Relationship between biomass and enzymatic activity of a bloom-forming dinoflagellate (Dinophyceae) in southern Chile (41{degrees} S): a field approach

In a coastal area of southern Chile (41° S), the major ammoniumassimilating enzyme glutamine synthetase (GS) was detected ina green dinoflagellate bloom during April 2003. High chlorophylla concentrations (1000 µg L^–1) attributable to Gymnodiniumcf. chlorophorum in surface waters were associated with highand very low nitrate reductase activities. Coincident with thebloom, dissolved inorganic nitrogen concentrations were nearthe detection limit (NO₃^– + NH₄⁺ <0.5 µM). SinceGS correlates with the use of ammonium as an external nitrogensource, we suggest that GS activity seems to be a good indicatorof ammonium utilization in a period dominated by a single dinoflagellatespecies.     

A Comparative Study of the Influence of Salt Type and Concentration on 14CO2 Fixation in Potato Slices at 25{degrees} C and 0{degrees} C

Dark fixation of ¹⁴CO₂ was followed in potato disks under varyingsalt treatments at 0° C and 25° C. It is shown thatthe specific activity of the ¹⁴CO₂ supplied is heavily dilutedby endogenously produced CO₂ and that the apparently greaterfixation of ¹⁴CO₂, at 0° C as compared with that at 25 °C is due to the lower respiration rate at 0° C, with consequentlyless dilution of the ¹⁴CO₂. supplied. At 25° C organic acidformation in response to different salt treatments fulfils thecommon expectation, ¹⁴CO₂ fixation increasing in the presenceof K₂SO₄ and decreasing in CaCl₂ relative to that in KCl. Therole of organic acids in maintaining ionic balance within thecell at 25° C is thereby indicated but at 0° C organicacid adjustments did not follow the normal pattern. At 25°C but not at o° C increasing external concentration of KCIresulted in an increased level of ¹⁴CO₂ fixation.     

Size-fractionated mesozooplankton biomass, metabolism and grazing along a 50{degrees}N-30{degrees}S transect of the Atlantic Ocean

Size-fractionated mesozooplankton grazing and metabolism wereinvestigated along the wide latitudinal range (50°N–30°S)covered during the Atlantic Meridional Transect (AMT) 11 cruise.Five different oceanic provinces were traversed in this cruise:North Atlantic Drift (NADR), North Atlantic Subtropical Gyral(NAST), Canary Coastal (CNRY), Eastern Tropical Atlantic (ETRA),and South Atlantic Gyral (SATL). CNRY and ETRA were affectedby the upwelling Mauritanian and equatorial respectively andprimary production in these provinces was higher than in theoligotrophic subtropical gyres (NAST and SATL). Both mesozooplanktonand phytoplankton biomass were highest around the equator. Theamount of chlorophyll a ingested daily by copepods was noticeablyhigher in mesotrophic than in oligotrophic provinces as shownby the spatial distribution of gut content values and the highabundances of copepods recorded at the equator. Grazing impactalong the transect ranged from 0.2 to 5.6% of the phytoplanktonstanding stock and from 1.6 to 14.5% of primary production.If only phytoplankton >2 µm are considered, the rangesare 1.0–19.4% (stock) and 3.4–44.7% (primary production).Grazing impact upon both phytoplankton biomass and primary productionfollowed a spatial distribution similar to that of chlorophylla ingestion, with higher values in upwelling zones than in thegyres. Weight-specific rates of respiration and NH₄⁺ and PO₄^3–excretion showed large variability both along the transect andwithin provinces, but did not differ between provinces. Therefore,zooplankton assemblages inhabiting the different provinces visitedin the AMT 11 seem to be adapted to the prevailing thermal conditions.Given the substantial proportion of nitrogen and phosphorusthat are supplied to primary producers through the excretoryactivity of mesozooplankton (the percentage of nitrogen andphosphorus requirements of phytoplankton accounted for by mesozooplanktonexcretion was >30% in all the provinces) it follows thatthey may play a crucial role as nutrient regenerators, especiallyin the oligotrophic gyres where regenerated production dominates.     

Nitrate Effects on Pre-emergence Growth and Emergence Percentage of Wheat (Triticum aestivum L.) from Different Sowing Depths

The effects of a range of applied nitrate (NO₃^–) concentrations(0–20 mol m3) on germination and emergence percentageof Triticum aestivum L. cv. Otane were examined at 30, 60, 90and 120 mm sowing depths. Germination percentage was not affectedby either sowing depth or applied NO₃^– concentration whereasemergence percentage decreased with increased sowing depth regardlessof applied NO₃^– concentration. Nitrate did not affectemergence percentage at 30 mm sowing depth, but at 60 to 120mm depth, emergence percentage decreased sharply with an increasedapplied NO₃^– concentration of 0 to 1·0 mol m^–3then decreased only slightly with further increases in appliedNO₃^– of about 5·0 mol m^–3. Root and shoot growth, NO₃^– accumulation and nitrate reductaseactivity (NRA) of plants supplied with 0, 1·0 and 1·0mol m^–3 NO₃^– at a sowing depth of 60 mm were measuredprior to emergence. The coleoptile of all seedlings opened withinthe substrate. Prior to emergence from the substrate, shootextension growth was unaffected by additional NO₃^– butshoot fr. wt. and dry wt. were both greater at 1·0 and1·0 mol m^–3 NO₃^– than with zero NO₃^–.Root dry wt. was unaffected by NO₃^–. Nitrate concentrationand NRA in root and shoot were always low without NO₃^–.At 1·0 and 10 mol m3 NO₃^–, NO₃^– accumulatedin the root and shoot to concentrations substantially greaterthan that applied and caused the induction of NRA. Regardlessof the applied NO₃^– concentration, seedlings which failedto emerge still had substantial seed reserves one month afterplanting. Coleoptile length was substantially less for seedlingswhich did not emerge than for seedlings which emerged, but wasnot affected by NO₃^–. It is proposed that (a) decreasedemergence percentage with increased sowing depth was due tothe emergence of leaf I from the coleoptile within the substrateand (b) decreased emergence percentage with additional NO₃^–was due to the increased expansion of leaf 1 within the substrateresulting in greater folding and damage of the leaf. Key words: Triticum aestivwn L., nitrate, sowing depth, seedling growth, seedling emergence     

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     

Ontogenetic vertical distribution and diel migration of the copepod Eucalanus inermis in the oxygen minimum zone off northern Chile (20-21{degrees} S)

The vertical and ontogenetic distribution, and diel verticalmigration (DVM), of Eucalanus inermis in relation to the strongvertical gradient in oxygen concentration associated with anintense oxygen minimum zone (OMZ) were studied at a coastalarea off northern Chile (20–21° S). A close relationshipbetween the abundance of the whole copepod population and lowoxygen waters was found, with most developmental stages remainingnear the base of the oxycline (30–80 m) and within theupper zone of the OMZ (30–200 m). All stages performedDVM but not at all the stations, mainly between the 30–60and 60–200 m strata; a small fraction (<20%) appearedin the surface layer (0–30 m) mostly at night. This strategyof movement would result in a better utilization of food resourcessince the strong physical and chemical gradients at the baseof the oxycline and upper OMZ boundary might serve as a siteof particle accumulation. A secondary fluorescence peak was,in fact, found at all the stations, coinciding with minimaldissolved oxygen (DO, <1 mL O₂ L^–1) at the base ofthe oxycline or in the upper OMZ boundary. The relevance ofthe biogeochemical flux involved in this diel migration patternwas assessed by calculating the potential active input of carbonand nitrogen from the upper layers into deeper the OMZ.     

Distribution of spring phytoplankton (mainly diatoms) in the upper 50 m of the Southwestern Atlantic Ocean (30-61{degrees}S)

This is the first study on diatom spatial patterns in relationto major oceanographic features along a megascale transect inthe Southwestern Atlantic Ocean and provides a comparison withdiatom distribution in surface sediments. Absolute abundancesof diatoms, silicoflagellates and dinoflagellates (>10-µmfraction) were assessed in 80 bottle samples from 5 to 50 m,retrieved in November 1993 at 20 stations (30–61°S)along 53°W. Siliceous phytoplankton were scarce in the northernhalf of the transect and in the south of 57°S (100–150cells L^–1), with a strong peak in the vicinity of thePolar Front (200 000 cells L^–1), whereas dinoflagellateswere more abundant at the northern stations (up to 24 000 cellsL^–1). In the south of 50°S phytoplanktonic cell densitieswere loosely (but significantly, r = 0.54, P < 0.01) associatedwith chlorophyll a, whereas in the north of this latitude, thisrelationship disappeared (r = 0.018, P > 0.1). In total,191 diatoms and 4 silicoflagellates were recorded. Changes indiatom assemblage compositions along the transect allowed identificationof five discrete areas: Subtropical (29°S), Northern Transitional(34–41°S), Southern Transitional (43–48°S),Subantarctic (49–54°S) and Antarctic (55–59°S),each characterized by a set of typical species. Diversity changedlittle with latitude, but numbers of species were higher inthe north of 40°S. Comparison of diatom assemblage makeupin the plankton and in the surface sediments shows very strongdisagreements, whereby cold water species are very significantlyover-represented in the sedimentary record, suggesting enhancedpreservation and strong subsurface equatorward advection ofthe cold water taxa.     

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     

Influence of the Form of Nitrogen Supply on Root Uptake and Translocation of Cations in the Xylem Exudate of Maize (Zea mays L)

Concentrations of inorganic cations are often lower in plantssupplied with NH₄⁺ as compared with NO₃^–. To examine whetherthis is attributable to impaired root uptake of cations or lowerinternal demand, the rates of uptake and translocation of K,Mg, and Ca were compared in maize plants (Zea mays L.) withdifferent growth-related nutrient demands. Plants were grownin nutrient solution with either 1·0 mol m^–3 NO₃^–or NH₄⁺ and the shoot growth rate per unit weight of roots wasmodified by varying the temperature of the shoot base (SBT)including the apical shoot meristem. The shoot growth rate per unit weight of roots, which was takenas the parameter for the nutrient demand imposed on the rootsystem, was markedly lower at 12°C than at 24°C SBT.As a consequence of the lower nutrient demand at 12°C SBT,uptake rates of NO₃^– and NH₄⁺ declined by more than 50%Compared with NO₃^– supply, NH₄⁺ nutrition depressed theconcentrations of K and particularly of Ca in the shoot, bothin plants with high and with low nutrient demand. This indicatesa control of cation concentration by internal demand ratherthan by uptake capacity of the roots. Translocation rates of K, Mg and Ca in the xylem exudate werelower in NH₄⁺- than in NO₃^–-fed plants. Net accumulationrates of Ca in the shoot were also decreased, whereas net accumulationrates of K in the shoot were even higher in NH₄⁺-fed plants.It is concluded that reduced cation concentrations in the xylemsap of plants supplied with NH₄⁺ are due to the lower demandof cations for charge balance. The lower K translocation tothe shoot is compensated by reduced retranslocation to the roots.For Ca, in contrast, decreased translocation rates in NH₄⁺-fedplants result in lower shoot concentration. Key words: Nitrogen form, cation nutrition, charge balance, xylem exudate, recirculation     

Effect of Photoperiod and Irradiance Changes Upon Development of Freezing Tolerance and Accumulation of Soluble Carbohydrate in Seedlings of Lolium perenne Grown at 2 {degrees}C

Two contrasting cultivars of Lolium perenne were exposed toa range of daily radiation integrals during hardening at 2°Cfor 15 d. The maximum induced freezing tolerance measured asLT₅₀ (temperature for 50 % kill) differed markedly between thecultivars. The observed LT₅₀ values were unaffected by changesin the radiation integral above 10 mol m^–2 d^–1,whereas accumulation of water-soluble carbohydrate showed astrong positive correlation with the radiation integral overthe entire range of the experiment. The correlation betweenLT₅₀ and soluble carbohydrate content at the end of the hardeningperiod was poor and showed no obvious connection with genotype.Fructan polymers and sucrose were the major components of thesoluble carbohydrates in both cultivars. The depression of freezingpoint attributable to the accumulation of soluble, osmoticallyactive carbohydrate was not sufficient to account for the observedchanges in LT₅₀ in the hardy genotype. These results are discussedin relation to the interactions between growth, photosynthesisand assimilate partitioning during hardening. Lolium perenne, hardening, freezing tolerance, irradiance, carbohydrate, fructan     

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     

The Effects of Temperature and Form of Nitrogen Supply on the Relative Contribution of Root Uptake and Remobilization in Supplying Nitrogen for Laminae Regrowth of Lolium perenne L.

Plants of Lolium perenne L. cv. S23 were grown in sand culturesupplied with either ammonium (NH₄⁺) or nitrate (NO₃^–)in an otherwise complete nutrient solution at 12°C or 20°C.Three weeks after germination, plants were clipped weekly tosimulate grazing. After 10 weeks growth all nitrogen (N) wassupplied enriched with ¹⁵N to quantify the effects of form ofN supply and temperature on the relative ability of currentroot uptake and remobilization to supply N for laminae regrowth. The form of N supply had no effect on the dry matter partitioning,while at 20°C more dry weight was allocated to laminae regrowthand less to the remaining plant material. The current root uptakeof N, which subsequently appeared in the laminae regrowth, wassimilar for plants supplied with NH₄⁺ or NO₃^–, and bothwere equally reduced at the lower temperature of growth. Remobilizationof N to laminae regrowth was greater for plants receiving NH₄⁺than NO₃^–; remobilization with either form of N supplywas reduced at the lower temperature of growth. Remobilizationwas reduced to a lesser extent at 12°C than current rootuptake. It was concluded that remobilization became relativelymore important in supplying N for regrowth of laminae at lowertemperatures. Key words: Lolium perenne, ammonium, nitrate, temperature, remobilization     

Function and contribution of the root tip in the induction of NO3- uptake along the barley root axis

The seminal roots of N-free-grown barley seedlings were ableto take up NO₃^– immediately upon initial exposure; theuptake rate in the tip was half of that in the older root zones(middle and base). A lag of 60 min was required in all rootzones before the uptake rates started to increase during inductionwith external NO₃^–. This increase could be prevented bythe addition of pFPA; we thus assume that additional NO₃^–transport proteins were synthesized during NO₃^– induction.During the time-course of NO₃^– induction different uptakerates were measured in morphologically different regions ofthe tip (1 mm segments) indicating a regulation of NO₃^–induction on a narrow local scale. In NO₃^– grown plants, NO₃^– uptake as well as NO₃^–content increased basipetally along the root axis concomitantlywith increasing vacuolization of the cells. Although NO₃^–uptake into the tip was only half of that into the older rootzones, this NO₃^– uptake was very important for the entireroot. Firstly, it provided the substrate for protein biosynthesisin the meristematic region: nitrate reductase activity and totalsoluble protein were highest in the first apical mm of the tip.Secondly, 3% of the NO₃^– taken up by the tip was foundin the base where it induced NO₃^– uptake: NO₃^– wastranslocated almost exclusively basipetally and as little as20nmolg^–1 root fr. wt. translocated from the tip weresufficient for acceleration of NO₃^– induction in the rootbase of N-free-grown plants. This clearly shows that the inductionof NO₃^– uptake does not depend exclusively on the availabilityof external NO₃^–, but can be mediated also with internallytranslocated NO₃^–.The root tip, therefore, may be consideredthe NO₃^– sensing region of the root. Key words: Barley, Hordeum vulgare L, internal NO₃^–, NO₃^– uptake, root zones     

The role of faecal material in the particulate organic carbon flux in the northern Humboldt Current, Chile (23{degrees}S), before and during the 1997-1998 El Nino

Sedimentation rates of faecal material, phytoplankton and microzooplanktonand production rates of faecal material from crustaceans andpelagic tunicates were estimated during the austral summer andwinter 1997, and summer 1998, in the northern Humboldt Current(23°S, off Antofagasta, Chile). Sampling periods coveredpre-El Niño (January 1997) and El Niño 1997–98(July 1997 and January 1998). Samples were collected using floatingsediment traps deployed at 65, 100, 200 and 300 m depth in oceanicand coastal areas. Sedimentation rates during January 1997 were,on average, 152 ± 23 and 85 ± 57 mg C m^–2day^–1 at 65 and 300 m depth, respectively. During July,these rates averaged 93 ± 56 mg C m^–2 day^–1at 65 m depth and 35 ± 12 mg C m^–2 day^–1at 300 m depth, while in January 1998 they were 98 and 109 ±37 mg C m^–2 day^–1 at 65 and 200 m depth, respectively.Recognizable faecal material made up the bulk of the sedimentingmatter, accounting for 8 ± 5% (n = 14), 31 ± 26%(n = 16) and 8 ± 5% (n = 5) of the average total organiccarbon recorded from all sediment trap samples collected duringJanuary and July 1997 and January 1998, respectively. However,at300 m depth, the contribution of recognizable faecal materialto total sedimented organic carbon increased to 43 ±33% (n = 4) during July 1997. The remaining sedimenting particlesconsisted mainly of tintinnids, crustacean exuviae, heterotrophicdinoflagellates (both thecated and athecated) and diatom cells.During this study, we estimated that only a minor fraction (average± SD = 5 ± 8%) of the copepod faecal materialproduced within the photic zone sedimented down to 300 m depth,suggesting an efficient recycling within the overlaying watercolumn. On the other hand, an important fraction (47 ±30%) of the euphausiid faecal strings was collected in the 300m depth trap, suggesting that this material would enhance thedownward flux of particulate organic matter (POC). POC fluxesto 65 and 300 m depth traps were in the range of 4–20%and 3–8% of the estimated primary production during thewhole study period. It is postulated that the overall verticalflux of particulates and, in particular, faecal pellets wasdetermined by a combination of three factors. The first wasthe composition of the zooplankton assemblages in the studyarea. When the dominant group was calanoid copepods, their faecesseemed to contribute poorly to the vertical flux of particulates.On the other hand, when the dominant group was euphausiids,a significant proportion of their faecal material was collectedin the sediment trap located at 300 m depth. The second wasthe relatively high abundance of cyclopoid copepods from thegenera Oncaea, Corycaeus and Oithona, which are reported tofeed on aggregates of phytodetritus and faecal pellets producedby calanoid copepods, suggesting that they may act as a naturalfilter to sedimenting particulates. The third was the compositionand size spectrum of the phyto- and microzooplankton assemblageswhich are potential food sources for the meso- and macrozooplankton.These factors were partially modulated by both the 1997–1998El Niño and seasonality.     

Plant Growth in Relation to the Supply and Uptake of NO3-: A Comparison Between Relative Addition Rate and External Concentration as Driving Variables

Two approaches to quantifying relationships between nutrientsupply and plant growth were compared with respect to growth,partitioning, uptake and assimilation of NO₃^– by non-nodulatedpea (Pisum sativum L. cv. Marma). Plants grown in flowing solutionculture were supplied with NO₃^– at relative addition rates(RAR) of 0·03, 0·06, 0·12, and 0·18d^–1, or constant external concentrations ([NO₃^–)of 3, 10, 20, and 100 mmol m^–3 over 19 d. Following acclimation,relative growth rates (RGR)approached the corresponding RARbetween 0·03–0.12 d^-1, although growth was notlimited by N supply at RAR =0.18 d^-1. Growth rates showed littlechange with [NO₃–] between 10–100 mmol m^–3(RGR=0·15 –0·16 d^-1). The absence of growthlimitation over this range was suggested by high unit absorptionrates of NO₃^–, accumulation of NO₃^– in tissues andprogressive increases in shoot: root ratio. Rates of net uptakeof NO₃^– from 1 mol m^–3 solutions were assessed relativeto the growth-related requirement for NO₃^–, showing thatthe relative uptake capacity increased with RGR between 0·03–0·06d^–1 , but decreased thereafter to a theoretical minimumvalue at RGR     

Nitrate transport in intact wheat roots:II. Long-term effects of NO3- concentration in the nutrient solution on NO3- unidirectional fluxes and distribution within the tissues5

We have examined the long-term effects of NO₃^– concentrationson NO₃^– (¹⁵NO₃^–) fluxes and cellular pool sizesin roots of intact 30-d-old wheat (Triticum aestivum cv. Courtot)grown hydroponically. Compartmental analysis was performed understeady-state conditions at five different levels of NO₃^–concentration (from 0.1 up to 5 mol m^–3 taking into accountmetabolism and secretion into the xylem (Devienne et al., 1994).Nitrate and reduced nitrogen levels in the tissues were largelyindependent of external NO₃^– concentration although below1.5 mol m^–3 NO₃^–; concentration limited plant growth.In the chamber, marked diurnal variations in net uptake occurredand, in the light, higher NO₃^– concentrations yieldedhigher NO₃^– uptake rates. After transfer of the plantsto the laboratory, the increase in net uptake linked to elevationof NO₃^–; concentrations was even larger (from 0.1 to 8.8µmolh^–1 g^–1 FW) as a result of a marked increase (x10–11) in the unidirectional influx at the plasmalemmawhile NO₃^– efflux was less enhanced (x 4–5). Underthese conditions, influx into the vacuole was also higher (x2–4) while efflux from the vacuole was little affected(x 1–3). NO₃^– concentrations within the cell compartmentswere estimated under the clas sical assumptions. The vacuolarconcentration was a little modified by NO₃^– availabilitywhereas that in the cytosol increased from about 10 mol m^–3to about 20 mol m^–3 indicating that (1) the absolute valuefor the cytosol was high and (2) it displayed only a small increasedespite very large changes in NO₃^– fluxes. NO₃^–distribution within the cells did not seem to involve an activeaccumulation of NO₃^– in the vacuole. Key words: Wheat, ion transport, nitrate, ¹⁵N, compartmentation     

O2bullet production at 37{degrees}C plays a critical role in depressing tetanic force of isolated rat and mouse skeletal muscle

To find out whether the decrease in muscle performance of isolated mammalian skeletal muscle associated with the increase in temperature toward physiological levels is related to the increase in muscle superoxide (O₂^–) production, O₂^– released extracellularly by intact isolated rat and mouse extensor digitorum longus (EDL) muscles was measured at 22, 32, and 37°C in Krebs-Ringer solution, and tetanic force was measured in both preparations at 22 and 37°C under the same conditions. The rate of O₂^– production increased marginally when the temperature was increased from 22 to 32°C, but increased fivefold when the temperature was increased from 22 to 37°C in both rat and mouse preparations. This increase was accompanied by a marked decrease in tetanic force after 30 min incubation at 37°C in both rat and mouse EDL muscles. Tetanic force remained largely depressed after return to 22°C for up to 120 min. The specific maximum Ca²⁺-activated force measured in mechanically skinned fibers after the temperature treatment was markedly depressed in mouse fibers but was not significantly depressed in rat muscle fibers. The resting membrane and intracellular action potentials were, however, significantly affected by the temperature treatment in the rat fibers. The effects of the temperature treatment on tetanic force, maximum Ca²⁺-activated force, and membrane potential were largely prevented by 1 mM Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl), a membrane-permeable superoxide dismutase mimetic, indicating that the increased O₂^– production at physiological temperatures is largely responsible for the observed depression in tetanic force at 37°C by affecting the contractile apparatus and plasma membrane. intact mammalian muscle; physiological temperature; superoxide; excitation-contraction coupling; maximum Ca²⁺-activated force; muscle excitability; cytochrome c assay     

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     

Using neural networks to predict surface zooplankton biomass along a 50{degrees}N to 50{degrees}S transect of the Atlantic

Four Atlantic transects between the UK and the Falkland Islandswere carried out during spring and autumn as part of the AtlanticMeridional Transect (AMT) programme. These 50°N to 50°Stransects cross several ocean regions. An optical plankton counter(OPC-1L) sampled continuously along the transects from the ship'suncontaminated seawater supply, giving a surface distributionof zooplankton abundance and size. Measurements of underwayfluorescence-derived chlorophyll, sea surface temperature andsalinity were also taken from the uncontaminated seawater supply.The relationship between zooplankton biomass and these variableswas investigated using multiple linear regression and neuralnetwork techniques. In the analysis, log_e-transformed biomasswas used to reduce the influence of extreme values. Two transectswere used to develop the models, and two to test the generalizationcapabilities of the models. Multiple linear regression couldexplain up to 55% of the observed variation in the transformedbiovolume, and demonstrated the association of hydrographicvariables and diel migration within the surface zooplanktoncommunity. Neural networks, starting with the same set of variables,were able to explain up to 78% of the variability, showing anincreased performance over the multivariate analysis. An optimizedmodel accounted for 77% of the variance in the original data.However, it showed greater generalization capabilities (R² =0.47) when applied to new data sets than either the originalneural network model (R² = 0.31) or the multiple linear regressionmodel (R² = 0.34). This study highlights the non-linear natureof the parameters' associations with the zooplankton biomassand their variability between oceanographic regions.     

The Influence of Composition of the Basal Medium on the Growth and Morphogenesis of Cultured Sitka Spruce (Picea sitchensis) Tissues

In vitro culture of Picea sitchensis (Bong.) Carr. needle explantson a number of basal culture media indicated that a complexmixture of organic additives was neither essential nor stimulatory.Adventitious bud production occurred at inorganic nitrogen levelsof 15–60 x 10^–3M and 7.5–30 x 10^–3 Min the adventitious bud induction and elongation media, respectively,when a well-balanced ratio of NH₄⁺:NO₃^– was maintained.However, necrosis was increased at the highest level of inorganicnitrogen. Organogenesis was more sensitive to changes in theratio of NH₄⁺:NO₃^–. Complete replacement of NH₄⁺ withNO₃^– during the adventitious bud induction passage severelyinhibited organogenesis, indicating that a reduced form of nitrogenmay be essential for adventitious bud differentiation. Conversely,a high proportion of NH₄⁺ in either the adventitious bud inductionor elongation medium increased tissue necrosis and inhibitedbud induction, reflecting the potential toxicity of this ion.Explants from different individual trees were found to varyconsiderably in their morphogenetic responses, but NH₄⁺:NO₃^–ratios of 1:5 or 1:2 were o ptimal for all individual treeswhich developed adventitious buds from needle cultures. Picea sitchensis, Sitka spruce, tissue culture, nitrogen