changesin photosynthetic activity in the source leaf due to photorespiratoryactivity need not markedly alter, at least during the shortperiod, the loading and translocation of either [¹¹C ] or [¹³N]-labelledleaf products. Key words: Translocation, CO₂, NH₃, Leaves, Helianthus annuus, Lupinus albus     

Nitrogen-13 Studies of Nitrate Fluxes in Barley Roots: II. EFFECT OF PLANT N-STATUS ON THE KINETIC PARAMETERS OF NITRATE INFLUX

Influx of nitrate into the roots of intact barley plants wasfollowed over periods of 1–15 min using nitrogen-13 asa tracer. Based on measurements taken over 15 min from a rangeof external nitrate concentrations (0·2–250 mmolm^–3), the kinetic parameters of influx, I_max and K_m, werecalculated. Compared with plants grown in the presence of nitrate throughout,plants that had been starved of N for 3 d showed a significantlygreater value ofI_max for ¹³N-nitrate influx (by a factor of1·4–1·8), but a similar value of K_m (12–14mmol m^–3). Pre-treating N-starved plants with nitratefor about 5 h further increased the subsequent rate of ¹³N-nitrateinflux, but had little effect in the unstarved controls. Allowingfor this induction of additional nitrate transport, the differencein rates of nitrate influx in control and N-starved plants wassufficient to account for the previously-observed differencein net uptake by the two groups of plants. In barley plants grown without any exposure to nitrate, butwith ammonium as N-source, both I_max and K_m for subsequent ¹³N-nitrateinflux were significantly decreased (by about one-half) comparedwith the corresponding nitrate-grown controls. The importance of changes in the rate of influx in the regulationof net uptake of nitrate is discussed. Key words: Ion transport, nitrate, influx, kinetic parameters, N-deficiency     

Relationships between Acetylene Reduction Activity, Hydrogen Evolution and Nitrogen Fixation in Nodules of Acacia spp.: Experimental Background to Assaying Fixation by Acetylene Reduction under Field Conditions

Hansen, A. P., Pate, J. S. and Atkins, C. A. 1987. Relationshipsbetween acetylene reduction activity, hydrogen evolution andnitrogen fixation in nodules of Acacia spp.: Experimental backgroundto assaying fixation by acetylene reduction under field conditions.—J.exp. Bot. 38: 1–12 Glasshouse grown, symbiotically-dependent seedlings of Acaciaalata R.Br., .A. extensa Lindl., and A. pulchella R.Br. wereexamined for acetylene reduction in closed assay systems usingundisturbed potted plants, excavated whole plants, nodulatedroots or detached nodules. Nitrogenase activity declined sharplyover the first hour after exposure of detached nodules to acetylene(10% v/v in air), less steeply or not at all over a 3 h periodin assays involving attached nodules. Using detached nodules,rates of acetylene reduction, nitrogen (¹⁵N₂) fixation, andhydrogen evolution in air (¹⁵N₂) and acetylene-containing atmosphereswere measured in comparable 30 min assays. Total electron flowthrough nitrogenase in air was determined from rates of nitrogen(¹⁵N₂) fixation ( ? 3) plus hydrogen evolution, that in thepresence of acetylene from rates of acetylene reduction andhydrogen evolution in air: acetylene. Values for the ratio ofelectron flow in air: acetylene to that in air ranged from 0?43to 0?83 in A. pulcheila, from 0?44 to 0?66 in A. alala and from0?37 to 0?70 in A. extensa, indicating substantial inhibitionof electron flow through nitrogenase of detached nodules byacetylene. Relative efficiencies of nitrogenase functioningbased on hydrogen evolution and acetylene reduction were from0?15 to 0?79, those based on nitrogen (¹⁵N₂) fixation and hydrogenevolution from 0?53 to 0?87. Molar ratios of acetylene reducedto nitrogen (¹⁵N₂) fixed were 2?82 ? 0?24, 201 ? 0?15, and 1?91? 0?11 (?s.e.; n = 7) for A. pulcheila,A. extensa and A. alata respectively A standard 5–10 min acetylene reduction assay, conductedon freshly detached unwashed nodules in daytime (12.00–14.00h), was calibrated for field use by comparing total N accumulationof seedlings with estimated cumulative acetylene reduction overa 7-week period of glasshouse culture. Molar ratios for acetylenereduced: nitrogen fixed using this arbitrary method were 3?58for A. alata, 4?82 for A. extensa and 1?60 for A. pulchella.The significance of the data is discussed. Key words: Acacia spp, nitrogenase functioning     

Effectiveness of Lotus Root Nodules: II. RELATIONSHIP BETWEEN ROOT NODULE EFFECTIVENESS AND 'IN VITRO' SENSITIVITY OF FAST-GROWING LOTUS RHIZOBIA TO FLAVOLANS

An extract from the roots of Lotus pedunculatus plants was foundto contain a compound toxic towards fast-growing Lotus rhizobia.This compound was identified as a flavolan, which has a prodeiphinidin:procyanidin ratio of 75:25. A fast-growing strain of Rhizobium(NZP2213) which forms ineffective root nodules on L. pedunculatuswas four times more sensitive to this flavolan (ED₅₀ = 25 ?gml^–1) than another strain (NZP2037, ED₅₀ = 100 ?g ml^–1)which forms effective root nodules on this species. The rootsof another Lotus species, L. tenuis, on which both strains ofRhizobium form effective root nodules, also contained a flavolan( 95% procyanidin) but both strains were relatively insensitiveto this flavolan (EDED₅₀ = 350 to 500 ?g ml^–1) L. pedunculatusplants bearing ineffective root nodules contained two to threetimes more flavolan in their roots (5–7 mg g^–1 fr.wt.)than uninoculated control plants. Experiments with seven otherLotus species and with hybrid plants developed between L. pedunculatusand L. tenuis showed a relationship between the prodeiphinidin:procyanidin ratio of the flavolan in their roots and the effectivenessof root nodules formed on these plants by NZP2213. Quantitativebinding studies of the flavolan from L. pedunculatus to NZP2037and NZP2213 indicated that, while the affinity constants forbinding were similar for both strains, the surface of strainNZP2037 contained four times more binding sites than NZP2213,possibly correlating with this strain's ability to toleratehigher concentrations of this flavolan. It is suggested thatthe differential sensitivity of these two strains of Rhizobiumto flavolans is related to their ability to form effective rootnodules on Lotus species.     

Assimilation of 15N2 and 15NO3- by Partially Nitrate-Tolerant Nodulation Mutants of Soybean

Growth-chamber studies were conducted to evaluate nitrogen assimilationby three hypernodulated soybean [Glycine max (L.) Merr.] mutants(NOD1–3, NOD2–4, NOD3–7) and the Williamsparent. Seeds were inoculated at planting and transplanted atday 7 to nutrient solution with 1 mol m^–3 urea (optimizesnodule formation) or 5 mol m^–3 NO₃^– (inhibits noduleformation). At 25 d after planting, separate plants were exposedto ¹⁵NO₂ or ¹⁵NO₃^– for 3 to 48 h to evaluate N₂ fixationand NO₃^– assimilation. Plant growth was less for hypernodulatedmutants than for Williams with both NO₃^– and urea nutrition.The major portion of symbiotically fixed ¹⁵N was rapidly assimilated(30 min) into an ethanol-soluble fraction, but by 24 h aftertreatment the ethanolinsoluble fraction in each plant part wasmost strongly labelled. Distribution patterns of ¹⁵N among organswere very similar among lines for both N growth treatments aftera 24 h ¹⁵N₂ fixation period; approximate distributions were40% in nodules, 12% in roots, 14% in stems, and 34% in leaves.With urea-grown plants the totalmg ¹⁵N fixed plant^–1 24h^–1 was 1·18 (Williams), 1·40 (N0D1-3),107 (NOD2-4), and 0·80 (NOD3-7). The 5 mol m^-3 NO₃^- treatmentresulted in a 95 to 97% decrease in nodule mass and ¹⁵N₂ fixationby Williams, while the three mutants retained 30 to 40% of thenodule mass and 17 to 19% of the ¹⁵N₂ fixation of respectiveurea-grown controls. The hypernodulated mutants, which had restrictedroot growth, absorbed less ¹⁵NO₃^- than Williams, irrespectiveof prior N growthcondition. The ¹⁵N from ¹⁵NO₃^- was primarilyretained in the soluble fraction of all plant parts through24 h. The ¹⁵N incorporation studies confirmed that nodule developmentis less sensitive to external NO₃^- in mutant lines than in theWilliams parent, and provide evidence that subsequent metabolismand distribution within the plant was not different among lines.These results further confirm that the hypernodulated mutantsof Williams are similar in many respects to the hyper- or supernodulatedmutants in the Bragg background, and suggest that a common mutationalevent affectingautoregulatory control of nodulation has beentargeted. Key words: Glycine max (L.) Merr., soybean, N₂fixation, nitrate assimilation, nodulation mutants, ¹⁵N isotope     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 3. Time-course Study on 13C Incorporation into Soluble Metabolites and Respiratory Evolution of 13CO2 from Roots and Nodules

Well-nodulated soya bean (Glycine max L.) plants were allowedto assimilate ¹³CO₂ for 10 h in the light, under steady-stateconditions in which CO₂ concentration and ¹³C abundance wereboth strictly controlled at constant levels. The respiratoryevolution of ¹³CO₂ from roots and nodules and ¹³C incorporationinto various metabolic fractions were measured during the ¹³CO₂feeding and subsequent 48 h chase period. CO₂ respired from nodules was much more rapidly labelled with¹³C than that from roots. The level of labelling (percentageof carbon currently assimilated during the ¹³COM₂ feeding period)of CO₂ respired from nodules reached a maximum of about 87 percent after 4 h of steady-state ^l3CO₂ assimilation and thereafterremained fairly constant. The absolute amount of labelled carbonevolved by the respiration of the nodules during the 10 h ¹³CO₂feeding period was 1·5-fold that of root respiration.These results demonstrated that the currently assimilated (labelled)carbon was preferentially used to support nodule respiration,while root respiration relied considerably on earlier (non-labelled)carbon reserved in the roots. Sucrose pools were mostly composed of currently assimilatedcarbon in all tissues of the plants, since the levels of labellingaccounted for 86–91 per cent at the end of the ¹³CO₂ feeding.In the nodules, the kinetics and levels of sucrose labellingwere in fairly good agreement with those of respired CO₂, whilein the roots, the level of labelling of respired CO₂ was significantlylower than that of sucrose. Succinate and malate were highly labelled in both roots andnodules but they were labelled much more slowly than sucroseand respired CO₂. The kinetics and levels of labelling of theseKrebs cycle intermediates resembled those of major amino acidswhich are derived directly from Krebs cycle intermediates. Itis suggested that large fractions of organic acids in noduleswere physically separate from the respiration site. Glycine max L., Soya bean, ¹³CO₂ assimilation, respiratory evolution of ¹³CO₂, carbon metabolism in root nodules     

Long Distance Transport of Abscisic Acid in NaCI-Treated Intact Plants of Lupinus albus

Plants of Lupinus albus were grown for 51 d under control (1.1mol m^–3 NaCl) and saline (40 mol m^–3 NaCl) conditions.Plants were harvested and changes of carbon, nitrogen and abscisicacid (ABA) contents of individual organs were determined 41d and 51 d after germination. In the period between the twoharvests xylem and phloem saps were collected and respirationand photosynthesis of individual organs were measured. Usingflows of carbon, C/ABA ratios and increments of ABA flows ofABA in phloem and xylem and rates of biosynthesis and degradationof ABA were calculated. Both under control and saline conditionsnet biosynthesis occurred in the root, the basal strata of leavesand in the inflorescence. Metabolic degradation of ABA tookplace in the stem internodes and apical leaf strata. Salt stress increased xylem transport of ABA up to 10-fold andphloem transport to the root up to 5-fold relative to that ofthe controls. A considerable amount of ABA in the xylem saporiginated from biosynthesis in the roots, i.e. 55% in salt-treatedand smaller than 28% in control plants. The remaining part ofABA in the xylem sap originated from the shoot: it was translocatedin the phloem from fully differentiated leaves towards the rootand from there it was recirculated back to the aerial partsof the plant. The data suggest that ABA may serve as a hormonalstress signal from the root system. Key words: Lupinus albus, salt stress, abscisic acid, long distance transport     

The Effect of Irradiance on the Recovery of Soybean Nodules from Sodium Chloride-Induced Senescence

Soybean (Glycine max L. Merr) cv. Clarke plants inoculated withBradyrhizobium japonicum strain RCR3407 were grown either ina greenhouse with a low irradiance (200–400)µmolm^–2 s^–1) or in a controlled-environment growth cabinetwith a higher irradiance (600 µimol m^–2 s^–1).At 42 d plants were given a nitrogen-free nutrient solutioncontaining 50 mol m^–3 sodium chloride for 2 weeks andthen allowed to recover from salt-stress for a further 2 weeks. Salt treatment reduced plant growth by at least half in bothgrowth regimes, however, the controlled environment-grown (CEG)plants were five times larger than the greenhouse-grown (GG)plants in terms of dry weight and number/weight of nodules perplant, regardless of treatment. The structure of nodules, from both growth regimes, harvestedat the end of the 2 week salt-stress was similar to unstressedcontrol nodules. However, nodules harvested 1 week later fromboth CEG and GG plants had structural changes including degradationof bacteria in vacuoles around host cell nuclei, particularlyin the outer cell layers of the infected tissue. In addition,meristematic activity was seen in the cortex of some nodulesfrom GG plants. Young cells here contained infection threadsand newly-released bacteria. Nodules harvested 2 weeks after removal of the salt-stress fromCEG plants showed an apparent recovery from the stress. However,there was a very marked increase in the amount of starch inthe cortex which was not seen in equivalent GG nodules. In contrast,nodules from GG plants contained many vacuolate infected cellsand, consequently, a lowered bacteroid population. Further,meristematic activity was seen in a zone concentric to the infectedzone, newly-formed cells contained many large infection threadsand were interspersed with intercellular bacteria. The meristematicactivity increased the relative volume of cortical to infectedcells in these nodules. Growth conditions did not affect control nodule specific nitrogenaseactivity or oxygen diffusion resistance (R) and these parameterswere also not altered in CEG nodules exposed to salt plus the14 d recovery period. However, nitrogenase activity was greatlyreduced, and R increased by more than eight times in equivalentGG nodules exposed to salt plus recovery. It is hypothesized that the gross morphological changes werean attempt to counter salt toxicity and/or oxygen damage underconditions of reduced photosynthate supply to the nodules dueto the poor light levels in the greenhouse. However, soybeannodules supplied with adequate photosynthate were able to withstandand recover from long-term salt-stress with little alterationto their structural integrity. Key words: Soybean, sodium chloride, nitrogen fixation, light intensity, oxygen diffusion resistance     

Diurnal regulation of NO3- uptake in soybean plants I. Changes in NO3- influx, efflux, and N utilization in the plant during the day/night cycle

The effect of light on NO₃^– utilization was investigatedin non-nodulated soybean (Clycine max L. Merr., cv. Kingsoy)plants during a 14/10 h light/dark period at a constant temperatureof 26C. A 30–50% decrease of net NO₃^– uptake ratewas observed 2–6 h after the lights were turned off. Thiswas specifically due to an inhibition of NO₃^– influx asmeasured by ¹⁵N incorporation during 5 min. The absolute valuesof NO₃^– efflux depended on whether the labelling protocolinvolved manipulation of the plants or not, but were not affectedby illumination of the shoots. Darkness had an even more markedeffect in lowering the reduction of ¹⁵NO₃^– in both rootsand shoots, as well as xylem transport of ¹⁵NO₃^– and reduced¹⁵N. Concurrently with this slowing down of transport and metabolicprocesses, accumulations of NO₃^– and Asn were significantlystimulated in roots during the dark period. These data are discussedin view of the hypothesis that darkness adversely affects NO₃^–uptake through specific feedback control, in response to alterationsin the later steps of N utilization which are more directlydependent on light. Key words: Glycine max, light/dark cycles, nitrate uptake, nitrate reduction     

The Influence of NO-3 and NH+4 Nutrition on the Gas Exchange Characteristics of the Roots of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

Respiratory oxygen consumption by roots was 1·4- and1·6-fold larger in NH⁺₄-fed than in NO^-₃-fed wheat (Triticumaestivum L.) and maize (Zea mays L.) plants respectively. Higherroot oxygen consumption in NH⁺₄-fed plants than in NO^-₃-fedplants was associated with higher total nitrogen contents inNH⁺4-fed plants. Root oxygen consumption was, however, not correlatedwith growth rates or shoot:root ratios. Carbon dioxide releasewas 1·4- and 1·2-fold larger in NO⁺3-fed thanin NH⁺₄-fed wheat and maize plants respectively. Differencesin oxygen and carbon dioxide gas exchange rates resulted inthe gas exchange quotients of NH^-₄-fed plants (wheat, 0·5;maize, 0·6) being greatly reduced compared with thoseof NO^-₃-fed plants (wheat, 1·0; maize, 1·1). Measuredrates of HCO^-₃ assimilation by PEPc in roots were considerablylarger in 4 mM NH⁺₄-fed than in 4 NO^-₃ plants (wheat, 2·6-fold;maize, 8·3-fold). These differences were, however, insufficientto account for the observed differences in root carbon dioxideflux and it is probable that HCO^-₃ uptake is also importantin determining carbon dioxide fluxes. Thus reduced root extension in NH⁺₄-fed compared with NO^-₃-fedwheat plants could not be ascribed to differences in carbondioxide losses from roots.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize assimilation, ammonium assimilation, root respiration     

Modification of Stomatal Conductance by Sulphur Dioxide

The mechanism of SO₂-induced changes in stomatal conductance(g) of alder was examined to determine if SO₂ affects guardcell function directly or indirectly through the SO₂-inducedchanges in photosynthesis. During experimental fumigations at SO₂ concentrations of 3–3µmol m^–3 (0.08 µl l^–1), stomatal closurepreceded declines in net photosynthetic rate (A), indicatingthat SO₂ can directly affect guard cells. From these and otherstudies it appears that the sequence of A and g responses maybe influenced by SO₂ concentration as well as by species. Fumigation with SO₂ did not cause increases in g, even whenthe intercellular substomatal CO₂ concentration (c_i) was reducedby 50 µmol mol^–1. Increases in g are not attributableto SO₂ effects on the CO₂-based stomatal control system. Key words: Air pollution, Alnus serrulata, gas exchange, stomata, sulphur dioxide     

C3 and CAM Photosynthetic Characteristics of the Submerged Aquatic Macrophyte Littorella uniflora: Regulation of Leaf Internal CO2 Supply in Response to Variation in Rooting Substrate Inorganic Carbon Concentration

The relationships between CO₂ concentrating mechanisms, photosyntheticefficiency and inorganic carbon supply have been investigatedfor the aquatic macrophyte Littorella uniflora. Plants wereobtained from Esthwaite Water or a local reservoir, with thelatter plants transplanted into a range of sediment types toalter CO₂ supply around the roots. Free CO₂ in sediment-interstitial-waterranged from 1–01 mol m^–3 (Esthwaite), 0.79 mol m^–3(peat), 0.32 mol m^–3 (silt) and 0–17 mol m^–3(sand), with plants maintained under PAR of 40 µmol m^–2s^–1. A comparison of gross morphology of plants maintained underthese conditions showed that the peat-grown plants with highsediment CO₂ had larger leaf fresh weight (0–69 g) andtotal surface area (223 cm² g^–1 fr. wt. including lacunalsurface area) than the sand-grown plants (0.21 g and 196 cm²g^–1 fr. wt. respectively). Root fresh weights were similarfor all treatments. In contrast, leaf internal CO₂ concentration[CO₂], was highest in the sand-grown plants (2–69 molm^–3, corresponding to 6.5% CO₂ in air) and lowest inthe Esthwaite plants (1–08 mol m^–3). Expressionof CAM in transplants was also greatest in the low CO₂ regime,with H⁺ (measured as dawn-dusk titratable acidity) of 50µmolg^– fr. wt., similar to Esthwaite plants in natural sediment.Assuming typical CAM stoichiometry, decarboxylation of malatecould account largely for the measured [CO₂]₁ and would makea major contribution to daytime CO₂ fixation in vivo. A range of leaf sections (0–2, 1–0, 5–0 and17–0 mm) was used to evaluate diffusion limitation andto select a suitable size for comparative studies of photosyntheticO₂ evolution. The longer leaf sections (17.0 mm), which weresealed and included the leaf tip, were diffusion-limited witha linear response to incremental addition of CO₂ and 1–0mol m^–3 exogenous CO₂ was required to saturate photosynthesis.Shorter leaf sections were less diffusion-limited, with thegreatest photosynthetic capacity (36 µmol O₂ g^–1 fr. wt. h^–1) obtainedfrom the 1.0 mm size and were not infiltrated by the incubatingmedium. Comparative studies with 1.0 mm sections from plants grown inthe different sediment types revealed that the photosyntheticcapacity of the sand-grown plants was greatest (45 µmolO₂ g^–1 fr. wt. h^–1) with a K_0.5 of 80 mmol m^–3.In terms of light response, saturation of photosynthesis intissue slices occurred at 850–1000 µmol m^–2s^–1 although light compensation points (6–11 µmolm^–2s^–1) and chlorophyll a: b ratios (1.3) were low.While CO₂ and PAR responses were obtained using varying numbersof sections with a constant fresh weight, the relationshipsbetween photosynthetic capacity and CO₂ supply or PAR were maintainedwhen the data were expressed on a chlorophyll basis. It is concludedthat under low PAR, CO₂ concentrating mechanisms interact inintact plants to maintain saturating CO₂ levels within leaflacunae, although the responses of the various components ofCO₂ supply to PAR require further investigation. Key words: Key words-Uttorella uniflora, internal CO₂ concentration, crassulacean acid metabolism, root inorganic carbon supply, CO₂ concentrating mechanism     

Developmental and Biochemical Regulation of 'Constitutive' Nitrate Reductase Activity in Leaves of Nodulating Soybean

The developmental profile of ‘constitutive’ nitratereductase activity (cNRA) in leaves of soybean (Glycine max(L.) cv. Bragg) plants at different ages is described. The youngestleaves had most cNRA and the activity dropped off as a newerleaf developed above it. Each leaf had its distinct active periodof in vivo cNRA. This pattern was different in urea-grown andsymbiotically-grown plants (inoculated with Bradyrhizobium japonicumstrain USDA 110), where the latter had no detectable in vivocNRA in older leaves. Urea-grown plants maintained considerablein vivo NRA in such older leaves. When symbiotically-grown plantshad their nodules removed, in vivo cNRA reappeared in olderleaves within 1 d of removal, nearly reaching levels of youngleaves at 3 d after nodule excision. Allantoic acid (ALL), oneof the known transport ureides of soybeans, was implicated asa possible signal molecule from nodules to leaves. Allantoicacid (100 µM) inhibited in vitro c₁ NRA significantly,with 400 µM ALL resulting in complete inhibition. In contrast,allantoin (ALN) had no inhibitive effect on NRA. Inhibitionof c₁NRA by ALL was by a competitive process, judging from Lineweaver-Burkeplots against nitrate. Kinetics showed a constant Vmax of around105 nmol NO₂ mg^–1 protein h^–1 and a K_m for nitrateof 15 mM, which increased to 60 mM in the presence of 200 µMallantoic acid. Non-specific (ionic and pH-related) influenceswere eliminated. Allantoic acid also had a slight stimulatingeffect of in vitro NRA (up about 25% at 400 µM). Thesefindings suggest that c1NRA may be involved in ureide metabolism,rather than in vivo nitrate metabolism. Key words: Root-shoot interaction, nitrogen metabolism, nodulation, symbiosis     

The Role of the Stem in Phloem Loading of Minerals in Lupinus albus L. cv. Ultra

Collections of phloem sap made over a 40-day period from a varietyof locations on nodulated white lupin plants (Lupinus albusL. cv. ultra) showed considerable enrichment with K⁺ and Mg²⁺in the phloem streams destined for the shoot apices or fruitsrelative to the streams arising from the leaflets (up to 5.5times). Sodium showed enrichment in the streams destined forthe roots (up to 2.5 times) but only when present in the watersupply at a high level (3 mM). The stem, in view of its centrallocation in the transport pathway, is seen as an organ capableof redistributing minerals in the phloem independently of photosynthate. Lupinus albus L., lupin, phloem loading, magnesium, potassium, sodium, mineral elements     

Responses of Noogoora Burr (Xanthium occidentale Bertol.) to Nitrogen Supply and Carbon Dioxide Enrichment

We studied the responses of Xanthium occidentale (Bertol.) (cockleburor Noogoora burr), a noxious weed, to atmospheric CO₂ enrichmentand nitrate-N concentrations in the root zone ranging from 0.5to 25 mM. CO₂ enrichment (1500 cm³ m^–3) increased dry-matterproduction to about the same extent (18 per cent) at all levelsof supplied N: most of the increment in dry matter was distributedequally between leaves and roots so that there was little effecton shoot-to-root dry-weight ratios. Growth was stimulated greatlyby N and plateaued at 12 mM supplied N. Shoot-to-root dry-weightand total N ratios increased with increasing N supply. CO₂ enrichmenthad no effect on the total amount of N accumulated by plants,but increased the N-use efficiency of leaves. Enriched plantshad lower concentrations and quantities of N in their leavesthan controls, and therefore lower shoot-to-root total N ratios.Little free NO₃^– accumulated in organs of control or enrichedplants. NO₃^– was the major form of N in xylem sap fromdetopped plants at low supplied NO₃-N, but amino N was equalin importance at high supplied NO₃-N in control and enrichedplants. Concentrations of NO₃^– were lower in the xylemsap of CO₂ enriched plants. It was concluded that the betterN-use efficiency of CO₂ enriched plants could result in increasedgrowth of X. occidentale in regions of marginal soil fertilityas atmospheric levels of CO₂ increase. CO₂ enrichment, nitrogen, Xanthium, Noogoora burr, cocklebur