A multi-site field evaluation of granular inoculants for legume nodulation

The most common method of inoculating legume crops in Australia is the application of peat slurry inoculant to seed. The recent introduction of granular (solid) formulations of inoculants into the Australian market has provided the potential to apply rhizobia with greater ease, but their efficacy has not been independently evaluated. Here, we compare the efficacy of a range of experimental and commercially-available granular inoculants on chickpea, faba bean, lentil, lupin and pea crops in comparison with un-inoculated treatments, and with conventional seed-applied peat slurry inoculants. Thirty-seven field experiments were established in Victoria, South Australia and southern New South Wales over five years. Peat slurry inoculants provided effective nodulation of all legumes. Granular inoculants varied markedly in their ability to improve grain legume nodulation. The size of response depended inversely on background nodulation from soil rhizobial populations. At sites with median background nodulation, peat granules and attapulgite clay granules placed with seed resulted in nodulation similar to peat-slurry-based inoculation, but treatments with bentonite clay granules did not increase nodule numbers much above those in un-inoculated treatments. The generally lower numbers of rhizobia g⁻¹ in the bentonite granules, translated to lower rhizobia application rate to the soil. However, differences in number of rhizobia g⁻¹ granule did not fully explain the nodulation differences between granules. Granule moisture content and granule particle size differed markedly between granule types but their influence on nodulation was not tested. Grain yields did not differ between attapulgite granules placed with seed, peat granules and peat slurry inoculants (all well-nodulated treatments), but were lower with bentonite granule inoculants. Yield differences within sites were related to nodulation and the differences between treatments attenuated as background nodulation increased. Overall, these studies demonstrate that certain granule types have the potential to be used in Australia with grain legumes, particularly in circumstances when seed-applied inoculants are problematic, such as where seed fungicides or insecticides need to be applied. However, granular inoculant formulations differ substantially in their potential to produce nodules on a range of grain legumes.     

Legumes in temperate Australia: A survey of naturalisation and impact in natural ecosystems

Some of the most damaging weeds of natural ecosystems in temperate Australia and similar Mediterranean environments are legumes. A study was undertaken to determine the level of naturalisation and impact of exotic legumes in temperate Australia. This was the first step in attempting to develop a method of evaluating the weed risk of future legume species introductions. A study of nursery catalogues indicated that legumes were widely cultivated throughout temperate Australia in the early part of the 20th century. A number of these species were able to escape from cultivation and have become part of the region’s naturalised flora. Non-climbing herbaceous legume species were the most common type of legume recorded as naturalised. A smaller number of naturalised species were reported as being present in natural ecosystems. A higher introduction pressure increased the chance of legume species being able to naturalise and also establish in natural ecosystems. The results of a questionnaire showed that of the legume species that established in natural ecosystems there were differences in regard to their perceived level of impact. Woody perennial legumes had the highest reported impacts and non-climbing herbaceous species the lowest. The higher impact species also had a history of use as garden ornamentals, while lower impact species were largely agricultural. The results of this survey suggest that until greater accuracy is achieved with regard to the prediction of weeds and their impact, a precautionary approach should be taken to the importation, sale and cultivation of woody legume species.     

Conditions for successful Rhizobium-legume symbiosis in saline environments

Summary The Rhizobium-legume symbiosis in arid ecosystems is particularly important for locations where the area of saline soils is increasing and becoming a threat to plant productivity. Legumes, which are usually present in arid ecosystems, may be adapted to fix more N₂ under saline conditions than legumes grown in other habitats.Legumes are known to be either sensitive or moderately resistant to salinity. The salt sensitivity can be attributed to toxic ion accumulations in different plant tissues, which disturb some enzyme activities.Among the basic selection criteria for salt-tolerant legumes and rhizobia are genetic variability within species with respect to salt tolerance, correlation between accumulations of organic solutes (e. g., glycine betaine, proline betaine, and proline) and salt tolerance, and good relationships between ion distribution and compartmentation, and structural adaptations in the legumes.Salt stress reduces the nodulation of legumes by inhibiting the very early symbiotic events. Levels of salinity that inhibit the symbiosis between legumes and rhizobia are different from those that inhibit the growth of the individual symbionts. The poor symbiotic performance of some legumes under saline conditions is not due to salt limitations on the growth of rhizobia.Prerequisites for a successful Rhizobium-legume symbiosis in saline environments include rhizobial colonization and invasion of the rhizosphere, root-hair infection, and the formation of effective salt-tolerant nodules.The possibility of exploring the Rhizobium-legume symbiosis to improve the productivity of saline soils is reviewed in this paper.     

‘Ecological forestry’ and eucalypt forests managed for wood production in south-western Australia

The model of ‘ecological forestry’ has evolved as a part of the development of the concept of ecosystem management. ‘Ecological forestry’ emphasises that manipulation of a forest ecosystem should consider, and as far as practicable work within the limits of, natural disturbance patterns prior to extensive human alteration of the landscape. This paper evaluates the extent to which forest management practices in jarrah (Eucalyptus marginata) and karri (Eucalyptus diversicolor) forests of south-western Australia align with this view of the characteristics and appropriate silviculture of ‘ecological forestry’. Characteristics and appropriate silviculture of ‘ecological forestry’ are evaluated in relation to (i) the stand level decisions of stand structure and harvest timing and (ii) the landscape level decisions of harvest levels and age structures, and spatial patterns of harvest. Forest management in south-western Australia is found to align with appropriate silviculture under this model of ‘ecological forestry’. Additionally, the landscape triad of areas managed to ‘ecological forestry’ principles, conservation reserves and areas managed to production forestry is in place in the south-western forests of Australia. Strengths and weaknesses in the model of ‘ecological forestry’ and the ability to interpret consistency of practices in the forests of south-western Australia with the characteristics of ‘ecological forestry’ are identified.     

Inoculant rhizobia suppressed root-knot disease,and enhanced plant productivity and nutrient uptake of some field-grown food legumes

The potential effect of rhizobial inoculation on root knot nematodes in chickpea, mungbean and pigeonpea were studied under field condition. The seed treatment with respective rhizobium strains increased the nodulation, leghemoglobin content, bacteriod population, plant growth, yield and nitrogen uptake of three three food legumes compared to the plants without the rhizobium treatment. The nematode (1500?juveniles/kg soil) incited oval galls on the roots of the three legumes, and suppressed plant growth and yield. The galling, egg mass production and soil population of the nematode was greater on the plants without the rhizobium treatment. The pure culture and culture filtrate of the rhizobium strains suppressed the egg hatching and induced mortality to the juveniles of M. incognita over control. The nematode infection reduced the nodulation, bacteroid population and leghemoglobin contents of the nodules and NPK uptake by the plants. Hence, the rhizobia treatment shall be integrated to common agronomic practice of food legume cultivation so as to enhance crop productivity and to protect roots from nematode attack.     

Role of rhizobial EPS in the evasion of peanut defense response during the crack-entry infection process

Roles of rhizobial exopolysaccharides (EPS) in symbiotic nodulation have been most thoroughly studied in legumes infected by the infection thread (IT) mechanism. Peanut (Arachis hypogaea L.) differs from other legumes in that rhizobial penetration and spreading inside the nodule occur without IT formation but rather by crack-entry infection. By using a defined mutant (NET30-M1024) affected in the EPS production, we have previously shown that peanut symbionts require these molecules for efficient nodulation. In this work, we monitored the relationship between the symbiotic behavior of this mutant and the EPS level production, and evaluated ex planta if these molecules could play a role in protecting the microsymbiont against plant defense reactions.     

Involvement of autoregulation in the interaction between rhizobial nodulation and AM fungal colonization in soybean roots

Soybean plants autoregulate to suppress excessive nodulation. It has been revealed recently that the autoregulation of various legumes controls both nodulation and arbuscular mycorrhizal (AM) fungal colonization. We investigated the involvement of autoregulation in the interaction between rhizobial nodulation and AM fungal colonization. We used a wild-type soybean cv. Enrei and its hypernodulating mutant Kanto100, defective in the autoregulation. We included four different treatments: an uninoculated control, inoculation with rhizobium Bradyrhizobium japonicum alone, inoculation with AM fungus Gigaspora rosea alone, and dual inoculation with rhizobium and AM fungus. In both Enrei and Kanto100, AM fungal colonization enhanced the weight and N₂ fixation of nodules, suggesting that autoregulation of host plant is not involved in the stimulatory effect of AM fungal colonization on rhizobial nodulation. In plants with the AM fungus alone, the AM fungal colonization of Enrei was comparable to that of Kanto100. In plants with dual inoculation, however, this was significantly (P?<?0.05) lower than in Kanto100. To confirm the control of AM fungal colonization by the autoregulation of host plant, a reciprocal grafting experiment was performed between Enrei and Kanto100. In plants with the AM fungus alone, AM fungal colonization was comparable among Enrei (shoot)/Enrei (root), Enrei/Kanto100, Kanto100/Enrei, and Kanto100/Kanto100 grafts. In plants with dual inoculation, however, AM fungal colonization of Enrei/Enrei and Enrei/Kanto100 grafts was significantly (P?<?0.05) lower than that of Kanto100/Enrei and Kanto100/Kanto100. These results indicate that rhizobial nodulation suppresses AM fungal colonization, and the autoregulation of host plant, initiated by nodulation, is involved in this phenomenon.     

Legume-based farming in Southern Australia: developing sustainable systems to meet environmental challenges

Development of legume-based farming systems has resulted in Australian agriculture being globally competitive. There is now political pressure for agriculture to become accountable for ‘off-site’ environmental consequences. Farming systems relying on annual species are unsustainable because of a mismatch between the supply and demand of water and N, resulting in N leakage to streams or groundwater. Rainfall in excess of plant requirements coupled with N build-up, permeable soils, limited opportunities for reduction and proximity to surface or groundwater present risks for leakage of NO₃-N. We present examples of N leakage from legume systems in southern Australia, where rainfall exceeds 450 mm yr⁻¹, and the evidence suggesting that leakage contributes to stream and groundwater pollution. N build-up in autumn through mineralisation of organic-N from legume-based systems often exceeds 100 kg N ha⁻¹ and N leakage losses can be 15-35 kg N ha⁻¹ yr⁻¹. Stream and groundwater N pollution issues are emerging. Surface water quality problems are already apparent in Victoria although the contribution from legumes, N fertilisers and point sources remains unresolved. Examples of groundwater problems where legumes are a contributing factor have been recorded in New South Wales (NSW), South Australia (SA), Western Australia (WA) and Victoria. In Victoria, areas at risk of N groundwater contamination are found along the Great Dividing Range and in southern Victoria. Groundwater pollution causes concern because once problems are found they take decades to reverse. Stores of N in the unsaturated zone combined with limited N monitoring in groundwater suggests that early detection is unlikely. Solutions for reducing off-site consequences are outlined and include management to prevent water and N leakage happening, capture of N before it reaches waterways or groundwater and low input systems including land retirement. For scientists interested in N fixation and biological mediation, future research areas include increasing the proportion of perennials in farming systems, better control of N supply and demand through improved technology and us of N fertiliser, use of nitrification inhibitors and studies of the potential for N immobilisation and reduction through denitrification, both within and below the root zone. Integrated management strategies that address environmental implications from point/micro-scale to paddock and catchment scales are needed as are considerations of other environmental consequences. Research priorities will change from maximising N fixation for profitability towards balancing profitability and environmental goals for more sustainable systems.     

Nodulation of tree legumes and the ecology of their native rhizobial populations in tropical soils

A legume introduced into a new area will only form nodules and fix nitrogen if compatible rhizobia are present in the soil. Using 25 (60 in the case of Sesbania sesban) soils sampled from tropical areas of Africa, Asia and Latin America, we examined the nodulation of four agroforestry tree species (Calliandra calothyrsus, Gliricidia sepium, Leucaena leucocephala and S. sesban), their symbiotic interactions with the native rhizobial populations, and some of the ecological indicators of rhizobial population dynamics. Rhizobial population sizes estimated by the legume species ranged from undetectable numbers to 3.16×10⁴ cells per g of soil depending on the trap host species. Although C. calothyrsus had the highest nodulation rate in the soils used, inoculation tests showed L. leucocephala to be the most promiscuous species while G. sepium had the most effective symbiosis. S. sesban was the most specific for both nodulation and symbiotic effectiveness. Symbiotic effectiveness did not bear any close relationship with specific soil parameters, but rhizobial numbers were highly correlated with soil acidity, particle size and exchangeable bases. Soil acidity was also the main factor that was highly correlated with genetic diversity among the rhizobial populations.     

Legume seed inoculation technology—a review

Inoculation of legume seed is an efficient and convenient way of introducing effective rhizobia to soil and subsequently the rhizosphere of legumes. However, its full potential is yet to be realised. Following widespread crop failures, the manufacture of high quality inoculants revolutionised legume technology in Australia in the 1960s. Many improvements to inoculants and the advent of an inoculant control service ensured that quality was optimised and maintained. Minimum standards for the number of rhizobia per seed were set after consideration of several factors including seed size and loss of viability during inoculation. Despite manufacturers' recommendations for storage and application of inoculants, there is a distinct lack of control over the inoculation process; hence the full potential of high quality products may not always be achieved. The efficacy of inoculation varies depending on several factors, all of which affect the number of viable rhizobia available for infection of legume roots. Increased numbers of viable rhizobia per seed by application of inoculant above the commercially recommended rate, results in a continued linear increase in nodulation and yield. Several studies have reported yield increases of up to 25%. However, applying higher quantities of inoculant is uneconomical and technically difficult. Alternatively, higher numbers of viable rhizobia per seed may be achieved by improving survival during seed inoculation. Despite recognition of the factors affecting survival of rhizobia on seed and a substantial demand for commercially pre-inoculated legume seed, poor survival is still a major concern. Desiccation, temperature and seed coat toxicity all influence survival of rhizobia on seed. Their adverse effects may be ameliorated by selecting tolerant rhizobial strains and legume seed cultivars with low toxicity or artificially, by the use of additives in the seed coating. The accumulation of the desiccant protectant trehalose in strains of rhizobia, may result in better survival under desiccation stress. Similarly, the accumulation of exopolysaccharide (EPS) may act as a barrier reducing excessive water loss. Polymeric adhesives such as gum arabic, methyl cellulose and polyvinyl pyrollidone (PVP) have improved survival. However, studies of additives used in inoculation have been ad hoc and little of their mode of action is understood. A better understanding of the mechanisms involved in the protection of rhizobia from adverse conditions will assist in defining the optimum conditions for seed inoculation and storage to ensure a higher quality product for farmers at the time of sowing.     

Nature and mechanisms of aluminium toxicity,tolerance and amelioration in symbiotic legumes and rhizobia

Recent findings on the effect of aluminium (Al) on the functioning of legumes and their associated microsymbionts are reviewed here. Al represents 7% of solid matter in the Earth’s crust and is an important abiotic factor that alters microbial and plant functioning at very early stages. The trivalent Al (Al³⁺) dominates at pH <?5 in soils and becomes a constraint to legume productivity through its lethal effect on rhizobia, the host plant and their interaction. Al³⁺ has lethal effects on many aspects of the rhizobia/legume symbiosis, which include a decrease in root elongation and root hair formation, lowered soil rhizobial population, and suppression of nitrogen metabolism involving nitrate reduction, nitrite reduction, nitrogenase activity and the functioning of uptake of hydrogenases (Hup), ultimately impairing the N₂ fixation process. At the molecular level, Al is known to suppress the expression of nodulation genes in symbiotic rhizobia, as well as the induction of genes for the formation of hexokinase, phosphodiesterase, phosphooxidase and acid/alkaline phosphatase. Al toxicity can also induce the accumulation of reactive oxygen species and callose, in addition to lipoperoxidation in the legume root elongation zone. Al tolerance in plants can be achieved through over-expression of citrate synthase gene in roots and/or the synthesis and release of organic acids that reverse Al-induced changes in proteins, as well as metabolic regulation by plant-secreted microRNAs. In contrast, Al tolerance in symbiotic rhizobia is attained via the production of exopolysaccharides, the synthesis of siderophores that reduce Al uptake, induction of efflux pumps resistant to heavy metals and the expression of metal-inducible (dmeRF) gene clusters in symbiotic Rhizobiaceae. In soils, Al toxicity is usually ameliorated through liming, organic matter supply and use of Al-tolerant species. Our current understanding of crop productivity in high Al soils suggests that a much greater future accumulation of Al is likely to occur in agricultural soils globally if crop irrigation is increased under a changing climate.     

Nodulation and Symbiotic Nitrogen Fixation of Cowpea Genotypes as Affected by Fertilizer Nitrogen

Abstract

Intercropping with legumes and non‐legumes is commonly practiced in many parts of the world to maximize productivity per unit area of land. In India, cowpea [Vigna unguiculate (L.) Walp] is a popular pulse legume component of intercropping farming systems. Often, however, potential production is compromised, particularly in high fertilizer input systems, because legume component competes with the non‐legume component of the system for nitrogen (N) in the soil. An experiment was conducted in order to identify lines of cowpea that could obtain the majority of their nitrogen requirements from symbiotic fixation of atmospheric nitrogen rather than from uptake of soil nitrogen. Twenty‐nine genotypes of cowpea were screened for tolerance to (applied) nitrogen in soil in field condition. The parameters used to appraise tolerance were extent of root nodulation, the amount of nitrogen fixed, nitrate reductase activity (NR) in roots and nodules, and nitrite content of roots and nodules. There were two nitrogen treatments applied as urea, 40 kg N per ha (N40), and 120 kg N per ha (N120). There were three genotypes whose nitrogen‐fixing effectiveness was apparently unimpaired by applications of nitrogen to the soil. Genotype EC‐170442‐3 nodulated and fixed atmospheric nitrogen satisfactorily at higher levels of applied nitrogen. At N40, genotypes EC‐244390 and EC‐240900 formed a great abundance of large nodules effective in nitrogen fixation; even at N120, EC‐240900 had better symbioses than the majority of the 29 cowpea lines originally screened. These three genotypes are deemed worthy of further examination for their suitability for intercropping systems. How this might be achieved is discussed.     

Nodulation status of native woody legumes and phenotypic characteristics of associated rhizobia in soils of southern Ethiopia

The nodulation of provenances of Acacia seyal, Acacia tortilis and Faidherbia albida, and other indigenous multipurpose tree species were tested in 14 different soil samples collected from diverse agro-ecological zones in southern Ethiopia. Associated rhizobia were isolated from these and from excavated nodules of field standing mature trees, and phenotypically characterized. Indigenous rhizobia capable of eliciting nodules on at least one or more of the woody legume species tested were present in most of the soils. Tree species were markedly different in nodulation in the different site soils. Sesbania sesban and Acacia abyssinica showed higher nodulation ability across the different sites indicating widespread occurrence of compatible rhizobia in the soils. The nodulation patterns of the different provenances of Acacia spp. suggested the existence of intraspecific provenance variations in rhizobial affinity which can be exploited to improve N fixation through tree selection. Altogether, 241 isolates were recovered from the root nodules of trap host species and from excavated nodules. Isolates were differentiated by growth rate and colony morphology and there were very fast-, fast-, slow-, and very slow-growing rhizobia. The bulk of them (68.5%) were fast-growing acid-producing rhizobia while 25.3% were slow-growing alkali-producing types. Fast-growing alkali-producing (2.9%) and slow-growing acid-producing strains (3.3%) were isolated from trap host species and excavated nodules, respectively. All isolates fell into four colony types: watery translucent, white translucent, dull glistering and milky (curdled) type. The diversity of indigenous rhizobia in growth rate and colony morphology suggested that the collection probably includes several rhizobial genera.     

Responses of legumes to rhizobia and arbuscular mycorrhizal fungi: A meta-analysis of potential photosynthate limitation of symbioses

Legumes are prized for their seed protein and lipid mass fractions. Since legumes spend up to 4-16% of photosynthesis on each of the rhizobial and arbuscular mycorrhizal (AM) fungal symbioses, it might be expected that positive responses in yield due to rhizobial and AM symbioses are accompanied by decreases in seed protein and lipid mass fractions due to a photosynthate (C) limitation. We performed a meta-analysis of 348 data points from published studies with 12 legume species to test whether yield, harvest index, and seed protein and lipid mass fractions are affected by symbioses. There was a significant increase in yield due to rhizobial inoculation (16% in the field; 59% in pot experiments). There were no responses of yield to AM fungi and rhizobial + AM fungi inoculations in the field (presumably because an AM fungi-free control cannot be ensured), but significant responses in pots (45% with AM fungi; 44% with rhizobial + AM fungi). Rhizobial inoculation improved seed protein mass fraction by 7% in the field; AM fungi increased this parameter by 14% in pots. There were no discernable effects of symbioses on seed lipid mass fraction. Rhizobial symbioses in the field increased harvest index (+5%), but AM fungi did not affect harvest index. In conclusion, increases in yield due to symbioses also resulted in increases in seed protein and constant lipid mass fractions, indicating that legumes are not C-limited under symbiotic conditions.     

INTERACTIONS BETWEEN COMMON BEAN GENOTYPES AND RHIZOBIA STRAINS ISOLATED FROM MOROCCAN SOILS FOR GROWTH,PHOSPHATASE AND PHYTASE ACTIVITIES UNDER PHOSPHORUS DEFICIENCY CONDITIONS

The improvement of common bean production requires the selection of effective rhizobia strains and Phaseolus vulgaris genotypes adapted to available soil phosphorus limitations. The interactions between bean genotypes and rhizobia were studied in hydroponic culture using six genotypes and four strains, CIAT899 as reference and three strains isolated from nodule of farmer's fields in the Marrakech region. The phosphorus (P) sub-deficiency caused a significant reduction on shoot biomass in some bean genotype-rhizobia combinations. Nodule biomass is significantly more reduced under P limitation for several combinations tested. Bean plants inoculated with these local rhizobial strains showed higher nodulation and an increase of nodules phytase and phosphatase activities under phosphorus sub-deficiency especially for RhM11 strain. It was concluded that the studied bean-rhizobia symbiosis differ in their adaptation to phosphorus sub-deficiency and the nodule phosphatases and phytases activities may constitute a strategy of nodulated bean plants to adapt their nitrogen fixation to P deficiency.     

Beneficial effects of silicon on abiotic stress tolerance in legumes

Soil salinity, drought, metal toxicity, and ultraviolet-B radiation were major abiotic stresses that limit plant growth and productivity by disrupting the plants' cellular ionic and osmotic balance; legumes, a diverse plant family, suffered from these abiotic stresses. Although silicon (Si) is generally considered non-essential for plant growth and development, Si uptake by plants could facilitate plant growth by reducing biotic and abiotic stresses. There is however, a lack of systematic study on Si uptake benefits and mechanism on legumes because legumes reject Si uptake. Here, we reviewed the beneficial role of Si in enhancing abiotic stress tolerance in legumes and highlighted the mechanisms through which Si could improve abiotic stress tolerance in legumes. Future research needs for Si mediated alleviation of abiotic stresses in legumes are also discussed.     

活性氧调控豆科植物早期结瘤的研究进展

活性氧（reactive oxygen species,ROS）是一类具有高反应活性的氧衍生物,包括超氧阴离子（·O₂^—）、羟自由基（·OH）、过氧羟自由基（·HO₂）以及过氧化氢（H₂O₂）等。植物在进行有氧代谢或遭遇生物与非生物胁迫时会产生ROS,它不仅仅是有氧代谢的有毒副产物,同时能作为信号分子调节体内代谢过程,对抗外界环境。豆科植物形成根瘤时同样会产生ROS,这种ROS的变化区别于病原体入侵,而是作为一种信号物质参与结瘤过程。结瘤因子（nod factor,NF）诱导下ROS的产生参与了浸染线形成时细胞壁的重建、植物基质糖蛋白（matrix glycoprotein,MGP）的交联和肌动蛋白微丝的成核和延长过程。细胞质膜NADPH氧化酶（respiratory burst oxidase homologue,RBOHs）是共生过程中ROS产生的主要途径,Rboh基因的过表达会促进根瘤菌浸染和根瘤形成,同时根瘤中的共生微粒体数量增加,固氮效率提高,而表达受抑制后会减少ROS的产生,同时下调结瘤相关基因RIPs、NIN、ENOD2的表达,抑制固氮酶活性。此外,ROS时空上的变化与Ca²⁺相关联,协同调控根系结瘤。ROS的产生是植物与微生物早期的识别信号,通过认识ROS在早期结瘤过程中的作用有助于我们进一步理解共生关系建立的特异性。本文就ROS在早期结瘤过程中的产生及其发挥的作用做了综述,指出ROS通过直接或间接作用诱导结瘤基因的表达,是豆科植物根瘤形成以及功能固氮的重要信号分子。     

Effects of resident rhizobial communities and soil type on the effective nodulation of pulse legumes

Communities of resident rhizobia capable of effective nodulation of pulse crops were found to vary considerably over a range of soil environments. These populations from soils at 50 sites in Southern Australia were evaluated for nitrogen fixing effectiveness in association with Pisum sativum, Vicia faba, Lens culinaris, Vicia sativa, Cicer arietinum and Lupinus angustifolius. The values for nitrogen fixing effectiveness could be related to soil pH as determined by soil type and location. It was found that 33% of paddocks had sufficient resident populations of Rhizobium leguminosarum bv viciae for effective nodulation of faba bean, 54% for lentils, 55% for field pea and 66% for the effective nodulation of the vetch host plant. Mesorhizobium cicer populations were very low with only 7% of paddocks surveyed having sufficient resident populations for effective nodulation. Low resident rhizobial populations (<10 rhizobia g⁻¹ soil) of R. leguminosarum bv viciae and M. cicer were found in acid soil conditions. In contrast, Bradyrhizobium populations increased as soil pH decreased. Inoculation increased faba bean yields from 0.34 to 4.4 t ha⁻¹ and from 0.47 to 2.37 t ha⁻¹ for chickpeas on acid soils. On alkaline soils, where resident populations were large there was no consistent response to inoculation. Observations at experimental field sites confirmed the findings from the survey data, stressing the importance of rhizobial inoculation, especially on the acid soils in south-eastern Australia.     

The abundance and efficacy of Rhizobium leguminosarum bv. viciae in cultivated soils of the eastern Canadian prairie

For optimum production, the use of commercial rhizobial inoculant on pea (Pisum sativum L.) at seeding is necessary in the absence of compatible rhizobial strains or when rhizobial soil populations are low or symbiotically ineffective. Multiple site experiments were conducted to characterize the abundance and effectiveness of resident populations of Rhizobium leguminosarum bv. viciae (Rlv) in eastern Canadian prairie soils. A survey of 20 sites across a broad geographical range of southern Manitoba was carried out in 1998 and was followed by more intensive study of five of the sites in 1999 and 2000. Appreciable nodulation of uninoculated pea was observed at all sites which had previously grown inoculated pea. However, uninoculated pea grown at two sites, which had not previously grown pea, had negligible nodulation. Likewise, wild Lathyrus sp. and Vicia sp. plants collected from uncultivated areas adjacent to agricultural sites were poorly nodulated. In the more intensively studied sites, there was a tendency towards higher nodulation in pea plants receiving commercial inoculant containing Rlv strain PBC108 across all site-years (e.g., 4.7% in nodulation and 22% in nodule mass), but the effect was significant at only 2 of 10 site-years. Despite a relatively high range of soil pH (6-8), regression analysis indicated that decreasing soil pH resulted in lower nodulation rates. Likewise, electrical conductivity (EC) was correlated to nodulation levels, however the effect of EC was likely more indicative of the influence of soil texture and organic matter than salinity. As with nodulation, commercial inoculation tended to increase above-ground dry matter (DM) and fixed-N (estimated by the difference method) at the early pod-filling stage, but again the effects were significant at only 2 of 10 site-years. Specifically, above-ground DM and fixed-N levels were up to 29 and 51% greater, respectively, in inoculated compared to non-inoculated treatments at these sites. Addition of N-fertilizer at a rate of 100 kg N ha⁻¹ decreased nodulation at almost all site-years (by as much as 70% at one site), but rarely resulted in increases in above-ground DM compared to inoculated plots. The study indicates for the first time that populations of infective, and generally effective strains of Rlv occur broadly in agricultural soils across the eastern Canadian prairie, but that there is a tendency for increased symbiotic efficiency with the use of commercial inoculant.     

High and low nodulation in relation to molecular diversity of chickpea mesorhizobia in Indian soils

Chickpea (Cicer arietinum L.) nodulation variants of two cultivars ICC 4948 and ICC 5003 were used as trap plants to isolate 385 native rhizobia from CCS Haryana Agricultural University, Hisar farm soil. After authentication and considering growth characteristics, selected 110 rhizobia revealed immense molecular diversity using the profiles of DNA fragments generated by Polymerase chain reaction (PCR) with enterobacterial repetitive intergeneric consensus (ERIC) sequences. Low nodulating variants of cvs ICC 4948 and ICC 5003 were able to trap more numbers of rhizobial genotypes, namely seven as compared four to five by high nodulating variants of these cultivars. Overall eight rhizobial genotypes were trapped by the chickpea cultivars. Rhizobial isolates from same nodule or same plants were present in the same or different clusters and few isolates showed 100% similarity also. Based on nodules from a plant, nodulation variant or cultivar, rhizobia could not be differentiated and no exclusive cluster was formed by either rhizobial isolates from low or high nodulating variants of both the cultivars. Two most efficient rhizobial isolates LN 707b and LN 7007 were characterized by amplification and sequencing of 16S rRNA gene. Rhizobial isolate LN 707b showed more than 98% similarity with Mesorhizobium sp SH 2851 and Mesorhizobium mediterraneum. Another isolate LN 7007 showed more than 99% similarity with the sequence of 16S r RNA gene of Mesorhizobium sp STM 398, and M. mediterraneum. So the chickpea rhizobia from Northern Indian subcontinent are proposed to be kept under M. mediterraneum strain LN707b and LN 7007.