Arachis hypogaea (PROSEA)
- Protologue: Sp. Pl. 2: 741 (1753).
- Family: Leguminosae
- Chromosome number: 2n = 4x = 40
- Groundnut, peanut (En)
- Arachide (Fr)
- Indonesia: kacang tanah, kacang jawa, kacang manila
- Malaysia: kacang china, kacang jawa, kacang goreng
- Philippines: mani, batung china
- Thailand: thua lisong, thua din
- Papua New Guinea: galip bilong giraun, kasang
- Vietnam: dâu phong, lac
- Cambodia: sânndaèk dèi
- Laos: thwàx din, thwàx ho
Origin and geographic distribution
Groundnuts originated in the southern Bolivia - northwestern Argentina area of South America. It is believed the Portuguese took them from Brazil to west Africa and then to southwest India in the 16th century. Africa is now regarded as a secondary centre of diversity. At the same time also the Spaniards introduced groundnuts from Mexico to the western Pacific from where they spread to China, Indonesia and to Madagascar. The Dutch also probably took the groundnut from Brazil to Indonesia by the middle of the 17th century. Groundnuts are thought to have been introduced to the USA on slave ships from Africa, although they may have been introduced directly from the Caribbean Islands. Groundnut is now grown in most tropical, subtropical and temperate countries between 40°N and 40°S latitude. They are most important in Africa, Asia, North America and South America. In Asia the groundnut is a major crop in India, China, Indonesia, Burma, Thailand and Vietnam.
A majority of the world crop is crushed for oil, which is used mainly for cooking. The press cake after oil extraction is a high protein animal feed but is also used to produce groundnut flour which is used in many foods for human consumption. Most of the Burmese crop, about 20% of the Indonesian crop, and 30% of the Thai crop is crushed for oil. However, the majority of the crop in most countries of South East Asia is used for direct human consumption. The seeds or kernels are eaten raw, boiled or roasted, made into confectionery and snack foods, and are used in soups or made into sauces for use on meat and rice dishes. The vegetative crop residues are excellent forage.
Production and international trade
Groundnuts are a major world oilseed and food legume. The following data are based on FAO statistics for the period 1981-1985. In this period world area under cultivation averaged 20 million ha and in shell production averaged 19 million t. From trade figures over this period, world price was estimated to be US$ 860/t on a fresh, shelled basis.
By country, Indonesia produced 792 000 t from 500 000 ha, Burma produced 600 000 t from 585 000 ha, Thailand 149 000 t from 127 000 ha, Vietnam 92 000 t from 136 000 ha, the Philippines 48 000 t from 55 000 ha, Malaysia 21 000 t from 6 000 ha, Laos 10 000 t from 12 000 ha and Cambodia 9 000 t from 8 000 ha. Production in Papua New Guinea was less than 1 000 t. The Indonesian crop is produced entirely by small landholders.
Most of the production in each country is consumed locally. Only Vietnam (21 000 t) and Thailand (5 800 t) were significant exporters of groundnut while Indonesia (30 000 t) and Malaysia (9 400 t) imported groundnuts during the period 1981-1985.
Per 100 g edible portion, groundnuts contain approximately: water 5.4 g, protein 30.4 g, fat 47.7 g, carbohydrates 11.7 g, fibre 2.5 g, ash 2.3 g. The energetic value averages 2457 kJ/100 g. Groundnuts are good sources of vitamins B and E.
Seed weight varies between 25 and 115 g/100 seeds.
Groundnut kernels are noted for their high oil and protein contents. Crude protein content of whole dry kernels ranges between 22 and 30% and varies according to type, cultivar, location, year and physiological maturity of the seed. Aspartic acid, glutamic acid and arginine account for about 45% of total amino acids present. Deficient amino acids tend to be lysine, methionine and threonine.
Oil content of whole dry kernels ranges between 44 and 56% with an average of 50%. Virginia type cultivars tend to have a lower oil content than Spanish types. Groundnut oil is an unsaturated liquid susceptible to oxidation and hence rancidity. Oleic and linoleic acids are unsaturated acids and together account for about 80% of the fatty acid composition. Palmitic acid accounts for a further 10% and the remaining fatty acids range in concentration from 0.01 to 4%. The higher the ratio of oleic to linoleic acids (O/L) in groundnut oil, the more stable is the oil; that is, it is less prone to oxidative rancidity. The O/L ratio in mature kernels can range from less than 1.0 to greater than 3.0, with values in excess of 1.3 generally being considered satisfactory by processors. Spanish type cultivars tend to have lower O/L ratios than Virginia type cultivars.
- A monoecious, prostrate to erect annual herb, usually 15-70 cm high. Root system consisting of a well developed taproot with many lateral roots, able to penetrate to depths in excess of 2 m; root hairs absent, nitrogen fixing nodules present. The main stem or central axis develops from the epicotyl and bears a cotyledon at each of the first two nodes.
- Branching dimorphic, with vegetative branches and contracted reproductive branches. All vegetative branches characterized by the presence of scale leaves, called cataphylls, subtending the first two nodes of the branch. Secondary and tertiary vegetative branches can develop from the primary vegetative branches.
- Leaves on the central stem spirally arranged in a 2/5 phyllotaxy, on the primary vegetative branches arrangement is distichous; leaves are 4 foliolate with two opposite pairs of obovate leaflets of 3-7 cm x 2-3 cm; petiole 3-7 cm long; pulvini at the base of the petiole and at the base of each leaflet cause characteristic night movements in which the petiole folds downwards and the leaflets fold upwards until they touch.
- The contracted reproductive branches or inflorescences are formed singly at both cataphyllar and ordinary leaf axils on vegetative branches and, in some forms, at higher nodes on the central stem; each inflorescence bears 2-5 flowers; short (axillary) branches develop in the axils of simple bracts which occur along the central axis of the inflorescence; these axillary branches of the inflorescence terminate after the production of one leaf, the bifid bract, in the axil of which the flower is borne; an inflorescence never occurs in the same leaf axil as a vegetative branch.
- Flowers sessile, consisting of a 4-6 cm long tubular hypanthium (fused lower parts of calyx, corolla and staminal tube), at the top of which are borne expanded lobes of the 5 sepals and petals (pale yellow through to orange red), and 10 short filaments with anthers; superior ovary of single sessile carpel with 2-6 ovules, situated at the base of the hypanthium tube; style free within the tube, ending in a club shaped stigma among the anthers.
- After fertilization a stalk like structure called 'peg' or gynophore elongates by means of an intercalary meristem at the base of the ovary. The peg curves to grow down towards the soil, carrying the ovary at its tip which becomes hardened into a protective cap as the peg enters the soil. The length of the peg depends on the initial distance of the flower from the soil, but if this is more than 15 cm it usually fails to reach the ground and the tip dies. When the peg has penetrated the soil to a depth of 2-7 cm it turns horizontal and fruit development begins as the tip swells rapidly.
- Mature fruits (pods) cylindrical, 1-8 cm x 0.5-2 cm, containing 1-6 seeds. The pod surface, or pericarp, may be constricted to varying degrees between the seeds and is reticulated because of the presence of veins in the hardened mesocarp.
- The mature seeds, or kernels, are cylindrical to ovoid, 1-2 cm x 0.5-1 cm. Each seed is enclosed in a thin papery testa with colours ranging from white to pink, red, purple, tan and shades of brown. Each seed has two large cotyledons, an epicotyl with leaf and bud primordia, a hypocotyl and the primary root.
- Upon epigeal germination the primary root elongates rapidly, reaching 10-12 cm before lateral roots appear. As growth proceeds, the outer layer of the primary root of a seedling is sloughed off so that root hairs do not form.
Growth and development
Following harvest, seed of Virginia type cultivars have a dormant period of 1 6 months depending on temperature and storage conditions. Heat treatment or ethylene can break the dormancy. Spanish and Valencia type cultivars do not possess seed dormancy. After sowing, seedling emergence occurs in 5-10 days depending on sowing depth and soil temperature. A period of vegetative development follows for 20-35 days after which the first flowers appear. Groundnuts are capable of flowering over an extended period (20-60 days) depending on plant water status, irradiance, temperature and photoperiod. This flowering period is considerably shorter in Spanish and Valencia cultivars than in Virginia cultivars. During this period, daily flower production typically increases to a peak and then declines. On each inflorescence only one flower opens on one day and a period of one to several days elapses before another flower opens on that inflorescence. The flower bud elongates rapidly during the night prior to anthesis. Anthesis and self pollination within the closed flower take place around sunrise the following day. Normally 60-80 days are required for individual pod development from flowering to maturation in Virginia types and slightly less than this (50-60 days) in Spanish types. As groundnuts are botanically indeterminate, vegetative development can continue during the flowering and pod filling periods, although typically vegetative development declines or ceases during pod filling. In warm tropical environments, short season cultivars of the Spanish Valencia group are maturing in 85 100 days whereas longer season cultivars of the Virginia group require 110-130 days. In cooler subtropical environments or at elevation, corresponding periods are 115-130 days for Spanish types and 140-160 days for Virginia types.
Other botanical information
There is considerable variation in Arachis hypogaea and two subspecies have been proposed: subsp. hypogaea and subsp. fastigiata Waldron. Breeding programs have crossed cultivars from the two subspecies and in cultivars developed from such crosses, the subspecific distinctions for some traits such as branching pattern are no longer distinct.
- Subsp. hypogaea, known as the Virginia group, is characterized by plants having a central axis which never bears inflorescences and has lateral branches in which two vegetative branches alternate regularly with two inflorescences or reproductive branches; cultivars are late maturing; plants have a dark green colour and prostrate to spreading bunch growth habit; pods are typically two seeded and seeds show marked dormancy.
- Subsp. fastigiata, known as the Spanish Valencia group, is characterized by plants having inflorescences on the central axis and without a regular pattern in the sequence of reproductive and vegetative branches; cultivars are earlier maturing; plants have a lighter green colour (Spanish) and erect growth habit; pods are concentrated around the central axis, contain 2-6 seeds which have no dormancy. Spanish types have mainly two seeded pods; Valencia types typically have 3-6 seeded pods, thicker stems and considerably fewer secondary and tertiary branches than Spanish types.
Groundnuts are grown between 40°N and 40°S latitudes in warm tropical and subtropical environments and in temperate humid climates with sufficiently long warm summers. Optimum mean daily temperature for growth is around 30°C, while growth ceases at about 15°C. The phenology of groundnuts is determined primarily by temperature, with cool temperatures delaying flowering. In controlled environments, photoperiod has been shown to influence the proportion of flowers producing pods and assimilate distribution between vegetative and reproductive structures (harvest index) in some cultivars. Generally long photoperiods (greater than 14 hrs) increase vegetative growth and short photoperiods (less than 10 hrs) increase reproduction growth. However, the significance of these effects in crop communities in the tropics and subtropics has yet to be established.
Between 500 and 600 mm of water reasonably well distributed through the growing season will enable satisfactory groundnut production. Nevertheless, groundnuts are a drought tolerant species and are able to withstand high internal water deficits, although yield reductions will generally result.
Because pods are developed underground and must be recovered at harvest, friable well drained soils are preferred, although adequate plant growth and development will occur on heavier clay soils. For optimum growth, soil pH should be in the range of 5.5-6.5, although Spanish types will tolerate more acid conditions (to pH 4.5) and some cultivars will grow well in alkaline soils to pH 8.5.
Commercial crops are grown from seed. Ideally the seed bed should be deep and friable with an even particle size and be weed free. In some countries, seed is routinely dressed with a protective fungicide and sown with mechanical planters. Cloddy and uneven seed beds, combined with hand planting can result in uneven emergence and heavy seed bed losses of plants. In South East Asia, groundnuts are grown mainly by smallholders either as a rainfed crop during the wet season or in lowland areas after rice as a second or third crop with either supplementary irrigation or on residual moisture. They are grown as a sole crop and also intercropped with maize, soyabeans and cassava. In some areas, groundnuts are grown under perennial tree crops such as coconut, oil palm or rubber.
Recommended plant populations are in the vicinity of 200 000 250 000 plants per ha for the typically short season Spanish cultivars. This contrasts with tropical areas of Australia where populations of 100 000 - 125 000 plants per ha are recommended. In most countries cultivation is in rows with plant spacings ranging from 40 x 20 cm to 30 x 20 cm.
To achieve maximum economic yields, the competitive effects of weeds must be largely eliminated. In South East Asia this is often attempted by hand weeding. However, a range of pre emergence and post emergence herbicides are available to control grasses and many broad leaf weeds.
The only peculiar nutrient requirement is for Ca in the podding zone. Ca is absorbed directly by the pods if soil moisture is adequate. A shortage of Ca in this zone will result in empty pods, particularly in Virginia type cultivars. The nitrogen needs of the crop should be satisfied through symbiotic fixation by strains of Rhizobium of the cowpea group. Hence nitrogen fertilizers generally should not be required. In some acid soil areas, lime is applied to raise the pH to more suitable levels.
Moisture stress during flowering or pod filling causes yield reductions so that irrigation during these periods to minimize or eliminate the stress will increase production and seed quality.
Diseases and pests
The most serious foliage fungal diseases are the leafspots and rust. Early leafspot (Cercospora arachidicola, Mycosphaerella arachidis), late leafspot (Cercosporidium personatum, M. berkeleyii) and rust (Puccinia arachidis) can cause significant yield losses, particularly during the wet season. The diseases can be controlled by appropriate fungicides. Resistant cultivars are generally not available at this stage. The soil borne fungus Aspergillus flavus (and related species) is widespread in the region and infected groundnuts can be contaminated with the carcinogen aflatoxin. Data from Indonesia and Thailand suggest that A. flavus as a mould contaminant and toxin producer is much less serious during crop growth than during subsequent storage of kernels. Minimizing moisture stress during crop growth can reduce A. flavus invasion and toxin production. Bacterial wilt (Pseudomonas solanacearum) is serious in Indonesia, Malaysia and China. Control is available by use of resistant cultivars.
Peanut stripe virus (PStV) is a serious disease in Indonesia, Thailand and the Philippines. It can cause serious yield losses if a crop is infected early. The virus is transmitted by aphids (Aphis craccivora) and seed (up to 30% seed transmission). No control measures are currently available. Peanut mottle virus (PMV) is widespread but is considered less serious because of the lower incidence of seed transmission and less severe effects on yield. Peanut yellow spot virus (PYSV) is widespread in Thailand.
Nematodes, particularly species in the genera Meloidogyne and Pratylenchus, are widespread but the seriousness of their effect on yield are unknown. Control measures include nematicides and crop rotations.
There is a wide range of insects which can attack groundnuts and in some cases yield losses from insect damage can be severe. Leaf miners (Aproaerema modicella) have caused substantial foliage damage in Indonesia, Thailand and the Philippines. Other insects often present in high numbers include leafhoppers (Empoasca spp.), leaf eating caterpillars (Heliothis spp.) and thrips (Frankliniella schultzei, Scirtothrips dorsalis). Control by insecticides is possible. Aphids (Aphis craccivora) as virus vectors, particularly in the dry season, are a serious insect pest.
Harvesting occurs 85 100 days after sowing for Spanish cultivars and 110 130 days after sowing for Virginia cultivars, in the warm tropics. Heavy foliage disease pressure sometimes results in harvesting before seeds are fully mature. Much of the harvesting is by hand in South East Asia. Plants are pulled from the ground and pods removed from the bushes. Pods are then sun dried to about 10% moisture. Where mechanization is available, the tap roots are cut and plants lifted mechanically from the soil. After 2-10 days of sun drying, pods are mechanically threshed from the bushes.
Average yield of pods in South East Asia is around 1 t/ha, although there is considerable variation in reported yields among countries: 0.7-0.9 t/ha for Vietnam, Laos, the Philippines and Papua New Guinea; 1.0-1.2 t/ha for Thailand, Burma and Cambodia; 1.6 t/ha for Indonesia; 3.6 t/ha for Malaysia. World average yield is 0.95 t/ha, although in the USA average yield is around 2.9 t/ha.
Handling after harvest
To minimize the development of A. flavus and subsequent toxin production, groundnuts should be dried to less than 14% moisture. Interruption and retardation of sun drying by showers or overcast humid weather, or moisture uptake during storage, can result in A. flavus growth and aflatoxin contamination. Seeds can be protected from mechanical damage by storage and transport in the shell. In many areas pods are sold directly to consumers at a local level to provide cash flow for farmers. Hand shelling at a local level is also common. In Thailand, mechanized systems are used for shelling, oil extraction, and grinding. Processing factories produce dried or boiled groundnuts or groundnut products.
The International Crops Research Institute for the Semi Arid Tropics (ICRISAT) at Hyderabad in India maintains the largest world germplasm collection of over 12 000 accessions. ICRISAT has been designated as a principal repository for Arachis germplasm by the International Board for Plant Genetic Resources. The United States Department of Agriculture also maintains an extensive germplasm collection. The Bogor Research Institute for Food Crops (BORIF) in Indonesia maintains a collection of local Indonesian germplasm as well as some introductions.
Development of higher yielding cultivars adapted to environments and production systems in the region is the major objective of national and international (ICRISAT) breeding programs. Earliness (less than 80 days) and drought tolerance are objectives in rice based farming systems in Indonesia and Thailand. Other objectives include resistance to rust, leafspots, bacterial wilt (Indonesia) and A. flavus (Thailand); seed dormancy (Indonesia and Thailand) and acid soil tolerance (Philippines and Indonesia). Currently the world germplasm collection is being screened in Indonesia to find sources of resistance to peanut stripe virus.
A. hypogaea is an allotetraploid. Several diploid wild species, including A. cardenasii and A. chacoense, are being used as sources of disease resistance at ICRISAT and in the USA.
The prospects for groundnut appear bright. As a short season annual tropical legume, it is capable of contributing to the nitrogen economy of the associated farming systems as well as providing a valuable protein source in human diets. Because groundnut is a relatively valuable commodity, it can also contribute to the cashflow of small landholders.
Despite groundnuts, as well as other food legumes, generally being perceived by farmers as a low yielding, low return proposition and hence normally given relatively low inputs and marginal cultural conditions compared to the cereals, the importance of the crop in farming systems and as a source of additional income is well recognized. The crop is well established in ecological niches of various cropping systems, and there are good prospects of expanding the role of groundnuts in these systems as well as inclusion of groundnuts in new cropping systems. Considerable efforts are being made to increase groundnut production in most South East Asian countries.
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- ICRISAT, 1980. Proceedings of the International Workshop on Groundnuts, 13 17 October 1980. Patancheru, A.P., India. 325 pp.
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R. Shorter & A. Patanothai