Primary production in lakes Huron and Michigan: in vitro and in situ comparisons

Oxygen- and carbon-14-based primary production estimates from9–16 h in vitro incubations were compared in lakes Huronand Michigan. For surface mixing layer compansons, gross O₂/¹⁴Cphotosynthetic quotients (gross PQ) averaged 2.2, and net O₂/¹⁴Cphotosynthetic quotients (net PQ) averaged 1.4. The mean grossPQ is consistent with a theoretical P0 based on the CO₂ andNO₃ assimilation ratio. However, within the deep chlorophylllayer, gross PQ and net PQ averaged 4.9 and 2.8 respectively.These higher values were likely due to excess NO₃ reductionat the expense of CO₂ uptake. Thus, during short experimentsunder low light conditions, oxygen evolution and CO₂ uptakemay not be tightly coupled. In vitro and in situ O₂ productionestimates were compared in four diurnal (dawn to dusk) experimentsin Lake Huron. In situ production estimates were determinedby measuring water-mass oxygen changes and oxygen transfer acrossthe air-water interface. In situ production estimates were approximatelytwice in vitro production estimates for both surface mixinglayer and deep chlorophyll layer comparisons. The differencebetween estimates was attributable to containment effects manifestin 13–16 h bottle incubations. Short-term (1–2 h)in vitro production was also compared to diurnal in vitro production.Rates of short-term production were {small tilde}1.6 times higherthan rates of diurnal production, suggesting that short-termin vitro production experiments may provide reasonable estimatesof in situ primary production.     

Mechanism of Electron Flow through the QB Site in Photosystem II. 4. Reaction Mechanism of Plastoquinone Derivatives at the QB Site in Spinach Photosystem II Membrane Fragments

Interactions of externally added plastoquinone (PQ) derivatives(PQ₀-PQ₃) with the photosystem II (PSII) acceptor side wereinvestigated in PSII membrane fragments prepared from spinachby measuring the photoreduction rates of PQ derivatives at variousPQ concentrations, and the following results were obtained. From the kinetic analysis, all the PQ derivatives (PQ₀-PQ₃)except PQ₃ were shown to accept electrons at two sites (theQ_B site and the PQ site) as in the case of Synechococcus vulcanusPSII particles with benzoquinone derivatives [Satoh et al. (1995)Plant Cell Physiol. 36: 597]. Affinities of PQ derivatives at the Q_B site increased as thelength of the isoprene side chain got longer, while those atthe PQ site were not very much different for all the PQ derivativestested in this study. The inhibitory effect of DCMU was noncompetitive, and, therefore,the affinity of PQ₃ for the PQ site was determined while thatfor the Q_B site could not be estimated presumably due to itsfairly high affinity to the site. Based on the results obtained using PQ derivatives, the mechanismof interaction of an authentic PQ, PQ₉, at the Q_B site is discussed. (Received May 2, 1996; Accepted July 24, 1996)     

The Partitioning of Nitrate Assimilation Between Root and Shoot of a Range of Temperate Cereals and Pasture Grasses

Nitrate reductase activity (NRA, in vivo assay) and nitrate(NO^-₃) content of root and shoot and NO^-₃ and reduced nitrogencontent of xylem sap were measured in five temperate cerealssupplied with a range of NO^-₃ concentrations (0·1–20mol m^–3) and three temperate pasture grasses suppliedwith 0·5 or 5 0 mol m^–3 NO^-₃ For one cereal (Hordeumvulgare L ), in vitro NRA was also determined The effect ofexternal NO^-₃ concentration on the partitioning of NO^-₃ assimilationbetween root and shoot was assessed All measurements indicatedthat the root was the major site of NO3 assimilation in Avenasatwa L, Hordeum vulgare L, Secale cereale L, Tnticum aestivumL and x Triticosecale Wittm supplied with 0·1 to 1·0mol m^–3 NO^-₃ and that for all cereals, shoot assimilationincreased in importance as applied NO^-₃ concentration increasedfrom 1.0 to 20 mol m^–3 At 5.0–20 mol m^–3 NO3,the data indicated that the shoot played an important if notmajor role in NO^-₃ assimilation in all cereals studied Measurementson Lolium multiflorum Lam and L perenne L indicated that theroot was the main site of NO^-₃ assimilation at 0.5 mol m^–3NO^-₃ but shoot assimilation was predominant at 5.0 mol m^–3NO^-₃ Both NRA distribution data and xylem sap analysis indicatedthat shoot assimilation was predominant in Dactylis glomerataL supplied with 0.5 or 5.0 mol m^–3 NO^-₃ Avena sativa L., oats, Hordeum vulgare L., barley, Secale cereale L., rye, x Triticosecale Wittm., triticale, Triticum aestivum L., wheat, Dactylis glomerata L., cocksfoot, Lolium multiflorum Lam., Italian ryegrass, Lolium perenne L., perennial ryegrass, nitrate, nitrate assimilation, nitrate reductase activity, xylem sap     

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     

Effects of nitrogen nutrition on salt-stressed Nicotiana tabacum var. Samsun in vitro plantlets

Tobacco shoots were grown in vitro for 35 d, in MS culture mediummodified to include various sources (nitrate-N, ammonium-N ora mixture) and levels (0–120 mM) of N, and in the presenceof 0–180 mM NaCI or iso-osmotic concentrations of mannitol.Growth of control plantlets was significantly inhibited whenNH₄⁺-N was the sole N source, and at high (120 mM) NO^–₃-N supply. Under conditions of salt stress (90 and 180 mM NaCI)growth was repressed, with roots being more severely affectedthan shoots. Salinity also inhibited root emergence in vitro.The only alleviation of the salt stress by nitrate nutritionobserved in this study was on shoot growth parameters of plantletsgrown on 60 mM NO^–₃-N and 90 mM NaCI. Although both weresignificantly inhibited by NaCI, nitrate reduc-tase activitywas more severely affected than nitrate uptake. When mannitolreplaced NaCI in the culture medium, similar Inhibition of growth,nutrient uptake and enzyme activity were recorded. These observations,together with the relatively low recorded values for Na⁺ andCI^– uptake, indicate that under in vitro salt stress conditionsthe negative effects of NaCI are primarily osmotic. Key words: Growth, nitrogen metabolism, osmotic stress, salinity     

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 Apparent Induction of Nitrate Uptake by Chara corallina Cells following Pretreatment with or without Nitrate and Chlorate

Nitrate provision has been found to regulate the capacity forChara corallina cells to take up nitrate. When nitrate was suppliedto N sufficient cells maximum nitrate uptake was reached after8 h. Prolonged treatment of the cells in the absence of N alsoresulted in the apparent ability of these cells to take up nitrate.Chlorate was found to substitute partially for nitrate in the‘induction’ step. The effects on nitrate reductionwere separated from those on nitrate uptake by experiments usingtungstate. Tungstate pretreatment had no effect on NO^–₃uptake ‘induced’ by N starvation, but inhibitedNO^–₃ uptake associated with NO^–₃ pretreatment. Chloridepretreatment similarly had no effect on NO^–₃ uptake ‘induced’by N deprivation, but inhibited NO^–₃ uptake followingNO^–₃ pretreatment. The data suggest that there are atleast two mechanisms responsible for the ‘induction’of nitrate uptake by Chara cells, one associated with NO^–₃reduction and ‘induced’ by CIO^–₃ or NO^–₃and one associated with N deprivation. Key words: Nitrate, Chlorate, Chara corallina, Induction     

Orthophosphate Relations of Root: NO-3 Effects on Orthophosphate Influx, Accumulation and Secretion into the Xylem

Lamaze, T., Sentenac, H. and Grignon, C. 1987. Orthophosphaterelations of root: NO^–₃effects on orthophosphate influx,accumulation and secretion into the xylem.—J. exp. Bot.38: 923–934. Orthophosphate (Pi) accumulation by barley (Hordeum vulgareL.) roots was specifically inhibited by NO^–₃ as comparedto Cl^– and SO₄^{2 –}, and Pi secretion into the xylemwas stimulated. The inhibition of Pi accumulation by NO^–₃was also observed in roots of intact photosynthesizing horsebean(Vicia faba L.), rice (Oryza sativa L.) and soybean (Glycinemax L.) plants. NO^–₃ effects on Pi transport by rootswere more thoroughly investigated with corn (Zea mays L.). Theywere due to intracellular NO^–₃. Pi secretion was stillstimulated by NO^–₃ after Pi withdrawal from the absorptionsolution. ³²Pi influx decreased during Pi accumulation, supportingthe hypothesis that this ion allosterically regulated its owntransport system by feedback control. This control was modulatedby other anions: the decrease was more pronounced in the presenceof nitrate. Chronologically, the depressive effect of NO^–₃on ³²Pi influx appeared after the inhibition of Pi accumulation.Furthermore, under conditions where Pi accumulation was notaffected by NO^–₃, ³²Pi influx and Pi secretion into thexylem became insensitive to the presence of nitrate. Our hypothesisis that the stimulative effect of NO^–₃ on Pi secretionand the depressive one on ³²Pi influx are the repercussionsof an increase in the Pi cytosolic concentration due to an NO^–₃-induced decrease in Pi uptake by the vacuoles. Key words: Root, orthophosphate fluxes, orthophosphate accumulation, nitrate, ionic interaction     

Tracing uptake and assimilation of NO2 in spruce needles with 13N

For the first time, spruce shoots (Picea abies [L.] Karst.)were fumigated in vivo with ¹³N-labelled NO₂ with the aim ofelucidating the mechanism of NO₂^– trapping in the apoplastof the substomatal cavity. Uptake by the needles could be monitoredon-line, and a quantitative analysis of the activity recordsdelivered a deposition velocity in agreement with the commondry deposition estimates and ruled out rapid export processes.A fast extraction procedure was applied which revealed thatNO₂ did not produce any detectable traces of nitrite. In needlesin which the enzymes of nitrate reduction were not induced byprior fumigation with NO₂, incorporation of NO₂ was partiallyinhibited as compared to the fully induced shoots which tookup and assimilated NO₂ as expected from a constant influx. Theonly labelled inorganic species found in the extracts was nitrate(60%), whereas the rest of the label (40%) was assimilated organicnitrogen.A quantitative analysis of the data shows that thereaction of NO₂ in the apoplast yields at least three timesmore nitrate than nitrite, so that the existing models aboutthe apoplastic trapping reaction, disproportionation or antioxidantscavenging, which both postulate substantial production of nitrite,have to be reconsidered. Key words: ¹³N, nitrogen dioxide, spruce, air pollutants, deposition     

Nitrate Stimulation of Mobilization of Seed Reserves in Temperate Cereals: Importance of Water Uptake

Relationships between nitrate (NO^-₃) supply, uptake and assimilation,water uptake and the rate of mobilization of seed reserves wereexamined for the five main temperate cereals prior to emergencefrom the substrate. For all species, 21 d after sowing (DAS),residual seed dry weight (d.wt) decreased while shoot plus rootd.wt increased (15–30%) with increased applied NO^-₃concentrationfrom 0 to 5–20 mM . Nitrogen (N) uptake and assimilationwere as great with addition of 5 mM ammonium (NH⁺₄) or 5 mMNO^-₃but NH⁺₄did not affect the rate of mobilization of seedreserves. Chloride (Cl^-) was similar to NO^-₃in its effect onmobilization of seed reserves of barley (Hordeum vulgare L.).Increased rate of mobilization of seed reserves with additionalNO^-₃or Cl^-was associated with increases in shoot, root and residualseed anion content, total seedling water and residual seed watercontent (% water) 21 DAS. Addition of NH⁺₄did not affect totalseedling water or residual seed water content. For barley suppliedwith different concentrations of NO^-₃or mannitol, the rate ofmobilization of seed reserves was positively correlated (r >0.95)with total seedling water and residual seed water content. Therate of mobilization of seed reserves of barley was greaterfor high N content seed than for low N content seed. Seed watercontent was greater for high N seed than for low N seed, 2 DAS.Additional NO^-₃did not affect total seedling water or residualseed water content until 10–14 DAS. The effects of seedN and NO^-₃on mobilization of seed reserves were detected 10and 14 DAS, respectively. It is proposed that the increasedrate of mobilization of seed reserves of temperate cereals withadditional NO^-₃is due to increased water uptake by the seedlingwhile the seed N effect is due to increased water uptake bythe seed directly. Avena sativa L.; oat; Hordeum vulgare L.; barley; Secale cereale L.; rye; xTriticosecale Wittm.; triticale; Triticum aestivum L.; wheat; nitrate; seed; germination; seed reserve mobilization     

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     

A comparison of net and gross rates of oxygen production as a function of light intensity in some natural plankton populations and in a Synechococcus culture

In vitrorates of gross and net oxygen production were measuredas a function of light intensity in some plankton communitiescollected from Bedford Basin, Nova Scotia, and in a monoclonalculture of Synechococcus. The rate of gross oxygen productionwas measured by a technique in which the stable oxygen isotope,¹⁸O, serves as a photosynthetic tracer Net oxygen productionwas measured by automated Winkler technique. The rate of communityrespiration in the light was then determined by the differencebetween gross and net rates of oxygen production. In the naturalpopulations examined, neither gross nor net oxygen productionrates were significantly inhibited at the highest light intensitymeasured (500–800 µE m^–2 s^–1) In a samplein which the dark respiration rate was small relative to themaximal rate of production [P_max;sensu Platt et al (1980) JMar. Res., 38, 687–701] the rates of ‘light’respiration were 3 times greater. In two other communities,with high rates of dark respiration relative to P_maxthe ratesof ‘light’ respiration were closer to rates of darkrespiration. In the Synechococcus clone, both gross and netoxygen production rates were inhibited at high light intensities.Rates of ‘light’ respiration were found to varyas a function of light intensity. The greatest rates of respirationwere measured in samples incubated at light intensities thatwere just saturating (100 µE m^–2 s^–1). Therates of ¹⁴C production were also measured as a function oflight intensity The photosynthetic quotients, based on ¹⁴C productionrates and gross oxygen production rates, average 1 9     

Glucose and Pyruvate Metabolism in Daucus carota Cells: The Effect of the Ammonium Ion

In Daucus carota cells cultivated in vitro, the ammonium ionstimulates the incorporation of radioactivity from labelledglucose and labelled pyruvate into CO₂ and into the residueinsoluble in 60 per cent (v/v) ethanol. There is a higher ¹⁴CO₂production from [6-¹⁴C₂] glucose than from [6-¹⁴C] glucose.These results suggest a possible stimulation of glycolysis bythe ammonium ion.     

Joint field experiments for comparisons of measuring methods of photosynthetic production

During the 1st GAP Workshop at Konstanz in April 1982 comparativemeasurements of phytoplankton primary production by severaltechniques were conducted simultaneously at an offshore stationin Lake Konstanz and an experimental algal pond. Suspended glassbottle exposure techniques using ¹⁴C and ¹³C uptake gave P_z(mg C m^–3 h^–1) values which varied considerablynear-surface, but estimates of areal rates for the euphoticzone P_cu(mg C m^–3 h^–1) which were reasonably close.In the lake, P_z, from a vertical tube exposure (with ¹⁴C uptake)was greater than rates derived for integrated bottle samples.The oxygen bottle method permitted a good estimate of compensationdepth, corresponding to in situ growth studies. There were difficultiesin direct comparison between O₂ and carbon methods. Correlationbetween them for P_z was good in the lake but poor in the pond,both for suspended bottle and vertical tube methods. This seriesdemonstrates that despite reasonable overall estimates, comparativelyminor methodological differences in experimental technique cancause large variation. ⁺ Coordinator of the group for comparative measurements of photosyntheticproduction at the GAP Workshop, Konstanz, April 1982. ^*This paper is the result of a study made at the Group for AquaticPrimary Productivity (GAP) First International Workshop heldat the Limnological Institute, University of Konstanz, in April1982.     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. II. A general review

The causes of interspecific differences in the µ-l relationshipare examined in the context of a mechanistic model which relatesµ to irradiance in terms of six factors:, k_c photosyntheticquotient (PQ), Chl a:C, respiration and excretion. The effectof cell size on the light saturated growth rate is also considered.It is shown that photosynthetic efficiency and PQ exhibit remarkablylittle interspecific variability, and average 0.024 ±0.005 µg C(µg Chl a)^–1 h^–1 (µEm^–2 s^–1)^–1 and 1.5 ± 0.2 mol 02 molC^–1 (when NO₃^– is the nitrogen source) respectively.Two useful relationships were derived: (i) between growth efficiency,_g and Chl a:C at µ. = 0; (ii) between the compensationintensity, I_c and the Chl a-specific maintenance respirationrate. Both relationships were independent of temperature anddaylength. Species best adapted to growth at low light werefound to exhibit high Chl a:C ratios and low maintenance respirationrates. As a group, diatoms were consistently the best adaptedfor growth at low irradiance. Chiorophytes, haptophytes, chrysophytesand cryptophytes were intermediate in their performance at lowirradiance. Dinoflagellates exhibited extreme diversity, withspecies spanning the spectrum from very good performance atlow irradiance to very poor. A new µ_max-cell carbon relationshipis given based on growth rates normalized to 15°C. Evidenceis presented to show that noise in this relationship can besignificantly reduced by using only carbon-specific growth ratesand using only data for species grown at the same daylength.     

f-Ratio and its relationship to ambient nitrate concentration in coastal waters

The relationship between thef-ratio [NO₃^– uptake/(NO₃^–+ NH₄⁺) uptake] and ambient nitrate concentration was evaluatedfor eight data sets from coastal waters. The f-ratio increasedasymptotically with increase in nitrate concentration in mostdata sets. However, the rate at which f-ratio increased at lownitrate concentration (slope = m) and the maximum attained f-ratio(f_max) varied among regions; the initial slope varied most withvalues ranging in excess of an order of magnitude. The datawere analyzed in relation to environmental factors and methodologicalconsiderations known to influence the f-ratio. Ambient ammoniumconcentration was important in accounting for regional differencesin the f versus NO₃^– relationship. A further analysisof the data, relating f-ratio to the ratio of NO₃^–/(NO₃^–+ NH₄⁺) concentrations yielded a much more regionally consistentand approximately linear relationship; slopes varied by lessthan a factor of two in the extreme cases. Inclusion of knownalternative (aside from NH₄⁺) sources of reduced-N (e.g. urea)and correction for methodological/computational errors (isotopedilution) systematically reduce f-ratio estimates. Other factors,e.g. reduced-N uptake by microheterotrophs, may systematicallyincrease the f-ratio.     

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     

Xylem exudation is related to nitrate assimilation pathway in detopped maize seedlings: use of nitrate reductase and glutamine synthetase inhibitors as tools

The xylem exudation of detopped 7-d-old seedlings of Zea maysL. doubled when KCI was present in the root medium comparedto seedlings maintained on water. It was further enhanced whenKCI was replaced by nitrogen compounds such as nitrate, ammoniumand glutamine. The role of the nitrate assimilation pathwayon the enhancement of xylem exudation rate was investigatedusing tungstate, an inhibitor of nitrate reductase (NR) activity,and phosphinothricin or methionine sulphoximine, inhibitorsof glutamine synthetase (GS) activity. The sap levels of NO₃^–,NH₄⁺, glutamine, and asparagine was used to ascertain the invivo inhibition of both enzymes. The tungstate effects werealso checked by measuring leaf in vitro NA activity and NR proteincontent. Xylem exudation rate of detopped seedlings fed withKNO₃ decreased when the nitrate assimilation pathway was blockedeither at the NR or at GS sites. This decrease was preventedwhen urea (acting as NH₄⁺ supply) was given simultaneously withtungstate. KNO₃ does not act directly on exudation, but throughthe involvement of NH₄⁺. The involvement of glutamine was alsoshown since GS inhibition resulted in a cancellation of theenhancing effect of KNO₃ on exudation. As change of exudationrate was not linked to change in sap osmolarity, it is assumedthat the assimilation chain could modify root water conductance.The role of glutamine was discussed. Key words: Exudation, maize, nitrate, conductance, NR, GS     

Nitrate reductase from Dunaliella tertiolecta, purification and properties

The purification and properties of a nitrate reductase fromthe green alga Dunaliella tertiolecta are described. The enzymeis soluble, with a molecular weight greater than 500,000 andhas Km values of 0.26, 0.18, 0.10 and 0.06 m for NO₃^–,NADH, NADPH and FADH₂ respectively. Even at the highest specificactivity obtained, (0.86 µmoles NO₃^– reduced min^–1mg protein^–1) the enzyme retains the capacity to acceptelectrons from both NADH and NADPH. Unlike other nitrate reductasesit does not appear to be able to use reduced viologens as electrondonors. Its other properties are consistent with its being amolybdoflavoprotein of high molecular weight, which is alsoable to function as a cytochrome C reductase. ¹ Supported in part by the National Research Council of Canada. (Received June 18, 1972; )     

Metabolism of Glycollate by Lemna minor L. Grown on Nitrate or Ammonium as Nitrogen Source

Marques, I. A., Oberholzer, M. J. and Erismann, K. H. 1985.Metabolism of glycollate by Lemna minor L. grown on nitrateor ammonium as nitrogen source.—J. exp. Bot. 36: 1685–1697. Duckweed, Lemna minor L., grown on inorganic nutrient solutionscontaining either NH₄⁺ or NO₃^– as nitrogen source wasallowed to assimilate [1-¹⁴C]- or [2-¹⁴C]glycollate during a20 min period in darkness or in light. The incorporation ofradioactivity into water-soluble metabolites, the insolublefraction, and into the CO₂ released was measured. In additionthe extractable activity of phosphoenolpyruvate carboxylasewas determined. During the metabolism of [2-¹⁴C]glycollate in darkness, as wellas in the light, NH₄⁺ grown plants evolved more ¹⁴CO₂ than NO₃^–grown plants. Formate was labelled only from [2-¹⁴C]glycollateand in NH₄⁺ grown plants it was significantly less labelledin light than in darkness. In NO₃^– grown plants formateshowed similar radioactivity after dark and light labelling.The radioactivity in glycine was little influenced by the nitrogensource. Amounts of radioactivity in serine implied that thefurther metabolism of serine was reduced in darkness comparedwith its metabolism in the light under both nitrogen regimes.In illuminated NH₄⁺ plants, serine was labelled through a pathwaystarting from phosphoglycerate. After [1-¹⁴C]glycollate feedingNH₄⁺ grown plants contained markedly more radioactive aspartateand malate than NO₃^– plants indicating a stimulated phosphoenolpyruvatecarboxylation in plants grown on NH₄⁺. Key words: Photorespiration, glycollate, nitrogen, Lemna