长江三角洲地区雨水中NH4+-N/NO3--N和δ15NH4+值的变化

2003年6月至2005年7月,利用自行设计的雨水收集器对位于长江三角洲地区的常熟、南京和杭州3个观测点进行了全年性雨水观测,分析了雨水中NH₄⁺-N/NO₃^--N和铵态氮自然丰度(δ¹⁵NH₄⁺)值的变化.结果表明：研究区3个观测点雨水中NH₄⁺-N/NO₃^--N和δ¹⁵NH₄⁺值均呈相似的季节性变化规律,两者的规律性变化在以田间农事耕作为主的常熟观测点尤其明显,而位于市区的南京观测点和位于城乡结合部的杭州观测点的规律性次之;雨水中NH₄⁺-N/NO₃^--N的峰值出现在6月下旬到8月上旬,然后逐渐下降,冬季降到最低;雨水中δ¹⁵NH₄⁺值在6月下旬到8月中旬为负值,在8月下旬到11月中下旬为正值,12月至翌年3月又变为负值,5至6月中旬又转变为正值.雨水中NH₄⁺-N/NO₃^--N和δ¹⁵NH₄⁺值的季节变化与不同作物生育期间氮肥的施用、当地气候的季节性变化以及其他NH3释放源的NH3挥发有关（人和动物排泄物、氮污染水体及有机氮源中的氨挥发）,其对大气湿沉降中NH₄⁺的来源、形态组成及陆地不同NH₃排放源的强度具有明显的指示意义.     

氮肥施用时期及基追比例对土壤矿质氮含量时空变化及小麦产量和品质的影响

采用田间试验方法,研究了不同氮肥施用时期和基追比例对土壤硝态氮和铵态氮含量变化及小麦产量和品质的影响.结果表明：土壤硝态氮和铵态氮含量随着土层深度的增加而降低,不同氮肥施用时期和基追比例对0～20 cm土层土壤硝态氮和铵态氮含量均有显著影响;与氮肥全部基施处理相比,氮肥施用时期后移和基追比例的增加,明显提高了氮肥吸收利用率,减少了小麦全生育期土壤氮素的表观盈余量,同时显著改善了小麦籽粒品质;但对籽粒产量影响不显著,孕穗期追施比例过大导致产量显著降低.在本试验条件下,综合考虑产量、品质和生态效益,以基肥∶拔节肥∶孕穗肥为5∶3∶2为最佳氮肥运筹方式.     

Ammonium Nutrition in Ricinus communis: Its Effect on Plant Growth and the Chemical Composition of the Whole Plant, Xylem and Phloem Saps

Allen, S. and Smith, J A. C. 1986. Ammonium nutrition in Ricinuscommunis: its effect on plantgrowth and the chemical compositionof the whole plant, xylem and phloem saps.—J. exp. Bot.37: 1599–1610. The growth and chemical composition of Ricinus communis cultivatedhydroponically on 12 mol m ^{– 3} NO₃^–-N were comparedwith plants raised on a range of NH₄⁺-N concentrations. At NH₄⁺-Nconcentrations between 0·5 and 4·0 mol m^–3,fresh- and dry-weight yields of 62-d-old plants were not significantlydifferent from those of the NO₃^–-N controls. Growth wasreduced at 0·2 mol m^–3 NH₄⁺-N and was associatedwith increased root. shoot and C: organic N ratios, suggestingthat the plants were N-limited. At 8·0 mol m^–3NH₄⁺-N, growth was greatly restricted and the plants exhibitedsymptoms of severe ‘NH₄⁺ toxicity’. Plants growingon NH₄⁺-N showed marked acidification of the rooting medium,this effect being greatest on media supporting the highest growthrates. Shoot carboxylate content per unit dry weight was lower at mostNH₄⁺-N concentrations than in the NO₃^–-N controls, althoughit increased at the lowest NH₄⁺-N levels. Root carboxylate contentwas comparable on the two N sources, but also increased substantiallyat the lowest NH₄⁺-N levels. N source had little effect on inorganic-cationcontent at the whole-plant level, while NO₃^– and carboxylatewere replaced by Cl as the dominant anion in the NH₄⁺-N plants.This was reflected in the ionic composition of the xylem andleaf-cell saps, the latter containing about 100 mol m^–3Cl^– in plants on 8·0 mol m^–3 NH₄⁺. Xylem-saporganic-N concentration increased more than threefold with NH₄⁺-N(with glutamine being the dominant compound irrespective ofN source) while in leaf-cell sap it increased more than 12-foldon NH₄⁺-N media (with arginine becoming the dominant species).In the phloem, N source had little or no effect on inorganic-cation,sucrose or organic-N concentrations or sap pH, but sap fromNH₄⁺-N plants contained high levels of Cl^– and serine. Collectively, the results suggested that the toxic effects ofhigh NH₄⁺ concentrations were not the result of medium acidification,reduced inorganic-cation or carboxylate levels, or restrictedcarbohydrate availability, as is commonly supposed. Rather,NH₄⁺ toxicity in R. communis is probably the result of changesin protein N turnover and impairment of the photorespiratoryN cycle. Key words: Ricinus, ammonium nutrition, nitrate, whole-plant composition, xylem, Phloem, amino acids, carboxylate     

Determination of Levels of NO-3, NO-2 and NH+4 Ions in Leaves of Various Plants by Capillary Electrophoresis

Levels of NO^-₃, NO^-₂ and NH⁺₄ ions in leaves of six plant specieswere determined by capillary electrophoresis. Levels of NO^-₃ions differed by a factor of more than 350 in the six species.NO^-₂ and NH⁺₄ ions were detected in all species examined butat lower and more similar respective levels than NO^-₃ ions. (Received March 8, 1996; Accepted June 24, 1996)     

Influence of nutrients on the development of gibbosity in fronds of the duckweed Lemna gibba L.

The duckweeds Lemna gibba L. and Lemna minor L. only grew wellin undisturbed culture under axenic conditions in low lightintensity when provided with a suitable energy source such asglucose. In media containing N0₃-N gibbosity (a convex ventralsurface) was induced in the presence of the chelating agentethylene-diamine-di-o-hydroxyphenylacetic acid (EDDHA). In nutrientsolutions containing NO₃-N as the only N source, but withoutEDDHA, L. gibba occasionally exhibited gibbosity in culturesolutions of 40 cm³ volumes. More fronds were induced to exhibitgibbosity when the volume of the culture medium was increasedfrom 40 cm³ to 200 cm³. Gibbosity was never induced in L. minor,neither was it induced in L. gibba in media containing NH₄-N,even in the presence of NO₃-N. There was no direct correlationbetween the occurrence of gibbosity and frond growth rate, butgibbosity occurred only when there was good frond growth. In the absence of a sugar, frond growth was enhanced by bubblingair through the culture solution in the light. Increasing theCO₂ concentration in the air up to 1% enhanced growth and inducedgibbosity. Carbon dioxide did not induce gibbosity in mediacontaining NH₄-N. Key words: Ammonium-N, carbon dioxide, gibbosity, Lemna, nitrate-N     

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.     

Chemical Composition of Bleeding Xylem Sap from Kiwifruit Vines

A study of the chemical composition and charge balance was madeof bleeding xylem sap collected from excised one-year-old extensionshoots of healthy, Mn-deficient, Mn-toxic and Zn-deficient kiwifruitvines (Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson)immediately prior to leafburst. The exudates were analysed formacronutrient cations and anions, trace elements, amino acids,organic acids and sugars. Major charged species measured wereCa (13.3 mM), K (8.9 mM), Mg (5.6 mM), malate (12.5 mM) andphosphate (5.8 mM). Glutamine (12 mM) was the predominant Ncarrier identified, accounting for 58 per cent of the totalN followed by NO₂^–-N (4.5 per cent), NH₄⁺-N (3.5 per cent)and arginine-N (2.9 per cent). Approximately 22 per cent ofthe N was in a hydrolysable proteinaceous fraction comprisingmainly glutamine and glutamate. Eighteen free proteinaceousamino acids were idetified in sap, the most abundant being glutamine,glutamic acid, valine, isoleucine and phenylalanine. Computersimulation of the chemical composition predicted that in additionto hydrated cations, ion pairs formed between inorganic components(SO₄^2–, HPO₄^2–, H₂PO₄^–) and cations (Ca²⁺,Mg²⁺, Mn²⁺), plus metal-organic ligand complexes (Ca Malate,Zn Malate, FeCit, CuHis, CuGln) are important species involvedin translocation. The solubility product of hydroxyapatite wasexceeded in all exudates although in vitro precipitation wasnot observed. To achieve electroneutrality with the componentsmeasured, however, formation of precipitate precursors (hydroxyapatitenuclei) had to be assumed. Irregularities in Mn nutrition (butnot Zn) were clearly indicated by the elemental compositionof exudate suggesting the use of sap analysis as a possiblepre-season indicator of nutritional status for this species. Actinidia deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson, kiwifruit, xylem sap composition, trace metals, amino acids, organic acids     

Relationship between planktonic dinoflagellate abundance, cysts recovered in sediment traps and environmental factors in the Gullmar Fjord, Sweden

In order to study the relationship between planktonic dinoflagellates,cyst production and environmental factors, a sediment trap studywas conducted in the Gullmar Fjord, Swedish west coast, during21 days in May–June 1998. Five locations for sedimenttraps were randomly selected every third day. The traps weremoored at the five locations and moved to new locations after3 days. At every location, a CTD depth profile was obtainedand water samples were collected for plankton, chlorophyll aand nutrient analysis. Meteorological and hydrographic datafor the period were obtained from continuous monitoring. Threedinoflagellate species, which have not previously been recordedfrom the Kattegat or the Skagerrak (Scrippsiella crystallina,Scrippsiella lachrymosa and Scrippsiella trifida), were encounteredduring the analysis of cysts from the sediment traps. The abundanceof the different species in the motile form encountered in thewater column and cyst form encountered in the sediment trapsvaried greatly. The discrepancy between the number and speciesencountered in traps and water samples is discussed. No density-dependentrelationship between the abundance of planktonic cyst-formingdinoflagellates and the number of cysts recovered could be observed.A multiple regression showed that the variation in cyst yieldfrom the traps for the most abundant species was correlatedwith water surface temperature, ambient light radiation andthe depth of the halocline. The nutrient concentrations (NH₄⁺,NO₂^–, NO₃^– and PO₄^3–), which are known toplay a crucial role in induction of sexuality and cyst formationunder laboratory conditions, correlated poorly with the numberof dinoflagellate cysts encountered in the traps.     

Growth and Preferences for Ammonium or Nitrate Uptake by Barley in Relation to Root Termperature

Barley plants (Hordewn vulgare L. cv. Atem) were grown fromseed for 28 d in flowing solution culture, during which timeroot temperature was lowered decrementally to 5?C. Plants werethen subjected to root temperatures of 3, 5, 7, 9, 11, 13, 17or 25 ?C, with common air temperature of 25/15 ?C (day/night).Changes in growth, plant total N, and NO₃^– levels, andnet uptake of NH₄⁺ and NO₃^– from a maintained concentrationof 10 mmol m^–3 NH₄NO₃ were measured over 14 d. Dry matterproduction increased 6-fold with increasing root temperaturebetween 3–25 ?C. The growth response was biphasic followingan increase in root temperature. Phase I, lasting about 5 d,was characterized by high root specific growth rates relativeto those of the shoot, particularly on a fresh weight basis.During Phase I the shoot dry weight specific growth rates wereinversely related to root temperature between 3–13 ?C.Phase 2, from 5–14 d, was characterized by the approachtowards, and/or attainment of, balanced exponential growth betweenshoots and roots. Concentrations of total N in plant dry matterincreased with root temperature between 3–25 ?C, moreso in the shoots than roots and most acutely in the youngestfully expanded leaf (2?l–6?9% N). When N contents wereexpressed on a tissue fresh weight basis the variation withtemperature lessened and the highest concentration in the shootwas at 11 ?C. Uptake of N increased with root temperature, andat all temperatures uptake of NH₄⁺, exceeded that of NO₃^–,irrespective of time. The proportions of total N uptake over14 d absorbed in the form of NH₄⁺ were (%): 86, 91, 75, 77,76, 73, 77, and 80, respectively, at 3, 5, 7, 9, Il, 13, 17,and 25 ?C. At all temperatures the preference for NH₄⁺ overNO₃^– uptake increased with time. An inverse relationshipbetween root temperature (3–11 ?C) and the uptake of NH₄⁺as a proportion of total N uptake was apparent during PhaseI. The possible mechanisms by which root temperature limitsgrowth and influences N uptake are discussed. Key words: Hordeum vulgare, root temperature, ammonium, nitrate, ion uptake, growth rate     

Plant Characteristics and Nutrient Composition and Mobility of Broccoli (Brassica oleracea var. italica) Supplied with NH4+, NO3"" or NH4NO3

Shelp, B. J. 1987. Plant characteristics and nutrient compositionand mobility of broccoli (Brassica oleracea var. italica) suppliedwith NH⁺₄, NO₃ or NH₄NO₃.—J. exp. Bot. 38: 1603–1618. The effects of varying NH⁺₄, NO₃ or NH₄NO₃ concentration onthe final plant characteristics, element composition, and accumulationof NO₃-N, NH⁺₄-N and organic-N were evaluated in broccoli (Brassicaoleracea var. italicacv. Futura and Premium Crop) plants culturedin vermiculite under greenhouse conditions supplemented withlight. NH⁺₄-grown plants were stunted and exhibited signs ofmarginal necrosis on the old leaves, accompanied by an accumulationof NH4. The tissue levels of N, P, Mn, Cu, Zn and B were generallyincreased by NH⁺₄ versus NO₃ nutrition whereas the reverse wastrue for Ca; Mg and K were only slightly affected, if at all.These results are attributed to: changes in element availabilityresulting from reduced rhizosphere pH due to NH⁺₄uptake ratherthan NO ₃uptake; competition of Ca uptake by NH⁺₄; and dilutionof N by increased vegetative growth with NO₃-nutrition. Theelement concentrations of N, P or K were similar in all tissueswhereas Ca, B and Mn were markedly less in the florets and youngleaves compared to mature leaves; this supports literature indicatingthat the former elements are phloem-mobile whereas the latterare not. Assuming that the nutrient supply for mature leavesis delivered principally via the xylem stream, the data suggestthat nutrients for developing leaves and florets are suppliedpredominantly in the phloem. If so, under our experimental conditions.Zn and Cu were also readily mobile in the phloem whereas Mgmovement was restricted. NH₄⁺ versus NO₄⁺ J nutrition alteredthe distribution of these elements. The two broccoli cultivarstested under the greenhouse environment differed in NH⁺₄ toleranceand in the distribution of K and Cu suggesting there was a geneticbasis for cultivar variation in mineral acquisition and redistribution. Key words: Plant nutrition, phloem mobility, elemental composition.     

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

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

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     

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     

Diurnal K+ and Anion Transport in Phaseolus Pulvinus

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

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     

Temporal-spatial variations of chlorophyll a and primary production in Meiliang Bay, Lake Taihu, China from 1995 to 2003

There are few long-term data sets on primary production in alake, which can be used to validate the output from a productionmodel. To address this need, we determined the temporal–spatialvariations of chlorophyll a (Chl a) and primary production (PP_eu),based on the vertically generalized production model (VGPM)by using 742 samplings at seven sites in Meiliang Bay in LakeTaihu from 1995 to 2003. An empirical model estimating primaryproduction (PP_in) was used to validate VGPM PP_eu and the dominantfactors controlling PP were determined. Markedly higher Chla and PP_eu values were recorded in Meiliang Bay in 1996 and1997 than in other years and a marked decrease in Chl a andPP_eu was found between 2001 and 2003. Peaks of Chl a typicallyappeared in summer (June–August) and minima occurred inwinter (January). The highest daily mean PP_eu usually occurredin summer (June); the exception was at site 1, where peak dailymean PP_eu occurred in spring (April). The lowest daily meanPP_eu was recorded in winter (January). In Meiliang Bay, 43.0%of annual PP occurred in only 3 months, from June to August.The relative difference of maximum and minimum PP_eu was markedlylarger than the corresponding difference in Chl a. Levels ofboth Chl a and PP_eu were markedly decreased from the inner tothe outer areas of Meiliang Bay; the highest annual integratedPP_eu was found at site 1, close to the inflow of the River Liangxiand this level was 1.85 and 2.14 times higher than at sites3 and 6, respectively, that were located in outer Meiliang Bay.The estimated daily mean PP_eu variation closely matched withthat of the Chl a concentration, implying that Chl a concentrationcan account for the considerable variation of PP. The annualintegrated PP_eu of the euphotic zone in Meiliang Bay rangedfrom 3.44 x 10⁴ tC year^-1 to 8.59 x 10⁴ tC year^-1 with an overallmean of 5.65 x 10⁴ tC year^-1. A significant positive linearrelationship was found between VGPM PP_eu and empirical modelPP_in [PP_in = 0.826 (±0.015)PP_eu + 272.0 (±25.0),r² = 0.80, n = 742, P < 0.0001]. By considering the effectof water temperature, photosynthetically active radiation andphotoperiod on PP, the VGPM- generated PP_eu more accuratelycaptured monthly variations than did the empirical model thatonly included Chl a concentration.     

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     

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     

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

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

Effects of NO2 and O3 Alone or in Combination on Kidney Bean Plants (Phaseolus vulgaris L.): Growth, Partitioning of Assimilates and Root Activities

Ten-day old kidney bean plants (Phaseolus vulgaris L. cv. Shin-edogawa)were exposed to 2.0 and 4–0 parts 10^–6 NO2, and0.1, 0.2, and 0.4 parts 10^–6 O3 alone or in combinationfor 2, 4, and 7 d. The effects of these air pollutants wereexamined with respect to the growth, partitioning of assimilates,nitrogen uptake, soluble sugar content, and root respiration. Decreased dry matter production was significant with all treatmentsexcept 2.0 parts 10^–6 NO₂ and 0.1 parts 10^–6 O₃.Exposure to mixtures of the gases produced more severe suppressionof growth than exposure to the single gases. Root/shoot ratiowas significantly lowered at 7 d by the gas treatments otherthan 2.0 parts 10^–6 NO₂ and 0.1 parts 10^–6 O₃. Thetotal nitrogen content of plants was increased by all treatments;the higher percent of nitrogen found with O₃ exposure will resultfrom the growth retardation which increases the concentrationof nitrogen in the plants because the absorption of nitrogenby roots was unaffected. The combination of O₃ with NO₂ significantlydecreased the assimilation of NO₂ by the plants. The concentration of soluble sugars in roots was decreased bythe gas treatments. There was a strong positive correlationbetween soluble sugar content and dry weight of the roots harvestedat 7 d. Root respiration was relatively unchanged until 5 dand then decreased significantly at 7 d by 2.0 parts 10^–6NO₂ and 0–2 parts 10^–6 O₃. Retarded growth of theroots and the decreased root respiration may be due to diminishedtranslocation of sugars from leaves to roots caused by exposureto air pollutants. The uptake of soil nitrogen was not closelyrelated with root respiration in the case of O₃ exposure. Key words: NO₂, O₃, Phaseolus vulgaris, Growth, Sugars, Root respiration