Grain Legumes

1. INTRODUCTION
2. PRIMARY GRAIN LEGUMES
3. SECONDARY GRAIN-LEGUMES
4. TERTIARY GRAIN LEGUMES
5. REMARKS
6. REFERENCES
7. Table 1
8. Table 2
9. Table 3
10. Table 4
11. Table 5
12. Table 6
13. Table 7
14. Table 8
15. Fig. 1

INTRODUCTION

In this paper, I will present an overview of the Fabaceae grown for food focusing upon those plants used as grain legumes. The grain legumes are those plants used as food in the form of unripe pods, immature seed or mature dry seed, directly or indirectly (Table 3).

Not only do the grain legumes provide variety to the human diet but they also supply dietary protein for many populations lacking animal or fish protein. In general, the grain legumes are rich in lysine but poor in methionine content, thereby complementing the reverse amino acid pattern found in cereals. Additionally, virtually all of the grain legumes fix their own nitrogen, thereby reducing, in many situations, the cost of nitrogen inputs by farmers. The grain legumes, especially soybeans and peanuts, are excellent sources of vegetable oils used in the production of cooking oil, margarine, mayonnaise, and salad dressings (Table 4).

For convenience I have divided the grain legumes into three categories-primary, secondary and tertiary grain legumes. The legumes in each category will be discussed in the following sections.

PRIMARY GRAIN LEGUMES

• The crops primarily belong to two tribes of the subfamily Papilionoideae that is, the Vicieae (Lens, Pisum, Vicia) and Phaseoleae (Cajanus, Phaseolus, Vigna). Exceptions include Arachis which is placed in the Aeschynomeneae and Cicer which is now placed in its own tribe the Cicereae. The Phaseoleae are distributed in the tropics or subtropics while the Vicieae are distributed in the Sino-India region or the Fertile Crescent.
• During the domestication process, the primary grain legumes coevolved with the major cereal crops. Thus, the soybean is paired with rice in China; peas, lentils and chickpeas with wheat and barley in the Fertile Crescent; beans with maize in the New World; and cowpeas with sorghum in Africa.
• The crops are grown over wide geographical areas and traded in world commerce. Major comprehensive germplasm collections are maintained for each crop. They all have at least one full time curator.
• Cultivars have been developed through modern plant breeding procedures. Land races of these crops are rapidly disappearing.
• Except for Vicia faba, the conspecific wild relative of each cultigen has been identified.
• For each crop, there is at least one major national or international center of research excellence. A critical mass of plant breeders, pathologists, entomologists, soil scientists, etc. are available to solve production problems. In addition, fundamental research has been conducted on the crops. For example, at the University of Illinois the genomic relationships of species in the genus Glycine and the idiogram of soybean chromosomes have been elucidated (Table 6, Fig. 1).

SECONDARY GRAIN-LEGUMES

• The crops primarily belong to the tribe Phaseoleae (Canavalia, Lablab, Macrotyloma, Phaseolus, Psophocarpus, Vigna). Exceptions include Cyamopsis which is placed in the Indigofereae, Lathyrus is a member of the Viceae and Lupinus which is placed in the Genisteae. Cyamopsis, Lathyrus and all of the species in the genera assigned to the Phaseoleae, except for Phaseolus coccineus, are distributed in the tropics and subtropics. Phaseolus coccineus comes from the Central American highlands. At present, the Lupinus species utilized in the temperate zones originate from the Andean highlands or temperate Europe.
• Guar and the lupins are unusual members of the secondary grain legumes. Several million acres of guar are cultivated in India, Pakistan, the U.S. and elsewhere for use as a source of gum. The gum is extracted from the seed for utilization in the paper, oil well drilling, explosive, and mining industries as well as in many processed food and beverage products. The use of guar as a grain legume is limited to certain regions of India and Pakistan.

  The lupins, Lupinus albus, L. angustifolia, L. cosentinii, L. luteus and L. mutabilis primarily are used as green manures, fodder crops or forage for livestock. In the past, the lupins rarely were used as grain legumes because the seeds were rich in alkaloids. Only in the past 20-30 years, have cultivars been developed with reduced alkaloid content in seed and non-dehiscent fruits. These are the so-called "sweet" lupins. Lupins and winged beans are next to soybeans among the grain legumes with regard to protein content in seed (28-48% depending on species). The use of sweet lupins as a food for humans remains to be fully explored.

• As grain legumes, the crops are not grown over wide geographical areas and overall are not important in world trade.
• National germplasm collections of each crop are available, however, the curators usually have several crops to maintain or curate a specific collection on a part-time basis. Funds to maintain the collections usually come from short term grants.
• Except for guar and lupins, simple selection is the main plant breeding technique used for cultivar development. Thus, the distinction between cultigens and wild forms are often blurred. A high proportion of the total production is derived from land races.
• Instead of centers of research excellence, the research conducted on these crops is usually based upon the interests of one or two vigorous scientists employed at a specific location. These scientists usually enlist the assistance of a professional colleague working in another discipline on another crop to help them solve a particular problem.

TERTIARY GRAIN LEGUMES

REMARKS

An ubiquitous complaint about grain legumes concerns their low yield level when compared to the cereals. Unfortunately, the internal plumbing of grain legume plants is not well understood. In all grain legumes three processes operate against high grain yields, that is, photorespiration, nitrogen fixation and photosynthetic energy relationships. Photorespiration occurs in the light and consumes about 30% of the products of photosynthesis in all grain legumes. At present, there is no known benefit of photorespiration. For example, maize functions without photorespiration. Secondly, the symbiotic relationship between the legume plant and the Rhizobium bacterial organism does not come free of charge. The diversion of carbohydrates by the plant for use by the bacterial organism to fix nitrogen reduces potential grain production by about 10%. Lastly, it takes more energy by a plant to produce a given amount of oil and protein than starch. This energy intensive process ultimately manifests itself in lower yields in grain legumes.

Another complaint about grain legumes is that they are susceptible to many pests and pathogens. Unfortunately, the relationship between the impact of germplasm collections and the funding of scientific personnel is not well understood. The germplasm collections of the major cereals are from 2 to 8 times larger than the primary legume collections. Secondly, in the U.S. there are at a minimum 10 times as many maize breeders than soybean breeders. Most probably these comparisons are quite similar for plant pathologists and entomologists. Thus, it is not that cereals are less susceptible to pests and pathogens than grain legumes but rather, that the genetic resources (i.e. germplasm collections) are available to an enormous pool of scientific personnel to solve specific problems.

In summation. the grain legumes exhibit an enormous amount of variation and this variation is silently awaiting commercial exploitation. Funds lacking in the past for extensive and intensive research on grain legumes are becoming increasingly available. Lastly, recent advances in yield increase of wheat, rice, and maize has raised hopes that similar results may be possible with the grain legumes by classical plant breeding techniques or by newer genetic engineering approaches.

REFERENCES

• Isely, D. 1982. Leguminosae and Homo sapiens. Econ. Bot. 36:46-70.

• National Academy of Sciences. 1979. Tropical legumes: Resources for the Future. Natl. Acad. Sci. Washington, DC.

• Plunknett, D.L., N.J.H. Smith, J.T. Williams, and N. Murthi Amishetty. 1987. Gene banks and the world's food, Princeton Univ. Press, Princeton, NJ.

• Polhill, R.M. and P.H. Raven (eds.). 1981. Advances in legume systemics. 2 vols. Royal Botanic Gardens, Kew, England.

• Rachie, K.O. and L.M. Roberts. 1974. Grain legumes of the lowland tropics. Adv. Agron. 26:1-132.

• Singh, R.J. and T. Hymowitz. 1985a. The genomic relationships among six wild perennial species of the genus Glycine subgenus Glycine Willd. Theor. Appl. Genet. 71:221-230.

• Singh, R.J. and T. Hymowitz. 1985b. Intra- and interspecific hybridization in the genus Glycine, subgenus Glycine Willd.: Chromosome pairing and genome relationships. Z. Pflanzenzucht. 95:289-310.

• Singh, R.J., K.P. Kollipara, and T. Hymowitz. 1988. Further data on the genomic relationships among wild perennial species (2n = 40) of the genus Glycine Willd. Genome 30:166-176.

• Smartt, J. 1978. The evolution of pulse crops. Econ. Bot. 32:185-198.

• Smartt, J. 1980. Evolution and evolutionary problems in food legumes. Econ. Bot 34:219-235.

• Smartt, J. 1983. Gene pools in grain legumes. Econ. Bot 38:24-35.

• Summerfield, R.J. and A.H. Bunting (eds.). 1980. Advances in Legume Science. Royal Botanic Gardens, Kew, England.

• Summerfield, R.J. and E.H. Roberts (Eds.). 1985. Grain Legume Crops. Collins Professional and Technical Books, London, Great Britain.

• Witcombe, J. R. and W. Erskine. 1984. Genetic Resources and Their Exploitation-Chickpeas, Faba beans and Lentils. Martinus Nijhoff/Dr. W. Junk Publishers, The Hague, Netherlands.

Fig. 1. Proposed idiogram of the pachytene chromosomes of the soybean. Arrow indicates centromere location (2400x).