Comparative ability of grain legumes to fix N2
It is difficult to make useful generalizations as to the N2-fixing ability of the different grain legumes. Across all of the grain legumes for which data is presented in Table 8.2, optimal rates of N2-fixation are 1–2 kg N ha-1day-1, with all of the legumes recorded to fix more than 120 kg N ha-1within a cropping season in at least one case.
As discussed in Chapter 5, these fast rates should be considered as thepotentialof the grain legumes for N2-fixation within a given environment. The wide variations in N2-fixation within a given crop are found for a variety of reasons.
Cases where no N2-fixation has been observed, or very small %N from N2-fixation, are largely due to drought (C. arietinum), some other environmental constraint such as high temperatures (P. vulgaris), or perhaps nutrient limitations where the measurements were made in farmers’ crops. The largest amounts of N2-fixation have been recorded where there have been long, favourable growing seasons, generally on research stations.
Early-maturing pigeonpea varieties appear to nodulate and fix N2poorly com- pared with long-duration varieties (Kumar Raoet al., 1995, 1996b). In Australia, pigeonpea exhibits highly variable nodulation even when inoculated and is often con- sidered to fix N2poorly (Brockwellet al., 1991), although estimates from India and Africa show that it can often fix large amounts of N (Table 8.2). The smaller amounts of fixation withV. unguiculataandP. vulgaristend to be for shorter-season, determite types and the larger amounts are found with spreading or climbing varieties.
P. vulgaris has often been judged to be poor in N2-fixation (e.g. Piha and Munns, 1987a); yet under optimal conditions estimates of N2-fixation of up to 72%
of N derived from fixation have been obtained (Table 8.2) and in longer growing seasons amounts up to 125 kg N ha-1fixed have been recorded (Rennie and Kemp, 1983). These are comparable to estimates for soybean, which is considered to fix N2
abundantly. Under controlled conditions in growth rooms,P. vulgarisnodulates well and fixes N2at similar high rates to other grain legume species (Eaglesham, 1989). In relatively cool, long seasons in Austria, three climbing varieties ofP. vulgarisfrom the African highlands were among the best in terms of largest amounts of N accumulation and N2-fixation (Hardarsonet al., 1993). The success of grain legumes in N2-fixation in the field will be strongly influenced by the prevailing environmental conditions, and sensitivity of grain legumes to environmental stresses may be the overriding factor influencing the amount of N2fixed. This may partly explain why P. vulgarishas been classed as a poor N2-fixer. However, whereas soybean may often be able to meet all its requirements for growth and high yields from N2-fixation (e.g. Hungriaet al., 2000),P. vulgarismay still respond to N fertilizer even under conditions where it grows and fixes N2well (Redden and Herridge, 1999).
which the crops were originally domesticated. Lie (1981; Lieet al., 1987) highlighted the importance of these centres as a source of genes involved in symbiotic N2-fixation in both host plants and rhizobial strains. Wild legumes that are capable of forming effective N2-fixing symbioses are invariably nodulated in the field in their centres of diversity, even when they fail to nodulate when imported into other regions. This is presumably due to coevolution of adapted host plant genotypes and compatible rhizobial strains (Lieet al., 1987). Inoculation is therefore most likely to be necessary when legumes are introduced into new regions, although the need for inoculation will of course be conditioned by the requirements of the introduced legume for its own specific strains of rhizobia. If the introduced legume crop can nodulate effectively with rhizobia that are present in the soil in sufficient numbers, then inoculation may not be necessary. The contrasting requirements of grain legumes for inoculation with rhizobia can best be examined by comparing the examples of soybean, which often requires inoculation, and cowpea, which has rarely been found to respond to inoculation.
The soybean story
Soybean was domesticated in China and has been grown traditionally in many parts of Southeast Asia. The crop was first grown in North America in the 18th century but production has increased since the early part of the 20th century to the extent that soybean is now the most important grain legume crop in North America (Smith and Huyser, 1987). Soybean is a relatively specific host and does not nodulate when grown in the field for the first time in many parts of Africa, the Americas, Europe and some parts of Asia. Yields of soybean in North America were poor until soil containing compatible rhizobia was introduced from Japan (Allen and Allen, 1981).
Thus soybean crops routinely respond well to inoculation and a substantial soybean inoculum production industry has grown up, particularly in Brazil and the USA (Eaglesham, 1989).
However, the story is by no means so simple. In the centre of diversity of soy- bean and in countries of Southeast Asia, where the crop has been grown for centuries, soybean usually nodulates without inoculation. Strains ofBradyrhizobium isolated from nodules of other legume hosts, such as cowpea, grown in the same soils, can also nodulate the local soybean genotypes effectively. In some Chinese soils, fast-growing Sinorhizobiumspecies dominate the indigenous populations of soybean rhizobia.
Promiscuous varieties in southern Africa
Soybean cultivation in East and southern Africa appears to be first documented in the early 20th century (see review by Mpeperekiet al., 2000) but it is likely that the crop was introduced much earlier through the extensive trade around the Indian ocean.
Corby (1965, 1967) first described nodulation of soybean by rhizobia indigenous to African soils. He found that one variety, ‘Hernon 147’, nodulated effectively at five out of six sites in Zimbabwe and Zambia and did not respond to rhizobial inocu- lation. At the other site, in a more arid area of Zimbabwe, plants were ineffectively
nodulated and inoculation resulted in strong increases in crop yields. In earlier inocu- lation trials in the 1950s with a different variety, ‘Hernon 237’, crop yield responses to inoculation had been observed (Davis, 1986). Corby concluded that inoculation was unnecessary, as long as varieties able to nodulate with indigenous rhizobia were grown. The ‘Hernon’ varieties are ‘hay’ types, with luxuriant growth but relatively low yield potential, which were grown largely for fodder.
Nodulation of soybean varieties with indigenous rhizobia was also reported in South Africa (Van Rensburget al., 1976) and Tanzania (Chowdhury, 1977). Indeed, all varieties evaluated in Tanzania formed some nodules, but only those bred locally from ‘Hernon’ varieties or an Asian variety, ‘Malayan’, formed many nodules. More recent investigations have revealed a similar pattern in Zimbabwe, where even highly specific North American genotypes such as ‘Bragg’ nodulated in at least one of the soils tested, while ‘Hernon 147’ nodulated in most of the soils (Mpeperekiet al., 2000).
Observations by a farmer in 1977 of good nodulation and yields of uninoculated soybean (again ‘Hernon 147’) in a field in southern Zambia with no history of rhizobial inoculation led to further detailed investigations (described in Javaheri and Joshi, 1986; Javaheri, 1996). Out of 400 varieties evaluated, more than 40 formed nodules and, without exception, the varieties that nodulated shared common parentage with either ‘Hernon’ varieties or ‘Gilbert’, an introduction from Australia.
The link between promiscuous nodulation and the variety ‘Gilbert’ was surprising, until it was realized that both parental lines of ‘Gilbert’ were largely unimproved selections taken to Australia from Africa (Mpeperekiet al., 2000). Further screening led to the identification of one exceptionally promiscuous variety, which was named
‘Magoye’ and released in Zambia in 1981. ‘Magoye’ was selected from a cross between
‘Gilbert’ and ‘K53’, which themselves were both selections with the same parents.
These parental lines were ‘Avoyelles’ (a selection from a farmer’s field in Taiwan) and Mamloxi (a cross between selections from China and Japan), initially introduced to Africa from the USA (Mpeperekiet al., 2000). So although these varieties had almost circled the earth by the time Javaheri evaluated them in Zambia, they had been through a very limited breeding programme since their initial collection in East Asia.
‘Magoye’ nodulates readily in virtually all soils in southern Africa where it has been tested, and rarely shows any response to inoculation in Zambia and Zimbabwe, whereas ‘Hernon 147’ responds to inoculation more often (Mpeperekiet al., 2000).
The lack of response to inoculation, together with good and consistent yields of
‘Magoye’, have led to its widespread promotion in southern Africa (Chapter 14).
Breeding for promiscuity in soybean
Asian varieties also nodulated well in the field in Nigeria, whilst American varieties formed very few nodules (Nangju, 1980). As in southern Africa, Asian varieties – in this case ‘Orba’ and ‘Malayan’ – had been introduced around the turn of the century and were grown sporadically by smallholder farmers. The American varieties probably show restricted nodulation for two reasons: firstly, the genetic base from which they have been bred is limited (Hartwig, 1973; Gizliceet al., 1994); and secondly, only a limited range of inoculant strains ofB. japonicumwere introduced to North America, leading to increased cultivar-strain specificity.
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Inoculation of the varieties of Asian origin seldom increased yields in the field in Nigeria although yields of American varieties were more than doubled in most cases (Pulveret al., 1982). On the basis of these results, a breeding programme was initi- ated at IITA, Nigeria, in 1978 to reintroduce the ability to nodulate with indigenous strains of rhizobia into the American varieties, as they had far greater yield potential and better resistance to diseases (Kuenemanet al., 1984; Pulver et al., 1985). The programme was based on selection of progeny from crosses between Asian and Amer- ican varieties with good nodulation in local soils (using visual scores for nodule mass) and was partially successful. As a result a number of varieties were released that nodulate without inoculation in soils not previously cropped with soybeans, but few nodules are produced and the nodules are widely distributed over the root system rather than clustered around the tap root – the situation characteristic of small (< 102 cells g-1soil) populations of compatible rhizobia. Moreover, these ‘promiscuously nodulating’ soybeans are not in fact nodulated by the same range of rhizobia as cowpea when grown in the same soil (Bromfield and Roughley, 1980; Eaglesham, 1985; Abaidoo et al., 1999). Hence their nodulation can be very poor even in soils where cowpeas of the same age are abundantly nodulated (Eaglesham, 1989).
When the promiscuously nodulating soybean varieties were inoculated, yields were sometimes increased, at least in the first season, indicating again that the initial nodulation with indigenous bacteria is poor in comparison with the plant’s capacity for nodulation (Ranga Raoet al., 1985; Okereke and Eaglesham, 1993; Sanginga et al., 1996a). It is likely that over a few seasons the number of compatible bacteria may increase in response to the presence of the host, at least where soil conditions are favourable. On the basis of these results, Eaglesham (1989) concluded that it may be safer to rely on effective inoculant strains than to breed for the ability to nodulate with indigenous strains of unknown potential – a case of ‘better the devil you know’.
Despite the problems with initial nodulation in some soils, promiscuous soybean varieties from the IITA breeding programme have been widely adopted by farmers in parts of Nigeria (Manyong et al., 1998; Sanginga et al., 1999). The breeding programme at IITA has continued and recent materials have substantially improved ability to nodulate and fix N2in farmers’ fields without inoculation (Sangingaet al., 1997, 2000), as well as having higher yield potential (Sangingaet al., 2001).
Breeding for symbiotic specificity in soybean
In the USA, by contrast, exactly the opposite approach has been adopted, namely that of breeding for highly specific nodulation. In North America, strains of B. japonicum serogroup USDA 123 are prevalent in many soils, and are highly competitive in nodulation with the improved American soybeans (Moawadet al., 1984; Zdor and Pueppke, 1988). Unfortunately, serogroup USDA 123 is less effective in symbiosis with most American soybean cultivars than otherB. japonicum strains now present in the soils or used as inoculants (Caldwell and Vest, 1970). To enable establishment of more effective soybean symbioses, therefore, attempts have been made to breed cultivars that specifically exclude the less effective serogroup USDA 123 from nodulation (Cregan and Keyser, 1986; Keyser and Li, 1992).
Restricted nodulation ability of soybean genotypes with other strains has been
identified but whether this is a wise strategy remains a matter of debate. Kipe-Nolt et al. (1992) identified accessions ofP. vulgaristhat had been collected in the wild and were resistant to nodulation with R. tropici, which indicates that it may be possible to breed for nodulation specificity in P. vulgaris. As Herridge and Rose (2000) suggested, success of this approach will depend on the continual development of effective and compatible host genotype/strain combinations as infective but ineffective populations may build up in the soil.
‘Promiscuous’ grain legumes
Other grain legumes are less specific in their nodulation with rhizobia than soybean.
Cowpea is a very promiscuous legume host (Ahmadet al., 1981; Ranga Raoet al., 1985; Lewinet al., 1987) andBradyrhizobiumstrains with which it can form effective nodules are normally present. Thus cowpea, and some other tropical legumes, have rarely been found to respond to inoculation unless they are grown in soils where the conditions are not conducive to the survival of rhizobia (Lewinet al., 1987). For example, a response to inoculation was found in cowpea and black gram in only one of three experiments conducted in the field (Bushbyet al., 1983) and inoculation of pigeonpea resulted in a yield increase in only two of 12 field experiments conducted at the ICRISAT centre in India (Thompsonet al., 1980). Chickpea is more exacting in the specificity of its requirements for rhizobia and both nodulation and plant growth can commonly be increased by inoculation where it is introduced for the first time, but not in traditional growing areas (Smithsonet al., 1985). Problems for the survival of rhizobia in soil, inoculant technology and screening of rhizobial strains for use as inoculants are considered further in Chapters 13 and 14.
Phaseolus– a special case?
One of the most perplexing problems in research on improvement of N2-fixation in grain legumes has been the generally poor nodulation ofP. vulgarisin the field (Graham, 1981). Frustratingly, although poor nodulation is frequently observed, P. vulgarisrarely responds to inoculation. For example, in Cameroon inoculation did improve nodulation in trials over 3 years but improved grain yield in only one season (Salez and Saint Macary, 1987).
It has been shown in Colombia and in eastern Africa that, even in soils where nodulation ofP. vulgariswith indigenous rhizobia is sparse, these soils often contain large numbers (> 103g-1soil) of compatible and effective rhizobia. This poor nodulation is not due to an intrinsic inability ofP. vulgaristo nodulate, as profuse nodulation can occur in controlled conditions in the glasshouse. Therefore, it indicates either that some environmental constraint is limiting nodulation in the field, or that some factor other than N is limiting crop growth, or a combination of the two (Giller, 1990). In eastern Africa, although the nodule mass of Phaseolus plants was often increased by inoculation alone, application of phosphorus was the
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only treatment that gave consistent responses in N2-fixation and grain yield (Ssali, 1988; Amijee and Giller, 1998; Gilleret al., 1998). In Tanzania, the improvement in growth ofPhaseoluswhen phosphorus was added revealed that the ability of the soil to supply potassium was chronically deficient, and small potassium additions doubled grain yields in farmers’ fields (Smithsonet al., 1993).
Where P. vulgaris has responded to inoculation this is where effective, compatible rhizobia were absent, or present only in small numbers (Amijee and Giller, 1998). Absence of compatible rhizobia is particularly unlikely in the case of P. vulgaris, due to its promiscuity of nodulation. Although for a long time it was presumed thatP. vulgarisnodulated only withR. leguminosarumbv.phaseolistrains, it is now known that it can nodulate with many different species of rhizobia (see above). The wide range of rhizobia able to infectP. vulgarisincreases the likelihood that nodulation may occur with strains that are ineffective or poorly effective in N2-fixation. Further research is required to clarify the particular reasons why nodulation is poor in different environments, but there is no clear evidence to suggest thatP. vulgarisis particularly unusual in its response to inoculation.
Potential for strain selection for grain legumes
The vast majority of rhizobial strains that are recommended for use as inoculants have not been subjected to rigorous selection in soil in competition with native strains. Thus inoculation is rarely beneficial if populations of effective, compatible rhizobia are already present in the soil (Singleton and Tavares, 1986) but, despite the poor record of success, there remains considerable scope for strain improvement in the future (Chapter 14). A large proportion of rhizobia indigenous to Kenyan soils were more effective in N2-fixation withP. vulgaris than the widely recommended strain CIAT899 (Anyangoet al., 1995). Selection of adapted strains forP. vulgaris sown directly in pots of soils containing large populations of indigenous, compatible rhizobia has resulted in many cases in yield increases when these strains were tested in the field (Pineda and Kipe-Nolt, 1990; Sylvester-Bradley and Kipe-Nolt, 1990).
In some cases particular strains have shown promising results with grain legumes considered to be highly promiscuous in nodulation with rhizobia. An interesting example is found with groundnut, where a particular cultivar (Robut 33–1) gave increased pod yields on inoculation with a specific strain, Bradyrhizobium sp.
(Arachis) strain NC92 (Nambiar et al., 1984). Several experiments at a range of locations in India indicated the advantage of this host/strain combination but later work failed to achieve significant increases in yield and interest in this combination appears to have waned.