High Atmospheric Nitrate Inputs and Nitrogen Turnover in Semi-arid Urban Catchments

The influx of atmospheric nitrogen to soils and surfaces in arid environments is of growing concern due to increased N emissions and N usage associated with urbanization. Atmospheric nitrogen inputs to the critical zone can occur as wet (rain or snow) or dry (dust or aerosols) deposition, and can lead to eutrophication, soil acidification, and groundwater contamination through leaching of excess nitrate. The objective of this research was to use the δ¹⁵N, δ¹⁸O, and Δ¹⁷O values of atmospheric nitrate (NO₃ ^?) (precipitation and aerosols) and NO₃ ^? in runoff to assess the importance of N deposition and turnover in semi-arid urban watersheds. Data show that the fractions of atmospheric NO₃ ^? exported from all the urban catchments, throughout the study period, were substantially higher than in nearly all other ecosystems studied with mean atmospheric contributions of 38% (min 0% and max 82%). These results suggest that catchment and stream channel imperviousness enhance atmospheric NO₃ ^? export due to inefficient N cycling and retention. In contrast, catchment and stream channel perviousness allow for enhanced N processing and therefore reduced atmospheric NO₃ ^? export. Overall high fractions of atmospheric NO₃ ^? were primarily attributed to slow N turn over in arid/semi-arid ecosystems. A relatively high fraction of nitrification NO₃ ^? (~30%) was found in runoff from a nearly completely impervious watershed (91%). This was attributed to nitrification of atmospheric NH₄ ⁺ in dry-deposited dust, suggesting that N nitrifiers have adapted to urban micro niches. Gross nitrification rates based on NO₃ ^? Δ¹⁷O values ranged from a low 3.04 ± 2 kg NO₃-N km^?2 day^?1 in highly impervious catchments to a high of 10.15 ± 1 kg NO₃-N km^?2 day^?1 in the low density urban catchment. These low gross nitrification rates were attributed to low soil C:N ratios that control gross autotrophic nitrification by regulating gross NH₄ ⁺ production rates.     

Influence of medium salt strength and nitrogen source on biomass and metabolite accumulation in adventitious root cultures of Pseudostellaria heterophylla

We investigated the effects of medium salt strength and ammonia/nitrate ratio on biomass production and metabolites accumulation of adventitious root. The medium with full-strength Murashige and Skoog (MS) reached the highest growth rate (16.77), and the contents of saponin and polysaccharide reached the peak (i.e., 0.65 and 24.85 %) at 3/4 MS and 1 MS, respectively. In case of ammonia/nitrate ratio, a NH₄ ⁺/NO₃ ^? ratio of 20:40 was optimal for the production of biomass and polysaccharide (23.27 %). In contrast, the content of saponin achieved the optimum (0.74 %) at a NH₄ ⁺/NO₃ ^? ratio of 30:30. In 5-L balloon-type bubble bioreactor (BTBB) cultures, an approximately 23-fold increase in biomass was recorded. The fresh weight (FW) and dry weight (DW) were 72.78 and 6.79 g per bioreactor with the contents of saponin (0.62 %) and polysaccharide (17.32 %), respectively. It indicated potential application to produce adventitious roots of pseudostellaria heterophylla with bioreactors on large scale in commercial.     

Content of Oxalate in Actinidia Deliciosa Plants Grown in Nutrient Solutions with Different Nitrogen Forms

Kiwifruit plants (Actinidia deliciosa cv. Hayward) were grown in Hoagland nutrient solution with calcium nitrate, potassium nitrate, ammonium nitrate or ammonium chloride as the nitrogen source. Plants grown in the solution with nitrate nitrogen displayed a higher oxalate content, greater shoot length and leaf area, and higher content of ascorbic acid and NO₃ ^– ions in the leaves. Plants grown in the solution with ammonium nitrate, and particularly with ammonium chloride, showed low oxalate content, low content of ascorbic acid and NO₃ ^–, high content of Cl^– and Na⁺, low shoot length and leaf area. Oxalate formation appeared to be connected with the assimulation of nitrate, more precisely with nitrate reduction, while ammonium nitrogen assimilation did not induce the synthesis of oxalic acid.     

RAPID NITRATE UPTAKE RATES AND LARGE SHORT‐TERM STORAGE CAPACITIES MAY EXPLAIN WHY OPPORTUNISTIC GREEN MACROALGAE DOMINATE SHALLOW EUTROPHIC ESTUARIES1

We quantified the effects of initial macroalgal tissue nitrogen (N) status (depleted and enriched) and varying pulses of nitrate (NO₃^?) concentration on uptake and storage of nitrogen in Ulva intestinalis L. and Ulva expansa (Setch.) Setch. et N. L. Gardner using mesocosms modeling shallow coastal estuaries in Mediterranean climates. Uptake of NO₃^? (μmol · g dry weight [dwt]^?1 · h^?1) was measured as loss from the water after 1, 2, 4, 8, 12, and 24 h and storage as total tissue nitrogen (% dwt) and nitrate (ppm). Both species of algae exhibited a high affinity for NO₃^? across all N pulses and initial tissue contents. There was greater NO₃^? removal from the water for depleted than enriched algae across all time intervals. In the low‐N‐pulse treatment, U. intestinalis and U. expansa removed all measurable NO₃^? within 8 and 12 h, respectively, and in the medium and high treatments, removal was high and then decreased over time. Maximum mean uptake rates of nitrate were greater for U. expansa (～300 μmol · g dwt^?1 · h^?1) than U. intestinalis (～100 μmol · g dwt^?1 · h^?1); however, uptake rates were highly variable over time. Overall, U. expansa uptake rates were double those of U. intestinalis. Maximum tissue NO₃^? for U. expansa was >1,000 ppm, five times that of U. intestinalis, suggesting that U. expansa has a greater storage capacity in this cellular pool. These results showed that opportunistic green algae with differing tissue nutrient histories were able to efficiently remove nitrate from the water across a wide range of N pulses; thus, both are highly adapted to proliferate in estuarine environments with pulsed nutrient supplies.     

Nitrogen input together with ecosystem nitrogen enrichment predict nitrate leaching from European forests

The IFEF database (Indicators of Forest Ecosystem Functioning), consisting of nitrogen deposition, nitrate leaching fluxes, and soil and ecosystem characteristics, is analysed to evaluate the C/N ratio in the organic horizon as an indicator of nitrate leaching. One hundred and eighty one forests are examined, from countries across Europe ranging from boreal to Mediterranean regions, encompassing broadleaf and coniferous sites and plot and catchment studies. N input in throughfall ranges from less than 1 kg N ha^?1 y^?1 in northern Norway and Finland to greater than 60 kg N ha^?1 y^?1 in the Netherlands and Czech Republic. The amount of NO₃^– leached covers a smaller range, between 1 and 40 kg N ha^?1 y^?1. Nitrate leaching is strongly dependent on the amount of nitrogen deposited in throughfall (N input) and simply adding the C/N ratio in the organic horizon to a regression equation does not improve this relationship. However, when the data are stratified based on C/N ratios less than or equal to 25 and greater than 25, highly significant relationships (P < 0.05) are observed between N input and NO₃^– leached. The slope of the relationship for those sites where C/N ratio is ≤ 25 (′nitrogen enriched′ sites) is twice that for those sites where C/N ratio is > 25. These empirical relationships may be used to identify which forested ecosystems are likely to show elevated rates of nitrate leaching under predicted future nitrogen deposition scenarios. Elevated NO₃^– leaching also shows a relationship with soil pH, with high rates of NO₃^– leaching only observed at sites with a pH < 4.5 and N inputs > 30 kg N ha^?1 y^?1. Tree age and species have no significant impact on the ecosystem response to N input at a regional scale.     

Risk assessment and source identification of coastal groundwater nitrate in northern China using dual nitrate isotopes combined with Bayesian mixing model

Due to the intensive and complicated human activities, the identification of nitrate pollution source of coastal aquifer is usually a challenge. This study firstly adopted stable isotope technique and stable isotope analysis in R (SIAR) model to identify the nitrate sources and contribution proportions of different sources in typical coastal groundwater of northern China. The results showed that about 91.5% of the groundwater samples illustrated significantly high nitrate concentrations exceeding the maximum WHO drinking water standard (50 mg/l), reflecting the high risk of groundwater nitrate pollution in the coastal area. A total of 57 sampling sites were classified into three groups according to hierarchical cluster analysis (HCA). The δ¹⁵N-NO₃^? and δ¹⁸O-NO₃^? values of groundwater samples from Group C (including nine samples) were much higher than those from Group A (including 40 samples) and Group B (including 8 samples). SIAR results showed that NH₄⁺ fertilizer was the dominant nitrate source for groundwater samples of Groups A and B while manure and sewage (M&;S) served as dominant source for Group C. This study provided essential information on the high risk and pollution sources of coastal groundwater nitrate of northern China.     

Variations of the nitrate isotopic composition in the St. Lawrence River caused by seasonal changes in atmospheric nitrogen inputs

We present 42 dual-isotope nitrate analyses of fresh water samples collected in the St. Lawrence River between June 2006 and July 2008. Measured δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? values correlate negatively, while δ¹⁸O–NO₃ ^? displays no negative correlation with nitrate concentration. This suggests that nitrate uptake and/or elimination by denitrification is not the main driver of observed variations in nitrate concentration and isotopic signature in the St. Lawrence River. In addition, δ¹⁸O–NO₃ ^? is negatively correlated with the seasonally variable δ¹⁸O of ambient water, indicating that the variation in the isotopic signature of nitrate is barely modulated by in-stream nitrate regeneration (nitrification). It rather is constrained by along-river changes in the external sources of nitrate. Given the distinct nitrogen (N) and oxygen (O) isotopic signature of atmospheric nitrate, we argue that observed seasonal variations of δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? in the St. Lawrence River are due to variable contributions of snowmelt-derived water. Based on a N and O isotope mass balance, we show that total nitrate loading in the St. Lawrence River is dominated by a N input from the Great Lakes (47 ± 28 %) and from nitrate regeneration of both internal and external N (48 ± 22 %). While temporal nitrate N and O isotope dynamics in the St. Lawrence River are mainly influenced by the atmospheric N input fluctuations, with an increase in atmospheric loading during spring, atmospheric N plays overall a rather insignificant role with regards to the N budget (5 ± 4 %).     

Effect of two nutrient solution temperatures on nitrate uptake,nitrate reductase activity,NH4+ concentration and chlorophyll a fluorescence in rose plants

The effect of two nutrient solution temperatures (cold (10 °C) and warm (22 °C)) during two flowering events of rose plants (Rosa × hybrida cv. Grand Gala) were examined by measuring chlorophyll (Chl) a fluorescence, ammonium (NH₄⁺) content and nitrate reductase (NR) activity in four different leaf types, that is, external and internal leaves of bent shoots and lower and upper leaves of flowering stems. Besides, nitrate (NO₃^?) uptake and water absorption, total nitrogen (N) concentration in the plant, dry biomass, and the ratios of shoot/root and thin-white roots/suberized-brown roots were determined. Generally, cold solution increased NO₃^? uptake and thin-white roots production but decreased water uptake, so plants grown at cold solution had to improve their NO₃^? uptake mechanisms to obtain a higher amount of nutrient with less water absorption than plants grown at warm solution. The higher NO₃^? uptake can be related to an increase in NR activity, NH₄⁺ content and total N concentration at cold solution. Nutrient solution temperature also had an effect on the photosynthetic apparatus. In general terms, the effective quantum yield (?_PSII) and the fraction of open PSII reaction centres (q_L) were higher in rose plants grown at cold solution. These effects can be associated to a higher NO₃^? uptake and total N concentration in the plants and were modulated by irradiance throughout all the experiment. Plants could adapt to cold solution by enhancing their metabolism without a decrease in total dry biomass. Nevertheless, the effect of nutrient solution temperature is not simple and also affected by climatic factors.     

The effect of some ammonium salts on nitrate reductase level,onin vivo nitrate reduction and on nitrate content in excisedpisum sativum roots

The effect of some ammonium salts on nitrate reductase (NR) level, onin vivo nitrate reduction and on nitrate content was followed in the presence of nitrate in the medium, under changing experimental conditions, in excisedPisum sativum roots, and their effect was compared with that of KNO₃, Ca(NO₃)₂ and NaNO₃ at 15 mM NO₃ ^- concentration, i.e. at a concentration which considerably exceeded the level of saturation with nitrate with respect to nitrate reductase. The effect of ammonium salts on NR level is indirect and changes from a positive one to a strongly negative one which is dependent on the time of action of the salt, on the presence of other cations, on pH of the solution of the ammonium salt and on the nature of the anion of the ammonium salt. A positive effect on the enzyme level can be observed in the presence of other cations than NH₄ ⁺ at suitable concentrations of those ammonium salts, the solutions of which have their pH values in the acid region (i.e. NH₄H₂PO₄, (NH₄)₂SO₄ and NH₄NO₃). However their positive effect is independent of the presence of NH₄ ⁺ ions, and it is obviously the result of an increased concentration of H⁺ ions. A clear-cut negative effect on NR level can be observed after 24 h in one-salt NH₄NO₃ solution where NH₄ ⁺ is not balanced with other cations and thus certainly can adversely influence many metabolic processes, and in the solutions containing neutral (pH 6.2) and dibasic ammonium phosphates in which dissolved undissociated ammonia [(NH₃). (H₂O) which can also affect many metabolic processes incl. proteosynthesis] probably has a toxic influence. Thein vivo nitrate reduction is always depressed in excised pea roots in the presence of ammonium salts in the medium, regardless of the level of nitrate reductase. Under the described conditions, no relationship could be established between the enzyme level and the so-called metabolic NO₃ ^- pool (i.e. NO₂ ^- production under anaerobic conditions), nor between NR level and the total nitrate content in the roots. One-salt solutions of NaNO₃, Ca(NO₃)₂ and KNO₃ exert different effects on the level of nitrate reductase and on the content of NO₃ ^- in the roots, but the in vivo NO₃ ^- reduction shows the same trend as NR level in the roots influenced by these salts. Cl^- ions, supplied in NH₄C1, depress both NR level and NO₃ ^- content in the roots at higher concentrations, but they do not significantly affect the in vivo nitrate reduction in comparison with other ammonium salts. These results indicate that NR level,in vivo nitrate reduction, and nitrate uptake can be regulated in pea roots independently of each other.     

The Synergistic Effects of High Nitrate Concentrations on Sediment Bioreduction

Groundwaters at nuclear sites can be characterized by low pH and high nitrate concentrations (10–100 mM). These conditions are challenging for bioremediation, often inhibiting microbial Fe(III)-reduction which can limit radionuclide migration. Here, sediment microcosms representative of the UK Sellafield site were used to study the influence of variable pH and nitrate concentrations on microbially-mediated TEAP (terminal electron accepting processes) progression. The rate of reduction through the terminal electron accepting cascade NO^? ₃ > NO^? ₂ > Mn(IV)/Fe(III) > SO^2? ₄ at low pH (～5.5) was slower than that in bicarbonate buffered systems (pH ～ 7.0), but in the low pH systems, denitrification and associated pH buffering resulted in conditioning of the sediments for subsequent Fe(III) and sulfate-reduction. Under very high nitrate conditions (100 mM), bicarbonate buffering (pH ～ 7.0) was necessary for TEAP progression beyond denitrification and the reduction of 100 mM nitrate created alkaline conditions (pH 9.5). 16S rRNA gene analysis showed that close relatives of known nitrate reducers Bacillus niacini and Ochrobactrum grignonense dominated the microbial communities in this reduced sediment. In Fe(III)-reducing enrichment cultures from the 100 mM nitrate system, close relatives of the Fe(III)-reducing species Alkaliphilus crotonatoxidans and Serratia liquifaciens were observed. These results highlight that under certain conditions and contrary to expectations, denitrification may support bioreduction via pH conditioning for optimal metal reduction and radionuclide immobilization.     

Nitrate reduction in marine sediment: Pathways and interactions with iron and sulfur cycling

Abstract

In coastal marine sediments, the interactions between NO₃ ^? reduction and transformations of Fe and S compounds often occur in a strong gradient of electron activity ("redoxcline"). Denitrification activity is observed throughout the NC₃ ^?‐containing surface zone, although the reduction step from N₂O to N₂ can be inhibited by H₂S in the “redoxcline.”; Survival of denitrifiers is generally poor in NO₃ ^?‐free, reduced sediment; such populations are likely to employ Fe³⁺ reduction in their energy metabolism. At depth, the sediments often contain a larger capacity for “nitrate ammonification”; (dissimilatory NO₃ ^? reduction to NH₄ ⁺) than for denitrification. The “nitrate ammonification”; is found commonly among fermenting bacteria, although SO₄ ^2? reducers may also be involved. In situ activities observed in whole sediment cores indicate that “nitrate ammonification”; may account for as much as one‐third of the carbon oxidation in organic‐rich sediments. The control of partitioning between denitrification and “nitrate ammonification”; at low NO₃ ^? concentrations is poorly investigated, but the larger metabolic capacity of fermenting and S O₄ ^2?‐reducing baceria in relatively reduced sediment could be important. In addition to bacterial reduction, chemical NO₃ ^? reduction is possible where significant amounts of Fe²⁺ (or H₂S) accumulate in the “redoxcline.”;     

The activation state of nitrate reductase is not always correlated with total nitrate reductase activity in leaves

The relation between nitrate reductase (NR; EC 1.6.6.1) activity, activation state and NR protein in leaves of barley (Hordeum vulgare L.) seedlings was investigated. Maximum NR activity (NRA_max) and NR protein content (Western blotting) were modified by growing plants hydroponically at low (0.3 mM) or high (10 mM) nitrate supply. In addition, plants were kept under short-day (8 h light/16 h dark) or long-day (16 h light/8 h dark) conditions in order to manipulate the concentration of nitrate stored in the leaves during the dark phase, and the concentrations of sugars and amino acids accumulated during the light phase, which are potential signalling compounds. Plants were also grown under phosphate deficiency in order to modify their glucose-6-phosphate content. In high-nitrate/long-day conditions, NRA_max and NR protein were almost constant during the whole light period. Low-nitrate/long-day plants had only about 30% of the NRA_max and NR protein of high-nitrate plants. In low-nitrate/long-day plants, NRA_max and NR protein decreased strongly during the second half of the light phase. The decrease was preceded by a strong decrease in the leaf nitrate content. Short daylength generally led to higher nitrate concentrations in leaves. Under short-day/low-nitrate conditions, NRA_max was slightly higher than under long-day conditions and remained almost constant during the day. This correlated with maintenance of higher nitrate concentrations during the short light period. The NR activation state in the light was very similar in high-nitrate and low-nitrate plants, but dark inactivation was twice as high in the high-nitrate plants. Thus, the low NRA_max in low-nitrate/long-day plants was slightly compensated by a higher activation state of NR. Such a partial compensation of a low NR_max by a higher dark activation state was not observed with phosphate-depleted plants. Total leaf concentrations of sugars, of glutamine and glutamate and of glucose-6-phosphate did not correlate with the NR activation state nor with NRA_max. Received: 24 March 1999 / Accepted: 31 May 1999     

Influence of bioturbation on denitrification and dissimilatory nitrate reduction to ammonium (DNRA) in freshwater sediments

Intensive agriculture leads to increased nitrogen fluxes (mostly as nitrate, NO₃ ^?) to aquatic ecosystems, which in turn creates ecological problems, including eutrophication and associated harmful algal blooms. These problems have focused scientific attention on understanding the controls on nitrate reduction processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Our objective was to determine the effects of nutrient-tolerant bioturbating invertebrates (tubificid oligochaetes) on nitrogen cycling processes, specifically coupled nitrification–denitrification, net denitrification, DNRA, and biogeochemical fluxes (O₂, NO₃ ^?, NH₄ ⁺, CO₂, N₂O, and CH₄) in freshwater sediments. A mesocosm experiment determined how tubificid density and increasing NO₃ ^? concentrations (using N¹⁵ isotope tracing) interact to affect N cycling processes. At the lowest NO₃ ^? concentration and in the absence of bioturbation, the relative importance of denitrification to DNRA was similar (i.e., 49.6 and 50.4 ± 8.1 %, respectively). Increasing NO₃ ^? concentrations in the control cores (without fauna) stimulated denitrification, but did not enhance DNRA, which significantly altered the relative importance of denitrification compared to DNRA (94.6 vs. 5.4 ± 0.9 %, respectively). The presence of tubificid oligochaetes enhanced O₂, NO₃ ^?, NH₄ ⁺ fluxes, greenhouse gas production, and N cycling processes. The relative importance of denitrification to DNRA shifted towards favoring denitrification with both the increase in NO₃ ^? concentrations and the increase of bioturbation activity. Our study highlights that understanding the interactions between nutrient-tolerant bioturbating species and nitrate contamination is important for determining the nitrogen removal capacity of eutrophic freshwater ecosystems.     

Changes in amino acid content of an algal feed species (Navicula sp.) and their effect on growth and survival of juvenile abalone (Haliotis rubra)

The growth and survival of juvenile Haliotis rubra, when fed with the diatom Navicula sp. cultured in f/2 medium containing combined nitrogen at 24.71 mg NO₃-N L^–1 (high), 12.35 mg NO₃-N L^–1 (standard) or 2.47 mg NO₃-N L^–1 (low), were compared in a 33-day trial. The alga in the low nitrogen medium contained 37% less total amino acid than that in the high and standard nitrogen media. There was a slightly greater reduction in essential amino acids (40%) compared to non-essential amino acids (35%). Juvenile abalone feeding on Navicula grown in medium with low nitrate and lower total amino acid content grew more slowly than when fed on the same species grown in standard or higher nitrogen medium with a higher amino acid content. The growth rate of juveniles was highest (43 m d^–1) in the high nitrate treatment followed (40 m d^–1) by the standard nitrate treatment and lowest (31 m d^–1) in the low nitrate treatment. The survival of the juveniles was also effected by the diet. Survival was better in the high and standard nitrogen media (88%) than the low nitrogen medium (75%). The results suggest that in order to achieve uniformity in nutritional quality of diatoms and good growth of abalone juveniles in commercial abalone nurseries, the nitrogen concentration in tanks should be monitored and additional nitrate added to provide an optimum concentration of between 2 and 12 mg NO₃-N L^–1.     

Seed germination in Chenopodium album L.: further evidence for the dependence of the effects of growth regulators on nitrate availability

Abstract Chenopodium album L. plants, grown under controlled environmental conditions on different levels of soil nitrate, produced seeds with proportionately different NO^?₃ contents. Regardless of the endogenous NO^?₃ content, few seeds germinated in water or upon treatment with KNO₃. Ethylene promoted germination, and the extent of germination was positively correlated with the endogenous seed NO^?₃ content. Combined application of ethylene and KNO₃ in the dark had a synergistic effect on NO^?₃ -deficient seed. The synergism between ethylene and KNO₃ was attributable to the NO^?₃ moiety of the nitrate salt. Ethylene and light showed moderate synergism in seeds with low or high endogenous nitrate. Addition of nitrate, however, masked the interaction between ethylene and light. Gibberellic acid₄₊₇ (GA₄₊₇) or red light, each alone or combined with KNO₃, had little effect on germination. When applied together in the dark, ethylene and GA₄₊₇ synergistically enhanced the germination of NO^?₃-deficient seed. The combined effects of the two hormones on this seed were further enhanced by the addition of KNO₃. There was no synergism between ethylene and GA₄₊₇ in NO^?₃-rich seed. These interactions among GA₄₊₇, ethylene and KNO₃ were not affected by light. The results confirm and further elaborate our earlier finding that the sensitivity of C. album seeds to ethylene may depend on nitrate availability.     

Sink-stimulated photosynthesis and sink-dependent increase in nitrate uptake: nitrogen and carbon relations of the parasitic association Cuscuta reflexa–Ricinus communis

Ricinus communis L. was grown under limiting N supply in quartz sand culture, fed with 0.2, 1 or 5 mol m^?3 NO₃^?, or in liquid culture with 0.022, 0.05 or 0.5 mol m^?3 NO₃^?. Some of the plants were infected with Cuscuta reflexa Roxb. As occurred for the host, dry matter production and growth of C. reflexa were severely depressed with decreasing N supply to the host. When parasitized by C. reflexa, the shoot and root dry weight of Ricinus was diminished at all levels of N nutrition, but the total dry weight of host plus parasite was almost the same as that of uninfected Ricinus. In contrast to the situation in Lupinus albus (Jeschke et al. 1994b), infection by Cuscuta resulted in increased tissue N levels in the host and the N content of the system Ricinus plus C. reflexa was the same or even somewhat larger than that of uninfected plants. This indicated a sink-dependent stimulation of nitrate uptake. As a result of decreased root weights, nitrate uptake g^?1 FW was stimulated by 80, 60 or only 40% at 0.2, 1 or 5 mol m^?3 nitrate supply. Increased nitrate uptake was reflected, particularly at low N supply, in xylem transport; xylem sap nitrate concentrations were substantially elevated, while those of amino acids were decreased in parasitized plants. This indicated an inhibition of nitrate assimilation in roots of parasitized plants under limiting N supply. Besides these effects on N relations, C. reflexa induced a substantial sink-dependent stimulation of net photosynthesis in host leaves and a concomitant increase in stomatal opening and transpiration. This stimulation depended on the relative sink size induced by Cuscuta, on nitrogen nutrition and on leaf age, indicating that delayed senescence of leaves contributes to the overall effects of Cuscuta on its host. The Cuscuta-induced inhibition of nitrate assimilation in the roots and the increase in nitrate uptake suggest that nitrate reduction was shifted towards the leaves in the presence of C. reflexa. The stimulating effects of C. reflexa in the Ricinus-Cuscuta association are compared with the strongly inhibitory effects occurring in the tripartite association L. albus–Rhizobium–Cuscuta reflexa.     

Fate of nitrate during initial nitrate utilization by nitrogen-depleted dwarf bean

The fate of nitrate and nitrogen-15 was followed during the apparent induction phase (6h) for nitrate uptake by N-depleted dwarf bean (Phaseolus vulgaris L. ev. Witte Krombek). Experiments were done with intact plants and with detached root systems. Qualitatively and quantitatively, xylem exudation from detached roots was a bad estimate of the export of NO^?₃ or NO^?₃-¹⁵N from roots of intact plants. In vivo nitrate reductase activity (NRA) agreed well with in situ reduction, calculated as the difference between uptake and accumulation in whole plants, provided NRA was assayed with merely endogenous nitrate as substrate (‘actual’ NRA). The majority (75%) of the entering nitrate remained unmetabolized. Both nitrate reduction and nitrate accumulation occurred predominantly in the root system. Some (< 25%) of the root-reduced nitrate-N was translocated to the shoot. Nitrate uptake occurred against the concentration gradient between medium and root cells, and probably against the gradient of the electro-chemical potential of nitrate. Part of the energy expended for NO^?₃ absorption came from the tops, since decapitation and ringing at the stem base restricted nitrate uptake.     

GROWTH RESPONSE OF A NITROGEN FIXER (ANABAENA FLOS-AQUAE,CYANOPHYCEAE) TO LOW NITRATE1

Anabaena flos-aquae (Lyngb.) Bréb. was grown in varying concentrations of nitrate. Specific growth rates, as estimated in batch culture, were constant and approached the maximum rate at all concentrations of NO₃^?-N tested bewteen 0 and 400 μ/L. Steady-state biomass, as determined in semicontinuous culture, did not vary with NO₃^? at slower dilution rates. However at a faster dilution rate, significantly less biomass occurred in intermediate concentrations of NO₃^? than in either higher or lower concentrations. The results indicate that both growth rate and standing crop are maximized by either N₂ fixation or NO₃^? assimilation, but extracellular NO₃^? reduces the rate of N₂ fixation. Consequently, at very low NO₃^? concentrations, growth is virtually maximized by N₂ fixation alone, and at high concentrations of NO₃^?, N₂ fixation is inhibited but growth is maximized by assimilation of NO₃^?. At intermediate concentrations of NO₃^?, growth becomes a function of NO^3? assimilation augmented by N₂ fixation. In this case, full growth potential is realized only if hydraulic residence time is sufficiently long to compensate for the reduced rate of N₂ fixation. Growth rate and standing crop are not diminished in response to the large amount of energy allocated to N₂ fixation. Instead, other cellular processes are probably affected negatively during N₂ fixation.     

The influence of enriched rhizosphere CO2 on N uptake and metabolism in wild-type and NR-deficient barley plants

Positive influences of high concentrations of dissolved inorganic carbon (DIC) in the growth medium of salinity-stressed plants are associated with carbon assimilation through phosphoenolpyruvate carboxylase (PEPc) activity in roots; and also in salinity-stressed tomato plants, enriched CO₂ in the rhizosphere increases NO^?₃uptake. In the present study, wild-type and nitrate reductase-deficient plants of barley (Hordeum vulgare L. cv. Steptoe) were used to determine whether the influence of enriched CO₂ on NO^?₃ uptake and metabolism is dependent on the activity of nitrate reductase (NR) in the plant. Plants grown in NH₄⁺and aerated with ambient air, were transferred to either NO₃^? or NH₄⁺ solutions and aerated with air containing between 0 and 6 500 μmol mol^?1 CO₂. Nitrogen uptake and tissue concentrations of NO₃^? and NH₄⁺ were measured as well as activities of NR and PEPc. The uptake of NO^?₃ by the wild-type was increased by increasing CO₂. This was associated with increased in vitro NR activity, but increased uptake of NO₃^? was found also in the NR-deficient genotype when exposed to high CO₂ concentrations; so that the influence of CO₂ on NO₃^? uptake was independent of the reduction of NO₃^? and assimilation into amino acids. The increase in uptake of NO₃^? in wild-type plants with enriched CO₂ was the same at pH 7 as at pH 5, indicating that the relative abundance of HCO₃^? or CO₂ in the medium did not influence NO₃^? uptake. Uptake of NH₄⁺ was decreased by enriched CO₂ in a pH (5 or 7) independent fashion. Thus NO₃^? and NH⁺₄ uptakes are influenced by the CO₂ component of DIC independently of anaplerotic carbon provision for amino acid synthesis, and CO₂ may directly affect the uptake of NO₃^? and NH₄⁺ in ways unrelated to the NR activity in the tissue.