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1.
Strains of Rhizobium tropici IIB, CIAT899 and F98.5, both showing good N2 fixation, and a R. etli strain W16.3SB were introduced into a field which had no history of bean culture. Plant dilution estimates showed that in the presence of its host (Phaseolus vulgaris cv. Carioca) during the cropping seasons and the subsequent fallow summer periods, the bean rhizobial populations increased from less than 30 to 103 g–1 dry soil after 1 year and to 104 g–1 dry soil after 2 years. In the 1st year crop, the inoculated strains occupied most of the nodules, which resulted in a higher nodulation and C2H2 reduction activity. Without reinoculation for the second and third crops, however, little R. tropici IIB was recovered from the nodules and the bean population consisted mainly of R. etli, R. leguminosarum bv. phaseoli, and R. tropici IIA. Reinoculation with our superior R. tropici IIB strains before the second crop resulted in R. tropici IIB occupying the main part of the nodules and a positive effect on nodulation and C2H2 reduction activity, but reintroduction of the inoculant strain in the third season did not have any effect.  相似文献   

2.
We examined the competitiveness of five effective Rhizobium leguminosarum biovar phaseoli strains in the nodulation of kidney beans (Phaseolus vulgaris L.), either alone or in pairwise combination, against the indigenous strains. The results showed that the introduced Rhizobium sp. strains (B2, B17, B36, T2, or CIAT 652) occupying 64–79% of the total nodules (as single inocula) were more competitive in nodulation than the native rhizobia. However, the competitiveness of the individual Rhizobium sp. strain either increased or decreased when used in a pairwise combination of double-strain inocula. For example, strain B17, although quite competitive against the indigenous population (68% nodule occupancy), became poorly competitive in the presence of strain B2 (reduced from 68 to 2.5%). A similar reduction in nodule occupancy by strain B17 was observed in the presence of B36 or CIAT 652, indicating that two competitive strains may not always be compatible. These results suggest that it is important to co-select competitive as well as compatible rhizobia for multistrain inoculant formulation.  相似文献   

3.
Summary Six Rhizobium leguminosarum bv. phaseoli strains (Ciat 151, Ciat 895, Ciat 899, CE3, H2C, Kim5s) were tested for nodule occupancy in different bean cultivars at two field sites (one fertile, one acid tropical soil) and in the greenhouse. The effects of several environmental factors such as low pH, high temperature, Al and Mn toxicity, iron deficiency, bean tannins, and bean phytoalexins were tested in vitro. Strain Kim5s was competitive under all tested conditions while strains CE3 and H2C had consistently low nodule occupancy levels. Strain Ciat 151 was superior to the other inoculant strains in the acid soil but competed poorly in the fertile soil. Strain Ciat 895 was more competitive in the fertile soil. There was a decline in nodule occupancy for all strains tested from the first trifoliate leaf stage to the pod-filling stage. No plant genotype effect on nodule occupancy was observed. There were significant (P<0.05) plant genotype and location effects, but no significant strain effect on acetylene reduction activity, plant dry weight, and nodule number. The greenhouse experiments confirmed, at least partially, the results from the field trials. In Leonard jars with an acid soil, strains Ciat 151 and Kim5s were amongst the most competitive strains. In coinoculation experiments, Kim5s was the most competitive strain, followed by Ciat 899 and Ciat 895. The competitiveness of a given strain was affected by the coinoculant strain. Tolerance in vitro to low pH, high growth temperature, Al or Mn toxicity, or Fe limitation was not related to competitiveness of the inoculum strains. The sensitivity of the strains towards bean tannins or bean phytoalexins also was not correlated with their competitiveness.  相似文献   

4.
Abstract

Methyl jasmonate (MeJA) has recently been shown to act as a plant-to-bacteria signal. We tested the hypothesis that pre-induction of Rhizobium leguminosarum bv. phaseoli cells with genistein and/or MeJA would at least partially overcome the negative effects of low root zone temperature (RZT) on bean nodulation, nitrogen fixation and plant growth. Otebo bean plants were grown at constant air temperature (25oC) and two RZT regimes (25 and 17oC) and inoculated with R. leguminosarum bv. phaseoli pre-induced with MeJA and/or genistein. Our results indicate that low RZT inhibited nodulation, nitrogen fixation and plant growth. The plants growing at low RZT began fixing nitrogen seven days later compared to those at higher RZT. The low RZT plants had fewer nodules, lower nodule weight, less N fixation, slower plant growth, fewer leaves, smaller leaf area, and less dry matter accumulation comared to plants at a higher RZT. Rhzobium leguminosarum bv. phaseoli cells induced with genistein and/or MeJA enhanced bean nodulation, nitrogen fixation and growth at both optimum and suboptimum RZTs. The results of this study indicate that MeJA improves bean nitrogen fixation and growth at both optimum and suboptimum RZTs, and can be used alone or in combination with genistein to partially overcome the low RZT induced inhibitory effects on nodulation and nitrogen fixation.  相似文献   

5.
Inoculation of beans (Phaseolus vulgaris L.) with strains of R. tropici IIB and R. etli resulted in the disappearance of the R. tropici IIB stains from the nodule population and their replacement by other (non R. tropici IIB) bean symbionts (Vlassak et al. 1996). Coinoculation studies in monoxenic conditions and in soil core microcosms with plants harvested at two different growth stages indicated that the inoculated R. tropici IIB strains CIAT899 and F98.5 possess a good intrinsic competitiveness which declines, however, at a later plant growth stage and in soil conditions. The poor saprophytic competence of R. tropici IIB strain CIAT899 was further demonstrated by its poor survival in soil core microcosms after bean harvest. Strains were isolated from the field plots with a 3-year bean-planting history, characterized and evaluated for their competitiveness against R. tropici IIB strain CIAT899. Isolates from field plots, which had been repeatedly inoculated with R. tropici IIB strain CIAT899, showed a higher nodule occupancy compared to R. tropici IIB strain CIAT899, and this higher competitiveness exhibited by the field isolates might be an additional reason for the poor performance of R. tropici IIB strain CIAT899 in the field study. Plots with and without a history of bean production revealed after 3-year bean cultivation an almost totally different population that also significantly differed in competitiveness. Received: 12 February 1996  相似文献   

6.
The effects of inoculating field peas (Pisum sativum L.) with Rhizobium leguminosarum and field beans (Phaseolus vulgaris L.) with R. phaseoli, alone or in combination with Pseudomonas syringae R25 and/or P. putida R105, were assessed under gnotobiotic conditions in growth pouches and in potted soil in a growth chamber. Inoculation of peas with P. syringae R25 or P. putida R105 alone had no effect on plant growth in pouches. In soil, however, the isolate R25 inhibited nitrogenase activity (as assessed by acetylene reduction assay) of nodules formed by indigenous rhizobia; strain R105 stimulated pea seedling emergence and nodulation. P. syringae R25 inhibited the growth of beans in either plant-growth system. P. putida R105, however, had no effects on beans in pouches, but reduced plant root biomass and nodulation by indigenous rhizobia in soil. Coinoculation of pea seeds with R. leguminosarum and either of the pseudomonads significantly (P<0.01) increased shoot, root, and total plant weight in growth pouches, but had no effect in soil. Co-inoculation of field beans with R. phaseoli and P. putida R105 had no effects on plant biomass in growth pouches or in soil, but the number of nodules and the acetylene reduction activity was significantly (P<0.01) increased in the soil. In contrast, co-inoculation of beans with rhizobia and P. syringae R25 had severe, deleterious effects on seedling mergence, plant biomass, and nodulation in soil and growth pouches. Isolate R25 was responsible for the deleterious effects observed. Although plant growth-promoting rhizobacteria may interact synergistically with root-nodulating rhizobia, the PGPR selected for one crop should be assessed for potential hazardous effects on other crops before being used as inoculants.  相似文献   

7.
Competition for nodulation is a complex problem where bacterial adhesins, which are required for root colonization, may play a role. However, the possible influence of adhesins on competitiveness was scarcely studied. In this work, the Rhizobium leguminosarum bv trifolii adhesion protein RapA1 was overproduced from a pHC60-derived plasmid and expressed in R200 strain. When an overproducing strain and a control-carrying empty vector were co-inoculated on clover plants, a positive effect of RapA1 on competition for nodule occupation was observed. Therefore, optimization of RapA1 expression may be considered for improvement of rhizobial competitiveness.  相似文献   

8.
Summary The competitive ability of inoculated and indigenous Rhizobium/Bradyrhizobium spp. to nodulate and fix N2 in grain legumes (Glycine max, Vigna unguiculata, Phaseolus vulgaris) and fodder legumes (Vicia sativa, Medicago sativa, and Trifolium subterraneum) was studied in pots with two local soils collected from two different fields on the basis of cropping history. The native population was estimated by a most-probable-number plant infectivity test in growth pouches and culture tubes. The indigenous rhizobial/bradyrhizobial population ranged from 3 to 2×104 and 0 to 4.4×103 cells g-1 in the two soils (the first with, the second without a history of legume cropping). Inoculated G. max, P. vulgaris, and T. subterraneum plants had significantly more nodules with a greater nodule mass than uninoculated plants, but N2 fixation was increased only in G. max and P. vulgaris. A significant response to inoculation was observed in the grain legume P. vulgaris in the soil not previously used to grow legumes, even in the presence of higher indigenous population (>103 cells g-1 soil of Rhizobium leguminosarum bv phaseoli). No difference in yield was observed with the fodder legumes in response to inoculation, even with the indigenous Rhizobium sp. as low as <14 cells g-1 soil and although the number and weight of nodules were significantly increased by the inoculation in T. subterraneum. Overall recovery of the inoculated strains was 38–100%, as determined by a fluorescent antibody technique. In general, the inoculation increased N2 fixation only in 3 out of 12 legume species-soil combinations in the presence of an indigenous population of rhizobial/bradyrhizobial strains.  相似文献   

9.
The aim of this work was to evaluate the competitive ability between Rhizobium leguminosarum bv trifolii strain U204 used as commercial inoculants in Uruguay for Trifolium repens L. and Trifolium pratense L. and two native strains isolated from inoculated pastures of T. pratense. T126 is an efficient nitrogen fixer and a melanin producer strain; T70 is inefficient and a melanin non-producer strain; and U204 is very efficient in both hosts but is a melanin non-producer strain. Competitiveness between the strains was determined in experiments in pots and in growth pouches under controlled conditions. In the last experiment, we evaluated pH of plant nutrient solution and inoculum ratios. Plant dry weight was determined, and the identification of nodule bacteria was done using melanin production and DNA fingerprinting (GTG5-PCR). The U204 symbiotic efficiency was not affected by the co-inoculation with the others two native strains. The T70 strain was a poor competitor when was co-inoculated with one of the effective strains in both experiments. Our results confirmed a “selective nodulation” because an effective symbiosis occurred preferentially over an ineffective one in Trifolium species. The native effective strain competed with U204 for nodule formation in both clovers species, but the nodule occupancy depended on the inoculum ratio. The pH of nutritive solution did not affect competition ability of the studied strains. It may be possible to isolate efficient, competitive, and genetically different native rhizobial strains to be used as inoculant strains for clover pastures in Uruguay. Both (GTG)5-PCR and melanin production were useful methods to identify nodulating bacteria in competition studies.  相似文献   

10.
Summary Six fast-growing soybean rhizobia (Rhizobium fredii) and thirteen slow-growing soybean rhizobia (Bradyrhizobium japonicum) were examined for resistance to 10 antibiotics. Axenic studies were carried out to determine the competitiveness of dual-strain inocula consisting of fast- and slow-growing rhizobia isolated from subtropical-tropical soils for nodule occupancy on a hybrid of Asian and US soybean cultivars. Nodule occupancy was determined by intrinsic resistance to erythromycin and neomycin. The results showed wide variability in resistance to 10 antibiotics for fast- and slow-growing rhizobia. The intrinsic antibiotic resistance of fast- and slow-growing rhizobia was extremely high against nalidixic acid (400 g ml–1) and penicillin (200 g ml–1). The competitive ability of inoculant strains for nodule occupancy varied for different combination sets and with the plant growing media. Our results show that fast-growing rhizobia nodulate a hybrid of Asian and US soybean cultivars. Fast-growing soybean rhizobia did not completely exclude nodulation by the slow-growing strains, which formed 0–79% nodules, depending on the strain used in the inoculum.  相似文献   

11.
The relationship between soil acidity and polyamine (putrescine, spermidine, spermine) concentrations in roots was evaluated and compared to (1) nodule numbers in common beans and (2) tannin concentrations in roots. Six Phaseolus vulgaris cultivars were grown in pots in a greenhouse in soil at pH 4.5 or 5.5 and inoculated with Rhizobium leguminosarum bv. phaseoli strains. At pH 4.5 nodule numbers were strongly reduced but polyamine concentrations were 50% higher than at pH 5.5. At both pH levels putrescine and spermidine were the dominant polyamines, while the concentration of spermine was less than half of the other two. There was also a cultivar effect on the polyamine concentration, but this was much less pronounced than the pH effect. By 22 days after planting the uninoculated control plants had about 13% lower levels of polyamines than the inoculated plants. The concentrations of putrescine, spermidine, and spermine were negatively correlated with nodule numbers, but positively correlated with the root tannin content. There were no significant correlations with either root or plant dry weight.  相似文献   

12.
Summary Sixteen strains ofRhizobium phaseoli were isolated from the Loukkos sandy soil and were compared with four selected strains ofR. phaseoli (CIAT 676, CIAT 57, Olivia, Viking 1) for their N2-fixing ability and for their serological affinities by means of the fluorescent antibody technique. Two indigenous strains were rated as highly effective, six as moderately effective, and eight were ineffective. Viking 1 was ranked as highly effective while the other inoculum strains were moderately effective. None of the 4 inoculum strains cross-reacted with the 16 indigenous strains. The indigenous strains were grouped into only two serogroups but showed a high degree of heterogeneity in regard to the strength of the immunofluorescence response. The inoculum strains outcompeted but did not eliminate the resident population for nodule occupancy on two French bean cultivars (Royalnel and Fetiche). The competitiveness, however, differed between inoculum strains. It was influenced by the host cultivar and by the host cultivar growth stage. Viking 1 was consistent in its high competitive ability whether it was inoculated singly or in a mixture with the other strains. It showed high persistence and formed more than 50% of the nodules 1 year after it was introduced. CIAT 57, Olivia, and CIAT 676 were mediocre in their persistence.  相似文献   

13.
In 1993 and 1994, 12 bacterial isolates were isolated from root nodules of cicer milkvetch (Astragalus cicer). In the tests for nodulation of A. cicer by these bacterial isolates, five were found to form hypertrophic structures, while only two formed true nodules. These true nodules were formed in a sterilized soil system. This system might be able to act as a DNA donor to provide residual DNA to other microbes in the soil. The rhizobial isolates were thought to have lost genetic material crucial to nodulation during the isolation process. This hypothesis was supported by an experiment in which isolate B2 was able to nodulate A. cicer in vermiculite culture after being mixed with heat-killed rhizobia, Rhizobium leguminosarum bv. trifolii and R. loti. The nodulation would not occur in vermiculite culture system without the heat-killed rhizobia. Based on the biochemical data, the B2 and 9462L, which formed true nodules with A. cicer, were closely related. The rhizobia type cultures that nodulate A. cicer include Bradyrhizobium japonicum, Rhizobium leguminosarum bv. trifolii, R. leguminosarum bv. viceae, and R. loti. All of these rhizobia were from different cross-inoculation groups. The B2 and 9462L isolates could only nodulate Medicago sativa, Phaseolus vulgaris, and Melilotus officinalis, but not these species within the genus from which they were isolated: Astragalus. The traditional cross-inoculation group concept obviously does not fit well in the classification of rhizobia associated with Astragalus. The rhizobia isolated from A. cicer can be quite different, and the rhizobia able to renodulate A. cicer also quite diverse. Received: 27 June 1996  相似文献   

14.
Summary Axenically grown alfalfa (Medicago sativa L. var. Peace) was simultaneously inoculated with Canadian commercial Rhizobium meliloti strains NRG-185 and BALSAC. The plants were grown for 7 weeks in sealed units at five different root temperatures (8°, 13°, 17°, 21°, and 25°C) and at a relatively constant air temperature (24°–30°C). Nodule occupancy by each strain was determined by enzyme-linked immunosorbent assay (ELISA). Nitrogenase activity, nodule fresh weight, and plant dry weight were also measured. The lowest root-temperature regime (8°C) resulted in substantially lower nodule numbers and weights, and plant dry weights, than the higher temperature regimes. Development of nitrogenase activity was completely inhibited at 8°C. The immunoassay of nodule-strain occupancy showed markedly different strain-nodulation responses to the various root-temperature regimes. At 8°C, 63% of nodules were occupied by both strains. Dual strain occupancy decreased from 63% to 2% with increasing root-growth temperature, while the proportion of nodules containing only strain NRG-185 increased from 9% to 75%. Nodules containing only strain BALSAC remained relatively constant at 25% from 8° to 21°C, decreasing slightly at 25°C. The results suggest that root-environment temperatures during the period of nodule formation may have major differential effects on the success of competing rhizobial strains. If this is so, then selection of Rhizobium strains with enhanced low-temperature nodulation capabilities should be possible.  相似文献   

15.
Summary We investigated the reliability of antibiotic resistance and colony morphology of clones of a Bradyrhizobium sp. (Lotus) strain for strain identification in nodulation competitiveness experiments in soil. There was no difference in nodulation competitiveness between the wild type strain and each of five mutants resistant to streptomycin and spectinomycin at the time of their isolation from antibiotic-containing media. However, these mutants were significantly less competitive when tested 4 months later. The apparent instability of the newly isolated mutants and their subsequently decreased nodulation competitiveness show that mutants must be examined carefully after being allowed time to stabilize. Two clones of the Bradyrhizobium sp. (Lotus) strain that differed in colony morphology on yeast mannitol medium did not differ in antigenic properties, whole cell protein electrophoresis profiles, mean cell generation times in yeast mannitol medium, N2-fixing ability, nodulation of Lotus pedunculatus in growth pouches, or in nodulation competitiveness. Both clones retained their colony morphology after numerous transfers on yeast mannitol agar over 3 years and after at least 6 months in soil. A limiting factor, which may restrict the use of colony morphology as a marker for strain identification in competition experiments, is the problem of detecting double-infected nodules when the small colony type comprises a relatively small portion of the total nodule population.  相似文献   

16.
The use of the GUS reporter gene was found to be very suitable for studying the competitiveness of the Rhizobium strain NGR234 and Bradyrhizobium strain CP283 for nodulation in siratro. However, the expression from the transposon mTn5SSgusA20 declined in the nodules of old plants, particularly the nodules at 35 d after inoculation and onward. Siratro inoculated with both gusA-marked NGR234 and CP283 strains showed a similar nodulation and acetylene reduction activity (ARA) to those of their parental strains. No major changes in nodulation and symbiotic properties in these marked strains were observed. When the Rhizobium strain NGR234 and Bradyrhizobium strain CP283 were inoculated separately to siratro, both of them were found equally effective for nodulation in the plant. But when the Rhizobium strain NGR234, and gusAmarked Bradyrhizobium strain CP283 were co-inoculated to the plants in a ratio of 1:1, the strain NGR234 occupied 75% of the nodules, while, the strain CP283 occupied only 25%, irrespective of the growth stages and inoculum concentrations. Similar results were also observed in the plants in which the Bradyrhizobium strain CP283 and gusA-marked Rhizobium strain NGR234 had been co-inoculated. Thus, the Rhizobium strain NGR234 was more competitive for nodulation in siratro.  相似文献   

17.
Summary To investigate the effect of single versus dual inoculation of peas (Pisum sativum L.) with Rhizobium leguminosarum biovar viceae and Penicillium bilaji (a soil fungi capable of solubilizing soil P) on N2 fixation an experiment was carried out under controlled conditions. A sandy loam soil was selected which contained low levels of available N and P. P fertilizer [Ca(H2PO4)2] and P. bilaji significantly increased dry matter production. Peas inoculated with R. leguminosarum showed only a small increase in dry matter, but the additional application of P significantly increased the yield. The total N accumulation was highly dependent on the presence of R. leguminosarum. Using the 15N method for estimating N2 fixation, the highest level of N2-fixing activity was observed in peas inoculated with R. leguminosarum and fertilized with inorganic P. Dual inoculation of peas with P. bilaji and R. leguminosarum significantly decreased the amount of N2 fixed. Total P uptake was solely dependent on the P fertilizer.  相似文献   

18.
We previously reported that commercial Rhizobium leguminosarum bv. trifolii inoculants failed to outcompete naturalized strains for nodule occupation of clover sown into an alkaline soil [Aust. J. Agric. Res. 53 (2002) 1019]. Two field isolates that dominated nodule occupancy at the field site were labeled with a PnifH-gusA marker. Marked strains were chosen on the basis that they were equally competitive and fixed similar amounts of nitrogen in comparison to their parental strain. The minitransposon insertions were cloned and sequence analysis revealed that neither lesion disrupted the integrity of any known gene. The marked strains were then used to follow nodule occupancy of Trifolium alexandrinum in competition against the commercial inoculant TA1 under a range of experimental conditions. In co-inoculation experiments in sand-vermiculite, TA1 outcompeted each marked field isolate for nodule occupancy. However, using TA1-inoculated seed sown into alkaline soil containing a marked field strain, it was demonstrated that by increasing the cell number of marked rhizobia in the soil and reducing the cell number of the commercial inoculant, the proportion of nodules occupied by TA1 was reduced. These studies indicate that the ability of the field isolates to dominate nodule occupancy in the alkaline field soils was most likely caused by poor commercial inoculant survival providing the advantage for naturalized soil rhizobia to initiate nodulation.  相似文献   

19.
It is generally accepted that there are two major centers of genetic diversification of common beans (Phaseolus vulgaris L.): the Mesoamerican (Mexico, Colombia, Ecuador and north of Peru, probably the primary center), and the Andean (southern Peru to north of Argentina) centers. Wild common bean is not found in Brazil, but it has been grown in the country throughout recorded history. Common bean establishes symbiotic associations with a wide range of rhizobial strains and Rhizobium etli is the dominant microsymbiont at both centers of genetic diversification. In contrast, R. tropici, originally recovered from common bean in Colombia, has been found to be the dominant species nodulating field-grown common-bean plants in Brazil. However, a recent study using soil dilutions as inocula has shown surprisingly high counts of R. etli in two Brazilian ecosystems. In the present study, RFLP-PCR analyses of nodABC and nifH genes of 43 of those Brazilian R. etli strains revealed unexpected homogeneity in their banding patterns. The Brazilian R. etli strains were closely similar in 16S rRNA sequences and in nodABC and nifH RFLP-PCR profiles to the Mexican strain CFN 42T, and were quite distinct from R. etli and R. leguminosarum strains of European origin, supporting the hypothesis that Brazilian common bean and their rhizobia are of Mesoamerican origin, and could have arrived in Brazil in pre-colonial times. R. tropici may have been introduced to Brazilian soils later, or it may be a symbiont of other indigenous legume species and, due to its tolerance to acidic soils and high temperature conditions became the predominant microsymbiont of common bean.  相似文献   

20.
Seed inoculation is frequently essential for annual Medicago establishment in Mediterranean dryland farming systems. As post-planting soil inoculation is often practiced when seed inoculation fails, the effect of delayed inoculation was investigated. Roots of Medicago rigidula (L.). All selection 716, and Medicago rotata Boiss. selection 2132, were initially pre-exposed to Rhizobium meliloti strains. Subsequently, roots were exposed to a secondary inoculum after 6, 48 and 168 h to simulate delayed inoculation. and subsequent establishment of others strains in the nodules were investigated. Combinations of highly effective and host compatible ICARDA strains (M15 and M53) and effective-ineffective strains (M15 and M28) were used to evaluate proportional nodulation responses. Plants were harvested after 6 weeks of growth. Nodules were assessed for distribution in the root system and for occupancy based on their differential resistance to kanamycin and streptomycin, and, in the case of ICARDA M28, on nodule characteristics. The strain ICARDA M15 was a better competitor than ICARDA M53 when applied in equal density in M. rotata, at zero time. While forming nodules with M. rigidula, ICARDA M53 was equally competitive under the same conditions. With ICARDA M15 as the primary inoculum, and ICARDA M53 delayed for 6, 48 and 168 h, the incidence of ICARDA M15 nodules increased on M. rigidula from 43% (at zero time) to 86, 94, and 97% and, on M. rotata, from 78% (at zero time) to 88%, 95%, and 98% for the three time delays, respectively. Pre-exposure of 2-day old M. rigidula seedlings to the ineffective strain ICARDA M28 as the primary inoculant was followed by significant nodule number increases (P 0.01) as compared with ICARDA M15. Nonetheless, when ICARDA M15 was the primary inoculum, M28 was able to produce about 35% of the nodules when applied at the 6 and 48 h time delays. However, with ICARDA M28 as the primary inoculum, followed by ICARDA M15, nodule occupancy of ICARDA M28 in M. rigidula increased from 40 (at zero time) to 75%, 80%, and 95% for the three time delays, respectively. The percentage of total nodulation by M28, applied at the three delay times, was markedly lower (10%, 5% and 1%, respectively) when M. rotata was pre-exposed to ICARDA M15. This suggested a host preference for ICARDA M15, even if applied as a late inoculum. Results indicate that the early events in the nodulation process of annual medics coupled with host-specificity factors are perhaps the most critical for competition among R. meliloti strains for nodule formation.  相似文献   

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