南美天胡荽及其根际土壤水浸提液化感成分分析

为探讨南美天胡荽对其他植物种子萌发的影响以及筛选影响其他植物的主要化合物,该文采用种子萌发试验、气相色谱-质谱联用以及液相色谱-质谱联用的方法,分析了南美天胡荽不同溶剂浸提液对种子萌发的影响、南美天胡荽植株及其根际土壤浸提液成分。结果表明:(1)南美天胡荽不同溶剂浸提物均具有一定程度的抑制种子萌发作用。(2)气相色谱-质谱分析下,南美天胡荽植株水浸提液中共分离鉴定了35种化合物,其中,邻苯二甲酸二丁酯(15.2%)、10,15-十八烷二元酸(8.58%)、2,4-二叔丁基苯酚(6.81%)相对含量最高; 根际土壤水浸提液中共分离鉴定了17种化合物,其中,油酸酰胺(26.47%)、正二十七烷(9.63%)、十六酸乙酯(4.83%)相对含量最高。(3)液相色谱-质谱分析下,南美天胡荽植株水浸提液共分离鉴定了109种化合物,ESI⁺模式下,L-苯丙氨酸(3 483.99 ng·mg^-1)、木犀草素(2 306.64 ng·mg^-1)含量最多,ESI^-模式下,右旋奎宁酸(21 827.71 ng·mg^-1)、绿原酸(12 589.25 ng·mg^-1)含量最多; 根际土壤水浸提液中共分离鉴定了93种化合物,ESI⁺模式下,丁酸(7 660.53 ng·mg^-1)、棕榈酰胺(3 200.36 ng·mg^-1)含量最多,ESI^-模式下,正二十八酸(18 605.35 ng·mg^-1)、蔗糖(12 183.23 ng·mg^-1)含量最多。(4)南美天胡荽的潜在化感物质主要为脂肪酸类、酰胺类、酯类、芳香酸类化合物,而土壤中直接起化感作用的物质可能为丁酸、正二十八酸、羟基乙酸、油酸酰胺、棕榈酰胺、十六酸乙酯、苯甲酸,其中脂肪酸类化合物输入可能来源于南美天胡荽、土壤微生物和土壤动物,酰胺类、酯类、芳香类化合物则更可能来源于南美天胡荽植株。     

The Effect of Darkness on the Leghemoglobin Content and Amino Acid Levels in the Root Nodules of Pea Plants

The dependence of the nitrogen fixing system in the root nodules of pea plants (Pisum sativum) L. cv. Torsdag II) on light induced reactions was studied. The pots of the inoculated pea plants, after the nolules had fixed nitrogen for a fornight, were transferred to a dark room. The control plants were kept under normal lighting conditions. The decay of leghemoglobin was measured after photosynthesis had ceased. In the dark the red nodules turned green in three days, when about half of the haem had been broken down. The plants in normal lighting conditions had maintained the red nodules. The appearence of leghemoglobin and bacteroids was simultaneouos. In normal lighting conditions the number of bacteroids was about 1.6 × 10⁸ per g fresh nodules. The appearance of leghemoglobin and bacteroids was simultaneous. In normal lighting conditons the number of bacteroids was bout 1.6 × 10⁸ per g fresh nodules. At the same time as the nodules turned green in the dark most of the bacteroids disappeared and the number of rod-shaped bacteria increased. After five days int the dark thenumber of bacteria of the green nodules was 2.2 × 10⁸ per g fresh nodules. A large increase of of bacteria in the nodules is one of the results after the termination of effective symbiosis. Quantitative estimations were made with an automatic amino acid analysator of the amino acid composition in the root nodules of pea plants grown in the light and of pea plants grown in the dark. Altogether 27 amino acids and amides and 3 unknown ninhydrin positive compounds were found in the free amino acid fraction. In the red N-fixing nodules asparagine, the amide of aspartic acid, was the most prominent (more than 50 per cent of the total amino acid fraction), indicating the energy charge of the nitrogen fixation. 5 days in the dark affected the proportions of the amino acids as follows. Asparagine, homoserine, γ-aminobutyric acid and ethanolamine were decreased and the most of the others increased. In the hydrolysate of the non-soluble protein fraction 25 amino acids could be detected. The proportions of the amino acids in the root nodules of light-grown and dark-grown pea plants were very similar. Hydroxyproline and α, γ-diaminopimelic acid (DAP) were found in these fraction. Most of the DAP was contained in the peptide fraction. Also hydroxyproline was found to a small extent. It was assumed that the amino acids in this fraction were derived from the peptides of both plant cells and rhizobia.     

Acid-labile amino acid precursors in the Murchison meteorite. II: A search for peptides and amino acyl amides.

The basic fraction of the acid-labile amino acid precursors found in hot water extracts of the Murchison meteorite was treated with ninhydrin under conditions that give destruction of free amino acids but allow nearly complete recovery to peptides and amino acylamides. After this treatment and a subsequent acid hydrolysis, the amount of amino acids found corresponded to less than 30 per cent of the precursor content of the basic fraction. Since only 30 percent of the total extract precursors are found in the basic fraction, these results indicate that such compounds, if present, account for no more than 9 mole per cent of the total acid-labile amino acid precursors of the meteorite hot water extract.     

THE EFFECTS OF AMINO ACIDS UPON THE DEVELOPMENT OF EXCISED FLORAL BUDS OF AQUILEGIA

Young excised floral buds of Aquilegia were grown on a chemically defined medium containing various concentrations of single amino acids or mixtures of amino acids. γ-Amino butyric acid significantly promoted floral development through the initiation and differentiation of carpels. These floral organs were generally absent on the basal medium. Alanine, glutamic acid, and aspartic acid had no effect upon floral development. All other amino acids were either ineffective at lower concentrations and inhibitory at higher concentrations or were inhibitory at all concentrations. Casein hydrolysate and a mixture of amino acids found in coconut milk were ineffective. The addition of both γ-amino butyric acid and alanine to the basal medium promoted development approaching that achieved on the coconut-milk medium. However, further growth factors appear to be required before development on coconut-milk medium is equalled or exceeded.     

Seasonal changes in the free amino acids of oat and barley phloem sap in relation to plant growth stage and growth of Rhopalosiphum padi

The concentrations and composition of free amino acids in phloem sap from two cultivars of oats and barley, both susceptible to the aphid Rhopalosiphum padi, were determined by means of high performance liquid chromatography. Sap was collected from excised aphid stylets at three developmental stages (seedlings, tillering plants and plants undergoing stem elongation) from plants given or not given fertiliser and grown outdoors. In connection, the growth of individual R. padi nymphs was estimated at the same phenological stages on plants grown in the greenhouse. The content of free amino acids was consistently higher in seedlings than in plants at the early tillering stage. Only in seedlings did the addition of fertiliser increase amino acid levels. Barley phloem sap contained more free amino acids than that of oats when fertiliser was added and at later developmental stages. Phloem sap of oats and barley showed similar patterns in their composition of free amino acids at the seedling stage, but as the plants grew older the patterns became increasingly different. Plants given fertiliser had higher amounts of dicarboxylic amino acids (glutamic and aspartic acid) than unfertilised plants. The concentrations of γ-amino butyric acid, glycine, histidine, and methionine were very low in all treatments. The relative growth rates of R. padi nymphs were low when amino acid content was low and vice versa. The results are discussed in relation to host plant suitability and plant resistance mechanisms.     

Reactions between amino acid compounds and phenols during oxidation

Summary About 30 per cent of organic soil nitrogen can be hydrolized with HCl to amino acids; about 30 per cent is nonhydrolizable. In contrast to this high content of amino acid nitrogen is the small availability of the nitrogen to micro-organisms. In light of the theory proposing a reaction between the -amino group of amino acids or peptides and quinones formed during oxidation of lignin degradation products or other phenolic compound, different types of phenols were oxidized by phenolases in presence of amino acid compounds.It could be shown that the reaction of binding of nitrogen started at pH values higher than 6.5, and that only such phenols reacted which had no methoxylated hydroxyl groups. The reaction of some phenols during oxidation in presence of amino acids was accompanied by deamination and decarboxylation of the latter.The reaction products of phenols with amino acids were stable against hydrolysis. Using peptides it was found that all amino acids, except the N-terminal which is bound to oxidized phenols, could be hydrolyzed normally.With serum albumin it could be shown that there is a reaction with the amino group of the N-terminal amino acid and also with the -amino group of lysine residues with phenols during oxidation. The reacted protein seemed to be degraded normally with a protease ofBacillus subtilis.Guest Scientist as Fulbright Research Scholar from the Agronomy Department of the Iowa State University, Ames, Iowa, U.S.A.     

Changes in γ-Methyleneglutamic acid, γ-Methyleneglutamine and other amino acids and amides during fruit growth of Tribulus terrestris

Abstract

Distribution and metabolism of γ-methyleneglutamic acid, γ-methyleneglutamine and other amino acids and amides has been studied during fruit growth of Tribulus terrestris. The largest concentration of free amino acids and amides has been observed in fruit stage 1. The marked decline in the amount of γ-methyleneglutamic acid and γ-methyleneglutamine after fruit stage 1 may indicate their rapid utilization along with asparagine and glutamine during fruit growth. In leaf and in different fruit growth stages, γ-methyleneglutamic acid dominated over γ-methyleneglutamine.     

Distribution of Nitrogen during Growth of Sunflower (Helianthus annuus L.)

The accumulation, distribution and redistribution of dry matterand nitrogen is described for Helianthus annuus L. cv. Hysun21 grown on 6 mM urea in glasshouse culture. Seed dry matterand nitrogen were transferred to seedlings with net efficienciesof 40 and 86 per cent respectively. At flowering, the stem hadmost of the plant's dry matter and the leaves most of its nitrogen.About 35 per cent of the plant's nitrogen accumulated afterthree-row anthesis. The amount of protein in vegetative parts,especially leaves, declined after flowering. Concentrationsof free amino compounds also decreased during growth. Matureseeds had 38 per cent of the total plant dry weight and 68 percent of the total nitrogen. Seeds acquired 33 per cent of theirdry matter and nitrogen from redistribution from above-groundplant parts. The stem was most important for storage of carbohydrate,leaves the most important for nitrogen. Over 50 per cent ofthe nitrogen in the stem and leaves was redistributed. Plantsthat received 6 mM nitrate accumulated more dry matter thanurea-grown plants. Seeds from nitrate-grown plants were heavier(58 mg) than those of urea-grown plants (46 mg), and their percentageoil was greater (50 and 41 respectively). The amount of nitrogenper seed was the same. Little or no urea was detected in xylem sap of plants suppliedwith 5 mM urea, but it was detected in sap of plants which received25 mM. Concentrations of urea and amino compounds in the sapdecreased up the stem. Plants supplied with nitrate had mostof the nitrogen in xylem sap as NO₂^–, suggesting littlenitrate reduction in roots. Plants grown on 6 mM nitrate andchanged to high levels of urea-nitrogen for 14 days still hadhigh levels of nitrate; little nitrate remained in plants receivinglow levels of urea. When urea is applied in irrigation waterto field-grown sunflower, the nitrogen is subsequently takenup as nitrate due to rapid nitrogen transformations in the soil. Helianthus annuus L., sunflower, urea, nitrate, nitrogen transport, xylem sap, nitrogen accumulation nitrogen distribution     

Effect of nitrogen nutrition on amino acid composition of xylem sap and stem wood in Alnus glutinosa

Citrulline was the major amino acid in root pressure sap, stem sap and stem wood from Alnus glutinosa L. Gaertn. plants relying on fixed nitrogen or, partly or wholly, on mineral nitrogen for growth. Glutamine increased in prominence in plants assimilating mineral nitrogen but asparagine remained a relatively insignificant component. Differences in the relative amounts of the free amino compounds of stem sap from nitrogen-fixing and mineral nitrogen-fed plants were usually small compared to differences between plants fed different sources of mineral nitrogen. In contrast, relatively high values for the ratios of citrulline/total free amino nitrogen compounds and particularly of citrulline/amides in root pressure sap distinguished nitrogen-fixing plants from those receiving mineral nitrogen. Although the amino acid ratios of stem wood extracts showed closer similarity to those for root pressure sap than stem sap, the seasonal accumulation of citrulline, possibly as a storage amino acid, in stem wood from field-grown plants negated the possibility of utilising stem wood analyses as an indicator of the form of nitrogen assimilation. Comparative data on the levels of citrulline or other free amino acids in Alnus glutinosa are unlikely to be useful as an index of nitrogen fixation, under most experimental conditions.     

High altitude acclimatization in fourArtemisia species: Changes in free amino acids and nitrogen contents in leaves

During high altitude acclimatization the highest number of amino acids were found in temperateArtemisia species (A. vestita) and the lowest one in tropical species (A. scoparia). The amount of free amino acids in temperateArtemisia species was higher when this was grown at 3600 m altitude. InA. scoparia, A. vulnaris andA. parviflora, the higher amount of individual amino acids was ascertained in plants grown at lower altitudes. InA. vestita, the nitrogen contents per unit dry matter was lower in plants grown at high altitude. In the other three species, the contents were significantly higher in the leaves of plants grown at a higher altitude. The nitrogen contents per unit leaf dry matter determined during active growth of plants were minimum inA. parviflora and maximum inA. vestita. With progressing plant senescence the concentration of nitrogen decreased in all species studied.     

The content of GABA and other amino acids in bovine cerebral cortex synaptic vesicles

The content of γ-amino butyric acid (GABA) and of other water soluble amino acids in bovine brain synaptic vesicles was determined by a modified automated amino acid analysis method. Following subcellular fractionation, GABA, glutamate and aspartate were distributed largely in the supernatant fractions and in the upper layer of the sucrose gradient. Only 10–20% of the total content was associated with the vesicular fraction. On the other hand, the other water soluble amino acids, such as serine, glycine and alanine, were evenly distributed between cytoplasmic and particulate fractions in a similar pattern to that observed with cytoplasmic enzyme markers. The results may indicate specific association of GABA, glutamate and aspartate with low density particles or cytoplasmic components.     

Changes in Mulberry Leaf Metabolism in Response to Water Stress

A series of experiments were conducted to characterize the water stress-induced changes in the activities of RuBP carboxylase (RuBPCO) and sucrose phosphate synthase (SPS), photosystem 2 activity, and contents of chlorophylls, carotenoids, starch, sucrose, amino acids, free proline, proteins and nucleic acids in mulberry (Morus alba L. cv. K-2) leaves. Water stress progressively reduced the activities of RuBPCO and SPS in the leaf extracts, the chlorophyll content, and PS2 activity in isolated chloroplasts. Plants exposed to drought showed lower content of starch and sucrose but higher total sugar content than control plants. While the soluble protein content decreased under water stress, the amino acid content increased. Proline accumulation (2.5-fold) was noticed in stressed leaves. A reduction in the contents of DNA and RNA was observed. Reduced nitrogen content was associated with the reduction in nitrate reductase activity. SDS-PAGE protein profile showed few additional proteins (78 and 92 kDa) in the water stressed plants compared to control plants.     

The effect of nitrogen source on levels of amino acids in peas

Summary When nitrogen fixation in peas was partially replaced by nitrate assimilation as the source of nitrogen, an increase was found in the amount of soluble nitrogen that could be extracted from the fruits of the plants, and within this soluble fraction, increases were found in the levels of some of the acidic amino acids. Levels of protein amino acids in the peas were generally unaffected by the type of nitrogen source except for the level of aspartic acid which was about 20 per cent lower in peas supplied with nitrate. No differences were found in the proportions of the essential amino acids.     

GABA/BABA priming: a means for enhancing abiotic stress tolerance potential of plants with less energy investments on defence cache

Abiotic stress is one of the major factors limiting plant growth and yield globally. Though substantial progress has been made in breeding and genetic manipulation of plants to enhance abiotic stress tolerance, the task remains as a challenge even today. Investigations on the priming activity of various chemicals in plants for enhancing abiotic stress tolerance have been undertaken over the past few years. Priming with γ-amino butyric acid (GABA) and β-amino butyric acid (BABA) gains greater attention, because priming with these non-protein amino acids equips the plants to resist abiotic stresses effectively without suffering costly energy investments in operating defence mechanisms. It is well documented that the protective effect of non-protein amino acids like BABA and GABA on plants is due to a potentiation of natural defence mechanisms against abiotic stresses but at the same time not activating the complete defence arsenal before the stress exposure. The exact mode of action of priming with GABA/BABA in plants is still a puzzle, though their importance as signaling molecules during stress is undoubtful. The better understanding of molecular, physiological, and ecological aspects of GABA/BABA priming might lead to the emergence of this technique as a successful strategy for enhancing the abiotic stress(es) tolerance potential of plants in the field, without compromising much on productivity.     

Free Amino Acids in Cigarette Smoke (I)–(II)

Free amino acids in the cigarette smoke, produced in constant-volume continuous smoking by the use of an artificial smoking-device, have been paper-chromatographically studied, and twelve amino acids, i.e., α-alanine, β-alanine, glycine, glutamic acid, glutamine, serine, γ-amino butyric acid, valine, leucine, aspartic acid, proline, and ornithine(?) were qualitatively identified. Besides these amino acids, the presence of the other two unidentified ninhydrin-positive substances was observed. The presence of ten amino acids, other than glutamic acid and glutamine, has not yet been reported in the literature concerning tobacco smoke.     

Tissue specificity of assimilation and transport of nitrogen in the root ofZea mays L. seedlings. II. Radial transport of14C-amino acids

The radial transport of organic nitrogen compounds was studied in maize seedling roots in relation to the metabolism of uniformly labelled¹⁴C-amino acids (alanine, arginine, dicarboxylic amino acids and their amides) in the cortex zone. Most active metabolism accompanying transport to the stele was observed for¹⁴C-glutamic acid of “primary” amino acids and for¹⁴C-glutamine of “reserve” nitrogen sources. The transport of¹⁴C-asparagine and¹⁴C-arginine to the conducting bundles is accompanied by weak metabolism. A distinguishing feature of nitrogen metabolism in the stele is intensive decarboxylation of glutamic acid, formed in the course of radial transport and metabolism, to gamma-amino butyric acid. This process is assisted by a highly active glutamate decarboxylase present in the conducting bundles.     

Accumulation,selection and covariation of amino acids in sieve tube sap of tansy (Tanacetum vulgare) and castor bean (Ricinus communis): evidence for the function of a basic amino acid transporter and the absence of a γ‐amino butyric acid transporter

Sieve tube sap was obtained from Tanacetum by aphid stylectomy and from Ricinus after apical bud decapitation. The amino acids in sieve tube sap were analyzed and compared with those from leaves. Arginine and lysine accumulated in the sieve tube sap of Tanacetum more than 10‐fold compared to the leaf extracts and they were, together with asparagine and serine, preferably selected into the sieve tube sap, whereas glycine, methionine/tryptophan and γ‐amino butyric acid were partially or completely excluded. The two basic amino acids also showed a close covariation in sieve tube sap. The acidic amino acids also grouped together, but antagonistic to the other amino acids. The accumulation ratios between sieve tube sap and leaf extracts were smaller in Ricinus than in Tanacetum. Arginine, histidine, lysine and glutamine were enriched and preferentially loaded into the phloem, together with isoleucine and valine. In contrast, glycine and methionine/tryptophan were partially and γ‐amino butyric acid almost completely excluded from sieve tube sap. The covariation analysis grouped arginine together with several neutral amino acids. The acidic amino acids were loaded under competition with neutral amino acids. It is concluded from comparison with the substrate specificities of already characterized plant amino acid transporters, that an AtCAT1‐like transporter functions in phloem loading of basic amino acids, whereas a transporter like AtGAT1 is absent in phloem. Although Tanacetum and Ricinus have different minor vein architecture, their phloem loading specificities for amino acids are relatively similar.     

The ectomycorrhizal fungus (Paxillus involutus) modulates leaf physiology of poplar towards improved salt tolerance

Ectomycorrhizas (EMs) are mutualistic associations between soil fungi and plant roots. Although the physical interaction occurs only in roots, mycorrhizas may alter the physiology of the whole plant, resulting in changes in host responses to abiotic stress. To elucidate the influence of an ectomycorrhizal fungus on leaf physiology and performance under salt stress, we analysed the levels of nutrient elements, phytohormones, carbohydrates, amino compounds and fatty acids in leaves of Populus × canescens. The poplars were cultivated either in the presence or absence of Paxillus involutus and either with or without salt stress imposed by 150 mM NaCl. Leaves of ectomycorrhizal plants displayed higher quantum yield of photochemistry (Φ_PSII), increased concentrations of phosphorus and potassium, decreased concentrations of galactose, increased concentrations of the stress metabolite γ-amino butyric acid and a lower unsaturated-to-saturated fatty acid ratios than those of non-ectomycorrhizal plants. Salt exposure of P. × canescens led to leaf chlorosis and shedding, decreases in Φ_PSII, K⁺-to-Na⁺ ratio, 9Z-hexadecenoic acid, 9Z-octadecenoic acid and unsaturated-to-saturated fatty acid ratio, and increases in ABA, glucose, fructose and some amino compounds. Under salinity leaves of ectomycorrhizal plants showed an alleviation of leaf chlorosis, improved water status, higher Φ_PSII and K⁺-to-Na⁺ ratio, less accumulation of major amino compounds and lower unsaturated-to-saturated fatty acid ratios than those of non-mycorrhizal plants. These results indicate that ectomycorrhizas attenuate salinity induced injury in leaves of P. × canescens, which may be ascribed to the improved nutrient status, osmo-regulation and changes in fatty acid composition in leaves of ectomycorrhizal plants.     

An Analysis of the Effects of Tobacco Mosaic Virus on Growth and the Changes in the Free Amino Compounds in Young Tomato Plants

Tomato plants at the four-leaf stage were inoculated on thefirst leaf with TMV in a growth room and the effects studiedin systemically-mfected leaves with reference to growth, virusmultiplication and changes in water, nitrogen, nitrate and chlorophyllcontents. Parallel changes in the free amino compounds werealso studied in the growth room (incident radiation 152 mwhcm^-2 day^-1) and in two experiments in a glasshouse (352 and226 mwh cm^-2 day^-1) Dry matter accumulation and leaf expansion in leaves 3, 4 and5 were checked by TMV 5–7 days after inoculation but notin leaf 2. In the period 7–25 days after inoculation therelative growth rates of whole plant and leaves 3 and 6 andnet assimilation rate were not affected by TMV. Stem heightand dry weight were not affected by TMV but ‘root’dry weight was reduced from days 5–15. Virus was presentin the stem and in leaves 2 and 6 by days 3, 5 and 15 respectivelyso that infection per se did not always check growth. Chlorophyllcontent of systemically-infected leaves was reduced 10 daysafter inoculation. Total N and ammonia contents were not affectedby TMV but infected leaves contained less nitrate. At the two lower levels of incident radiation the initial effectof TMV was to reduce the content of total free protein aminoacids and amides, which were minimal 5–7 days after inoculation.In the glasshouse experiment a reduction could be measured only1 day after inoculation or before virus was present in the youngerleaves. With high incident radiation there was no initial reductionbut an increase at day 13 when mottling symptoms were visible.Total non-protein amino acids, of which amino butyric acidwas the major constituent, were increased by TMV in all threeexperiments for up to 13 days after inoculation. It is suggested that inoculation of a leaf with TMV temporarilyinterferes with export of photosynthates and import of root-synthesisedamino acids and that the results reported above can be interpretedin this context. Evidence in support of this is adduced froman experiment in which 13 foliar sprays of gibberellic acid(2·5 ppm) were combined with TMV inoculation and thechanges in free amino compounds followed. It is concluded that analyses of the changes in individual freeamino compounds are unlikely to provide useful information concerningthe sources of virus coat protein.     

Nitrate Assimilation in Higher Plants with Special Reference to the Cocklebur (Xanthium pennsylvanicum Wallr.)

A soluble NADH-dependent nitrate reductase is described forthe shoot system of Xanthium. Young leaves and immature stemtissues contain high levels of the enzyme. They are relativelyrich in free amino acids and amides but store little free nitrate.The specific activity of the enzyme is lower in fully expandedleaves, although these leaves exhibit higher rates of fixationof carbon in photosynthesis than do younger leaves. Neithernitrate nor free amino acids accumulate in the mesophyll ofthe leaf. Older parts of the stem axis accumulate large amountsof soluble nitrogen, almost entirely as free nitrate. Reservesof nitrate in the shoot and root are rapidly depleted if nitrateis removed from the external medium. Nitrate reductase is apparently absent from roots of Xanthium.This finding is supported by analyses of bleeding sap from nitrate-fedplants which show that 95 per cent of the nitrogen exportedfrom roots is present as free nitrate. However, roots are capableof synthesizing and exporting large amounts of amino nitrogenif supplied with reduced nitrogen such as urea or ammonium. A scheme is presented summarizing the main features of the metabolismof nitrate in Xanthium and this is compared with the situationin nitrate-fed plants of the field pea (Pisum arvense L.), aspecies previously shown to be capable of reducing nitrate inits root system.