WORLD
CENTRES OF PRIMARY DIVERSITY OF DOMESTICATED PLANTS
Origin of crop plants
The
history of crops and man reveals that the evolution of man and the evolution of
crop plants went hand in hand. Man has become so utterly dependent on plants
such that the plants have 'domesticated' him (Harlan, 1975). The origins of
crop plants and their history reveal that the evolution of crop plants is a
gradual and slow process rather than sudden and rapid. As such, some cultivated
plants differ very little from their original wild relatives, while others
differ enormously from their progenitors.
To know
precisely about the present day crop plants there needs a thorough understanding
of what kinds of plants did man eat before today's crops were available? What
were the conditions that prompted him to domesticate the plants? How was his
life before agriculture?
To have a
rough idea about these questions from the hunter man to the present day
civilized agricultural society. Lee and Devore, (1968) cf. Harlan,
(1975) put it as “cultural man has been on earth for some 2 million years, for
over 99 percent of this period he has lived as a hunter gatherer. Only in the
last 10,000 years man has begun to domesticate plants and animals, to use
metals and to harness energy sources etc.”
Hunter man
was too ignorant or unintelligent to understand the life cycles of plants and
they were intellectually insensitive and incapable of 'improvement'.
Gatherers
used to collect grass seeds, legumes, roots and tubers, oilseeds, fruits and
nuts, vegetables and spices for their food, totaling about 1,400 species. The
list suggests that
(i)
many
more species have been gathered from the wild than have ever been domesticated,
(ii)
even
after agriculture is fully developed, gathering wild plant foods is still a
worthwhile effort, and
(iii)
wild
plant resources are of the same general kinds as domesticated plant resources
(Harlan, 1975).
GRASS
SEEDS like wild rice (Zizania
aquatica, Oryza barthii, Oryza /ongistaminata, O. subu/ata, Panicum,
Sporobolus, Digitaria, Avena barbata and A. fatua, etc.
LEGUMES include Acacia spp., Canavalia, Vigna,
Tephrosia, Phaseolus, Dolichos, etc.
ROOTS
AND TUBERS include Dioscorea,
Allium, Ipomoea, Cyperus rotundus, Eleocharis, Nymphaea etc.
OIL
PLANTS fruits of Arecaceae, E!aeis guineensis, Cocos
nucifera; Pistacia, Aleurites, Theobroma (cocoa), Olea,
Butyrospermum (Shea butter tree), Sesamum and Lilium.
FRUITS
AND NUTS Walnut (Juglans),
hickory (Carya), Chestnut (Castanea), Oak (Quercus), pine-nuts
(Pinus); Vitis, Ficus, Citrus, Musa, Artocarpus, Annona and Carica etc.
VEGETABLES
AND SPICES species of
wild Capsicum, Lycopersicon, Nicotiana, Cucumis, Momordica, Luffa, Lagenaria
etc.
Thus, the
gatherers exploited a wide range of plants wherever they found them. As a
result of this practice, independent domestications of different species of the
same genus occur and if the genus is wide spread, the different domesticates
originate in different continents of the world. Some examples of such wide
spread domestications include, Amaranthus, Annona, Cucurbita, Gossypium,
Phaseolus, Digitaria, Oryza, Solanum, Dolichos, Ipomoea, Panicum etc.
From the
ethaographic evidence (scientific description of races of mankind) it is
inferred that gatherers had some knowledge of planting seeds and sowed seeds of
wild plants like Chenopodium, Oryzopsis, Eleocharis etc. They also had
the knowledge of seed-bed preparation and irrigation to increase production.
Gatherers
had the knowledge of how to make use of poisonous foods safe, like seeds of Zamia
(a Cycad); certain
leguminous and Solanaceous fruits; Dioscorea spp.; Aroid tubers.
They also know about drugs, narcotics, medicines, fish poisons, arrow poisons,
gums, resins, glues, dyes and paints, bark cloth, woods for spears, arrows,
bows, shields, fire sticks and canoes. They had the knowledge of spinn1ng,
weaving, basket-making and construction of house hold utensils, fish traps and
ceremonial objects.
The
Australian aborigins used tobacco for chewing, and another masticatory, Duboisia
hopwoodi which contains hyoscyamine, norhyoscyamine and scopolamine.
Agriculture,
in mythologies of all civilizations reveal that it is a divine gift to humans
as can be seen in Egypt by Goddess Isis, Demeter in Greece and Ceres in
Rome. According to the Chinese mythology P’an Ku separated the heaven
and the earth, created the sun, moon and stars, and produced plants and animals
on earth.
Plant domestication
According
to Childe (1952, Cf. Harlan, 1975) the process of plant domestication,
by man went through a series of steps like hunter, herder and then a
cultivator, the so called Neolithic revolution i.e., a shift from
hunting and gathering to food production.
The origin
of domestication of plants by man seems to be that the gatherer took up,
agriculture or farming since it invo1ves more energy to obtain food by
intensive gathering and thought that cultivation is more advantageous than
gathering.
World centres of origin of domesticated plants
Milestone work : Two great works, historically, by Alphonse de Candolle and N.I. Vavilov are
worth mentioning in any discussion of the origins of cultivated plants.
De
Candolle was a French
botanist and systematist who produced a voluminous work called the 'Prodromus
Systematis Naturalis' Regni Vegetabilis 'Origin of Cultivated Plants (l886)
concerning the geography of plants, distributions of wild relatives, history,
names, linguistic derivatives, archeology, variation patterns etc. on the
cultivated plants.
N. I.
Vavilov, a Russian
geneticist and agronomist at the National Institute of Plant Industry, USSR,
relates to Studies on the Origin of Cultivated Plants published in 1926
and 1951. Vavilov launched an ambitious plant breeding programme that was ever
attempted to collect and assemble all of the useful germplasm of all crops. A
vigorous, world wide plant exploration, programme was launched and a systematic
survey for genetic resources of crop plants was started by him. He was
interested in the genetic diversity of crop plants and their centres of origin.
The centre
of origin could be determined by an analysis of patterns of variation, and
according to Vavilov, the geographic region in which one found the greatest
genetic diversity was the center of origin for that particular crop.
From, his
extensive studies, Vavilov, proposed eight centers of origin for most of the
cultivated plants of the world (Fig.1.1) :
Figure 1.1. Centres of origin for crop plants according to
Vavilov (1951). 1. Chinese Centre. 2. Indian Centre. 3. Central Asiatic Centre.
4. Near Eastern Centre. 5. Mediterranian Centre. 6. Abyssinian Centre. 7.
South-Mexican and Central American Centre. 8. South American Centre.
On the basis of a large amount of
supplementary data we have been enabled to locate more exactly the regions
of the origin of cultivated plants.
Southwestern Asia including Transcaucasia and the
northwestern portion of India originated soft wheats and rye as well as many
grain Leguminosae, alfalfa, Persian clover, etc. Here, especially in the
western part of this area, is the home of the most important fruit trees.
India is the native country of rice, sugar cane
and many tropical plants.
The mountains and foothills of Eastern China are
the home of many fruit trees, truck crops and the soybean. The vast regions of
Central Asia, investigated by us in detail in 1929, have proved alien to the
primary process of form origination. In spite of some former botanical
suppositions, Central Asia and Siberia have had no influence upon the origin of
cultivated plants.
Abyssinia, though economically a country of no
particular importance with its cultivated area of only several million acres,
shows a striking concentration of the diversity of the genes of wheat, barley
and many leguminous grain crops.
Certain countries bordering the Mediterranean are
the home of the olive tree, the carol tree, a series of original forage plants
and Egyptian clover.
The 7th
& 8th centers
must be sought in America. In the New World the primary process of form
origination is narrowly localized; the regions showing a striking species and
varietal diversity occupy comparatively small territories concentrated in
Southern Mexico and Central America as well as in Peru and Bolivia. The home of
corn and of the upland cotton in all probability is Mexico and Central America,
whereas that of the potato is in Peru and Bolivia.
These centers have developed on the basis of an
extremely rich wild flora. Here we find conditions especially favorable to the
development of species and varietal diversity. These regions have proved
equally favorable to civilizations and of course it is no accident that the map
showing the distribution of the chief sources of food plants essentially coincides
with that of the distribution of the first agricultural civilizations.
The mountain and foothill regions in the
subtropics are the most remarkable places for comprehending the evolution of
cultivated plants as well as of many wild species.
In these regions the beginnings of the
evolutionary process manifest themselves in a salient way especially when we
compare the evolution of different species and genera. The existence of such
group evolution of different species and genera facilitates greatly an understanding
of the evolutionary process.
But it
soon became apparent that the pattern is much more complex than what Vavilov
has thought of, since some crops do not have centers of diversity. (Fig. 1.2).
According
to Harlan (1971) there are three independent systems each with a center and a
non center. According to him no single model will explain agricultural origins
and he recognizes a humanistic no-model, model to explain the origins of
domesticated plants, which is mainly a humanistic problem ‘Man took the
initiative in modifying his environment, and plants responded genetically to
his activities’. Plant domestication is an evolutionary process operating under
the influence of human activities.
Vavilov thought that areas of maximum genetic diversity,
represented centers of origin and that the origin of a crop could be identified
by the simple procedure of analysing variation patterns and plotting regions
where diversity was concentrated. It turned out that centers of diversity are
not the same as centers of origin, yet many crops do exhibit centers of
diversity.
FIG. 1.2. Origin and evolution of plants according to
Harlan.
The
reasons for the origin of secondary centers of crop plants might be due to
(Harlan, 1975)
(i)
A
long history of continuous cultivation,
(ii)
Ecological
diversity,
(iii)
Human
diversity, different tribes are attracted to different races of a crop,
(iv)
Introgression
with wild or weedy relatives or between different races of the crop, which
leads to hybridization, segregation and selection, and
(v)
The
deliberate introduction of certain exotic plants by mail from one continent to
another during history.
Germplasm
collections of world crop plants are made continuously at the following
institutes in the world
§
Food
and Agricultural Organizations of the United Nations;
§
The
RockfelIer Foundation;
§
The
'Ford Foundation;
§
The
Consultative Group;
§
Eucarpia;
§
The
United States Development and Agriculture;
§
The
Vavilov Institute of the Soviet Union;
§
CSIRO
of Australia;
§
The
Kihara Institute of Japan;
§
The
National Bureau of Plant Genetic Resources of India etc.
Seed
storage facilities are available in large quantities at Fort Collins in the
United States of America at the National Seed Storage Laboratory. Other places
for storage of seed include : Bari (Italy); Braunschweig (West Germany);
Izamin- (Turkey); Japan, Bulgaria, Poland, United Kingdom and Australia.
Some of
the most important crops in the world are the following:
(1)
Wheat, (2)
Rice, (3)
Maize,
(4)
Potato, (5)
Barley, (6)
Manioc,
(7) Oats, (8) Sorghum, (9) Soybean,
(10) Cane
sugar, (11) Beet
sugar, (12) Citrus,
(13)
Cotton fiber, (14)
Cotton seed, (15) Bean,
Pea, Chickpea,
(16) Rye, (17) Banana, (18) Tomato,
(19)
Millets, (20)
Sesame, (21)
Palm oil,
(22)
Pea-nut, (23)
Sweet potato and yams, (24) Coffee,
(25)
Tobacco, (26)
Rubber (27)
Cocoa and
(28) Tea.
Process of
domestication of crop plants from their wild progenitors
Domestication
of cultivated plants is an evolutionary process through human intervention and
the process involves a slow and gradual progression from the wild state to
incipient domesticated forms or species. But, the process differs from crop to
crop, some species evolve directly and some indirectly by a series of steps in
their evolution.
Harvesting
wild grass seeds was the beginning of domestication by the gatherers. It is
always the selection that is associated with harvesting, which causes
domestication. Most of the seeds that do not shatter are harvested and most of
the seeds that shatter escape the harvest. The shattering character in cereals
is simple, one or two gene controlled. Domestication introduces into the crop
plants a non-shattering, annual habit with lack of seed dormancy from a wild,
shattering, perennial habit and with seed dormancy. This has been well
exemplified in the evolution of rice crop (Oryza sativa L.).
There has
been a trend in all cereals called the ‘Sunflower effect’ i.e., from
many small inflorescences to a few or a single large inflorescence, which is
usually accompanied by an increase in the seed size. The head of a commercial
cultivar of sunflower, an ear of maize, or a head of modern grain-sorghum or
grain-type, pearl millet are strikingly different from their wild progenitors.
(Harlan, 1975).
Domestication
also introduces a trend towards lower protein and higher carbohydrate content
of cereals; increasing seed size and hence endosperm. The embryo is richer in
protein and oil but does not increase in the same proportion as the endosperm.
This type of selection results in increased seedling vigour.
Cultivated
plants have the capacity to evolve rapidly. Rapid evolution is possible only
through some variation on the theme of the differentiation-hybridization cycle
in which variability already accumulated can be exploited. Mutations play an
important role as the sources of variability in crop plants. The crop-weed
interaction is the only system by which differentiation-hybridization cycles
can be set up in cultivated plants. This enhances variability and broadens the
base for plant selection.
Several
isolating barriers are known to exist which fragment the populations and are
kept genetically apart whereby differentiation occurs. They include: geographic
and ecological separation, difference in time of blooming, self-fertilization,
and translocation races, polyploid races, gametophytic factors, cryptic
chromosomal differences and meiotic irregularities. Differentiation again
largely depends upon genetic buffering i.e., the amount of redundancy of
genetic information.
Under domestication, changes occur, until the end products are radically
different in appearance from their wild progenitors. Thus, domestication
results in great morphological changes
without substantial change in the genetic background.
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