Population size and N2-fixing activity of native peanut rhizobia in soils of Thailand

Summary The objective of this study was to assess the number and effectiveness of peanut rhizobia in soils of the major peanut-growing areas of Thailand. Three cropping areas, (1) continuously cropped with peanuts, (2) continuously cropped with non-legumes, and (3) non-cultivated fields, were chosen in each region. Peanut rhizobia were found in the soil at 38 to 55 sites sampled. Cultivated fields with a peanut cultivation history contained (as estimated by most probable numbers) an average of 1.6×10³ cells g^-1 of soil. The numbers of peanut rhizobia in most of the fallow fields and some of the noncultivated shrub or forest locations were much the same as at the sites where Arachis hypogaea was cultivated. In contrast, there were no or few (28–46 cells g^-1 soil) peanut rhizobia in the majority of fields continuously cultivated with sugarcane, cassava, corn, and pineapple. It appears that in these areas the indigenous peanut rhizobial populations are not adequate in number for a maximal nodulation of peanuts. A total of 343 Bradyrhizobium isolates were tested for effectiveness and were found to vary widely in their ability to fix N₂. In some areas the majority of rhizobia were quite effective while in others they were less effective than the inoculum strain THA 205 recommended in Thailand.     

Genetic diversity of rhizobia associated with indigenous legumes in different regions of Flanders (Belgium)

We investigated the diversity of rhizobia isolated from different indigenous legumes in Flanders (Belgium). A total of 3810 bacterial strains were analysed originating from 43 plant species. Based on rep-PCR clustering, 16S rRNA gene and recA gene sequence analysis, these isolates belonged to Bradyrhizobium, Ensifer (Sinorhizobium), Mesorhizobium and Rhizobium. Of the genera encountered, Rhizobium was the most abundant (62%) and especially the species Rhizobiumleguminosarum, followed by Ensifer (19%), Bradyrhizobium (14%) and finally Mesorhizobium (5%). For two rep-clusters only low similarity values with other genera were found for both the 16S rRNA and recA genes, suggesting that these may represent a new genus with close relationship to Rhodopseudomonas and Bradyrhizobium. Primers for the symbiotic genes nodC and nifH were optimized and a phylogenetic sequence analysis revealed the presence of different symbiovars including genistearum, glycinearum, loti, meliloti, officinalis, trifolii and viciae. Moreover, three new nodC types were assigned to strains originating from Ononis, Robinia and Wisteria, respectively. Discriminant and MANOVA analysis confirmed the correlation of symbiosis genes with certain bacterial genera and less with the host plant. Multiple symbiovars can be present within the same host plant, suggesting the promiscuity of these plants. Moreover, the ecoregion did not contribute to the separation of the bacterial endosymbionts. Our results reveal a large diversity of rhizobia associated with indigenous legumes in Flanders. Most of the legumes harboured more than one rhizobial endosymbiont in their root nodules indicating the importance of including sufficient isolates per plant in diversity studies.     

Rhizobia nodulating African Acacia spp. and Sesbania sesban trees in southern Ethiopian soils are metabolically and genomically diverse

The diversity of 110 rhizobial strains isolated from Acacia abyssinica, A. seyal, A. tortilis, Faidherbia albida, Sesbania sesban, Phaseolus vulgaris, and Vigna unguiculata grown in soils across diverse agro-ecological zones in southern Ethiopia was assessed using the Biolog™ system and amplified fragment length polymorphism (AFLP) fingerprinting technique. By cluster analysis of the metabolic and genomic fingerprints, the test strains were grouped into 13 Biolog and 11 AFLP clusters. Twenty-two strains in the Biolog method and 15 strains in the AFLP analysis were linked to eight and four reference species, respectively, out of the 28 included in the study. Most of the test strains (more than 80% of 110) were not related to any of the reference species by both methods. Forty-six test strains (42% of 110) were grouped into seven corresponding Biolog and AFLP clusters, suggesting that these groups represented the same strains, or in some cases clonal descendants of the same organisms. In contrast to the strains from S. sesban, isolates from Acacia spp. were represented in several Biolog and AFLP clusters indicating the promiscuous nature of the latter and widespread occurrence of compatible rhizobia in most of the soil sampling locations. The results showed that indigenous rhizobia nodulating native woody species in Ethiopian soils constituted metabolically and genomically diverse groups that are not linked to reference species.     

Nodulation and estimation of symbiotic nitrogen fixation by herbaceous and shrub legumes in Guinea savanna in Nigeria

Twelve herbaceous and shrub legume species were grown in pot and field experiments in five sites representing three agroecological zones in moist savanna in Nigeria. The objectives were to: (1) assess natural nodulation of the legumes and characterize their indigenous rhizobia, (2) determine their need for rhizobia inoculation and (3) estimate the amount of N₂ fixed by each of these legumes. At 4 weeks after planting (WAP), Crotolaria verrucosa was not nodulated at any of the sites while Centrosema pascuorum had the highest number of nodules in all sites. At 8 WAP, all legumes were nodulated, with Mucuna pruriens having the least number of nodules and Stylosanthes hamata the highest. The number of nodules, however, was inversely correlated to the mass of nodules. Significant differences in nodulation of the legume species grown in the field also occurred between and within sites. Mucuna pruriens and Lablab purpureus produced more shoot and nodule biomass than the other legumes in all sites. Growth of most of these legumes responded to fertilizer application, except for C. verrucosa and Aeschynomene histrix. Except for C. verrucosa, average proportion of N₂ fixed was about 80% and this was reduced by about 20% with N fertilizer application. The majority of rhizobia isolates (60%) were slow growing, belonging to the Bradyrhizobia spp. group. Selected rhizobia isolates evaluated on Cajanus cajan, C. pascuorum, M. pruriens and Psophocarpus palustris varied from ineffective to highly effective in Leonard jar conditions. However, only growth of M. pruriens responded to inoculation in potted soils, whereas it was lower than that obtained with N fertilizer application. This indicated the need to screen more rhizobia in order to improve N₂ fixation and growth of legume species such as M. pruriens when it is introduced in soils deficient in N.     

Nodulation and nitrogen fixation of Lupinus species with Bradyrhizobium (lupin) strains in iron-deficient soil

 The effect of six Bradyrhizobium sp. (lupin) strains (WPBS 3201D, WPBS 3211D, USDA 3040, USDA 3041, USDA 3042 and CB 2272) and Fe supply on nodulation, N₂-fixation and growth of three lupin species (Lupinus termis, L. albus and L. triticale) grown under Fe deficiency in an alkaline soil, were examined in sterilized and non-sterilized pot experiments. When inoculated with USDA 3040, 3041, 3042 and CB2272 without Fe addition, the three lupin species had a very low nodule number and mass, low shoot and root dry matter accumulation and lower N yield. However, inoculation with WPBS 3201D and 3211D without Fe treatments increased all these parameters substantially. The ability of WPBS 3201D and 3211D to form nodules on the three lupin species under conditions of Fe stress could be attributed to their ability to scavenge Fe from Fe-deficient environments through their siderophore production. Addition of Fe to the other four strains significantly increased nodulation and N₂-fixation of the three lupin species, indicating that the poorer nodulation and N₂-fixation of these strains in the absence of Fe, resulted from a low ability to obtain Fe from alkaline soils. Bradyrhizobium strains WPBS 3201D and 3211D were superior to the other four strains in terms of promoting greater nodulation, N₂-fixation, plant growth and N accumulation of L. termis and L. albus. However, the other four strains were more efficient in symbiotic association with L. triticale. The greater variations in nodule efficiencies (specific nitrogenase activity) under different levels of Fe supply could be attributed to the quantities of bacteroid protein and leghaemoglobin in the nodules. The results suggested that Bradyrhizobium (lupin) strains differ greatly in their ability to obtain Fe from alkaline soils, and that the selection of bradyrhizobial strains which are tolerant of Fe deficient soils could complement plant breeding for the selection of legume crops for Fe-deficient soils. Received: 5 January 1998     

The tripartite symbiosis between legumes, rhizobia and indigenous mycorrhizal fungi is more efficient in undisturbed soil

We investigated how the rate of colonization by indigenous arbuscular mycorrhizal fungi (AMF) affects the interaction between AMF, Sinorrhizobium meliloti and Medicago truncatula Gaertn. To generate a differential inoculum potential of indigenous AMF, five cycles of wheat, each of 1 month, were grown in sieved or undisturbed soil before M. truncatula was sown. The early colonization of M. truncatula roots by indigenous AMF was faster in undisturbed soil compared with sieved soil, but by pod-fill the frequency of hyphae, arbuscules and vesicles was similar in both treatments. At this latter stage, M. truncatula grown in undisturbed soil had accumulated a greater biomass in aboveground tissues, had a greater P concentration and derived more N from the atmosphere than plants grown in disturbed soil, although soil compaction resulted in plants having a smaller root system than those from disturbed soil. The difference in plant P content could not be explained by modifications in hydrolytic soil enzymes related to the P cycle as the activity of acid phosphatase was greater in sieved than in undisturbed soil, and the activity of alkaline phosphatase was unaffected by the treatment. Thus, the results observed were a consequence of the different rates of AMF colonization caused by soil disturbance. Together with earlier results for soybean, this study confirms that soil disturbance modifies the interaction between indigenous AMF, rhizobia and legumes leading to a reduced efficacy of the bacterial symbiont.     

Physiological and symbiotic characteristics of Rhizobium fredii isolated from subtropical-tropical soils

Summary Physiological and symbiotic characteristics were identified in Rhizobium fredii isolated from subtropical-tropical soils. The generation times of R. fredii Taiwan isolated-SB 357 and -SB 682 were 1.7 and 2.5 h, respectively. These strains were associated with acid production in yeast-extract mannitol medium. They were able to use hexoses, pentose, sucrose, trehalose and raffinose. Strain SB 357 can resist a high concentration of kanamycin (100 g ml^–1 and penicillin (400 g ml^–1). It can tolerate up to 2.34% NaCl and 1031.3 mosmol kg^–1 (23.4 bars). The growth rate of R. fredii SB 357 under the concentration of approximately 450 mosmol kg^–1 (10.2 bars) was not affected by salinity, but responded to osmotic pressure. Both strains (SB 357 and SB 682) isolated from subtropical-tropical soils were able to form an effective N₂-fixing symbiosis with the US soybean cv Clark lanceolate leaflet.     

Species Diversity in Wheat Landrace Populations from two Regions of Ethiopia

Wheat (Triticum spp.) landrace populations in Ethiopia are mostly species mixtures. However, no quantitative data is available with regard to their species components. We studied here 32 wheat landrace populations originating from two regions (Bale and Wello). A total of 2559 individual plants, 45–110 plants representing each population, were classified into their species components. Five tetraploid (2n = 4x = 28) and one hexaploid (2n = 6x = 42) wheat species were found in mixtures of varying proportions. These included the tetraploids Triticum durum Desf., Triticum turgidum L., Triticum aethiopicum Jakubz., Triticum polonicum L., Triticum dicoccon Schrank and the hexaploid Triticum aestivum L. Also found, however in a rare frequency, in two populations from Wollo was T. durum Desf. convar. durocompactoides Flaksb. (Triticum pyramidale Percival), which is a very dense spiked durum. Discriminant analysis using seven qualitative traits revealed 91.5% correct classification of the wheat species, beak awn and awn length with the most significant importance. Single species were found in eight of the populations; six were for T. durum and two for T. aethiopicum. Two to three species-combinations were the most frequent; a maximum of four species was recorded in one population. The highest diversity index (H′) observed was 0.44. T. durum was the most predominant species. The hexaploid T. aestivum was found in nine of the Wollo populations and, in one population, its frequency reached up to 35.5%. On altitudinal basis, no clear trend of clinal variation was observed both from the frequency distributions and H′ estimates. The results confirmed that Ethiopian wheats, despite the morphological overlaps, could be classified into their species components with high degree of certainty. For the future, therefore, genetic diversity estimations should be dissolved into their species components for more expeditious utilization and conservation of this important genetic resource.     

Co-selection of compatible rhizobia and vesicular-arbuscular mycorrhizal fungi for cowpea in sterilized and non-sterilized soils

Summary We selected two isolates of Rhizobium for cowpea (Vigna unguiculata) with sterilized soil tests and two different isolates by non-sterilized soil testing. The four rhizobia were then paired individually with either Glomus pallidum, Glomus aggregatum, or Sclerocystis microcarpa in separate, sterilized, or non-sterilized soil experiments. The purpose of the experiments was to determine the effect of soil sterilization on the selection of effective cowpea rhizobia, and to see whether these rhizobia differed in their effects on cowpea growth when paired with various vesicular-arbuscular mycorrhizal (VAM) fungi. Our experiments showed that the rhizobia selected in sterilized soil tests produced few growth responses in the cowpea compared to the other introduced rhizobia, irrespective of pairing with VAM fungi in sterilized or non-sterilized soil. In contrast, the two rhizobia initially selected by non-sterilized soil testing significantly improved cowpea growth in non-sterilized soil, especially when paired with G. pallidum. Our results suggest that it is important to select for effective rhizobia in non-sterilized soil, and that pairing these rhizobia with specific, coselected VAM fungi can significantly improve the legume growth response.     

Displacement of native rhizobia nodulating chickpea (Cicer arietinum L.) by an inoculant strain through soil solarization

Summary Soil solarization greatly reduced the native chickpea Rhizobium population. With inoculation, it was possible to increase the population of the Rhizobium in solarized plots. In the 1st year, 47% nodulation was obtained with chickpea inoculant strain IC 59 when introduced with a cereal crop 2 weeks after the soil solarization and having a native Rhizobium count of <10 g^-1 soil, and only 13% when introduced 16 weeks after solarization at the time the chickpeas were sown, with 2.0×10² native rhizobia g^-1 soil. In the non-solarized plots inoculated with 5.6×10³ native rhizobia g^-1 soil, only 6% nodulation was obtained with the inoculant. In the succeeding year, non-inoculated chickpea was grown on the same plots without any solarization or Rhizobium inoculation. The treatment that showed good establishment of the inoculant strain in year 1 formed 68% inoculant nodules. Other treatments indicated a further reduction in inoculant success, from 1%–13% to 1%–9%. Soil solarization thus allowed an inoculant strain to successfully displace the high native population in the field and can serve as a research tool to compare strains in the field, irrespective of competitive ability. In year 1, Rhizobium inoculation of chickpea gave increased nodulation and increased plant growth 20 and 51 days after sowing, and increased dry matter, grain yield, and grain protein yield at maturity. These beneficial effects of inoculation on plant growth and yield were not measured in the 2nd year.Submitted as Journal Article No. JA 945 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, Andhra Pradesh 502 324, India     

Nematofauna associated with exotic and native leguminous plant species in West Africa: effect of<Emphasis Type="Italic"> Glomus intraradices</Emphasis> arbuscular mycorrhizal symbiosis

Ten leguminous trees, four exotic species (Australian Acacia) and six indigenous species (three Sahelian Acacia spp. and three Sesbania spp.), were grown for 4 months in a natural Sahelian soil inoculated with or without the endomycorrhizal fungus, Glomus intraradices. In control trials, the determinant factor structuring the soil nematode fauna was the plant species, related plants having a similar influence on the nematode community in the soil. Soil nematode abundance increased from exotic acacias (3.3 g^-1 dry soil) to native acacias (11.5 g^-1 dry soil) and Sesbania species (17.6 g^-1 dry soil). Plant feeding nematodes (mainly Scutellonema and Tylenchorhynchus) were significantly less abundant under exotic acacias (1.4 g^-1 dry soil) than under native acacias (7.2 g^-1 dry soil) or Sesbania species (7.3 g^-1 dry soil). Bacterial feeding nematode density increased from exotic acacias (1.2 g^-1 dry soil) to native acacias (3.0 g^-1 dry soil) and Sesbania species (7.7 g^-1 dry soil) as total densities. However, the differences in the structure of the nematode communities between plant groups were suppressed in the presence of the mycorrhizal fungus. In fact, no difference in nematode densities remained between plant groups when G. intraradices developed in several dominant taxa belonging to different trophic groups, particularly: Tylenchorhynchus, Apelenchina, Cephalobus and Dorylaimoidea. This study clearly indicated that inoculation with the arbuscular mycorrhizal fungus G. intraradices diminished the plant-specific effect on the structure of the soil nematode community.     

Genetic erosion of Ethiopian tetraploid wheat landraces in Eastern Shewa,Central Ethiopia

Ethiopia is a centre of diversity and hosts rich genetic resources of tetraploid wheats. Through time, the wheat materials were subject to genetic erosion. Closer investigation was made to assess the status of loss, and identify the possible causes by studying two districts from East Shewa. Information from primary and secondary sources was reviewed and analysed. Farmers identified 26 tetraploid wheat landraces (21 from Akaki and 17 from Ejere), which were once widely grown in the area. Of these, only six were currently available. Compared to the formerly available number of landraces, the loss of diversity in the study area was estimated to be 77%. In the Ejere locality, the loss was 100% before the launching of the on farm landrace conservation programme, and for Akaki it was 95%. Major factors that contributed to the loss include: (1) introduction and expansion of bread wheat varieties; (2) expansion of tef; (3) lack of a mechanism to re-supply seeds of tetraploid wheat landraces; (4) decline in size of landholdings; (5) changes in land use and cropping patterns; (6) lack of policy support; and (7) expansion of improved tetraploid wheat varieties. The lessons from this study underscore the importance of strengthening the local seed supply system as a prerequisite for sustaining on farm conservation of landraces. Moreover, it is necessary to initiate diversity studies focusing on the distribution and status of tetraploid wheat landraces across the country. Complementing these by molecular analyses is essential in order to assess the genetic distinctness of the landraces.     

Effect of temperature on flavonoid production in pigeonpea [Cajanus cajan (L) Millsp] in relation to nodulation

To ascertain the reasons for poor nodulation of pigeonpea, we studied the effect of high temperature on the production of flavonoids by the pigeonpea host. A high temperature affected flavonoid production by pigeonpea and mungbean. At 37°C pigeonpea root exudates contained four flavonoids and the root extract contained five. The proportion of the second flavonoid in the pigeonpea and the mungbean was higher and the proportion of the third flavonoid was lower at 37°C compared to 30°C. At the higher temperature the flavonoids exuded from pigeonpea roots were same those in the root homogenate.     

Rhizobium sp. exopolysaccharide production,nodulation, and acetylene reduction activity (N2-fixation) in pigeon pea (Cajanus cajan L.)

Summary There was no correlation between the quantity of exopolysaccharide produced and acetylene reduction activity by Rhizobium spp. or by Bradyrhizobium spp. (Cajanus). The exopolysaccharide-defective mutants of Rhizobium sp. strain P 116 either failed to nodulate or showed a decrease in effectiveness. The deficiency in exopolysaccharide production was corrected by the addition of purified exopolysaccharide from the parent strain, or from Bradyrhizobium sp. strain P 149 or S24 isolated from pigeonpea (Cajanus cajan) and mungbean (Vigna radiata), respectively. However, the nodules so formed were not fully effective compared to those formed by the parent strain.     

Effects of chitin amendment of soil on microorganisms,nematodes, and growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.)

Effects of soil amendment with crabshell chitin on the growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.), and on populations of soil bacteria, fungi, and plant-parasitic and free-living nematodes were investigated in a pot trial. Five soil samples were collected from Te Puke (Paengaroa Shallow Sand, a Typic Hapludand) and five from Hamilton (Bruntwood silt loam, an Aquic Hapludand), New Zealand. Subsamples of each soil were either amended with chitin or unamended and planted with white clover and ryegrass. The ryegrass shoot weight in amended soil was greater (P<0.01), most probably due to N mineralised from chitin. A significantly lower (P<0.01) root: shoot ratio of ryegrass in the amended soil also suggested improved N availability, and therefore less root mass was needed to support a given shoot mass. A reduction in nodulation was observed in 12-day-old white clover seedlings (P<0.05) and also in 6-week-old seedlings (P<0.01). The shoot weight of white clover was significantly lower (P<0.05) in amended soil, possibly due to phytotoxic effects of chitin. Chitin increased (P<0.01) the populations of bacteria and fungi by 13-fold and 2.5-fold, respectively. The cyst nematode of white clover, Heterodera trifolii, was significantly reduced in chitin-amended soil, possibly due to increased levels of chitinase produced by rhizosphere microorganisms. Two other plant-parasitic nematodes, Pratylenchus spp. and Tylenchus spp., were also reduced in ryegrass roots and in soil as a result of the chitin amendment. However, the total number of free-living nematodes increased 5.4-fold in amended soil.     

Competition between inoculated and indigenous Rhizobium/Bradyrhizobium spp. strains for nodulation of grain and fodder legumes in Pakistan

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.     

Characterization and evaluation of North American Trifolium ambiguum -nodulating rhizobia

Kura clover ( Trifolium ambiguum M. Bieb.) is a promising persistent forage legume, whose use is currently limited by establishment difficulties in part attributable to nodulation problems. In this study, we phenotypically characterized 18 Kura clover rhizobia including 12 newly isolated North American strains using 111 growth tests. The symbiotic performances of these 12 strains when used to inoculate hexaploid Kura clover plants were then evaluated in a growth chamber and compared to five strains commonly used in commercial Kura clover inoculants. Non-inoculated plants with or without N fertilization were used as controls. Field evaluations were also conducted in 2001 and 2002 in Montreal, Quebec and Becker, Minnesota. Hexaploid Kura clover was seeded with one of three North American strains or three commercial strains and compared to non-inoculated controls with or without N fertilizer. Phenotypic diversity observed among the 12 North American strains was limited. The North American strains were often more efficient than strains commonly used in commercial rhizobial inoculants in both growth chamber and field trials. In the growth chamber study, 60 days after seeding, shoot dry matter accumulation was overall 205% greater with North American strains than with commercial strains and 45% greater than with the best commercial strain tested. Some of the strains currently used in commercial inoculants failed to nodulate Kura clover plants effectively. North American strains tested in field trials (i.e., CT1-1, CT1-2, and WI4-4) overall increased total and shoot dry matter accumulation, 100 days after seeding, in three of four environments by 27% and 31% respectively, when compared to commercial inoculant strains. While these strains appear to have potential they still produced less dry matter than non-inoculated N-fertilized controls.     

Size, symbiotic effectiveness and genetic diversity of field pea rhizobia (Rhizobium leguminosarum bv. viciae) populations in South Australian soils

Field pea (Pisum sativum L.) is widely grown in South Australia (SA), often without inoculation with commercial rhizobia. To establish if symbiotic factors are limiting the growth of field pea we examined the size, symbiotic effectiveness and diversity of populations of field pea rhizobia (Rhizobium leguminosarum bv. viciae) that have become naturalised in South Australian soils and nodulate many pea crops. Most probable number plant infection tests on 33 soils showed that R. l. bv. viciae populations ranged from undetectable (six soils) to 32×10³ rhizobia g⁻¹ of dry soil. Twenty-four of the 33 soils contained more than 100 rhizobia g⁻¹ soil. Three of the six soils in which no R. l. bv. viciae were detected had not grown a host legume (field pea, faba bean, vetch or lentil). For soils that had grown a host legume, there was no correlation between the size of R. l. bv. viciae populations and either the time since a host legume had been grown or any measured soil factor (pH, inorganic N and organic C). In glasshouse experiments, inoculation of the field pea cultivar Parafield with the commercial Rhizobium strain SU303 resulted in a highly effective symbiosis. The SU303 treatment produced as much shoot dry weight as the mineral N treatment and more than 2.9 times the shoot dry weight of the uninoculated treatment. Twenty-two of the 33 naturalised populations of rhizobia (applied to pea plants as soil suspensions) produced prompt and abundant nodulation. These symbioses were generally effective at N₂ fixation, with shoot dry weight ranging from 98% (soil 21) down to 61% (soil 30) of the SU303 treatment, the least effective population of rhizobia still producing nearly double the growth of the uninoculated treatment. Low shoot dry weights resulting from most of the remaining soil treatments were associated with delayed or erratic nodulation caused by low numbers of rhizobia. Random amplified polymorphic DNA (RAPD) polymerase chain reaction (PCR) fingerprinting of 70 rhizobial isolates recovered from five of the 33 soils (14 isolates from each soil) showed that naturalised populations were composed of multiple (5-9) strain types. There was little evidence of strain dominance, with a single strain type occupying more than 30% of trap host nodules in only two of the five populations. Cluster analysis of RAPD PCR banding patterns showed that strain types in naturalised populations were not closely related to the current commercial inoculant strain for field pea (SU303, ≥75% dissimilarity), six previous field pea inoculant strains (≥55% dissimilarity) or a former commercial inoculant strain for faba bean (WSM1274, ≥66% dissimilarity). Two of the most closely related strain types (≤15% dissimilarity) were found at widely separate locations in SA and may have potential as commercial inoculant strains. Given the size and diversity of the naturalised pea rhizobia populations in SA soils and their relative effectiveness, it is unlikely that inoculation with a commercial strain of rhizobia will improve N₂ fixation in field pea crops, unless the number of rhizobia in the soil is very low or absent (e.g. where a legume host has not been previously grown and for three soils from western Eyre Peninsula). The general effectiveness of the pea rhizobia populations also indicates that reduced N₂ fixation is unlikely to be the major cause of the declining field pea yields observed in recent times.     

Nitrogenase activity and nitrogen-fixing bacteria associated with the roots ofAtriplex spp. growing in saline sodic soils of Pakistan

Summary TwoAtriplex spp. growing in low-fertility saline sodic soils were assayed for root-associated nitrogenase activity. The excised washed and unwashed root of the two species.A. lentiformis andA. amnicola, showed high root-associated nitrogenase activity. Acetylene-reducing activity seemed to be directly influenced by moisture. The highest number of diazotrophs, enumerated using a most probable number technique was observed on the root surface. Most of the isolated diazotrophs were identified asEnterobacter agglomerans. Root-associated nitrogenase activity inAtriplex spp. may explain the high protein and biomass content of these plants growing in low-fertility saline sodic soils.