Legumes – Pods for Protein

The Long History of Legumes

Legumes have been important constituents of human diets for many thousands of /years. Neolithic man in the Near East was certainly eating pods of wild kinds such as vetchlings and shauk at least 10,000 years ago. The predecessors of the Incas of Peru were eating kidney beans some 7,000 years ago. Broad beans have been eaten in Europe since the Iron Age—the ancient Greeks and Romans, however, believed that eating them would dull their intelligence, and Pythagoras regarded the eating of beans as a sign of moral weakness! The inclusion of leguminous plants in crop rotation over the past few hundred years was one of the great advances in agriculture, and today they represent a vital source of protein for an undernourished world.

The Botanical Classification of Legumes

Botanically the legumes belong to the order Fabales, an enormous group of some 700 genera and about 14,000 species (almost 5 percent of all the known flowering plants).

Subfamilies of legumes

This family is divided into three subfamilies. The Papilion-aceae (or Leguminosae) have flowers which are mostly bilaterally symmetrical, the petals often being unequal in size with a hood and a keel. In the other subfamilies, Mimosaceae and Caesalpinaceae, the flowers are quite different; but almost all produce the characteristic pods enclosing the seeds, though these may range from soft to woody, and from straight to spiral. They may be fleshy, as in runner beans, or relatively thin and fast to dry out, as in the gorse or furze, Ulex.

Characteristics of leguminous plants

Most species have divided leaves, sometimes into three rounded leaflets, as in clover, more often pinnately with a number of leaflets on each side of a mid-rib, as in vetches and acacias. Some genera, like gorse and many acacias, have well-developed spines which originate as modified leaves. In temperate regions most legumes are herbaceous, but in the tropics the greater proportion are trees.

Old Foods and New Foods

Ancient and traditional legumes

The garden pea, Pisum sativum, is known from Stone Age caves in central Europe and from Ancient Troy. The various species of bean, Phaseolus, are eaten in many parts of the world, as is the lentil, Lens esculenta, which is thought to have a western Asian origin. There are many ‘pulses’, legumes grown for the mature seeds which are suited to drying and storage as a food source, for example butter and Lima beans, black and green grain or mung beans.

Modern legumes and their origins

The origin of the peanut, Aracliis liypogaea, is not clear but it may have originated in South America, where a number of related species occur naturally, or in south China or Africa. The soya bean, or soybean, Glycine max, originated in Northeast Asia but, like many major crops, is now mainly grown away from its area of origin, today very extensively in the USA.

The Nutritional Importance of Legumes

Legumes are important foods partly because their seeds often have a relatively high protein content compared with other crop plants.

Protein content and quality

It is, however, not only the quantity of protein in the diet that is important, but also its quality; that is to say the proportions of the different amino-acids that make up the protein. Man requires over twenty different amino-acids to build the proteins he needs, but eight of them— the essential amino-acids—he cannot make for himself and must obtain from the protein in his food. All the essential amino-acids are plentiful in most meats and meat products such as milk and eggs, but proteins from plant seeds usually have four of them present in small amounts only—threonine, lysine, tryptophan and methionine. A diet based on plant seeds would thus require a large amount of protein in order to obtain sufficient of these amino-acids. On the other hand as a major supplement to animal proteins, seed proteins are of great benefit.

Efficient use of plant proteins

When a cow feeds on plants some of the plant proteins are used to build more cow, ie meat, but much will be lost by excretion. If the cow is eventually eaten for meat only a relatively small proportion of the original plant protein will be available, perhaps about 20 percent. If, however, man was to eat the plant directly all the protein would be potentially available, although in practice not all would be in an assimilable form. Thus, where there is a shortage of protein for human consumption eating the plant material directly is a much more efficient use of a scarce resource.

The Soya Bean: A Modern Protein Source

One legume which is being increasingly used as a protein source is the soya bean.

Soya protein and meat substitutes

The protein is extracted from the beans, concentrated and processed, and is then used as a substitute for meat, which it may, upon suitable preparation and cooking, be made to resemble; steak which has never seen a cow! It should, of course, not be forgotten that soya beans may be cooked and eaten directly, eliminating the need for industrial processing, providing that one does not need the illusion that it is meat that is being eaten.

Other potential protein sources

It is not only seeds that may be treated in this way. An important potential protein source comes from the leaves and seed pods of many plants, including legumes, which are at present discarded when the seeds have been separated. Protein may be extracted from these just as from soya bean.

Legumes and Nitrogen Fixation

Legumes are also important for their ability to metabolize nitrogen. They have the property of producing combined nitrogen compounds from atmospheric nitrogen, N2—a process known as nitrogen fixation.

The role of nitrogen in living organisms

Nitrogen is an essential component of the amino-acids which make up proteins, the enzymes and structural elements essential for the living cell. With the exception of the nitrogen-fixers, organisms can only take up nitrogen in some combined form: plants mainly as nitrate, Nos, or sometimes as ammonium compounds, NH 4 , taken up by the roots; animals as ammonium compounds such as amino-acids and proteins from plants or from other animals.

Sources of combined nitrogen

Some combined nitrogen may enter the soil from the weathering of rocks containing nitrates and some may be formed in the soil by lightning, reaching the soil dissolved in rain, but these two sources are small compared to the amounts formed by biological nitrogen fixation.

The symbiosis between legumes and rhizobium

Nitrogen fixation in legumes is the product of a symbiotic relationship between the plant roots and the bacterium Rhizobium. The bacteria occur freely in the soil, and may be attracted to the roots by a growth substance produced by the plant. The bacteria enter the root by penetrating the root hairs, and migrate into the cortex of the root. A growth substance secreted by the Rhizobium causes the root cells to divide and grow to form a root nodule. Nodules are usually roundish structures, sometimes branched, normally situated on the fine lateral roots. Vascular strands from the plant root run into the nodule, in the centre of which are the large number of cells containing Rhizobium.

The nitrogen fixation process

The biochemical mechanism of nitrogen fixation is complex and as yet not fully understood, but its net result is the splitting of molecular nitrogen and its incorporation into amino-acids. The plant thus obtains a supply of amino-acids and the bacterium receives organic compounds from the plant as energy sources. If an active root nodule is cut it will appear to ‘bleed’—the cut interior of the nodule being in places clearly red coloured. The substance concerned is none other than a haemoglobin, the compound responsible for the transport of oxygen in our blood. What its function is in a plant root is not certain, but it may be involved in controlling the oxygen concentration within the root nodule.

The ecological and agricultural importance of nitrogen fixation

The nitrogen so ‘fixed’ will initially all be within the plant. It will only become available to the other parts of the eco-system either when the plant is eaten by an animal or when the plant dies and its breakdown releases nitrogen compounds into the soil. This input of combined nitrogen will be especially important where legumes are growing in nitrogen-deficient soils, such as those of recently formed sand-dunes. Legumes are also important where the harvest of agricultural crops results in the loss of combined nitrogen from the soil. This is the reason for including legumes in many crop rotation schemes. The amounts of nitrogen added to the soil may be considerable, often as much as ioo-20okg per hectare (88—1761b per acre). The legumes, such as alfalfa, Medicago sativa, and clover, Trifolium, are often grown as a winter crop and then ploughed into the soil, or are grown in a mixture with grasses so that some combined nitrogen is also passed to grazing animals.

Nitrogen fixation beyond legumes

It should be remembered that legumes are not the only organisms that can fix nitrogen; there are a few non-legume higher plants that can do so, as can a number of microorganisms, mainly bacteria and blue-green algae.

Future possibilities in nitrogen fixation

Among the dreams of plant scientists is the possibility of producing crop plants such as wheat which are receptive to nitrogen-fixing bacteria; something which might be done by fusing or otherwise combining the cells or nuclei of, say, a wheat plant and a bean. Such nitrogen-fixing cereals would not only vastly improve soils they were grown in but immensely reduce the need for fertilizer application, notably of nitrates which have created so many side problems by their excess amounts draining into watercourses and lakes. That would be a revolution indeed.