|ORIGINS, DOMESTICATION, DISPERSAL AND CLASSIFICATION OF CULTIVATED CROPS
- Alphonse de Candole (1882) was the first scientist to trace the origins of cultivated plants
He concluded that the region where a species is abundant is not necessarily its centre of origin.
Indicated areas of earliest domestication – Agriculture originated from three regions.
ii) S.W Asia and Egypt
iii) Tropical America ( Mexico and Peru)
Nicolai Vavilov ( 1926)
A Russian botanist – developed the idea of centres of genetic diversity
Found great diversity in some areas but not in other areas, for certain crops.
- Diversity – many different forms of a spp. And in addition, wild relatives of the crop are available.
Vavilov suggested that the centre of diversity for a crop is also its centre of
origin. This was however disputed, since some crops had more than one centre of diversity. More recent work e.g. by Harlan (1992), suggests that a centre of origin for a given spp is not necessarily the centre of diversity
Vavilov concluded that a centre of origin was characterized by dominant alleles while towards the periphery, the frequency of recessive alleles increased and the genetic diversity decreased
- Vavilov's original concept was modified by Harlan J.R (1971) who proposed that crops may have originated from centres and non-centres
Centre: A delimited geographical area where a crop was domesticated and from which it was distributed to other areas
Non-centre: A broad geographical area where a crop was domesticated and from which it was distributed to other areas
Harlan recognized three centres and three non-centres (Which are they?)
Most centres of origin are in temperate climates at latitudes between 200 –450 Primary versus secondary centres of origin (what are these?)
Vavilov’s 8 centres of Origin
Centre of Origin Example of spp of the centre
1 China Soyabean
2. India Rice
3. Central Asia Wheat
4. Middle East Alfalfa
5. Mediterranean region White clover
6. Abyssinia (Ethiopia) Sorghum
7. Central America Corn
8. South America Potato
Vavilov 1926 and Zhukovsky (1970) reported the following centres of Origin
China Brassica campestris and related spp,
Camellia sinensis (tea); Glycine max (soya); Panicum miliaceum (); Raphanus sativus () and Setaria italica();
Also a secondary centre for Oryza sativa spp and Zea mays and other crops.
Indo-Malaya Wild oryza sativa, cocos nucifera(); colacasia esculenta(); Dioscorea spp(yams); Saccharum officinalis (sugar cane); .
India Oryza sativa; Phaseolus mungo(); Saccharum sinensis(); Vigna sinensis and Cucurbita sativa().
Central Asia Allium cepa(onion); Allium sativum(); Daucus carota(carrot); Spinacea oleracea(); Vicia faba
Near East Brassica oleracea; Hordeum vulgare (barley); Medicago spp; Secale spp; Triticum spp; Vicia sativa(); Vitis vinifera ()
Mediterranean Centre Avena spp, Beta vulgaris (sugar bean); Brassica napus; Brassica. Oleracea; Lupinus spp; Rhaphanus sativum;
Ethiopia/Africa Coffea spp; Cucumis spp; Gossypium spp; Hibiscus spp; Lablab purpureaus; oryza spp; Pennisetum spp (Pearl millet); Ricinus cucumis (castor bean); Sesamum indicum (sesame); setaria spp; Sorghum bicolour; Vignia unguiculata (cowpea).
Central America and Mexico Agave spp(sisal); Cucurbita spp; Gossypium spp; Ipomoea batatas (sweet potato); Phaseolus spp; Zea mays.
South America Centre for most tuberous crops e.g oxalis tuberosa and solanamum spp.
Amaranthus spp; Arachis hypogea (groundnuts) Capsicum spp(pepper); cucurbita maxima; Gossypium spp; Lupinus spp; Lycopersicum spp (tomato); Manihot esculenta; Nicotiana spp (tobacco); Phaseolus spp; solanum spp.
Also secondary centre for diversity for zea mays.
Some important tropical crop plants and their places of origin.
Place of origin
Cassava ( Manioc, Tapioca )
Tropical America, Africa and Asia
Why is it important to know the centres of origin and centres of diversity and wild and weed relatives of cultivated plants crops? Important in agricultural research, breeding in particular.
Diverse germplasm is found in centres of diversity
Plant breeders know where to search for variation in a given plant spp.
The wild ancestors are often the only sources of germplasm for continued development of new cultivars and therefore Collection of seed and protection of wild populations is now an international goal in maintaining genetic diversity of crop plants
Maintenance and use of world collections: International Plant Genetic Resources Institute (IPGRI), under the auspices of FAO, co-ordinates the international plant germplasm network. There is also a network of germplasm centres under the Consultative Group for International Agricultural Research (CGIAR). These are mandated to collect , preserve and distribute specific plant germplasm/resources. E.g
International Crop Research Institute for the Semi-Arid Tropics (ICRISAT) for sorghum, pearl millet, groundnut, chickpeas and pigeon peas.
International Centre for Maize and Wheat Improvement (CIMMYT)
DOMESTICATION OF CULTIVATED PLANTS
Domestication of plants arose originally from wild progenitors
Man selects for characteristics useful to him. (The process is driven by man’s desire for certain products e.g. domestication of strains of penicillium for production of antibiotics)
Through artificial selection, domesticated spp were selected for characterises entirely different from those for which the wild spp were selected in nature.
Through domestication, genetic variability within a spp is considerably
It brings about genetic, physiological and structural changes, which often render the plant less likely to survive in a wild state. These changes are brought about by both natural and artificial selection and the resulting crops are classified as distinct species
Examples of changes include:
i ) Crop may come to have a different ecological preference (a spp is spread
to diverse environments on domestication).
ii) Simultaneous seed bearing
iii) Absence of seed shattering mechanism or even complete loss of
dispersal mechanism e.g seeds dispersal of corn does not happen because of the way modern husked ears have been selected
iv) Increase in fruit size and seed size. This often reduces dispersal efficiency.
v) Conversion of perennials into annuals
vi) Loss of seed dormancy
vii) Loss of photoperiodic control
viii) absence of normal pollinating organisms
Changes in breeding systems: cross to self-fertilization
loss of defensive adaptation e.g. hairs, spines or thorns
Improvement in palatability and chemical composition – crops are rendered more prone to be eaten by animals.
Increased susceptibility to pests and diseases
Development of seedless parthenocarpic fruits.
Note: - above points should be considered in relation to individual crops.
- increased economic value was the most noticeable and most desirable change under domestication.
-An area in which a crop plant evolved is not necessarily the most ideal area for the crop. Optimal conditions may very well occur in a different hemisphere (Purseglove)
- Genotypes which could have failed in a natural environment
may survive because of man
- Recessive genes masked or having a low frequency in the centre of origin may attain a high frequency in the new areas. They may manifest themselves more frequently in the homozygous state and this is likely to be accelerated in self- pollinating crops
- under cultivation build-up of big populations growing in proximity and when grown in pure stand, chances of cross-pollination in outbreeding crops is increased.
- In addition crop is removed from competition with other species and more favourable habitat is provided through practices such as weeding, irrigation etc.
Domestication may be considered a form of plant breeding in the sense that if successfully done, it may provide domestic types that could be superior to those previously available.
Temporal sequence in crop domestication
From 10,000 to 9,000 BC on:
From 2,000 BC on:
Last 200 years:
blueberries, macadamia nuts
Note: A number of crops were initially taken into cultivation and some were
CENTRES OF PRODUCTION (TROPICAL CROPS) – (Onwueme)
The main areas of production of major economic crops are usually far removed from the regions in which they originated.
Crop Country of origin Country of highest production
Citrus S.E Asia United States
Coffee Ethiopia Central & South America
Groundnuts S. America India, China
Soyabean N.E Asia United States
Most important reason for above scenario: Introduction of crops into new areas without the major diseases and pests which attack them at their centre of origin
For an introduction to be successful, there must be a place for it in the agricultural system be it an economic or food plant.
Other factors that have influenced the present distribution of crops include:
Selection and breeding of new cultivars suited to the new environments and utilizations
Area of origin may not necessarily provide the optimum environmental conditions
Competition from other spp and weeds is removed when crops are grown in pure stands
Improved husbandry methods
Availability of capital
Existence of markets
Nearness of markets particularly with perishables
Advisory services to the farmers etc
Easy of mechanization
DISPERSAL OF DOMESTICATED CROPS
Was by both natural and man-made means
Example of natural means – Floating of coconuts across the pacicific ocean from Asia to the western coast of Central America
Man-made through the following:
With the expansion and migration of primitive agricultural communities (early stages)
Early trade and migrations between different parts of the world
Attempts by the Europeans to find a sea route to India. In the 15th century, they explored the routes to India, China, and other Asian countries and also discovered the Americas. Following these explorations and discoveries, was expanded world trade – (Early explorers were mostly responsible for introducing plants from around the world into western European culture)
16th and 17th centuries –Dispersal of most tropical crops to various parts of the world while,
Crops indigenous to the Americas (maize, G/nuts, potatoes, tomatoes & cassava) were spread to other parts of the tropics during the sixteenth century.
18th – 19th century : The demand from Europe, for agricultural raw materials for use in industry, stimulated the development of agricultural enterprises in the tropics
Recent developments in the dispersal of crops have been associated with the expansion of agricultural research with International Corporation. This work has and continues to involve exchange of seeds and at times large collections of germplasm among researchers,
Plants can be classified in several ways. Methods of classification important or related to Agriculture are given below:
BOTANICAL /SCIENTIFIC SSYSTEM OF CLASSIFICATION
System of classification was started by Carl Linnaeus (a Swedish botanist)
System adopted in the 18th century. and since at that time Latin was the written language of science, the classification is in Latin.
The Botanical system of classification
KINGDOM = planta
DIVISION = tracheophyta
CLASS = angiospermasida
Subclass: Monocotyledonae Subclass: Dicotyledonae
(One seed leaf) (two seed leaf)
Cyperales - ORDER - Rosales
Graminaeae - FAMILY - Leguminoseae
Zea - GENUS - Glycine
Mays - SPECIES - max
SC 513 - Variety - Soma
The Gramineae and Leguminosae families are the most important in food production. There are approximately 15 000 spp in the legume family and
approximately 7 500 in the grass family.
Major botanical families containing many important cultivated crop plants.
Family Important Tropical Crop Plants
Leguminosae Cowpea, groundnut, soya, pigeon pea
Gramineae Maize, rice, millet, sugar cane
Solanaceae Tomato, tobacco, peppers, Irish Potato
Malvaceae Cotton, olera, kenaf (Hibiscus cannabinus)
Euphorbiaceae Rubber, cassava, castor
Cucurbitaceae Pumpkin, melon, gourd
Palmae Oil palm, coconut palm
Cruciferae - cabagge ,mustad ,rape ,kale
Chenopodiaceae - sugarbeet
Linaceae - flax
Polygonaceae - buckwheat
Compostee - sunflower,Jerusalem artichoko
Moraceae - hemp
Tiliceae - jutae
Lillaceae - onion
The Binomial System of Naming plants
Linnaeus also developed a system of binomial nomenclature for plants
In this system the genus and species for the name for any plant. The generic name always starts with a capital letter while the spp is in lower case. The two names are always italicised or underlined. The initial of the classifier follows the species name.
Some important crops and their scientific names
Common name genus spcies classifier
Alfalfa medicago sativa L.
Bambara nuts Voandzeia subterranea -
Barley Horeum vulgare L.
Broadbean/Faba bean Vicia faba -
Cassava manihot esculenta Crantz
Castor bean Ricinus communis L.
Cotton Gossypium hirsutum L.
Cowpua Vigna ungucuilata or sinensis L.
Dry beans Phaseolus vulgaris L.
Field peas Pisum arvense L.
Finger millet Eleusine coracana (L.) Gaertn
Groundnuts(peanuts) Arachis hypogeae L.
Guar Gyamopsis tetragondoba L.
Jute carchorus capsularis & clitorius L.
Maize Zea mays L.
Oats Avena sativa L
Pape(oilseed) Brassica napus Koch
Pearl millet pennisetum typhoides (Burm f.) Stapf & Hubbard
Pigeon pea cajanus cajan L
Potato Solanum tuberosum L
Rice Oryza sativa L.
Rye Socale cereale L.
Safflower Corthamus tinctarius L.
Sisal Agave sisalana (Perrine ex Engelm).
Sorghum Sorghum bicolar L.
Soyabean Glycine max (L.) Merr
Sugar cane Saccharum officinarum L.
Sugarbeet Beta vulgaria L
Sunflower Helianthus annus L.
Sweet potato Ipomoea batatus (L.) Lam
Tobacco Nicotiana tabacum L.
Wheat Triticum aestivum L.
CLASSIFICATION ON THE BASIS OF PLACE OF ORIGIN
Two broad catergories:
Indigenous – plants that are native to the area in question
Exotic - plants that have been introduced into the area in question
( refer to centres of origin – Vavilov).
CLASSIFICATION BY LIFE CYLE
Annuals: complete their lentire life cycle in a single season and then die. Their growth cycle is always less than year and they always reproduce by cell.
Winter annuals: Utilize parts of two growing seasons in completing their life cycle. They germinate in the fall, live through winter, usually in dormant state, and then mature and produce seed in late spring and die in summer. They include winter wheat, winter rye, winter barley and winter oats – only in the temperate regions. The cold winter vernalizes(to initiate flowers) these crop spp.
Biennials – Require two years to complete their life cycles . Vegetative growth occurs in the first year ( vernalization occurs during winter) and then they flower and produce seed and die in the next year e.g sugarbeet, carrot and sweet clover.
Perennials have an indefinitelife period ranging from a few to many years. They do not die after reproduction. Some have herbaceous stems which die back to the soil surface in winter and resume growth from thje crown or taproot in spring e.g alfalfa. Other perennials such as trees and shrubs, add new growth every year from woody stems. E.gs. are cotton, castor bean and sorghum which can grow for several years in tropical regions but are killed by frost each fall in the temperates. Sugar cane and forage crops are important perennials.
CLASSIFICATION BASED ON WATER REQUIREMENTS OR
Hydrophytes – Plants adapted to live in water or in soil saturated with water. The only major hydropyte is is paddy or lowland rice.
Mesophytes – Thsde are intermediate in water use. They can tolerate water-saturated soils briefly and are also sensitive to extremely dry soil conditions. Most crops are mesophytes.
Xerophytes -require little water and can survive in dry soils for very long
periods. -have special features which enable them to conserve water e.g. fleshy leaves, stomata that only open at night, or an extremely short life span that allows them to flower and produce seed before the soil gets dry. -No common crops are xerophytes, e.gs are cactus, saltbrush (desert plants).
Halophytes – Can grow in soils with high concentrations of salts e.g some varieties of barley.
CLASSIFICATION ON THE BASIS OF TYPES OF SEED (Botanical
Higher plants are basically divided into monocots and dicots.
Monocots – have one seed leaf in the embryo and include grasses and sedges
Dicots – have two seed leaves in the embryo and include legumes and the other broadleaved plants.
CLASSIFICATION BASED ON AGRONOMIC USE
a) Cereal or grain crops - Crops of the Gramineae family/ grasses grown for their edible seeds. E.g. wheat, barley, grain sorghum. (Note: sugar cane is not a cereal although it belongs to the grass family).
b) Small grains – collective term used to include sorghum and millets (in the tropics) and wheat, rye, oats and barley (in the temperates)
c) Grain legumes - Belong to the leguminosae family and grown for their seed e.g. groundnuts, dry bean, cowpeas, soya, pigeon pea etc.
d) Pulse crops – those large-seeded legumes that are used as a source of protein for humans or livestock e.g groundnuts, dry bean, cowpeas, soya, lima bean , pigeon pea; usually high in protein content.
e) Oil crops – grown primarily for the oil content. Plant oils are are mostly used for food processing and in vegetable oils and shortenings, also as lubricants and in industrial processing e.g soya, groundnut, castor bean ( high heat resistant), oil palm. Mustard, rape and maize.
f) Root crops – the root is harvested for human food or livestock feed, e.g carrot, radish, sweet potato, cassava, yam and sugarbeet.
g) Tuber crops – grown for their enlarged underground tuber. E.g. Irish potato and
h) Sugar crops: grown for their sweet juice from which sucrose is extracted and refined. e.gs are syrup from sorghum, dextrose (maize sugar) from naize and sorghum; table sugar from sugar cane in tropics and sugar beet in temperate regions.
i) Fibre Crops
These are grown for the coarse or fine fibres derived from their fruits, stems or leaves. The fibre is used for making textiles, rope, twine, bags etc
E.g Cotton : fibre is from the seed.
Sisal : fibre is from the leaf
Kenaf, jute : fibre is from the stem
Other fibre crops are hemp and flax
E.g peppers, cloves, tobacco and mint
k) Forage Crops
Grown for use as ruminant animal feed. Can be fed to animals as fresh or may be preserved and used later (in dried form such as hay and silage)
E.g: guinea grass, cowpea, maize
Pasture crop: a forage crop that is harvested directly by livestock through grazing
Hay crop: a forage crop that is cut while still green, allowed to dry in the field, cured, stored and fed to livestock
Silage crop: a forage crop that is harvested in a green , succulent condition and then stored under unaerobic conditions where controlled fermentation occurs. Oxygen must be excluded from crop during storage.
‘Green chop’ – It is harvested in a green , succulent condition and fed directly to livestock.
l) Horticultural crops: These usually require very intensive cultural practices as compared to field crops such as maize and dry beans. They include orchard crops, vegetable crops, small fruit such as blackberry, raspberry and strawberry and gooseberry; ornamental plants and flowers.
m) Beverage Crops
For example: tea, coffee and cocoa
Are also a source of stimulants
n) Latex Crops
Grown for their milky sap or latex which they produce
E.g : Rubber
SPECIAL USE CROPS
Grown to provide adequate cover on a piece of land, especially during fallow periods.
Examples : Cowpea, Calopogonium – devil bean.