Plant breeding: objectives and methods
Plant breeding improves crops for higher yield, quality and disease resistance. Its methods are introduction, selection, hybridisation, mutation breeding and polyploidy.
Objectives of plant breeding
- Higher yield: more grain, fruit or fibre from the same land.
- Better quality: more protein, oil or vitamins; better taste, size and keeping quality.
- Resistance to diseases and pests.
- Tolerance to drought, salt, heat and cold.
- Early maturity, so that more crops can be grown in a year.
- Short, strong stems that do not fall over in wind and rain.
Methods of plant breeding
| Method | What is done | Example |
|---|---|---|
| Introduction | Plants are brought from another region or country. In primary introduction they are used directly. In secondary introduction they are first selected or crossed. | Potato, tomato, chilli and groundnut came to India from the Americas; Mexican dwarf wheat in the 1960s |
| Selection | The best plants are chosen from a mixed crop and their seeds are grown. Types: mass, pure line and clonal selection. | TMV-2 groundnut (by mass selection) |
| Hybridisation | Two genetically different plants are crossed to bring their good traits into one plant, the hybrid. | Triticale (wheat × rye); hybrid rice and maize |
| Mutation breeding | Mutagens (agents that cause mutations) create new traits. The useful mutants are selected. | Sharbati Sonora wheat (from Sonora 64 by gamma rays); Atomita-2 rice (tolerates salty soil) |
| Polyploidy breeding | The number of chromosome sets is raised above two using colchicine. Plants become larger, and triploids are often seedless. | Seedless watermelon and banana (triploid); Triticale |
Three kinds of selection
- Mass selection: seeds of many good-looking plants are collected, mixed and sown together. It is simple, but the crop stays mixed.
- Pure line selection: the offspring of one single self-pollinated plant are raised. All the plants are genetically alike, forming a pure line.
- Clonal selection: used for crops grown from stems, tubers or suckers, such as banana, potato and sugarcane. The best plant is multiplied into identical copies (a clone).
Steps in hybridisation
Crosses can be between two varieties, two species or even two genera. Radish × cabbage gave Raphanobrassica.
- 1Choose the parents with the wanted traits, for example one high-yielding plant and one disease-resistant plant.
- 2★Emasculation: remove the anthers from the flower of the female parent before they shed pollen. This stops self-pollination.
- 3Bagging: cover the emasculated flower with a paper bag to keep out unwanted pollen.
- 4Crossing: dust pollen from the male parent onto the stigma, then bag the flower again.
- 5Harvest the hybrid seeds and grow the first generation (F₁).
- 6Select the best hybrids, test them in fields and release a new variety.
★ key step
Mutation breeding and polyploidy
A mutation is a sudden change in a gene or a chromosome. Breeders use mutagens to cause many mutations at once. Physical mutagens include X-rays, gamma rays and UV rays. Chemical mutagens include mustard gas and nitrous acid. Most mutations are harmful, so breeders keep only the few useful ones. Polyploidy means having more than two sets of chromosomes. Colchicine, a chemical from the autumn crocus plant, stops chromosomes from separating during cell division. So their number doubles. Polyploid plants are often larger. Triploid plants (three sets) are usually seedless, like seedless watermelon and banana.
The seedless watermelon and banana you buy are triploid plants. Most of the rice and wheat we eat today came from hybridisation and selection.
2 marks: “What is emasculation? Why is it done?” Removing the anthers from the female parent's flower before they shed pollen. It prevents self-pollination during hybridisation.
2 marks: “What are mutagens? Give examples.” Agents that cause mutations: physical (X-rays, gamma rays, UV rays) and chemical (mustard gas, nitrous acid).
7 marks: “Describe the methods of plant breeding with examples.” Introduction (primary, secondary); selection (mass, pure line, clonal); hybridisation with its steps; mutation breeding (Sharbati Sonora); polyploidy with colchicine (seedless watermelon).
I See Healthy Mango Plants: Introduction, Selection, Hybridisation, Mutation, Polyploidy.