Phagotrophy and NH4+ regeneration in a three-member microbial food loop

In a series of batch experiments we compared the efficiencyof nitrogen regeneration of a two- and three-member microbialfood loop consisting of a mixed bacterial assemblage, a small(3–5 µm) heterotrophic flagellate (Paraphysomonassp.), and a large (7–12 µm) heterotrophic flagellate(Paraphysomonas imperforata). In the two-member system the nitrogenregeneration efficiency for NH₄⁺ (R_n) was 41% and the grossgrowth efficiency (GGE) was 57% during active grazing by thesmall flagellate on bacteria. Regeneration of NH₄⁺ continuedduring the stationary phase so that R_n was 75% after 6 daysincubation. When the larger flagellate was introduced at theend of exponential growth of the smaller grazer in the three-membersystem, initially there was rapid regrowth of bacteria, tyingup 15% of the nitrogen originally in the bacteria. The largerflagellate grazed the smaller one with a GGE of 55%. Total nitrogenregeneration efficiency through exponential growth of the largerflagellate was 73%. Because microbial food loops in naturalwaters are far more complicated and with more grazing stepsthan portrayed in this study, we would expect the bulk of nutrientswithin these systems to be recycled with little transfer tohigher trophic levels.     

Size structures of microplankton biomass and production in Jiaozhou Bay, China

Seasonal investigations of size-fractionated biomass and productionwere carried out from February 1992 to May 1993 in JiaozhouBay, China. Microplankton assemblages were separated into threefractions: pico- (0.7–2 µm), nano- (2–20 µm)and netplankton (20–200 µm). The biomass was measuredas chlorophyll a (Chi a), paniculate organic carbon (POC) andparticipate organic nitrogen (PON). The production was determinedby ¹⁴C and ¹⁵N tracer techniques. The seasonal patterns in biomass,though variable, were characterized by higher values in springand lower values in autumn and summer (for Chi a only). Theseasonal patterns in production, on the other hand, were moreclear with higher values occurring in summer and spring, andlower values occurring in autumn and winter. Averaged over thewhole study period, the respective proportions of total biomassaccounted for by net-, nano- and picoplankton were 26, 45 and29% for Chi a, 32, 33 and 35% for POC, and 26, 32 and 42% forPON. The contributions to total primary production by net-,nano- and picoplankton were 31, 35 and 34%, respectively. Therespective proportions of total NH₄⁺–N uptake accountedfor by net-, nano- and picoplankton were 28, 33 and 39% in thedaytime, and 10, 29 and 61% at night. The respective contributionsto total NO₃^–-N uptake by net-, nano- and picoplanktonwere 37, 40 and 23% in the daytime, and 13, 23 and 64% at night.Some comprehensive ratios, including C/N biomass ratio, Chla/C ratio, C uptake/Chl a ratio, C:N uptake ratio and the f-ratio,were also calculated size separately, and their biological andecological meanings are discussed.     

Summer nutrient dynamics of the Middle Atlantic Bight: nitrogen uptake and regeneration

Nitrate and ammonium uptake and ammonium regeneration rates(by zooplankton, microplankton and benthos) were measured onthe Atlantic continental shelf (Middle Atlantic Bight) duringsummer, 1980. Euphotic zone profiles of NO₃^– and NH₄⁺uptake rates were similar in magnitude and vertical structureover a large geographical area. Microplankton NH₄⁺ regenerationrates, although measured less frequently, also showed a relativelyconsistent vertical structure; rates were positively correlatedwith uptake rates. Nitrate assimilation (‘new’ production)was used to estimate vertical eddy diffusivity and paniculatesinking rates. Eddy diffusion estimates ranged from <0.1to >2.0 cm² s^–1 and were positively related to arealprimary production. Estimated particulate sinking rates averaged5 mg at Nm^–2d^–1 and compared favorably with sedimentationrates measured from free-floating and moored sediment traps.Benthic nitrogen regeneration rates represented <10% of thispaniculate nitrogen flux. Within the mixed layer, NH₄⁺ assimilation(‘regenerated’ production) represented 50–80%of the total (NO₃^– + NH₄⁺ ) nitrogen productivity and33% for the euphotic zone. Of this, 30% was attributed to zooplankton,63% to microplankton (<100 µm) and 7% to benthos. Onthe average, 74% of the microplankton NH₄⁺ regeneration wasassociated with organisms passing 1 µm filters.     

Ammonium and nitrate uptake by soybean during recovery from nitrogen deprivation

Soybean [Glycine max (L.) Merrill] plants that had been subjectedto 15 d of nitrogen deprivation were resupplied for 10 d with1.0 mol m^–3 nitrogen provided as NO₃^–, NH₄⁺, orNH₄⁺+NO₃^– in flowing hydroponic culture. Plants in a fourthhydroponic system received 1.0 mol m^–3 NO₃^– duringboth stress and resupply periods. Concentrations of solublecarbohydrates and organic acids in roots increased 210 and 370%,respectively, during stress. For the first day of resupply,however, specific uptake rates of nitrogen, determined by ionchromatography as depletion from solution, were lower for stressedthan for non-stressed plants by 43% for NO₃^- resupply, by 32%for NH₄⁺ + NO₃^– resupply, and 86% for NH₄⁺ resupply. Whenspecific uptake of nitrogen for stressed plants recovered torates for non-stressed plants at 6 to 8 d after nitrogen resupply,carbohydrates and organic acids in their roots had declinedto concentrations lower than those of non-stressed plants. Recoveryof nitrogen uptake capacity of roots thus does not appear tobe regulated simply by the content of soluble carbon compoundswithin roots. Solution concentrations of NH₄⁺ and NO₃^– were monitoredat 62.5 min intervals during the first 3 d of resupply. Intermittent‘hourly’ intervals of net influx and net effluxoccurred. Rates of uptake during influx intervals were greaterfor the NH₄⁺ -resupplied than for the NO₃^– -resuppliedplants. For NH₄⁺ -resupplied plants, however, the hourly intervalsof efflux were more numerous than for NO₃^– -resuppliedplants. It thus is possible that, instead of repressing NH₄⁺influx, increased accumulation of amino acids and NH₄⁺ in NH₄⁺-resupplled plants inhibited net uptake by stimulation of effluxof NH₄⁺ absorbed in excess of availability of carbon skeletonsfor assimilation. Entry of NH₄⁺ into root cytoplasm appearedto be less restricted than translocation of amino acids fromthe cytoplasm into the xylem. Key words: Ammonium, nitrate, nitrogen-nutrition, nitrogen-stress, soybean     

Inorganic nitrogen uptake and regeneration in perennially icecovered Lakes Fryxell and Vanda, Antarctica

The isotope ¹⁵N was used to examine nitrogen dynamics in LakesFryxell and Vanda, two permanently ice-covered Antarctic lakes.Half-saturation constants for NH₄⁺. uptake in the shallow watersof both lakes were <10 µg N l^–1; uptake kineticexperiments on populations forming the deep-chlorophyll layersof these lakes showed zero-order kinetics and could not be fittedwith the Michaelis-Menten equation. Elevated uptake within thefirst few minutes following pulses of NH₄⁺. and NO₃^– occurredin both lakes. NH₄⁺ regeneration, determined from isotope dilutionexperiments, exceeded uptake at 4.6 m in Lake Fryxell, was lessthan uptake at 9 m in Lake Fryxell and was equal to uptake at10 m in Lake Vanda under the experimental conditions. NO₃^–uptake was suppressed by NH₄⁺ levels as low as 2 µg NH₄⁺-N l^–1 in Lake Fryxell; the suppression was strongestin the near-surface populations. Substrate-saturated C:N uptakeratios (g:g) in Lake Fryxell decreased from 8.4 near the surfaceto 1.8 at the bottom of the trophogenic zone. Overall, the nitrogendynamics in these lakes are similar to other lakes and the openocean in that biological productivity during the austral summeris supported by regenerated nutrients.     

Potassium-Ammonium Uptake Interactions in Tobacco Seedlings

Short-term (< 12 h) uptake experiments were conducted with6–7-week-old tobacco (Nicotiana tabacum L. cv. Ky 14)seedlings to determine absorption interactions between K⁺ andNH₄⁺. At equal solution concentrations (0.5 mol m^–3) netK⁺ uptake was inhibited 30–35% by NH₄⁺ and NH₄⁺ uptakewas decreased 9–24%. Removal of NH₄⁺ resulted in completerecovery in K⁺ uptake rate, but NH⁴⁺ uptake rate did not recoverwhen K⁺ was removed. In both cases, inhibition of the uptakerate of one cation saturated as the concentration of the othercation was increased up to 0.5 mol m^–3. The relative effectof K⁺-NH₄⁺ interactions was not altered when Cl- was replacedwith SO₄^2–, but the magnitudes of the uptake rates wereless in the absence of Cl-. The V_max for NH₄⁺ uptake was reducedfrom 128 to 105 µmol g^–1 dry wt. h^–1 in thepresence of 0.5 mol m^–3 K⁺ and the K_m for NH₄⁺ doubledfrom 12 to 27 mmol m^–3 in the presence of K⁺. The resultsof these K⁺-NH₄⁺ experiments are interpreted as mixed-noncompetitiveinteractions. However, an enhanced efflux of K⁺ coupled to NH₄⁺influx via an antiporter cannot be ruled out as contributingto the decrease in net K⁺ uptake. Key words: Nicotiana tabacum, K⁺, NH₄⁺, Uptake interactions     

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     

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.     

Phytoplankton uptake of ammonium and urea during growth on oxidized forms of nitrogen

Uptake capabilities for ammonium (NH₄⁺) and urea by diatoms(Thalassiosira pseudonana and Skeletonema costatum) growingon oxidized forms of nitrogen were studied in short-term uptakeexperiments. Even when nutrient-saturated, an enhanced uptakecapability not coupled with the growth rate was present forNH₄⁺ and urea. No such enhanced uptake ability was seen forNO₂^– or NO₃^– under either nutrient-saturated ornutrient-depleted conditions. The presence of NH₄⁺ decreasedthe enhanced ability to take up urea, but the urea uptake ratein 5 min incubations remained greater than the growth rate evenwhen NH₄⁺ was present.     

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.     

Differences in nitrate and ammonia uptake among components of a phytoplankton population

We examined the NO₃^– and NH₄⁺ uptake capabilities of thedinoflagellate Peridinium cinctum and of the accompanying nanoplanktonduring the spring P. cinctum bloom in Lake Kin-neret. Throughoutthe Peridinium season, the smaller algae had greater affinitiesand faster specific uptake rates for both NO₃^–and NH₄⁺It appears that P. cinctum cannot directly compete with nanoplanktonfor nitrogen nutrients. Other factors such as the ability ofdinoflagellates to swim freely in the water column and low grazingpressures may explain their dominance in the lake.     

Uptake and regeneration of inorganic nitrogen in coastal waters influenced by the Mississippi River spatial and seasonal variations

The Mississippi and Atchafalaya Rivers introduce large amountsof nutrients to surface waters of the northern Gulf of Mexico.This paper reports the most complete data to date on inorganicnitrogen uptake and regeneration in a broad range of coastalenvironments influenced by the river water, along with informationon nutrient concentrations and including pico-, nano-, and microplanktonspecies composition. Nitrate in surface waters is greatly reducednear the river plume, at salinities between 5 and 25 PSU, wherethe largest variance in uptake rates was observed, and was coincidentwith peaks in surface chlorophyll. Despite the depletion ofnitrate, nitrogen limitation was a rare event during the study,because of relatively high ammonium concentrations (>1 µmolNH₄⁺ I^–1 and regeneration rates. Two contrasting situationscharacterize the seasonal nitrogen dynamics in surface shelfwaters. High nitrate input during the spring caused a largebloom in which the cells were well adapted to use nitrate.Thedominant phytoplankton species were chain forming diatoms, alsoreported in sediment-trap studies in the area. Ammonium regenerationonly accounted for a small fraction of the nitrogen requirementsduring the bloom. In contrast, the low flow of river water duringsummer resulted in low nitrate concentrations in surface water.In this case phytoplankton productivity was highly reduced andmay depend greatly on ‘in sita’ ammonium regeneration.     

Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000

Microbial potential uptake and regeneration rates of ammonium(NH₄⁺) were studied along a salinity gradient (salinities 0.2–34.4)in the Mississippi River plume during an extreme drought inspring 2000. Chlorophyll concentrations up to 30 µg L^–1were highest in the low- and mid-salinity regions (salinities8.5–28.2) and comparable to records of other years butextended over smaller areas than during periods of normal riverflow. Bacterial biomass (5.1–28.3 µg C L^–1)was at the low end of the range observed in normal flow years,decreased with distance from the river mouth and did not peakwith chlorophyll. Heterotrophic nanoflagellate abundance (1.4–4.0µg C L^–1) did not reflect phytoplankton and bacterialspatial distribution but peaked at 9.2 µg C L^–1at salinity 8.5. Microbial NH₄⁺ regeneration rates were estimatedby ¹⁵NH₄⁺ isotope dilution experiments for the whole microbialcommunity, under light and dark conditions, and for the <2µm bacterium-dominated size fraction. Microbial NH₄⁺ regenerationrates (0.018–0.124 µmol N L^–1 h^–1) werelow relative to previous reports and peaked at salinity 28.Total NH₄⁺ regeneration rates were higher than those in the<2 µm size fraction at only four stations, suggestingthat bacterial mineralization was a significant component ofNH₄⁺ recycling in some parts of the river plume. Higher NH₄⁺regeneration in whole-water samples versus <2 µm fractionsprovided evidence for microbial grazing in regions where chlorophylland regeneration rates peaked and at two full-salinity stations.     

The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

Influx and Efflux of Nitrate and Ammonium in Italian Ryegrass and White Clover Roots: Comparisons Between Effects of Darkness and Defoliation

Seedlings of Italian ryegrass (Lolium multiflorum Lam. cv. RVP)and clonal stolon cuttings of white clover (Trifolium repensL. cv. Blanca) were grown for 19 d in flowing solution culture,with N supplied as either 250 mmol m^–3 NO₃^– or NH₃⁺.Rates of net uptake, influx and translocation of NO₃^–and NH₄⁺ were then determined using ¹⁵N and ¹³N labelling techniques:between 3–5 h into the photoperiod following 8 h darknessfor white clover (CL), and for ryegrass plants that were eitherentire (IL) or with shoots excised 90 min prior to ¹³N influx(IC); and 75 min into the photoperiod following 37–39h darkness for ryegrass (ID). Rates of net uptake, influx andefflux of NH₄⁺ exceeded those of NO₃^– in IL and IC ryegrassplants: the opposite occurred in white clover (CL). The decreasein net uptake following defoliation of ryegrass was greaterfor NH₄⁺ (62%) than NO₃^– (40%). For NH₄⁺ this was associatedwith a large decrease in influx from 110 to 6.0µmol h^–1g^–1 root fr. wt; but for NO₃^–, influx only decreasedfrom 42 to 37 µmol h^–1 g^–1. Prolonged exposureto darkness (ID plants) also lowered net uptake of NO₃^–and NH₄⁺ by, respectively, 86% and 95% of IL levels. For NH₄⁺this was characterized by a large decrease in influx and a smalldecrease in efflux; whilst for NO₃^– the effect of a largedecrease in influx was reinforced by a smaller increase in efflux. The data were used to estimate the translocatory fluxes of NO₃^–(03–20µmol h^–1 g^–1) and NH₄⁺ (003–0.4µmolh^–1 g^–1), assimilation in the roots of NO₃^–(02–26µmol h^–1 g^–1) and NH⁺⁴ (05–89 µmolh^–1 g^–1), and the concentrations of NO₃^– (9–15mol m^–3) in the cytoplasmic compartment of the roots.The relevance of variable influx and efflux to models for theregulation of N uptake is discussed. Key words: Lolium multiflorum, Trifolium repens, influx, efflux, nitrate, ammonium, ¹³N     

Influence of nitrate on uptake of ammonium by nitrogen-depleted soybean: is the effect located in roots or shoots?

In non-nodulated soybean [Glycine max (L.) Merrill cv. Ransom]plants that were subjected to 15 d of nitrogen deprivation inflowing hydroponic culture, concentrations of nitrogen declinedto 1.0 and 1.4mmol Ng^–1 dry weight in shoots and roots,respectively, and the concentration of soluble amino acids (determinedas primary amines) declined to 40µmol g^–1 dry weightin both shoots and roots. In one experiment, nitrogen was resuppliedfor 10 d to one set of nitrogen-depleted plants as 1.0 mol m^–3NH₄⁺ to the whole root system, to a second set as 0.5 mol m^–3NH₄⁺ plus 0.5 mol m^–3 NO₃^– to the whole root system,and to a third set as 1.0 mol m^–3 NH₄⁺ to one-half ofa split-root system and 1.0 mol m^–3 NO₃^– to theother half. In a second experiment, 1.0 mol m^–3 of nitrogenwas resupplied for 4 d to whole root systems in NH₄⁺ : NO₃^–ratios of 1:0, 9:1, and 1:1. Nutrient solutions were maintainedat pH 6.0. When NH₄⁺ was resupplied in combination with NO₃^– to thewhole root system in Experiment I, cumulative uptake of NH₄⁺for the 10 d of resupply was about twice as great as when NH₄⁺was resupplied alone. Also, about twice as much NH₄⁺ as NO₃^–was taken up when both ions were resupplied to the whole rootsystem. When NH₄⁺ and NO₃^– were resupplied to separatehalves of a split-root system, however, cumulative uptake ofNH₄⁺ was about half that of NO₃^–. The uptake of NH₄⁺,which is inhibited in nitrogen-depleted plants, thus is facilitatedby the presence of exogenous NO₃^–, and the stimulatingeffect of NO₃^– on uptake of NH₄⁺ appears to be confinedto processes within root tissues. In Experiment II, resupplyof nitrogen as both NH₄⁺ and NO₃^– in a ratio of either1:1 or 9:1 enhanced the uptake of NH₄⁺. The enhancement of NH₄⁺uptake was 1.8-fold greater when the NH₄⁺: NO₃^–-resupplyratio was 1:1 than when it was 9:1; however, only 1.3 timesas much NO₃^– was taken up by plants resupplied with the1 :1 exogenous ratio. The effect of NO₃^– on enhancementof uptake of NH₄⁺ apparently involves more than net uptake ofNO₃^– itself and perhaps entails an effect of NO₃^–uptake on maintenance of K⁺ availability within the plant. Theconcentration of K⁺ in plants declined slightly during nitrogendeprivation and continued to decline following resupply of nitrogen.The greatest decline in K⁺ concentration occurred when nitrogenwas resupplied as NH₄⁺ alone. It is proposed that decreasedavailability of K⁺ within the NH₄⁺-resup-plied plants inhibitedNH₄⁺ uptake through restricted transfer of amino acids fromthe root symplasm into the xylem. Key words: Ammonium, Glycine max, nitrate, nitrogen-nutrition, nitrogen stress, split-root cultures     

The Influence of Nitrate and Ammonium Nutrition on the Growth of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

The effects of NO^-₃ and NH⁺₄ nutrition on hydroponically grownwheat (Triticum aestivum L.) and maize (Zea mays L.) were assessedfrom measurements of growth, gas exchange and xylem sap nitrogencontents. Biomass accumulation and shoot moisture contents ofwheat and maize were lower with NH⁺₄ than with NO^-₃ nutrition.The shoot:root ratios of wheat plants were increased with NH⁺₄compared to NO^-₃ nutrition, while those of maize were unaffectedby the nitrogen source. Differences between NO^-₃ and NH⁺₄-fedplant biomasses were apparent soon after introduction of thenitrogen into the root medium of both wheat and maize, and thesedifferences were compounded during growth. Photosynthetic rates of 4 mM N-fed wheat were unaffected bythe form of nitrogen supplied whereas those of 12 mM NH⁺₄-fedwheat plants were reduced to 85% of those 12 mM NO^-₃-fed wheatplants. In maize supplied with 4 and 12 mM NH⁺₄ the photosyntheticrates were 87 and 82% respectively of those of NO^-₃-fed plants.Reduced photosynthetic rates of NH⁺₄ compared to NO^-₃-fed wheatand maize plants may thus partially explain reduced biomassaccumulation in plants supplied with NH⁺₄ compared to NO^-₃ nutrition.Differences in the partitioning of biomass between the shootsand roots of NO^-₃-and NH⁺₄-fed plants may also, however, arisefrom xylem translocation of carbon from the root to the shootin the form of amino compounds. The organic nitrogen contentof xylem sap was found to be considerably higher in NH⁺₄- thanin NO^-₃-fed plants. This may result in depletion of root carbohydrateresources through translocation of amino compounds to the shootin NH⁺₄-fed wheat plants. The concentration of carbon associatedwith organic nitrogen in the xylem sap of maize was considerablyhigher than that in wheat. This may indicate that the shootand root components of maize share a common carbon pool andthus differences induced by different forms of inorganic nitrogenare manifested as altered overall growth rather than changesin the shoot:root ratios.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize, nitrogen, growth, photosynthesis, amino acids, xylem     

The Effect of Root Temperature on Growth and Uptake of Ammonium and Nitrate by Brassica napus L. cv. Bien venu in Flowing Solution Culture: II. UPTAKE FROM SOLUTIONS CONTAINING NH4NO3

Macduff, J. H., Hopper, M. J. and Wild, A. 1987. The effectof root temperature on growth and uptake of ammonium and nitrateby Brassica napus L. CV. Bien venu in flowing solution culture.II. Uptake from solutions containing NH₄NO₃.—J. exp. Bot.38: 53–66 The effects of root temperature on uptake and assimilation ofNH₄⁺ and NO₃^– by oilseed rape (Brassica napus L. CV. Bienvenu) were examined. Plants were grown for 49 d in flowing nutrientsolution at pH 6?0 with root temperature decrementally reducedfrom 20?C to 5?C; and then exposed to different root temperatures(3, 5, 7, 9, 11, 13, 17 or 25?C) held constant for 14 d. Theair temperature was 20/15?C day/night and nitrogen was suppliedautomatically to maintain 10 mmol m^–3 NH₄NO₃ in solution.Total uptake of nitrogen over 14 d increased threefold between3–13?C but was constant above 13?C. Net uptake of NH₄⁺exceeded that of NO₃^– at all temperatures except 17?C,and represented 47–65% of the total uptake of nitrogen.Unit absorption rates of NH₄⁺ and of 1?5–2?7 for NO₃^–suggested that NO₃^– absorption was more sensitive thanNH₄⁺ absorption to temperature. Rates of absorption were relativelystable at 3?C and 5?C compared with those at 17?C and 25?C whichincreased sharply after 10 d. Tissue concentration of N in theshoot, expressed on a fresh weight basis, was independent ofroot temperature throughout, but doubled between 3–25?Cwhen expressed on a dry weight basis. The apparent proportionof net uptake of NO₃^– that was assimilated was inverselyrelated to root temperature. The results are used to examinethe relation between unit absorption rate adn shoot:root ratioin the context of short and long term responses to change ofroot temperature Key words: Brassica napus, oilseed rape, root temperature, nitrogen uptake     

Changes in the growth rates of saline-lake diatoms in response to variation in salinity, brine type and nitrogen form

The growth rates of four saline-lake diatom taxa were measuredunder varying conditions of salinity (5, 8 and 11), brine type(sulfate- versus bicarbonate-dominated) and nitrogen form (NH₄⁺versus NO₃^–), using a full factorial design. With NO₃^–as the nitrogen source, Cyclotella quillensis, Cymbella pusillaand Anomoeoneis costata exhibited lower growth rates in thesulfate versus bicarbonate media. The strain of Chaetoceroselmorei used in these experiments, isolated from a sulfate-dominatedlake, was unable to grow on NO₃^– alone. In the NH₄⁺ treatments,neither salinity nor brine type affected the growth rates ofC.quillensis or C.elmorei. When supplied with NH₄⁺, C.pusillaand A.costata had higher growth rates in the bicarbonate versussulfate media, although for C.pusilla the difference on NH₄⁺was not as great as on NO₃^–. The impact of brine typeon NO₃^– use is consistent with the theory that sulfateinhibits molybdate uptake, as molybdenum is required for NO₃^–use but not NH₄⁺. Cymbella pusilla was the only taxon affectedby changes in salinity. The four taxa used in these experimentsare frequently found in saline lakes and saline-lake sediments,hence they are used in paleoclimate reconstructions; the resultspresented here provide additional information that may enhancethese diatom-based reconstructions.     

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