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1.
The use of the relative ureide content of xylem sap [(ureide-N/total N) × 100] as an indicator of N 2 fixation in soybeans (Merr.) was examined under greenhouse conditions. Acetylene treatments to inhibit N 2 fixation were imposed upon the root systems of plants totally dependent upon N 2 fixation as their source of N and of plants dependent upon both N 2 fixation and uptake of exogenous nitrate. Significant decreases in the total N concentration of xylem sap from plants of the former type were observed, but no significant decrease was observed in the total N concentration of sap from the latter type of plants. In both types of plants, acetylene treatment caused significant decreases in the relative ureide content of xylem sap. The results provided further support for a link between the presence of ureides in the xylem and the occurrence of N 2 fixation in soybeans. The relative ureide content of xylem sap from plants totally dependent upon N 2 fixation was shown to be insensitive to changes in the exudation rate and total N concentration of xylem sap brought about by diurnal changes in environmental factors. There was little evidence of soybean cultivars or nodulating strains affecting the relative ureide content of xylem sap. `Ransom' soybeans nodulated with Rhizobium japonicum strain USDA 110 were grown under conditions to obtain plants exhibiting a wide range of dependency upon N 2 fixation. The relative ureide content of xylem sap was shown to indicate reliably the N 2 fixation of these plants during vegetative growth using a 15N method to measure N 2 fixation activity. The use of the relative ureide content of xylem sap for quantification of N 2 fixation in soybeans should be evaluated further. 相似文献
2.
Xylem sap composition was examined in nodulated and nonnodulated cowpea ( Vigna unguiculata [L.] Walp.) plants receiving a range of levels of NO 3 and in eight other ureide-forming legumes utilizing NO 3 or N 2 as sole source of nitrogen. A 15N dilution technique determined the proportions of plant nitrogen derived from N 2 in the nodulated cowpeas fed NO 3. Xylem sap composition of NO 3-fed, nodulated cowpea varied predictably with the relative extents to which N 2 and NO 3 were being utilized. The ratios of asparagine to glutamine (N/N) and of NO 3 to ureide (N/N) in xylem sap increased with increasing dependence on NO 3 whereas per cent of xylem nitrogen as ureide and the ratio of ureide plus glutamine to asparagine plus NO 3 (N/N) in xylem sap increased with increasing dependence on N 2 fixation. The amounts of NO 3 and ureides stored in leaflets, stems plus petioles, and roots of cowpea varied in a complex manner with level of NO 3 and the presence or absence of N 2 fixation. All species showed higher proportions of organic nitrogen as ureide and several-fold lower ratios of asparagine to glutamine in their xylem sap when relying on N 2 than when utilizing NO 3. In nodulated (minus nitrate) cowpea and mung bean ( Vigna radiata [L.] Wilczek) the percentage of xylem nitrogen as ureide remained constant during growth but the ratio of asparagine to glutamine varied considerably. The biochemical significance of the above differences in xylem sap composition was discussed. 相似文献
3.
Experiments were conducted to characterize the distribution of N compounds in the xylem sap of nodulated and nonnodulated soybean plants through development and to determine the effects of exogenous N on the distribution of N compounds in the xylem. Xylem sap was collected from nodulated and nonnodulated greenhouse-grown soybean plants ( Glycine max [L.] Merr. “Ransom”) from the vegetative phase to the pod-filling phase. The sum of the nitrogen in the amino acid, nitrate, ureide (allantoic acid and allantoin), and ammonium fractions of the sap from both types of plants agreed closely with total N as assayed by a Kjeldahl technique. Sap from nodulated plants supplied with N-free nutrient solution contained seasonal averages of 78 and 20% of the total N as ureide-N and amino acid-N, respectively. Sap from nonnodulated plants supplied with a 20 millimolar KNO 3 nutrient solution contained seasonal averages of 6, 36, and 58% of total N as ureide-N, amino acid-N, and nitrate-N, respectively. Allantoic acid was the predominant ureide in the xylem sap and asparagine was the predominant amino acid. When well nodulated plants were supplied with 20 millimolar KNO 3, beginning at 65 days, C 2H 2 reduction (N 2 fixation) decreased relative to nontreated plants and there was a concomitant decrease in the ureide content of the sap. A positive correlation ( r = 0.89) was found between the ureide levels in xylem sap and nodule dry weights when either exogenous nitrate-N or urea-N was supplied at 10 and 20 millimolar concentrations to inoculated plants. The results demonstrate that ureides play a dominant role in N transport in nodulated soybeans and that the synthesis of ureides is largely dependent upon nodulation and N 2 fixation. 相似文献
4.
Xylem sap and phloem exudates from detached leaves and fruit tips were collected and analyzed during early pod-fill in nodulated soybeans ( Glycine max (L.) Merr. cv Wilkin) grown without (−N) and with (+N) NH 4NO 3. Ureides were the predominant from (91%) of N transported in the xylem of −N plants, while amides (45%) and nitrate (23%) accounted for most of the N in the xylem of +N plants. Amino acids (44%) and ureides (36%) were the major N forms exported in phloem from leaves in −N plants, but amides (63%) were most important in +N plants. Based on the composition of fruit tip phloem, ureides (55% and 33%) and amides (26% and 47%) accounted for the majority of N imported by fruits of −N and +N plants, respectively. C:N weight ratios were lowest in xylem exudate (1.37 and 1.32), highest in petiole phloem (24.5 and 26.0), and intermediate in fruit tip exudate (12.6 and 12.1) for the −N and +N treatments, respectively. The ratios were combined with data on fruit growth and respiration to construct a model of C and N transport to developing fruits. The model indicates xylem to phloem transfer provides 35% to 52% of fruit N. Results suggest the phloem entering fruits oversupplies their N requirement so that 13% of the N imported is exported from fruit in the xylem. 相似文献
5.
The vasculature of the dorsal suture of cowpea ( Vigna unguiculata [L.] Walp) fruits bled a sugar-rich exudate when punctured with a fine needle previously cooled in liquid N 2. Bleeding continued for many days at rates equivalent to 10% of the estimated current sugar intake of the fruit. A phloem origin for the exudate was suggested from its high levels (0.4-0.8 millimoles per milliliter) of sugar (98% of this as sucrose) and its high K + content and high ratio of Mg 2+ to Ca 2+. Fruit cryopuncture sap became labeled with 14C following feeding of [ 14C]urea to leaves or adjacent walls of the fruit, of 14CO 2 to the pod gas space, and of [ 14C] asparagine or [ 14C]allantoin to leaflets or cut shoots through the xylem. Rates of translocation of 14C-assimilates from a fed leaf to the puncture site on a subtended fruit were 21 to 38 centimeters per hour. Analysis of 14C distribution in phloem sap suggested that [ 14C]allantoin was metabolized to a greater extent in its passage to the fruit than was [ 14C] asparagine. Amino acid:ureide:nitrate ratios (nitrogen weight basis) of NO 3-fed, non-nodulated plants were 20:2:78 in root bleeding xylem sap versus 90:10:0.1 for fruit phloem sap, suggesting that the shoot utilized NO 3-nitrogen to synthesize amino acids prior to phloem transfer of nitrogen to the fruit. Feeding of 15NO 3 to roots substantiated this conclusion. The amino acid:ureide ratio (nitrogen weight basis) of root xylem sap of symbiotic plants was 23:77 versus 89:11 for corresponding fruit phloem sap indicating intense metabolic transfer of ureide-nitrogen to amino acids by vegetative parts of the plant. 相似文献
6.
Nodulated and denodulated roots of adzuki bean ( Vigna angularis), soybean ( Glycine max), and alfalfa ( Medicago sativa) were exposed to 14CO 2 to investigate the contribution of nodule CO 2 fixation to assimilation and transport of fixed nitrogen. The distribution of radioactivity in xylem sap and partitioning of carbon fixed by nodules to the whole plant were measured. Radioactivity in the xylem sap of nodulated soybean and adzuki bean was located primarily (70 to 87%) in the acid fraction while the basic (amino acid) fraction contained 10 to 22%. In contrast, radioactivity in the xylem sap of nodulated alfalfa was primarily in amino acids with about 20% in organic acids. Total ureide concentration was 8.1, 4.7, and 0.0 micromoles per milliliter xylem sap for soybean, adzuki bean, and alfalfa, respectively. While the major nitrogen transport products in soybeans and adzuki beans are ureides, this class of metabolites contained less than 20% of the total radioactivity. When nodules of plants were removed, radioactivity in xylem sap decreased by 90% or more. Pulse-chase experiments indicated that CO 2 fixed by nodules was rapidly transported to shoots and incorporated into acid stable constituents. The data are consistent with a role for nodule CO 2 fixation providing carbon for the assimilation and transport of fixed nitrogen in amide-based legumes. In contrast, CO 2 fixation by nodules of ureide transporting legumes appears to contribute little to assimilation and transport of fixed nitrogen. 相似文献
7.
During the period examined from 12 to 63 days after planting, the ureides, allantoin and allantoic acid, were the predominant nitrogenous solutes in the xylem exudate of soybeans ( Glycine max [L.]) growing solely on symbiotically fixed nitrogen, accounting for approximately 60% and greater than 95% of the total nitrogen in the xylem exudate before and after the onset of active nitrogen fixation, respectively. For plants between 18 and 49 days of age, the apparent rate of ureide export estimated from concentrations of ureides in xylem exudate collected over a period of one hour was closely related to the rate of nitrogen fixation estimated from measurements of C 2H 2 reduction by nodulated root systems. After this time, the apparent rate of ureide export per plant continued to increase, reaching a maximum value at day 63 of 12 micromoles per plant per hour, even though the rate of C 2H 2 reduction per plant declined approximately four-fold. The most probable pathway for the biosynthesis of ureides involves the catabolism of purines. The levels of phosphoribosylpyrophosphate ( PRPP) synthetase, which catalyzes the formation of the PRPP required for purine synthesis, increased in parallel with the rates of nitrogen fixation (C 2H 2) from day 18 reaching a maximum value of 13.9 micromoles per plant per hour at day 49, and then both activities declined rapidly. During the period of active nitrogen fixation the ratio of PRPP synthesis estimated from measurements of PRPP synthetase activity in cell-free extracts to the apparent rate of ureide export was between 1 and 2. The activities of the enzymes of purine catabolism, xanthine dehydrogenase, uricase, and allantoinase, increased in parallel with the increases in nodule mass and the export of ureides with maximum activities of 13, 119, and 79 micromoles per plant per hour, corresponding with apparent rates of ureide export in the range of 9.5 to 11.9 micromoles per plant per hour. These results demonstrate that there is a close association between nitrogen fixation, PRPP synthetase activity, and ureide export in soybeans and support the proposal that recently-fixed nitrogen is utilized in the de novo synthesis of purines which are subsequently catabolized to produce the ureides. 相似文献
8.
The ureides, allantoin and allantoic acid, are the major nitrogenous substances transported within the xylem of N 2-fixing soybeans ( Glycine max L. Merr. cv Amsoy 71). The ureides accumulated in the cotyledons, roots and shoots of soybean seedlings inoculated with Rhizobium or grown in the presence of 10 millimolar nitrate. The patterns of activity for uricase and allantoinase, enzymes involved in ureide synthesis, were positively correlated with the accumulation of ureides in the roots and cotyledons. Allopurinol and azaserine inhibited ureide production in 3-day-old cotyledons while no inhibition was observed in the roots. Incubation of 4-day-old seedlings with [ 14C]serine indicated that in the cotyledons ureides arose via de novo synthesis of purines. The source of ureides in both 3- and 4-day-old roots was probably the cotyledons. The inhibition of ureide accumulation by allopurinol but not azaserine in 8-day-old cotyledons suggested that ureides in these older cotyledons arose via nucleotide breakdown. Incubation of 8-day-old plants with [ 14C]serine suggested that the roots had acquired the capability to synthesize ureides via de novo synthesis of purines. These data indicate that both de novo purine synthesis and nucleotide breakdown are involved in the production of ureides in young soybean seedlings. 相似文献
9.
The synthesis, transport and assimilation of the ureides, allantoin and allantoic acid, in higher plants is reviewed. Evidence indicates that in nodulated legumes ureides are synthesized from products of N 2-fixation via purine synthesis and degradation. Their synthesis in other plants also appears to be via purine degradation but is dependent on the inorganic nitrogen source fed to the plant; greatest ureide production is associated with ammonium assimilation. The use of ureides rather than amides for N-transport from the root to the shoot via the xylem stream results in an improved carbon economy of the plant. Good evidence for the transport of ureides in the phloem is lacking for most species examined although it is assumed to be important, particularly in fruit and seed development. Ureides are stored and assimilated mainly in the shoot. The precise pathways, localization and regulation of ureide assimilation are poorly understood and require further investigation. Similarities exist between the properties of the enzymes involved in ureide assimilation in higher plants and in micro-organisms. However, the evidence that light appears to be involved in ureide assimilation in green tissues suggests that different regulatory mechanisms may exist in plants compared with micro-organisms. The economically important legume crops such as soybeans, cowpeas and Phaseolus sp. are all ureide producers. To aid our understanding of the productivity of these plants knowledge of how ureide-N is converted into seed protein is essential. 相似文献
10.
Abstract The ureide content of soybean ( Glycine max (L.) Merr.) nodules was unaffected by variations in the transpirational rate, while whole plant manipulations designed to decrease phloem supply to nodules resulted in lower rates of nitrogenase activity and an increase in the ureide content of the nodules. The rate of ureide export from the nodule was estimated from the exponential rate of decrease in the pool size of ureides in nodules, following exposure to an N 2-free atmosphere (Ar:O 2). Export was greatly reduced under treatments which reduced phloem supply to the nodule. A water budget for nodules suggested that the delivery of water to the nodule via mass flow in the phloem was comparable to that required for export of ureides from the nodule in the xylem from the nodule. Therefore, we suggest that xylem export from nodules is related to the phloem supply to the nodule rather than to the transpirational flux in the parent root. This suggestion is related to the reported decreases in nodule permeability to gases under conditions of phloem deprivation. 相似文献
11.
Nodulation, nitrogen (N 2) fixation and xylem sap composition were examined in sand cultured plants of Bambara groundnut ( Vigna subterranea L.) and Kersting's bean ( Macrotyloma geocarpum L.) inoculated with Bradyrhizobium strain CB756 and supplied via the roots for a 4 week period from the third week onwards with different levels of ( 15N)-nitrate (0–15 m M). The separate contributions of nitrate and N 2 to plant nitrogen were measured by isotope dilution. Increasing levels of nitrate inhibited nodule growth (measured as dry
matter or nodule N) of both species parallel with decreased dependence on symbiotically-fixed N. Specific nodule activity
(N 2 fixed g nodule dry −1 d −1 of nodules) was reduced progressively with time in V. subterranea at higher (5 or 15 m M) levels of NO 3, but this was not so for M. geocarpum. Root xylem bleeding sap of both species showed ureides (allantoin and allantoic acid) as predominant (>90%) solutes of nitrogen
when plants were relying solely on atmospheric N. Levels of ureide and glutamine decreased and those of asparagine and nitrate
in xylem increased with increasing level of applied nitrate. Relative levels of xylem ureide-N were positively correlated
(R 2=0.842 for M. geocarpum and 0.556 for V. subterranea), and the ratio of asparagine to glutamine in xylem exudate negatively correlated (R 2=0.955 for M. geocarpum and 0.736 for V. subterranea) with plant reliance on nitrogen fixation. The data indicate that xylem sap analyses might be useful for indirect field assays
of nitrogen fixation by the species and that Kersting's bean might offer some potential as a symbiosis in which N 2 fixation is relatively tolerant of soil N. 相似文献
12.
Utilising the 15N dilution technique the relationship between the proportion of N derived from N 2 fixation and relative abundance of ureides in xylem sap was evaluated for Phaseolus vulgaris L. cv. Mokcham during vegetative and reproductive development. In order to establish calibration curves for time integrated estimates of N 2 fixation, plants were raised in sand culture during the dry season in northern Thailand and continuously supplied with a N-free nutrient solution or the same solution amended with 0, 3, 6 or 9 mol m –3 nitrate. Large changes in plant dependence on N 2 fixation were concomitantly reflected by corresponding alterations in N solutes in xylem sap. Regression analyses of the data suggested high correlations between relative ureide content and N 2 fixation, but different slopes and line intercepts indicated the requirement for the use of calibration curves established for different phases of the development of the plant. Largest age related differences were noted between vegetative and reproductive development. Judging from 95% confidence limits, utilisation of appropriate calibrations can reduce errors of the technique to close to ±5%.A second experiment, involving similarly cultivated plants exposed to different sources of mineral N, indicated an effect of ammonium on xylem sap composition. This implies that calibrations, in which N 2 fixation is regulated only by applications of various concentrations of nitrate, may lead to errors in situations where a major proportion of the plant available soil nitrogen fraction is present in the form of ammonium. 相似文献
13.
Nitrogen fixation was estimated in `Bragg,' `Forrest,' and `Bethel' soybean ( Glycine max [L.] Merrill) from seven locations northwest of New South Wales, Australia, by relating ureide and nitrate contents of plant parts sampled at regular intervals during growth to standard curves derived under controlled nitrate regimes. Estimates were combined with data on crop growth and mineral N contents of soils to (a) determine the total requirements for N by the crops, (b) determine the contributions of N 2 fixation to crop growth, and (c) relate symbiotic dependence ([N 2 fixed/total plant N] × 100) of the crops to levels of mineral N in the soil at sowing. At two locations, Myall Vale and Glenara, levels of ureides in the shoot axes and roots of unnodulated seedlings were surprisingly high at the first time of sampling, perhaps reflecting effects of uptake of ammonium-N by the soybeans or breakdown and remobilization of cotyledonary protein. Ureides in plant parts declined significantly by the second (V5 to V7 growth stage) sampling. Subsequently, ureide contents increased whereas levels of nitrate in plant parts decreased. The relative abundance of ureides ([ureide-N/ureide-N + nitrate-N] × 100) in the shoot axes and nodulated roots of both crops increased linearly from almost zero during mid-vegetative growth (V5 to V7) to virtually 100% during late reproductive growth (R4 to R5, Myall Vale and R6, Glenara). The data suggest a steady transition in soybeans at both locations from dependence upon mineral N for early growth to complete reliance upon fixed N during late reproductive growth. Estimates of seasonal N 2 fixation for soybeans at the seven locations ranged from 73 to 288 kilograms per hectare N (shoot axes ureides) and from 147 to 337 kilograms per hectare N ha (nodulated roots ureides). 相似文献
14.
The yield of N in maize ( Zea mays L.) and ricebean ( Vigna umbellata [Thumb.] Ohwi and Ohashi) were compared on a Tropoqualf soil in North Thailand in 1984 and 1985. Both species were grown
in field plots in monoculture or as intercrops at a constant planting density equivalent to 8 maize or 16 ricebean plants
per m 2. The contribution of symbiotic N 2 fixation to ricebean growth was estimated from measurements of the natural abundance of 15N (δ 15N) in shoot nitrogen and from analysis of ureides in xylem sap vacuumextracted from detached stems.
The natural abundance of 15N in the intercropped ricebean was found to be considerably less than that in monoculture in both growing seasons. Using maize
and a weed ( Ageratum conyzoides L.) as non-fixing 15N reference plants the proportions ( P
15N) of ricebean shoot N derived from N 2 fixation ranged from 0.27 to 0.36 in monoculture ricebean up to 0.86 when grown in a 75% maize: 25% ricebean intercrop. When
glasshouse-derived calibration curves were used to calculate plant proportional N 2 fixation ( Pur) from the relative ureide contents of field collected xylem exudates, the contribution of N 2 fixation to ricebean N yields throughout the 1985 growing season were greater in intercrop than in monocrop even at the lowest
maize:legume ratio (25∶75). Seasonal patterns of sap ureide abundance indicated that N 2 fixation was greatest at the time of ricebean podset. The average Pur and P
15N in ricebean during the first 90 days of growth showed identical rankings of monocrop and intercrop treatments in terms of
N 2 fixation, although the two sets of P values were different. Nonetheless, seasonal estimates of N 2 fixation during the entire 147 days of legume growth determined from ureide analyses indicated that equivalent amounts of
N could be fixed by ricebean in a 75∶25 intercrop and in monoculture despite the former being planted at one-quarter the density. 相似文献
15.
The ureides, allantoin and allantoic acid, represented major fractions of the soluble nitrogen pool of nodulated plants of cowpea ( Vigna unguiculata [L.] Walp. cv. Caloona) throughout vegetative and reproductive growth. Stem and petioles were the principal sites of ureide accumulation, especially in early fruiting. Labeling studies using 14CO2 and 15N2 and incubation periods of 25 to 245 minutes indicated that synthesis of allantoin and allantoic acid in root nodules involved currently delivered photosynthate and recently fixed N, and that the ureides were exported from nodule to shoot via the xylem. From 60 to 80% of xylem-borne N consisted of ureides; the remainder was glutamine, asparagine, and amino acids. Allantoin predominated in the soluble N fraction of nodules and fruits, allantoin and allantoic acid were present in approximately equal proportions in xylem exudate, stems, and petioles. Extracts of the plant tissue fraction of nitrogen-fixing cowpea nodules contained glutamate synthase (EC 2.6.1.53) and glutamine synthetase (EC 6.3.1.2), but little activity of glutamate dehydrogenase (EC 1.4.1.3). High levels of uricase (EC 1.7.3.3) and allantoinase (EC 3.5.2.5) were also detected. Allantoinase but little uricase was found in extracts of leaflets, pods, and seeds. Balance sheets were constructed for production, storage, and utilization of ureide N during growth. Virtually all (average 92%) of the ureides exported from roots was metabolized on entering the shoot, the compounds being presumably used as N sources for protein synthesis. 相似文献
16.
During vegetative regrowth of Medicago sativa L., soil N, symbiotically fixed N 2 and N reserves meet the nitrogen requirements for shoot regrowth. Experiments with nodulated or non-nodulated plants were carried out to investigate the changes in N flows originating from the different N sources and in xylem transport of amino acids during regrowth. Exogenous N uptake, N 2 fixation and endogenous N remobilization were estimated by 15N labelling and amino acids in xylem sap were analysed. Removal of shoots resulted in great declines of exogenous N flows derived either from N 2 or from NH 4NO 3 during the first week of regrowth, thereafter recovery increased linearly. Mineral N uptake as well as N 2 fixation occurred mainly between the 10th and 18th day after removal of shoots while exogenous N assimilation in intact plants remained at a steady level. Nitrogen remobilization rates in defoliated plants increased by at least three to five-fold, especially during the first 10 days following shoot removal. Compared to control plants, contents of amino acids in xylem sap, during the first 10 days of regrowth, were reduced by about 72% and 82% in NH 4NO 3 grown and in N 2 fixing plants, respectively. Asparagine was the main amino acid transported in xylem sap of both treated plants. Its relative contents during this period significantly decreased from 75% to 59% and from 67% to 36% respectively in non-nodulated plants and in nodulated ones. This decline was accompanied by compensatory increase in the relative contents of aspartate and glutamine. 相似文献
17.
Nitrogen form has been shown to affect Zn uptake, translocation and storage in the Zn-hyperaccumulating plant Noccaea caerulescens but the biochemical processes are not fully understood. Organic acids and amino acids have been implicated in Zn transport and storage. This study aimed to examine the effect of N form on concentrations of organic acids and amino acids and how these metabolites correlated with Zn hyperaccumulation. Plants were grown in nutrient solution with NO 3−, NH 4NO 3 or NH 4+, supplied with 50 or 300 μM Zn, and buffered at either pH 4.5 or 6.5. The metabolomic profile was determined by gas chromatography mass spectroscopy. The concentration of Zn in shoots, xylem and roots was greatest for the NO 3−, pH 6.5 and 300 μM Zn treatments. For all N forms, the lower growth-medium pH raised xylem sap pH but had no influence on Zn concentration or exudation rate of the xylem sap. Nitrate enhanced organic acid production while NH 4+ increased amino acid production. Organic acids in the xylem were more responsive to changes in growth-medium pH than N form, and did not correlate with Zn concentration in shoots, roots or xylem. Serine might be directly involved in Zn hyperaccumulation. Phosphoric acid was associated with reduced Zn accumulation in the shoots. Malic acid was not detected in the shoots but responded to cation uptake more than to Zn specifically in the roots. Citric acid responded to cation uptake more than to Zn specifically in the shoots but did not correlate with Zn concentration in the roots or the xylem sap, or any other cations in the roots. In conclusion, organic acids in N. caerulescens are not specifically involved in Zn hyperaccumulation but are involved in regulating pH in the xylem and cation–anion balance in plants. 相似文献
18.
The response of nonnodulated white lupin ( Lupinus albus L. cv. Ultra) plants to a range of NO 3 levels in the rooting medium was studied by in vitro assays of extracts of plant parts for NO 3 reductase (EC 1.6.6.1) activity, measurements of NO 3-N in plant organs, and solute analyses of root bleeding (xylem) sap and phloem sap from stems and petioles. Plants were grown for 65 days with 5 millimolar NO 3 followed by 10 days with 1, 5, 15, or 30 millimolar NO 3. NO 3 reductase was substrate-induced in all tissues. Roots contained 76, 68, 62 and 31% of the total NO 3 reductase activity of plants fed with 1, 5, 15, and 30 millimolar NO 3, respectively. Stem, petioles, and leaflets contained virtually all of the NO 3 reductase activity of a shoot, the activity in extracts of fruits amounting to less than 0.3% of the total enzyme recovered from the plant. Xylem sap from NO 3-grown nonnodulated plants contained the same organic solutes as from nodulated plants grown in the absence of combined N. Asparagine accounted for 50 to 70% and glutamine 10 to 20% of the xylem-borne N. The level of NO 3 in xylem sap amounted to 4, 13, 12, and 17% of the total xylem N at 1, 5, 15, and 30 millimolar NO 3, respectively. Xylem to phloem transfer of N appeared to be quantitatively important in supplying fruits and vegetative apices with reduced N, especially at low levels of applied NO 3. NO 3 failed to transfer in any quantity from xylem to phloem, representing less than 0.3% of the phloem-borne N at all levels of applied NO 3. Shoot organs were ineffective in storing NO 3. Even when NO 3 was supplied in great excess (30 millimolar level) it accounted for only 8% of the total N of stem and petioles, and only 2 and 1% of the N of leaflets and fruits, respectively. 相似文献
19.
It is well established that nitrate is a potent inhibitor of nodulation and nitrogen fixation in legumes. The objective of
this study was to demonstrate the relative insensitivity of these processes to nitrate with Calopogonium mucunoides, a tropical South American perennial legume, native to the cerrado (savannah) region. It was found that nodule number was
reduced by about half in the presence of high levels of nitrate (15 mM) but nodule growth (total nodule mass per plant) and
nitrogen fixation (acetylene reduction activity and xylem sap ureide levels) were not affected. Other sources of N (ammonium
and urea) were also without effect at these concentrations. At even higher concentrations (30 mM), nitrate did promote significant
inhibition (ca. 50%) of acetylene reduction activity, but no significant reduction in xylem sap ureides was found. The extraordinary
insensitivity of nodulation and N 2 fixation of C. mucunoides to nitrate suggests that this species should be useful in studies aimed at elucidating the mechanisms of nitrate inhibition
of these processes. 相似文献
20.
The distribution of NO 3? reduction between roots and shoots was studied in hydro-ponically-grown peach-tree seedlings ( Prunus persica L.) during recovery from N starvation. Uptake, translocation and reduction of NO 3?, together with transport through xylem and phloem of the newly reduced N were estimated, using 15N labellings, in intact plants supplied for 90 h with 0.5 m M NH 4+ and 0.5, 1.5 or 10 m M NO 3?. Xylem transport of NO 3? was further investigated by xylem sap analysis in a similar experiment. The roots were the main site of NO 3? reduction at all 3 levels of NO 3? nutrition. However, the contribution of the shoots to the whole plant NO 3? reduction increased with increasing external NO 3? availability. This contribution was estimated to be 20, 23 and 42% of the total assimilation at 0.5, 1.5 and 10 m M NO 3?, respectively. Both 15N results and xylem sap analysis confirmed that this trend was due to an enhancement of NO 3? translocation from roots to shoots. It is proposed that the lack of NO 3? export to the shoots at low NO 3? uptake rate resulted from a competition between NO 3? reduction in the root epidermis/cortex and NO 3? diffusion to the stele. On the other hand, net xylem transport of newly reduced N was very efficient since ca 70% of the amino acids synthesized in the roots were translocated to the shoots, regardless of the level of NO 3? nutrition. This net xylem transport by far exceeded the net downward phloem transport of the reduced N assimilated in shoots. As a consequence, the reduced N resulting from NO 3? assimilation, principally occurring in the roots, was mainly incorporated in the shoots. 相似文献
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