Introduction
Maize (Zea mays L.) is one of the world’s most important cereal crops, belonging to the family Poaceae (Gramineae). It is commonly known as corn in many parts of the world. Maize serves as a staple food, livestock feed, and industrial raw material, making it an essential component of global food and nutritional security. Unlike wheat and rice, maize is a C4 plant with a high photosynthetic efficiency and wide adaptability, which explains its large-scale cultivation across diverse agro-climatic zones.
The somatic chromosome number of maize is 2n = 20, and the genome is large and complex, characterized by high genetic diversity and abundant transposable elements. As a cross-pollinated species with pronounced heterosis, maize has been central to the development of hybrid technology in plant breeding.
Centre of Origin
Maize is believed to have originated in southern Mexico and Central America, specifically the regions encompassing the Balsas River Valley. Archaeological evidence suggests its domestication occurred about 9,000 years ago from its wild ancestor teosinte (Zea mays ssp. parviglumis). Maize spread northward into the United States and southward into South America through human migration and trade. From the Americas, maize was introduced into Europe in the 15th century following Columbus’ voyages, and subsequently distributed to Africa and Asia, where it adapted remarkably well.
Distribution of Species
Maize has achieved global distribution due to its adaptability across different ecosystems ranging from temperate to tropical regions. Currently, maize is grown on about 200 million hectares worldwide, with a production exceeding 1.2 billion tonnes annually (FAO, 2023).
- Major producers: USA, China, Brazil, Argentina, Mexico, India, and South Africa.
- In India: Maize occupies about 9–10 million hectares, grown in states like Karnataka, Andhra Pradesh, Madhya Pradesh, Bihar, Rajasthan, Maharashtra, and Uttar Pradesh. It is cultivated during both kharif and rabi seasons.
- Ecological range: From sea level to high altitudes (up to 3,000 m in the Andes), maize tolerates diverse climates and soils.
Cultivated Species
The only cultivated species of maize is Zea mays L., but within this species, there are several morphological and agronomic races classified based on kernel type and endosperm composition. These cultivated races are utilized in breeding programs worldwide:
- Flint Corn (Zea mays indurata): Hard outer endosperm with rounded kernels; grown widely in Latin America, Europe, and parts of India; resistant to pests and stress but relatively lower yielding.
- Dent Corn (Zea mays indentata): Kernels with a characteristic dent on top; rich in starch; used for animal feed and starch industry; predominant in the USA.
- Sweet Corn (Zea mays saccharata): High sugar content due to sugary gene mutation; consumed as a vegetable; increasingly popular in urban markets.
- Popcorn (Zea mays everta): Small kernels with hard endosperm; “pops” when heated; used as a snack food.
- Waxy Corn (Zea mays ceratina): Contains waxy starch composed of amylopectin; used in adhesives and food industries.
- Pod Corn (Zea mays tunicata): Kernels enclosed in glumes; of botanical interest rather than economic use.
Wild Species
The genus Zea consists of the cultivated maize (Zea mays) and several wild relatives, which serve as valuable genetic resources:
- Teosinte (Zea spp.): Closest wild relative, native to Mexico and Central America; includes Z. mays ssp. parviglumis, Z. luxurians, Z. mexicana, and Z. diploperennis; sources of resistance to pests, diseases, and abiotic stress.
- Tripsacum spp. (Gamagrass): Another related genus, with species such as Tripsacum dactyloides; contributes genes for perenniality, drought tolerance, and disease resistance.
Wild relatives are critical for pre-breeding programs, enabling the transfer of novel alleles into cultivated maize through wide hybridization and molecular tools.
Botanical Description
Maize is a monocotyledonous, annual, cross-pollinated crop with distinct morphological features:
- Root system: Adventitious roots, including crown and prop roots, provide strong anchorage.
- Stem: Erect, solid, cylindrical, with nodes and internodes; height ranges from 1 m (dwarf) to over 4 m (tall varieties).
- Leaves: Large, alternate, linear-lanceolate with parallel venation.
- Inflorescence: Monoecious plant with separate male and female flowers:
- Male inflorescence (tassel) at terminal end.
- Female inflorescence (ear or cob) on axillary branches, covered by husks, with long silk stigmas.
- Pollination: Predominantly cross-pollinated by wind (anemophily).
- Fruit: Caryopsis (grain), variable in color—yellow, white, red, purple, or black.
Economic Importance
Maize plays a multifaceted role in global and national economies:
- Food: Staple diet in Africa and Latin America; consumed as flour (tortillas, bread), porridge, boiled/roasted kernels; sweet corn and popcorn have niche markets.
- Feed: About 65–70% of global maize production is used for animal feed, especially poultry, cattle, and pigs.
- Industrial Uses: Raw material for starch, glucose, fructose, corn syrup, dextrose, alcohol, ethanol; used in textile, paper, adhesive, and biofuel industries; maize germ oil has edible and pharmaceutical uses.
- Fodder: Green maize is an important fodder crop in India.
- Employment and Trade: Provides livelihood to millions and forms a major agricultural trade commodity.
Breeding Objectives
- Yield Enhancement: High-yielding hybrids and open-pollinated varieties.
- Abiotic Stress Tolerance: Resistance to drought, heat, salinity, cold.
- Biotic Stress Resistance: Resistance to turcicum leaf blight, downy mildew, rust, stalk rots, stem borer, fall armyworm.
- Quality Improvement: Nutritional enrichment with protein, lysine, tryptophan, pro-vitamin A, zinc, iron.
- Adaptability and Stability: Varieties suitable for diverse agro-ecologies.
- Maturity Duration: Early and extra-early maturing varieties to fit cropping systems.
Important Breeding Methods
1. Conventional Approaches
- Mass Selection: Selection from local landraces; maintains variability but modest yield gains; useful for open-pollinated varieties.
- Introduction and Acclimatization: Exotic germplasm from CIMMYT and other centers has been foundational for breeding.
- Hybrid Breeding: Exploits heterosis; includes single, double, three-way, and top-cross hybrids; single crosses are most productive.
- Composite Varieties: Developed by intercrossing several genotypes; broadly adapted but lower yielding.
- Synthetic Varieties: From selected inbreds with good combining ability; maintained by open pollination.
- Mutation Breeding: Induced mutations using radiation or chemicals for traits like earliness and disease resistance.
2. Modern and Innovative Approaches
- Marker-Assisted Selection (MAS): Uses DNA markers linked to important traits.
- Genomic Selection: Predicts performance using genome-wide markers; useful for complex traits.
- Genetic Engineering: Development of Bt-maize (insect resistance), HT-maize (herbicide tolerance), and biofortified maize (Pro-vitamin A).
- Genome Editing (CRISPR/Cas9): Precise editing for drought tolerance, nitrogen-use efficiency, and nutritional traits.
- Doubled Haploids (DH): Accelerates inbred development to 1–2 years versus 6–7 years by conventional methods.
- Climate-Smart Breeding: Developing resilience to climate change (heat, drought, efficiency).
Development of Hybrid Varieties
Hybrid maize breeding is the cornerstone of maize improvement programs.
- USA: Pioneer Hi-Bred developed the first commercial hybrid maize in the 1930s, starting the “Hybrid Corn Revolution.”
- India: ICAR institutions and state agricultural universities have released hybrids like Ganga 1, Deccan Hybrid, Vivek 9, HM-4, and biofortified HQPM-1 (Quality Protein Maize).
Steps in hybrid development: inbred line creation → combining ability testing → hybrid formation → multi-location trials for adaptability and stability.
Conclusion
Maize, with its diverse uses and adaptability, has undergone remarkable genetic improvement through centuries of selection, hybridization, and innovation. While hybrid technology transformed maize yields globally, the advent of molecular tools, biofortification, and genome editing promises a new era of climate-resilient, nutrient-rich, and sustainable maize varieties. Future crop improvement will emphasize integrated approaches, combining conventional breeding with advanced genomics, to ensure food, feed, and nutritional security in the face of population growth and climate change.
