Plant Breeding Practices in Maize (Zea mays L. 2n=20)
Maize is second leading crop after wheat. United status produces 50% of the world production. In India the total cropped area under maize is 6 million hectares with 1250 kg productivity. The status growing maize on large scale are Uttar Pradesh. Bihar, Rajastan, Madhya Pradesh, Panjab and Himachal Pradesh. The maize has C4 photosynthetic pathway and has high seed multiplication ratio.
Origin:
Maize is native of America, with two possible centres of origins.
a) Peru, Ecuador, Bolivia,
b) Southeren Mexico and Central America.
Based on excavations Mexico is accepted as centre of origin for maize.
Species:
1) Zae mays L. ( 2n=2x=20); corn
2) Zea maxicana ( 2n=2x=20) Annual teosinte
3) Zea Perinnis ( 2n=40) Perennianl tetraploid teosinte
4) Zea diploperinnis ( 2n=2x=20) Perennial diploid teosinte
Similarly, the genus Tripsicum is closely related to Zea and has 11 species. Crosses between maize and sugarcane are successful.
Botany and Floral Morphology:
It is tall annual plant growing to the height of 1.5 to 3.5 m stem is thick with distinct nodes and internodes. The leaves are broad and long, with parallel venation and having dark green colour. The roots are adventitious and fibrous. It is monoecious plant bearing male flower are the growing tip as tassel and female flower at the axial of the leaf on the shoot. The tassel is terminal with staminate flowers in several roots. Each pairs of flower consist of sessile and pedicillate spiklet. Each spiklet contains two similar glumes. The flower contains membranous pelia with three stamens and two lodicules. The pollens remain viable for 18 to 24 hours.
The female inflorescence is a spadex known as cob or ear. It is modified lateral branch developed from lateral bud. The shoot is composed of compressed internodes from which husk rise and terminates in an ear on which the sessile are borne. Spiklets are in pair. Each spiklet having two flowers, the lower one is reduced to lemma and pelia is non-functional, while upper one contained knob shaped ovary surrounded by broad lemma and thin pelia. One carpel is provided with long silky hair, which behaves as style and style stigma throughout the length.
Flowering:
Maize is cross pollinated crop. Blooming starts in the tassel near the tip of the central axil and it proceeds downward. It takes nearly 14 days to complete flowering. The pollen grains are viable for 24 hours. The pollen shedding begins 2 to 3 days before emergence of the skills.
Female flower i.e. the cob. The spiklets at the base elongate first and progress upward until emerge from the husk. The development of the sink takes about 2 to 5 days. Stigma is receptive for 14 days. Pollination is influenced by wind and gravity. The position of the cob has an advantage to trap the pollens shed by tassel.
Breeding Objective:
1) To develop high yielding hybrids and composite having various maturity duration.
2) To breed varieties for high grain and fodder yield.
3) To breed fertilize responsive varieties.
4) Breeding for wider adaptability to soil and climate.
5) Breeding for resistance to disease and logging.
6) Breeding for industrial quality (Starch or oil content etc)
Emasculation:
The tassels of the female plants are removed immediately as soon as appeared. The process is called as detasseling. It is always done in the morning. Ear shoot which emerging from the leaf sheath is bagged 1 to 2 days below the tip of the preveious day of pollination.
The tassels of selected male parents is also covered with bag on following day in the morning between 9.00 to 10.00 a.m. pollens from tassel bag is dusted over the silk of the female cob/eat. The bag covered ear shoot is torn and bag from the male parent may be placed over the cob. Care should be taken to avoid contamination of silk with foreign pollens.
Breeding Methods:
I) Mass Selection:
This is oldest method of selection of cross pollinated crops. It consist of selection of ears on the basis of plant and ear characteristics and bulking seed of harvested ears to grow next cycle of mass selection. During mass selection rigorous selection is avoided as it may create the problems of inbreeding depression. Varieties developed 1) KT-41. Basi and Jaunpuri.
II) Hybrid Breeding:
The Term hybrids used to designate F1 populations obtained by crossing genetically unlike parents.
Types of Hybrids:
A) Conventional Hybrids:
1) Single Crosses:
It is hybrid progeny from crosses between two unrelated inbreeds ( A XB) X C. Popular hybrids are Ganga 5- ( CM-202 X CM-111) X CM- 500 ; Ganga 11- (CM –202 X CM -111) X CM-501 ; high starch – (CM -400 X CM -300 ) X CM-601.
2) Double Crosses:
It is the hybrid progeny from crosses between two single crosses ( AX B) X ( C X D)
Popular Hybrids:
1) Deccan double hybrid: (CM-104 X CM -105) X ( CM-202 X CM -201)
2) Deccan 101- (CM-202 X CM -206) X (CM -115 X CM-114)
3) Deccan 103- (CM-120 X CM-118) X (CM -208 X CM-119)
B) Non Conventional Hybrids:
1. International Hybrid:
It is the hybrid progeny cross between two varieties. It is similar to synthetic and composites. E.g. Variety X Variety.
2. Top Crosses ( Inbreed Variety Cross):
It is the hybrid progeny from crosses between inbreed and variety E.g. A X variety.
3. Double Top Crosses:
It is the progeny of the cross between single cross of inbreds and variety. E.g. (AXB) X Variety.
Hybrid Seed Production:
Deta selling is common method used for producing hybrid seed in India. In USA detasseling and use of CMS line both are used. Male and female lines grown alternatively in the ratio 2:6. The female rows are detasseled before inflorescence shades the pollens and seed produced on detasseled lines is the F1 hybrid seed.
III) Pedigree Method:
In this method the pair of elite lines that complement to one another are crossed to produce F2 generation and selection is practised in F2 and on word generations.
IV) Back Cross Breeding:
In this method two inbreeds A and B are crossed, F1 is backcrossed with A followed by selection if desirable traits of B and F1 is also backcrossed with B where selection for desirable traits for A are made. After 3 backcrosses and selections selfing is done to fix the selected genes. This method is used to transfer characters like pest resistance, disease resistance and lodging resistance etc.
V) Population Improvement:
A) Synthetic Variety:
The term used to designate the variety that is maintained from open pollinated seed following synthesis by hybridization in all combinations amongst the genotypes selected for high general combining ability. Now a day no variety is popular developed by this method in Maharashtra.
B) Composite Varieties:
Composite varieties in maize are derived from varietal crosses and advanced generations, which are not tested for GCA. The components are selected on the basis of phenotype i.e disease and pest resitance, early maturity, resistance to lodging, ear characteristics etc. the equal quantity of seed of these phenotypically out standing lines is mixed sown in isolated field to obtain crosses in all possible combinations and mild selection is operated to maintain uniformity and homogeneity in the population. The selected bulk is evaluated in multilicational testing before release.
Popular Varieties:
Manjri Composite, Hunis, African tall, Vijay, Kisan, Panchganga, Ambar, Jwahar, etc.