Maize

Maize (corn)

Zea mays L.

Family- Poaceae

The most important maize growing countries of the world are: USA, Mexico, Brazil, Argentina, Yugoslavia, Rumania, Federal Republic of Germany, Italy, New Zealand, Belgium and China. In India maize is grown on a large scale in the following states: Uttar Pradesh, Bihar, Rajasthan, Punjab and Madhya Pradesh. Corn plant is native to America. 

History of Maize

According to the historical findings it is known that specimens of corn were taken to Spain by members of the second voyage of Columbus 1494 A.D., and a pamphlet was published in Italy giving a brief description the plant.  In Europe, it was known round about 1511 A.D. from the work of Peter Martyr called 'Decades of the New World' The Spanish writers applied the name maize, to corn from which the word maize was derived. Other European names applied to corn include, Triticum, Frumentum, Turkish corn, corn of Asia, Welsh corn, Bactrian corn, Indian corn and so on.

          It was soon introduced into France, Italy, Souht-Eastern Europe and Northern Africa. By 1575 A.D. corn made its way into China, Phillipines and East Indies. In the meantime, corn was gaining primary importance in America especially in Virginia and Massachusetts. By the end of the eighteenth century with the discovery of the closely related genera, Tripsacum and Euchlaena, corn was given greater importance in America. Corn was widely grown in states like Ohio, Indiana, Illinois, Iowa and adjacent states.

          Corn was by far, the most important cultivated plant in America in ancient times, when compared to other food plants like beans, squashes, cassava, peanuts, potatoes etc. This is because corn readily adapts to a wide range of soils and climates and had a genetic plasticity in evolving new varieties coupled with high yield.

Tripsacum and the origin of Maize

Tripascum is an important genus with which domesticated maize is known to cross to produce viable hybrids. The basic genome s of Tripascum (x=18) and Zea (x=10) are distinctly related. Brockhold (1964) and Mangledorf (1961, 1968) suggested that introgression from Tripsacum may have a played a role in the evolution of domesticated maize. According to David B. Walden (1978) there seems to be no possibility that Tripsacum could be a progenitor of maize. Introgression between maize and teosinte is conspicuous in Mexico and Guatemala.

Botanical relatives of Maize

Corn is a grass and belongs to the family Poaceae (Graminae). It belongs to the tribe Maydeae, which includes eight genera. Five of these are relatively unimportant oriental genera Coix, Schlerachne, Polytoca, Chinonachne and Trilibachne. These are native to area extending from India to Burma through the East Indied  and into Australia. Coix is the best known genus of the group. The three other American genera are, Zea which is the cultivated maize, Tripsacum (gama grass), which has some value as a forage crop, but none as a grain crop, and Euchlaena (teosinte) which appears to be the closest wild relative of corn.

In the recent years teosinte and Tripsacum have played a dominant role in the understanding of the evolution of maize.

Varieties of Maize

The maize varieties can be classified into flint, pod, dent, sweet, pop, waxy and floury on the basis of soft or hard starch and sugar content of the grain. India grows mostly flint maize, but dent varieties are also cultivated in some parts of Rajasthan and hill regions.

• Dent maize (Z. mays indenta Sturt): On account of the dent formation the top of the kernel, it is called as dent maize. Grains are yellow or white, popular in America.
• Flint maize (Z. mays indurata Sturt.): The endosperm is soft and starchy in the centre. The kernels are rounded on the top. It is cultivated in Europe, Asia, Central America and South America.
• Flour maize (Z. mays amylacea Sturt.): The kernels are composed entirely of soft starch. The kernels show variation in pigmentation. It is cultivated in drier parts of USA and in some pockets in South America and South Africa. The flour maize resembles flint maize.
• Pop maize (Z. mays ever Sturt.): Kernels small with hard corneous endosperm. It is cultivated in Europe, USA, Russia and Australia.
• Sweet maize (Z. mays saccharata Sturt.): The kernels are wrinkled at maturity, endosperm contains sugar and starch, unripe kernels have sweet taste, so it is called sweet maize.
• Waxy maize (Z.mays ceratina Kulash.): Kernels are waxy, starch is gummy and contains greater amount of amylopectin. It is cultivated for starch in northern Burma, Phillipines, Eastern China, Manchuria and USA.
• Pod maize (Z. mays tunicata Sturt.) The kernel is enclosed in the husk. The ear is also enclosed in the husk. It is not cultivated commercially.

Botany of the plant

Zea mays L. (Maize) is a tall annual grass having broad leaves arranged in two vertical ranks. The inflorescences are monoecious i.e., the tassel is staminate and sheds pollen while the ear shoot is pistillate producing silks or styles. The flowers of the tassel are borne in numerous spike-like racemes which together form large spreading panicles which terminated the stem. 

A pistillate inflorescence is borne in one or more axils of the leaves.   The spikelets are arranged in 8 to as high as 30 rows on a thickened almost woody axis known as the cob, the whole being enclosed in a foliaceous bracts or husks. The long styles or silks protrude from the tops of the bracts. 

The spikelets are unisexual. The staminate spikelets occur in pairs in the tassel and are two-flowered. The two glumes of the spikelets are membraneous, acute and covered by short hair. Inside the glume are present lemma and palea. The pistillate spikelets are sessile and occur in pairs, consisting of one fertile and one sterile floret. The glumes are broad and rounded or emarginate at the apex. The styles or silks are very long and slender.

 

Corn grain

 

Starch is the major component of the carbohydrate fraction in normal corn. The standard corn starch granule is usually a mixture of two polysaccharids- amylose and amylopectin. Major emphasis during the last 35-40 years has been o waxy maize, comprised of starch with 100% amylopectin, and more recently, on amylo maize, with amylose content 50% and higher. The average composition of corn whole grain consists of mostly carbohydrate and to some extent protein and fat. It also consists to a minor extent vitamins like scorbic acid, Thiamine, nicotinic acid, riboflavin, pyridine and Vitamine A.

Ecology

Maize is successively grown under highly varied environmental conditions. Maize can be cultivated in all regions of the world which have frost free nights and the temperature does not go below 15 C.

Soil: It grows best on fertile, well-drained soils that are neither too heavy nor too light. Maize plants are severely stunted in saline and alkaline soils. Yield is considerably reduced with increase in soil salinity and salinity. 

For hybrid maize the time of sowing for kharif crop is June and Rabi crop is October.

Cytogenetics of Maize

No other crop has ben subjected to such intensive genetic and cytogenetic studies as maize and nearly 500 different genes have been identified and their linkage maps have been constructed. Several fundamental discoveries in genetics have been made through maize like linkage, chromosome mapping, artificial mutations with x-rays and gamma rays, translocations, inversions, duplications and deficiencies, mutable loci, permutation, transposable elements, AcDs system of mutable loci, heterosis and hybrid vigour, polygenic inheritance, male sterility etc. Barbara McClintock has been awarded Nobel Prize in 1983 for her work on transposable elements in maize.

      Maize is a classical material for cytological studies like meiosis, since it ha s a low chromosome number (n-10) and the pachytene chromosomes are well spread and contracted for an easy identification of chromosome. Maize chromosomes possess stainable knobs which aid in easy identification of pachytene chromosomes apart from length measurements and centromere positions. Pachytene chromosomes of maize were first studied by McClintock in 1928 and were later studied by Longley, randolf and others.

Mutants of Maize

Several mutants like anthocyanin mutants in leaf, plant, stem cob, grains and morphological mutants in the plant, leaf, stem cob and grains; seedling and chlorophyll mutations have all been induced, studied and identified with their chromosome markers. Almost all the genes quoted above under cytogenetics of maize, have been mutated either spontaneously or induced by different mutagens.

   Several unstable genetic systems have been discovered in maize like the Ac-Ds system of transposable genetic elements by McClintock (1951) and Paramutation by R.A. Brink (1958) and mutable loci by Demeree, Randolph, Emerson and others.

Heterosis or Hybrid Vigour

The term heterosis, was proposed by G.H. Shull and is used synonymously with hybrid vigour. Hybrid vigour has been explained by dominance of growth-promoting genes, so that a heterozygous individual A/a should be as vigorous as one containing both dominant alleles A/A. This was proposed by several maize workers, but its general acceptance came into effect only after Jones (1917) who explained on the basis of dominance of linked genes.

Hybrid corn

Shull proposed the pure line method in corn breeding in 1909 and was later modified by D.F. Jones in 1917 as the double cross, which consisted of making a hybrid between two different unrelated single crosses. Inbred A*B produced a single cross called AB and also C*D produces single cross CD. The hybrid between AB and CD gives a double cross which combines in one hybrid the gene combinations from four separate inbred lines.

Diseases of Maize

1. Brown spot caused by Physoderma zeamaydis
2. Downy mildew caused by Sclerospora philippinensis
3. Head smut caused by Sphacelotheca reiliana
4. Leaf blight caused by Helminthosporium turcicum
5. Leaf rust caused by Puccinia sarghii
6. Smut caused by Ustilago zeae
7. Leaf spot caused by Cercospora sorghii

Insect pests

1. Maize stem borer caused by Child zonellus Swinh
2. Pink borer caused by Sesame infernos Wlk. (Phadka grasshopper)
3. Hieroglyphus nigrorepletus Boisd.
4. Leaf roller caused by Marasmia trapezalis Guen.
5. Aphid, Rhopalosiphum maids (Fitch) Sucks juice of young grains and feeds on stem, multiplies very fast

Uses of Maize

1. Forage and feed: Maize plant is extensively used as cattle feed both as fresh green fodder (entire fresh or cured plant), silage (properly cut, chopped and stored green fodder) and as stover (dried corn stalks minus ear). The digestibility of maize fodder is much higher than sorghum, bajra and other non-legume forage crops. Maize plant does not have the problem of hydrocyanic acid and prussic acid production, hence if necessary crop can be harvested and fed at any stage of its growth. Cattle , chicken, ducks and other domestic animals are extensively fed on maize.
2. Food: In India over 85% of the maize production is used as food in the form of chapatis, porridges, corn flakes and pop corn and roasted green ears. In Europe, bread is prepared from maize flour.
3. Industry: Maize is also processed in the industry to produce a large number of industrial products. The starch industry using the wet milling process produce starch, maize oil and gluten for feed. The starch in turn is further processed to produce a series of products like dextrose, corn syrup etc. Maize heats can be used as cattle feed when ground and fortified with molasses and urea. Industrial processing of maize heat also yields products like xylan, furfural etc. Corn is also used in distilling and fermentation industries in the manufacture of ethyl and butyl alcohol, acetone and whiskey. Maize oil is a poly-unsaturated oil. When refined it is highly valuable as a cooking media. It is also used in soap-making and paints.