In this essay we will discuss about:- 1. Distribution of Maize 2. Cultivated Species of Maize 3. Origin and Evolution 4. Reproduction and Pollination 5. Breeding Objectives 6. Breeding Procedures 7. Breeding Centres 8. Achievements.
Essay on Maize
Essay # 1. Distribution of Maize:
Maize is an important cereal crop of global importance, which belongs to the monocot family Gramineae (Poaceae). Maize is used for human consumption in developing and undeveloped countries and as livestock feed in developed countries. Moreover, it is also used as green fodder for livestock. Maize is grown in Central and South America, Africa, Central Europe and Asia.
In India, U.P., Bihar, Rajasthan, M.P., Punjab and H.P. are the major maize growing states. Maize is of two types, viz. flints (with round seeds) and dents (with flat seeds). The flour can be easily made from flints than from dents. Maize has C4 photosynthetic pathway and is more efficient than C3 plants.
Essay # 2. Cultivated Species of Maize:
There are five species of the genus Zea viz. Zea maxicana, Z. perennis, Z. luxurian, Z. diploperenis and Z. mays. The first four species are wild and commonly called Teosinte. The last species i.e, Z. mays is the only cultivated species. The teosinte species are easily crossed with maize. The diploid chromosome number in maize is 20.
The wild species, Z. perennis is tetraploid (4n = 40); rest are diploids. The other genus which is close to maize is Tripsacum. However, special techniques are required to cross maize and Tripsacum. There are 16 species of Tripsacum in which the somatic chromosome number varies from 36 to 108. The tetraploid wild species Z. perennis is believed to have originated from Z. mexicana.
Essay # 3. Origin and Evolution of Maize:
Central America (Mexico) is the place of origin of maize. The exact genetic origin of maize is not known so far.
There are four different views (hypotheses) about the possible genetic origin of maize which are summarised below:
i. Maize, Teosinte and Tripsacum have originated from a common ancestor (from high lands of Mexico and Guatemala) which is now extinct.
ii. Maize originated from a cross probably between Coix and Sorghum each with 10 chromosomes.
iii. Modern maize is believed to have originated from crosses between wild maize (Podcorn) and Tripsacum. The Teosinte probably developed from a cross between cultivated maize and Tripsacum in Central America.
iv. Some workers believe that maize has derived from Teosinte by direct selection.
Essay # 4. Reproduction and Pollination of Maize:
Maize is a seed propagated and cross pollinated crop. Under natural conditions, cross pollination is more than 99%. Cross pollination takes place by wind. Maize is a monoecious plant in which male and female inflorescences are separate but on the same plant. The male inflorescence is located at the top of the plant and is referred to as tassel.
The female inflorescence is located in the middle of the stalk and is called as ear or cob. Dichogamy also promotes cross pollination in maize. Generally, male flowers mature before the female flowers. Maize varieties are highly heterozygous and heterogeneous. As a result, maize has greater adaptability than self-pollinated cereals like wheat and rice.
The ear is covered by leafy structures called husks. Maize has long stigmas which emerge from the upper end of cob and husk. These stigmas are known as silk. In maize, pollen is very light and cross pollination occurs by wind. Hence isolation distance of 1600 metres is required for the production of breeder seed of open pollinated varieties.
Essay # 5. Breeding Objectives in Maize:
In maize, the main breeding objectives are higher yield, wider adaptability, better quality, early maturity, resistance to biotic (diseases and insects) and abiotic (drought, heat, salinity, lodging etc.) factors. The yield components include ear size, rows of kernel/ear, number of kernels per row and per ear, size of kernel and number of ears per plant.
Quality characters include grain colour, protein content and two amino acids, viz. lysine and tryptophan. The gene opaque 2 is the source of high lysine content. This gene is used for improving lysine content in maize. Seed oil and sugar contents are also considered important traits in some varieties.
The protein found in the endosperm is known as Zein which has inadequate amount of two amino-acids, viz. lysine and tryptophan. The main diseases are root, stalk and ear rots, leaf blight, leaf spot, downy mildew and mosaic virus. The stalk borer is the major insect pest in India.
Essay # 6. Breeding Procedures ion Maize:
In maize, there are three breeding approaches, viz.:
(1) Population improvement,
(2) Inbred improvement, and
(3) Cultivar development.
Population improvement refers to accumulation of desirable alleles in a population through various methods. The breeding methods which are used for population improvement include mass selection, progeny or ear to row selection, recurrent selection, disruptive mating and selection, and biparental mating.
These methods result in improvement of population for various polygenic characters. There are four methods, viz. pedigree selection, backcrossing, convergent improvement and gamete selection which are used for improving inbreds for various economic traits. Four methods are used for development of cultivars.
Mass selection is used for development of open pollinated cultivars, and their maintenance. Heterosis breeding is used for evolving high yielding hybrids. In maize, three types of hybrids are developed, viz. single cross hybrids, three ways cross hybrids and double cross hybrids. Synthetic and composite cultivars are developed by synthetic breeding and composite breeding procedures.
Tissue culture technique is expected to play an important role in future in creating somaclonal variation for maturity, insect and disease resistance, herbicide tolerance, salinity, resistance etc. Induced mutations have been rarely used in maize and that too for creating variability.
Essay # 7. Breeding Centres of Maize:
There are two types of maize improvement centres, viz. international centre and national centre. The International wheat and maize improvement centre (CIMMYT) is located in Mexico. This centre maintains global gene-pool of maize and takes up international programmes on maize breeding. Breeding material is tested in different countries by CIMMYT.
In India, maize breeding work is carried out by State Agricultural Universities and Directorate of Maize Research, IARI campus, New Delhi. Maize breeding work is also carried out by Vivekanand Parvatiya Krishi Anusandhan Shala, Almora, especially for hill regions. Maize varieties and hybrids are released by coordinated project after multi-location testing for 3 to 5 years.
Essay # 8. Practical Achievements of Maize:
In maize, several composite and hybrid cultivars have been released by AICMIP for various agro-climatic conditions of the country.
Some important high yielding composite and hybrid cultivars with wider adaptability are listed below:
Composite:
Jawahar, Vijay, Kisan, Amber, Sona, Vikram, Protina, Rattan, Shakti, Vikas, Navjot, Ageti 76, Kanchan, Diara 3, D 765, MCU 508, Kiran, Surya, Tarun, Arun, Renuka etc.
Hybrids:
Ganga safed, Ganga 5, Ganga 9, Deccan 103, Sartaj Makka 1, VL 42 etc. Hybrids Him 123 and Him 128 have been released for the hilly regions of UP, HP, Assam and Sikkim.
Both genetic and cytoplasmic genetic male sterility systems are available in maize. Usually, CGMS system is used for the development of single, three-way and double cross hybrids in maize.
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