Listing description
Peanut, also known
as groundnut (Arachis hypogaea) is a crop of
global importance. It is widely grown in the tropics and subtropics, being
important to both smallholder and large commercial producers. It is classified
as both a grain legume,[2] and, because of its high
oil content, an oil crop.[3] World annual production is
about 46 million tonnes per
year. Very unusually among crop plants, peanut pods develop under the ground.
Detailed description
As a legume, peanut belongs to
the botanical family Fabaceae (also known as Leguminosae, and
commonly known as the bean or pea family).[1] Like most other legumes, peanuts harbor symbiotic nitrogen-fixing bacteria in root
nodules.[4] This capacity to fix nitrogen means peanuts
require less nitrogen-containing fertilizer and improve soil
fertility, making them valuable in crop
rotations.
Peanuts are similar in taste
and nutritional profile to tree nuts such as walnuts and almonds, and are often
served in similar ways in Western cuisines. The botanical definition of a
"nut" is a fruit whose ovary wall becomes very hard at
maturity. Using this criterion, the peanut is not a nut,[5] but rather a legume.
However, for culinary purposes and in common English language usage, peanuts
are usually referred to as nuts.
History
Cultivated peanut (A.
hypogaea) has two sets of chromosomes from two different species, thought
to be A.
duranensis and A.
ipaensis.[6][7][8] The two species' chromosomes combined by hybridization
and doubling, to form what is termed an amphidiploid orallotetraploid.
Genetic analysis suggests this hybridization event probably occurred only once
and gave rise to A.
monticola, a wild form of peanut that occurs in a few restricted
locations in northwestern Argentina, and,
by artificial selection to A.
hypogaea.[6][7][9][10] The process of domestication through
artificial selection made A.
hypogaea dramatically
different from its wild relatives. The domesticated plants are more bushy and
compact, and have a different pod structure and larger seeds. The initial
domestication may have taken place in northwestern Argentina, or in
southeasternBolivia where
the peanut landraces with the most wild-like features are grown
today.[11][12] From this primary center of
origin cultivation
spread and formed secondary and tertiarycenters of diversity in Peru, Equador, Brazil, Paraguay and Uruguay. Over
time thousands of peanut landraces evolved, these are classified into six
botanical varieties and two subspecies (as listed in the Peanut Scientific
classification table). Subspecies fastigiata types are more upright in their growth
habit and have a shorter crop cycles. Subspecieshypogaea types spread more on the ground and
have longer crop cycles.[11][12]
It is difficult to estimate the
date of peanut's domestication. The oldest known archeological remains of pods
have been dated at about 7,600 years old. These may be pods from a wild species
that was in cultivation, or A.
hypogaea in the early phase
of domestication.[13] They were found in Peru, where
dry climatic conditions are favorable to the preservation of organic material.
It is almost certain that peanut cultivation predated this at the center of
origin where the climate is moister. Many pre-Columbian cultures, such as the Moche,
depicted peanuts in their art.[14] Cultivation was well established in
Mesoamerica before the Spanish arrived. There the conquistadors found the tlalcacahuatl (the plant's Nahuatlname,
whence Mexican
Spanish cacahuate,
Castillian Spanish cacahuete,
and French cacahuète)
being offered for sale in the marketplace of Tenochtitlan. The
peanut was later spread worldwide by European traders and cultivation is now
very widespread in tropical and subtropical regions. In West Africa, it
substantially replaced a crop plant from the same family, the Bambara
groundnut, whose seed pods also develop underground. In Asia it became an
agricultural mainstay and this region is now the largest producer in the world.[15]
In the English-speaking world peanut growing is most important in the U.S.A. Although it was mainly a garden crop for
much of the colonial period, it
was mostly used as animal
feed stock
until the 1930s.[16] The US Department of Agriculture initiated a program to encourage
agricultural production and human consumption of peanuts in the late 19th and
early 20th centuries. George Washington Carver developed hundreds of recipes for peanuts
during his tenure in the program.
Botany
Peanut is an annual herbaceous plant growing 30 to 50 cm (1.0 to
1.6 ft) tall.[17] As a legume, it belongs to the botanical family Fabaceae (also known as Leguminosae, and
commonly known as the bean or pea family).[1] Like most other legumes, peanuts harbor symbiotic nitrogen-fixing bacteria in root
nodules.[4] The leaves are opposite and pinnatewith
four leaflets (two opposite pairs; no terminal leaflet); each leaflet is 1 to
7 cm (⅜ to 2¾ in) long and 1 to 3 cm (⅜ to 1 inch) across. Like
many other legumes, the leaves arenyctinastic, that
is they have "sleep" movements, closing at night.
The specific name, hypogaea means "under the earth",
because peanut pods develop underground, a feature known as geocarpy. The
flowers are 1.0 to 1.5 cm (0.4 to 0.6 in) across, and yellowish orange
with reddish veining. They are borne in axillary clusters on the stems above
ground and last for just one day. In structure, they appear superficially
similar to the flowers of peas and beans. However, intriguing differences are
seen. The ovary is not positioned where expected, but is at the base of what
appears to be the flower stem (in fact, this "stem" is a highly
elongated floral
cup). After fertilization, a short stalk at the base of the ovary (termed
a pedicel) elongates to form a thread-like structure
known as a "peg". This pushes the ovary down into the soil where it
develops into a mature peanut pod.[18] Pods are 3 to 7 cm (1.2 to
2.8 in) long, normally containing 1 to 4 seeds.
Cultivation
Peanuts grow best in light,
sandy loam soil with a pH of 5.9–7. Their capacity to
fix nitrogen means that, providing they nodulate properly, peanuts benefit
little or not at all from nitrogen-containing fertilizer,[19] and they improve soil
fertility. Therefore, they are valuable in crop
rotations. Also, the yield of the peanut crop itself is increased in
rotations, through reduced diseases, pests and weeds. For instance, in Texas,
peanuts in a three-year rotation with corn yield 50% more than non-rotated
peanuts.[19] Adequate levels of phosphorus, potassium,
calcium, magnesium and micronutrients are also necessary for good yields.[19] To develop well, peanuts need warm weather
throughout the growing season. They can be grown with as little as 350mm of
water,[20] but for best yields need at least 500mm.[21] Depending on growing conditions and the
cultivar of peanut, harvest is usually 90 to 130 days after planting for
subspecies fastigiata types, and 120 to 150 days after
planting for subspecies hypogaea types.[20][22][23] Subspecies hypogaea types yield more, and are usually
preferred where the growing seasons are long enough.
Peanut plants continue to
produce flowers when pods are developing, therefore even when they are ready
for harvest, some pods are immature. The timing of harvest is an important
decision to maximize yield. If it is too early, too many pods will be unripe.
If too late, the pods will snap off at the stalk, and will remain in the soil.[24] For harvesting, the entire plant, including
most of the roots, is removed from the soil.[24] The fruits have wrinkled shells that are
constricted between pairs of the one to four (usually two) seeds per pod.
Harvesting occurs in two
stages:[citation needed] In mechanized systems, a machine is used to
cut off the main root of the peanut plant by cutting through the soil just
below the level of the peanut pods. The machine lifts the "bush" from
the ground and shakes it, then inverts the bush, leaving the plant upside down
on the ground to keep the peanuts out of the soil. This allows the peanuts to
dry slowly to a little less than a third of their original moisture level over
a period of three to four days. Traditionally, peanuts were pulled and inverted
by hand.
After the peanuts have dried
sufficiently, they are threshed,
removing the peanut pods from the rest of the bush.[24] It is particularly important that peanuts
are dried properly and stored in dry conditions. If they are too high moisture,
or if storage conditions are poor, they may become infected by
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