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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.
The role of lateral root nodules in N 2 fixation and the relationships between total shoot N and several traits which influence or control N 2 fixation in common bean ( Phaseolus vulgaris L.) i.e., acetylene reduction value, specific nodule activity, leghemoglobin concentration, total leghemoglobin and nodule mass, were investigated in field studies. Significant variation among bean lines was observed for all the traits measured. Lines varied for the proportion of total N accumulated up to the R3 growth state, thus measurements of total shoot N near maturity ( e.g., R7) provided a better estimate of total N 2 fixation than measurements taken at an early growth stage. Nodule mass was correlated with acetylene reduction and total leghemoglobin, and total leghemoglobin was correlated with acetylene reduction value. Total shoot N at R7 was correlated with seasonal means of nodule mass and number, acetylene reduction value and total leghemoglobin. For all traits except total leghemoglobin, values for lateral roots were more highly correlated with total shoot N than were values for either crown roots or the whole root system. Seed yield was most highly correlated with nodule mass of the lateral roots. These results will be useful in devising breeding strategies for improved N 2 fixation of the host plant. 相似文献
3.
野生大豆(Glycinesoja)酰脲含量与根瘤固氮活力的关系朱长甫,苗以农,刘学军,许守民(东北师范大学生命科学学院,长春130024)郑惠玉,徐豹(吉林省农业科学院大豆研究所,公主岭136100)关键词:野生大豆,固氮活力,酰脲,蛋白质根据固氮豆... 相似文献
4.
Non-nodular tissue of soybean ( Glycine max L. Merrill) plants grown hydroponically in the absence of added N have a 15N abundance close to that of atmospheric N 2. In contrast, nodules are usually enriched in 15N. In this paper, we report measurements of the 15N abundance of foliar tissue and nodules of soybeans inoculated with 11 variably efficient strains of Rhizobum japonicum and grown hydroponically with no added N. The efficiency of the 11 symbioses varied over a wide range as judged by a 16-fold difference in N content. The degree of 15N enrichment of nodules was closely correlated with N 2-fixing efficiency (milligrams N fixed per milligram N in the nodules). These results confirm prior preliminary data based on six variably efficient R. japonicum strains. The strong correlation between NN enrichment of soybean nodules and N2-fixing efficiency is consistent with the hypothesis that new nodule tissue is synthesized from a pool of recently fixed N within the same nodule. 相似文献
5.
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
6.
Nitrogen fixation by field-grown soybean ( Glycine max [L.] Merrill) was assessed by the natural 15N abundance and ureide methods. The field sites (five) and genotypes (six, plus two levels of inoculation on Bragg) were chosen to provide a range of proportions of plant N derived from nitrogen fixation ( P). Genotypes K466, K468, nts1007, and nts1116 and Davis were included on the basis of their reported tolerance of the suppressive effects of nitrate on nodulation and nitrogen fixation. Bragg was included as a `nitrate-sensitive' genotype. Seeds of all genotypes were inoculated at sowing with Bradyrhizobium japonicum CB1809 (USDA136). Amounts of nitrate in the soil profile (0-1.2 meter depth) at sowing ranged from 70 (site 3) to 278 kilograms per hectare (site 5), resulting in large effects on plant nodulation, on the δ 15N values of nodulated plants, on the relative abundance of ureide-N in vacuum-extracted sap (VES) and stem extracts, and finally on the estimates of P. There was no relationship between amount of soil nitrate at sowing and the δ 15N of the plant-available soil N. Correlation matrices of the measured and calculated parameters indicated generally weak correlations between crop growth (dry matter and N) and the parameters of symbiotic activity (nodule weight, δ 15N, relative ureide-N); correlations were strong and highly significant between nodulation and the measures of nitrogen fixation (δ 15N, relative ureide-N; r = 0.79-0.92). Estimates of P ranged between 0 and 68% (δ 15N) and between 6 and 56% (ureide) and were highly correlated ( r = 0.97). Results indicated that the ureide method can be used with confidence to assess P by field-grown crops of soybean. 相似文献
7.
We report experiments to quantify the relationships between the relative abundance of ureide-N in root-bleeding sap, vacuum-extracted sap, and hot water extracts of stems and petioles of nodulated soybean ( Glycine max [L.] Merrill cv Bragg) and the proportion of plant N derived from nitrogen fixation. Additional experiments examined the effects of plant genotype and strain of rhizobia on these relationships. In each of the five experiments reported, plants of cv Bragg (experiment 1), cv Lincoln (experiments 3, 4, 5), or six cultivars/genotypes (experiment 2) were grown in a sand:vermiculite mixture in large pots in a naturally lit, temperature-controlled glasshouse during summer. Pots were inoculated at sowing with effective Bradyrhizobium japonicum CB1809 (USDA 136) or with one of 21 different strains of rhizobia. The proportions of plant N derived from nitrogen fixation were determined using 15N dilution. In one experiment with CB1809, plants were supplied throughout growth with either N-free nutrients or with nutrients supplemented with 1, 2, 4, or 8 millimolar 15N-nitrate and harvested on eight occasions between V6 and R7 for root-bleeding sap, vacuum-extracted sap, stems (including petioles), and whole plant dry matter. Analyses of the saps and stem extracts for ureides (allantoin plus allantoic acid), α-amino-N, and nitrate, and of dry matter for N and 15N, indicated a positive effect of nitrate supply on concentrations of nitrate in saps and extracts and a negative effect on ureides and on the proportion of plant N derived from nitrogen fixation. The relative abundance of ureide-N in root-bleeding sap, vacuum-extracted sap (100 [ureide-N]/[ureide-N+ α-amino-N + nitrate-N]) and stem extracts (100 [ureide-N]/[ureide-N + nitrate-N]) and the proportion of plant N, derived from nitrogen fixation between successive samplings were highly correlated ( r = 0.97-1.00). For each variable, two standard curves were prepared to account for the shifts in the compositions of N solutes of xylem saps and extracts after flowering which were not related to a change in nitrogen fixation. Relationships between relative ureide-N and the proportion of plant N derived from nitrogen fixation were not affected by plant genotype or by strain of rhizobia. Therefore, assessment of nitrogen fixation by soybean using the ureide technique should now be possible with the standard curves presented, irrespective of genotype or strain of rhizobia occupying the nodules. 相似文献
8.
The effect of nitrate on N 2 fixation and the assimilation of fixed N 2 in legume nodules was investigated by supplying nitrate to well established soybean ( Glycine max L. Merr. cv Bragg)- Rhizobium japonicum (strain 3I1b110) symbioses. Three different techniques, acetylene reduction, 15N 2 fixation and relative abundance of ureides ([ureides/(ureides + nitrate + α-amino nitrogen)] × 100) in xylem exudate, gave similar results for the effect of nitrate on N 2 fixation by nodulated roots. After 2 days of treatment with 10 millimolar nitrate, acetylene reduction by nodulated roots was inhibited by 48% but there was no effect on either acetylene reduction by isolated bacteroids or in vitro activity of nodule cytoplasmic glutamine synthetase, glutamine oxoglutarate aminotransferase, xanthine dehydrogenase, uricase, or allantoinase. After 7 days, acetylene reduction by isolated bacteroids was almost completely inhibited but, except for glutamine oxoglutarate aminotransferase, there was still no effect on the nodule cytoplasmic enzymes. It was concluded that, when nitrate is supplied to an established symbiosis, inhibition of nodulated root N 2 fixation precedes the loss of the potential of bacteroids to fix N 2. This in turn precedes the loss of the potential of nodules to assimilate fixed N 2. 相似文献
9.
To estimate the N 2 fixation ability of the alder ( Alnus hirsuta (Turcz.) var. sibirica), we examined the seasonal variation in nitrogenase activity of nodules using the acetylene reduction method in an 18-year-old stand naturally regenerated after disturbance by road construction in Japan. To evaluate the contribution of N 2 fixation to the nitrogen (N) economy in this alder stand, we also measured the phenology of the alder, the litterfall, the decomposition rate of the leaf litter, and N accumulation in the soil. The acetylene reduction activity per unit nodule mass (ARA) under field conditions appeared after bud break, peaked the maximum in midsummer after full expansion of the leaves, and disappeared after all leaves had fallen. There was no consistent correlation between ARA and tree size (dbh). The amount of N 2 fixed in this alder stand was estimated at 56.4 kg ha ?1 year ?1 when a theoretical molar ratio of 3 was used to convert the amount of reduced acetylene to the amount of fixed N 2. This amount of N 2 fixation corresponded to the 66.4% of N in the leaf litter produced in a year. These results suggested that N 2 fixation still contributed to the large portion of N economy in this alder stand. 相似文献
10.
The relationship between the relative abundance of ureides ([ureide-N/ureide-N plus nitrate-N] × 100) in the shoot axis (stems plus petioles), nodulated roots and leaflets of “Bragg” soybean ( Glycine max [L.] Merrill) and the symbiotic dependence of these plants was examined under glass-house conditions. Plants, inoculated with effective Rhizobium japonicum CB1809, were grown with their roots exposed continuously to a nutrient solution containing either 0, 1.5, 3.0, 6.0 or 12.0 millimolar NO 3-N per liter. Nodulation and N 2-acetylene fixation were correlated inversely with the level of nitrate. Seasonal acetylene reduction profiles for each of the nitrate treatments were integrated and the symbiotic dependence ([N 2 fixed per total plant N] × 100) determined using a conversion ratio of 1.5:1 (acetylene reduced:N 2 fixed), calculated from the zero NO 3 treatment. Examination of the nitrogenous solutes of the shoot axis and nodulated roots showed linear relationships between the relative abundance of ureides and the symbiotic dependence of the plants. Two standard curves, depicting these relationships during vegetative and reproductive growth, were drawn for each plant part. The overriding effect of plant age invalidated any attempt to develop a standard relationship for leaflets. Data from two diurnal studies suggested that relative ureides were insensitive to diurnal fluctuations, thus simplifying sampling procedures. Plant material could be stored at ambient temperatures (20-30°C) for up to 24 h without affecting the relative concentration of ureides and nitrate. It is suggested that the shoot axis provides the most suitable target organ when using this technique as a quantitative assay for N 2 fixation because of ease of sampling of these tissues, especially with field-grown plants. 相似文献
11.
Controlled environment and field studies were conducted to determine relationships between various measurements of N 2 fixation using soybeans and to use these measures to evaluate a number of Bradyrhizobium japonicum strains for effectiveness in N 2 fixation in Thai soils. 15N dilution measurements of N 2 fixation showed levels of fixation ranging from 32 to 161 kg N ha −1 depending on bacterial strain, host cultivar and location. Midseason measures of N 2 fixation were correlated with each other, but not related measures taken at maturity. Ranking of B. japonicum strains based on performance under controlled conditions in N-free media were highly correlated with rankings based on soybean
seed yields and N 2 fixation under field conditions. This study showed that inoculation of soybeans with effective B. japonicum strains can result in significant increases in yield and uptake of N through fixation. The most effective strains tested
for use in Thai conditions were those isolated from Thai soils; however, effective strains from other locations were also
of benefit. 相似文献
12.
Nitrogen fixation activity in common bean is generally thoughtto be low and sensitive to soil drying and, consequently, droughtcan have important negative effects on N accumulation and yieldpotential. The objectives of this research were to examine theresponse of N 2fixation to drought stress in common bean, andto test the hypothesis that drought sensitivity of N 2fixationin common bean is linked to ureide levels in the plants. Twoglasshouse experiments were conducted to compare the responsesof leaf transpiration and acetylene reduction activity (ARA)to soil water contents. ARA decrease during soil dehydrationwas found to lag behind the decline in transpiration. This indicatesthat ARA is relatively less sensitive to soil dehydration comparedto leaf gas exchange. Further, in comparing two cultivars therewas no consistent difference in the relative response of ARAand transpiration to soil drying. The ureide concentrationsmeasured in common bean plants were low, ranging from 0.1 to1.0 mmol l -1in xylem sap exudates. Ureide concentrations inthe sap exudate varied significantly among the two genotypeseven though there was no difference in ARA response to drought.It was concluded that in common bean, the lower sensitivityof N 2fixation to drought compared to leaf gas exchange couldbe related to low ureide concentrations in petioles and xylemsap.Copyright 1998 Annals of Botany Company Phaseolus vulgaris,nitrogen fixation, drought stress, nodules, ureides. 相似文献
13.
Nodulation, acetylene reduction activity, dry matter accumulation, and total nitrogen accumulation by nodulated plants growing in a nitrogen-free culture system were used to compare the symbiotic effectiveness of the fast-growing Rhizobium fredii USDA 191 with that of the slow-growing Bradyrhizobium japonicum USDA 110 in symbiosis with five soybean ( Glycine max (L.) Merr.) cultivars. Measurement of the amount of nitrogen accumulated during a 20-day period of vegetative growth (28 to 48 days after transplanting) showed that USDA 110 fixed 3.7, 39.1, 4.6, and 57.3 times more N 2 than did USDA 191 with cultivars Pickett 71, Harosoy 63, Lee, and Ransom as host plants, respectively. With the unimproved Peking cultivar as the host plant, USDA 191 fixed 3.3 times more N 2 than did the USDA 110 during the 20-day period. The superior N 2 fixation capability of USDA 110 with the four North American cultivars as hosts resulted primarily from higher nitrogenase activity per unit nodule mass (specific acetylene reduction activity) and higher nodule mass per plant. The higher N 2-fixation capability of USDA 191 with the Peking cultivar as host resulted primarily from higher nodule mass per plant, which was associated with higher nodule numbers. There was significant variation in the N 2-fixation capabilities of the four North American cultivar-USDA 191 symbioses. Pickett 71 and Lee cultivars fixed significantly more N 2 in symbiosis with USDA 191 than did the Harosoy 63 and Ransom cultivars. This quantitative variation in N 2-fixation capability suggests that the total incompatibility (effectiveness of nodulation and efficiency of N 2 fixation) of host soybean plants and R. fredii strains is regulated by more than one host plant gene. These results indicate that it would not be prudent to introduce R. fredii strains into North American agricultural systems until more efficient N 2-fixing symbioses between North American cultivars and these fast-growing strains can be developed. When inoculum containing equal numbers of USDA 191 and of strain USDA 110 was applied to the unimproved Peking cultivar in Perlite pot culture, 85% of the 160 nodules tested were occupied by USDA 191. With Lee and Ransom cultivars, 99 and 85% of 140 and 96 nodules tested, respectively, were occupied by USDA 110. 相似文献
14.
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
15.
The
sensitivity of N 2 fixation to drought stress in soybean
( Glycine max Merr.) has been shown to be associated with
high ureide accumulation in the shoots, which has led to the hypothesis
that N 2 fixation during drought is decreased by a feedback
mechanism. The ureide feedback hypothesis was tested directly by
measuring the effect of 10 m m ureide applied by stem
infusion or in the nutrient solution of hydroponically grown plants on
acetylene reduction activity (ARA). An almost complete inhibition of
ARA was observed within 4 to 7 d after treatment, accompanied by
an increase in ureide concentration in the shoot but not in the
nodules. The inhibition of ARA resulting from ureide treatments was
dependent on the concentration of applied ureide. Urea also inhibited
ARA but asparagine resulted in the greatest inhibition of nodule
activity. Because ureides did not accumulate in the nodule upon ureide
treatment, it was concluded that they were not directly inhibitory to
the nodules but that their influence mediated through a derivative
compound, with asparagine being a potential candidate. Ureide treatment
resulted in a continual decrease in nodule permeability to
O 2 simultaneous with the inhibition of nitrogenase activity
during a 5-d treatment period, although it was not clear whether the
latter phenomenon was a consequence or a cause of the decrease in the
nodule permeability to O 2.The physiological basis of N 2 fixation
inhibition by water deficits in legume nodules is not clearly
understood. A potential physiological basis for this water-deficit
sensitivity may be that drought stress decreases the
Po ( Weisz et al., 1985), as has been shown with
other stresses such as temperature, salinity, or nitrate ( Hunt and
Layzell, 1993; Serraj et al., 1994; Denison and Harter, 1995). The role
of O 2 limitation in the response of nitrogenase
activity to drought stress has been discussed extensively ( Diaz del
Castillo and Layzell, 1995; Purcell and Sinclair, 1995; Serraj and
Sinclair, 1996b; Serraj et al., 1999). However, the mechanisms by which
drought affects Po have
not been elucidated. It is not clear whether drought stress has a
direct effect on Po, or whether the decrease in
Po is a consequence of a decrease in nodule
activity.An alternative explanation for the decrease in nitrogenase activity
under drought could be a feedback mechanism involving the accumulation
of N compounds. Pate et al. (1969) proposed that lower rates of water
movement out of the nodule during drought stress may restrict export of
products of N 2 fixation, and the accumulation of
these products would inhibit nitrogenase activity. Others have
suggested that N 2 fixation in legumes might be
regulated by a feedback mechanism involving N metabolism and the pool
of reduced N in the plant ( Silsbury et al., 1986; Parsons et al., 1993;
Hartwig et al., 1994). Oti-Boateng and Silsbury (1993) reported an
inhibition of nitrogenase activity in fava bean after plant uptake of
Asn or Gln.Soybean ( Glycine max Merr.) usually exports more
than 80% of the N compounds out of the nodules in the form of the
ureides Aln and Alac. They are transported in the xylem to the shoots,
where they are catabolized (Winkler et al., 1987). High accumulation of
petiole ureides has been measured during the inhibition of
N 2 fixation by drought in both controlled ( de
Silva et al., 1996; Serraj and Sinclair, 1996a) and field ( Purcell et
al., 1998) environments. Furthermore, in a comparison of grain legume
species, Sinclair and Serraj (1995) reported that those species
exporting ureides from the nodules had N 2
fixation that was drought sensitive. Those species that exported little
or no ureide had N 2 fixation that was relatively
drought tolerant.An important possibility is that the accumulation of ureides in soybean
nodules under soil-water deficits might trigger a feedback mechanism
that results in decreased N 2 fixation activity
( Sinclair and Serraj, 1995; Serraj et al., 1999). This paper reports a
series of experiments to investigate the hypothesis of a ureide
feedback inhibition of N 2 fixation in soybean.
First, ureide levels were measured in plant tissue (nodules, roots, and
shoots) upon the imposition of water deficits to confirm that ureide
levels increased in the nodules themselves, and not just in the shoot.
Second, the influence of ureides on nodule activity was examined by
introducing ureides, along with other compounds, into soybean plants.
These experiments were designed to examine the time course of the
response and to determine the concentration response. Third, data were
collected to determine if Po and the response of
N 2 fixation to pO 2 were
also sensitive to introduced ureides. 相似文献
16.
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
17.
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
18.
The combined effects of carbon dioxide (CO 2) enrichment and water deficits on nodulation and N 2 fixation were analysed in soybean [ Glycine max (L.) Merr.]. Two short-term experiments were conducted in greenhouses with plants subjected to soil drying, while exposed to CO 2 atmospheres of either 360 or 700 μmol CO 2 mol –1. Under drought-stressed conditions, elevated [CO 2] resulted in a delay in the decrease in N 2 fixation rates associated with drying of the soil used in these experiments. The elevated [CO 2] also allowed the plants under drought to sustain significant increases in nodule number and mass relative to those under ambient [CO 2]. The total non-structural carbohydrate (TNC) concentration was lower in the shoots of the plants exposed to drought; however, plants under elevated CO 2 had much higher TNC levels than those under ambient CO 2. For both [CO 2] treatments, drought stress induced a substantial accumulation of TNC in the nodules that paralleled N 2 fixation decline, which indicates that nodule activity under drought may not be carbon limited. Under drought stress, ureide concentration increased in all plant tissues. However, exposure to elevated [CO 2] resulted in substantially less drought-induced ureide accumulation in leaf and petiole tissues. A strong negative correlation was found between ureide accumulation and TNC levels in the leaves. This relationship, together with the large effect of elevated [CO 2] on the decrease of ureide accumulation in the leaves, indicated the importance of ureide breakdown in the response of N 2 fixation to drought and of feedback inhibition by ureides on nodule activity. It is concluded that an important effect of CO 2 enrichment on soybean under drought conditions is an enhancement of photoassimilation, an increased partitioning of carbon to nodules and a decrease of leaf ureide levels, which is associated with sustained nodule growth and N 2 rates under soil water deficits. We suggest that future [CO 2] increases are likely to benefit soybean production by increasing the drought tolerance of N 2 fixation. 相似文献
19.
The effect of short- and long-term changes in shoot carbon-exchange rate (CER) on soybean ( Glycine max [L.] Merr.) root nodule activity was assessed to determine whether increases in photosynthate production produce a direct enhancement of symbiotic N 2 fixation. Shoot CER, root + nodule respiration, and apparent N 2 fixation (acetylene reduction) were measured on intact soybean plants grown at 700 microeinsteins per meter per second, with constant root temperature and a 14/10-hour light/dark cycle. There was no diurnal variation of root + nodule respiration or apparent N 2 fixation in plants assayed weekly from 14 to 43 days after planting. However, if plants remained in darkness following their normal dark period, a significant decline in apparent N 2 fixation was measured within 4 hours, and decreasing CO 2 concentration from 320 to 90 microliters CO 2 per liter produced diurnal changes in root nodule activity. Increasing shoot CER by 87, 84, and 76% in 2-, 3-, and 4-week-old plants, respectively, by raising the CO 2 concentration around the shoot from 320 to 1,000 microliters CO 2 per liter, had no effect on root + nodule respiration or acetylene-reduction rates during the first 10 hours of the increased CER treatment. When the CO 2-enrichment treatment was extended in 3-week-old plants, the only measured parameter that differed significantly after 3 days was shoot CER. After 5 days of continuous CO 2 enrichment, root + nodule respiration and acetylene reduction increased, but such changes reflected an increase in root nodule mass rather than greater specific root nodule activity. The results show that on a 24-hour basis the process of symbiotic N 2 fixation in soybean plants grown under controlled environmental conditions functioned at maximum capacity and was not limited by shoot CER. Whether N 2-fixation capacity was limited by photosynthate movement to root nodules or by saturation of metabolic processes in root nodules is not known. 相似文献
20.
Soybean plants ( Glycine max [L.] Merr) were grown in sand culture with 2 millimolar nitrate for 37 days and then supplied with 15 millimolar nitrate for 7 days. Control plants received 2 millimolar nitrate and 13 millimolar chloride and, after the 7-day treatment period, all plants were supplied with nil nitrate. The temporary treatment with high nitrate inhibited nitrogenase (acetylene reduction) activity by 80% whether or not Rhizobium japonicum bacteroids had nitrate reductase (NR) activity. The pattern of nitrite accumulation in nodules formed by NR + rhizobia was inversely related to the decrease and recovery of nitrogenase activity. However, nitrite concentration in nodules formed by NR − rhizobia appeared to be too low to explain the inhibition of nitrogenase. Carbohydrate composition was similar in control nodules and nodules receiving 15 millimolar nitrate suggesting that the inhibition of nitrogenase by nitrate was not related to the availability of carbohydrate. Nodules on plants treated with 15 millimolar nitrate contained higher concentrations of amino N and, especially, ureide N than control nodules and, after withdrawal of nitrate, reduced N content of treated and control nodules returned to similar levels. The accumulation of N2 fixation products in nodules in response to high nitrate treatment was observed with three R. japonicum strains, two NR+ and one NR−. The high nitrate treatment did not affect the allantoate/allantoin ratio or the proportion of amino N or ureide N in bacteroids (4%) and cytosol (96%). 相似文献
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