Unit 1: Origin of Cultivated Plants
Centers of Origin and Vavilov’s Work
The concept of Centers of Origin refers to geographical areas where a group of organisms, either domesticated or wild, first developed its distinctive properties.
Nikolai I. Vavilov’s Contribution
Nikolai Vavilov, a Russian botanist, proposed that crops were not originated randomly across the globe but in specific regions characterized by high genetic diversity. He identified eight primary centers of origin after extensive world expeditions.
- China: Origin of soyabean, radish, and pear.
- India: Origin of rice, sugarcane, and mango.
- Central Asia: Origin of wheat, onion, and garlic.
- Near East: Origin of rye, alfalfa, and fig.
- Mediterranean: Origin of cabbage, olive, and lettuce.
- Abyssinia (Ethiopia): Origin of coffee, barley, and sorghum.
- Central America (Mexico): Origin of maize, cotton, and cacao.
- South America: Origin of potato, tomato, and tobacco.
Importance of Centers of Origin
Knowing the center of origin is crucial for plant breeders because these areas contain the wild relatives of our modern crops. These wild varieties often possess genes for disease resistance, drought tolerance, and nutritional quality that have been lost in modern monocultures.
Introduction and Domestication of Plants
Plant Introduction
This is the process of bringing a plant species or variety into a new area where it was not previously grown.
- Primary Introduction: When the introduced variety is well-suited to the new environment and released for cultivation without any changes.
- Secondary Introduction: When the introduced variety is subjected to selection or used in hybridization to create a new variety.
Domestication
Domestication is the process whereby wild plants are evolved into cultivated forms through human selection and management.
During domestication, plants undergo morphological and physiological changes known as the domestication syndrome. Key changes include:
- Elimination of seed dispersal: Development of non-shattering rachis so seeds stay on the plant for harvesting.
- Increase in size: Larger fruits, seeds, or tubers compared to wild ancestors.
- Loss of seed dormancy: Ensuring uniform germination when the farmer sows the seeds.
- Changes in reproduction: Shift from cross-pollination to self-pollination in many crops.
Loss of Crop Genetic Diversity
The loss of crop genetic diversity (also known as genetic erosion) is a major threat to global food security.
Causes of Diversity Loss
- Replacement of landraces: Traditional varieties (landraces) are being replaced by high-yielding modern varieties that are genetically uniform.
- Habitat Destruction: Expansion of urban areas and industrialization destroys the habitats of wild relatives of crops.
- Climate Change: Shifts in temperature and rainfall patterns can lead to the extinction of specialized local varieties.
- Modern Agriculture: The move towards large-scale monoculture reduces the overall number of species and varieties used in farming.
Evolution of New Crop Varieties
The evolution of new varieties is a continuous process driven by both natural selection and artificial selection by humans.
Mechanism of Variety Evolution
- Selection: Choosing plants with desirable traits (e.g., higher yield) for the next generation.
- Hybridization: Crossing two genetically different parents to combine their best traits.
- Mutation Breeding: Using chemicals or radiation to induce genetic changes that might provide new beneficial traits.
- Polyploidy: Increasing the number of chromosome sets, which often results in larger plant organs (e.g., seedless watermelon).
Importance and Conservation of Germplasm
Germplasm: The sum total of all the genes in a crop species and its related wild species.
Importance of Germplasm
Germplasm is the "raw material" for plant breeding. It is essential for developing varieties that can withstand new pests, diseases, and changing environmental conditions.
Methods of Conservation
Classification of Plant Resources
Plant resources are classified based on their primary uses to facilitate their study and management in Economic Botany.
- Food Plants: Cereals (Rice, Wheat), Legumes (Pulses), Vegetables, and Fruits.
- Industrial Resources: Fibers (Cotton, Jute), Rubber, Tannins, and Dyes.
- Medicinal Resources: Plants yielding drugs like Quinine (Cinchona) or Morphine (Poppy).
- Beverages: Tea, Coffee, and Cocoa.
- Spices and Condiments: Black pepper, Cardamom, Turmeric.
- Oil-Yielding Plants: Edible (Mustard, Groundnut) and Non-edible (Castor, Jatropha).
Exam Tips
- Vavilov's Centers: Memorize at least one major crop for each of the 8 centers; this is a frequent 5-mark question.
- Domestication vs. Evolution: Clearly distinguish that domestication involves human intervention, whereas evolution can be natural.
- Germplasm: Always mention the role of "Wild Relatives" when discussing the importance of germplasm.
Common Mistakes
- Confusing Primary Center of Origin with Secondary Center. A secondary center is where a crop shows high diversity but did not originally evolve.
- Ignoring the Wild Ancestors. Exams often ask for the wild relative of crops like Wheat (Triticum species) or Maize (Teosinte).
Frequently Asked Questions (FAQ)
Q: What is the 'Domestication Syndrome'?
A: It refers to a suite of traits that distinguish domesticated crops from their wild ancestors, such as non-shattering seeds and loss of defensive thorns or toxins.
Q: Why is ex-situ conservation preferred for crop seeds?
A: Because it allows for the storage of a vast amount of genetic diversity in a small space (Seed Banks) and protects the seeds from environmental disasters in the wild.