What is plant breeding and why do we need it?
Plant breeding means changing the genetic makeup of plants on purpose to get more useful varieties (also called cultivars).
Tasks of modern breeding:
- Higher yield per hectare.
- Better quality: more protein, vitamins (biofortification), taste, shelf life.
- Resistance to diseases (rust, blight) and pests.
- Tolerance of drought, salt, heat, cold, floods: important as the climate changes.
- Short duration, suitable for machines.
A new improved seed is tested in many fields for several years, then certified and multiplied before farmers get it.
Heredity and artificial selection
Traits pass from parents to offspring through genes (heredity). Plants differ a little: this variation is the raw material of breeding.
Artificial selection: people pick the plants with the traits they want and sow only their seeds. Over many generations the crop changes. This is how wild grasses became wheat, rice and maize, and how one wild cabbage gave cabbage, cauliflower and broccoli.
- Mass selection: seeds from many good plants are mixed and sown.
- Pure-line selection: offspring of a single self-pollinated plant are grown and tested.
- Clonal selection: best plants of crops grown from cuttings or tubers (potato, sugarcane).
Hybridisation, inbreeding and outbreeding
Hybridisation: crossing two genetically different plants. Steps: choose parents → remove stamens of the female flower (emasculation) → cover it with a bag → dust pollen from the male parent → bag again → collect seeds → select and test offspring.
Inbreeding: self-pollinating or crossing close relatives for many generations. It makes uniform pure lines, but in cross-pollinated crops like maize it causes inbreeding depression (weaker, smaller plants).
Outbreeding: crossing unrelated plants of the same species (intervarietal) or even different species.
Interspecific (distant) hybridisation: crossing two species. Example: wheat × rye = triticale. The hybrid is often sterile because the chromosomes cannot pair; doubling the chromosomes with colchicine restores fertility.
Heterosis (hybrid vigour)
When two different pure lines are crossed, the F1 hybrid is often taller, faster-growing and higher-yielding than both parents. This is heterosis or hybrid vigour.
It is strongest in F1 and gets weaker in F2 and later, because genes separate again. So farmers buy new F1 seed every season. Hybrid maize, sorghum, millet, sunflower, tomato and rice use heterosis.
Modern breeding methods and Vavilov's ideas
- Mutation breeding: radiation (gamma rays) or chemicals cause new mutations; useful ones are selected.
- Polyploidy: extra sets of chromosomes (3n, 4n, 6n). Gives larger fruits, seedless types (triploid watermelon, banana). Modern bread wheat is hexaploid (6n).
- Tissue culture (micropropagation): thousands of identical, virus-free plants from tiny pieces on a nutrient jelly.
- Marker-assisted selection: DNA tests find seedlings carrying a wanted gene early.
- Genetic engineering and gene editing: add or change specific genes (Bt cotton, Golden Rice).
Vavilov
Nikolai Vavilov collected crops worldwide and found that each crop has a centre of origin: a region where its wild relatives and greatest variety are found (for example, South Asia for rice and sugarcane, the Andes for potato, Central America for maize). Breeders search these regions for new genes. His law of homologous series: related species and genera show similar series of variation, so if a trait exists in one, it can be expected in its relatives.
Try it
Soak 20 moong or chana seeds and sprout them on wet cotton. After 5 days, keep the 5 tallest. Grow them in a pot, collect their seeds, and repeat. Note the average height each round. Then use the 3D slider to see what happens after 8 generations.
Key formulas and definitions
- Plant breeding: purposeful change of plant genes for better varieties
- Artificial selection: humans choose the parents
- Hybridisation: crossing two genetically different parents
- Inbreeding → pure lines (risk: inbreeding depression)
- Outbreeding: crossing unrelated plants
- Heterosis: F1 hybrid better than both parents
- Polyploidy: extra chromosome sets (3n, 4n, 6n)
- Triticale = wheat × rye (interspecific)
Worked examples
1. A farmer keeps seeds only from wheat plants that did not get rust disease, for 6 years. What method is this and what will happen?
Artificial (mass) selection. Over the years the share of rust-resistant plants rises because only resistant plants pass on their genes.
2. Parent A is tall with small grain; parent B is short with large grain. How would a breeder get a short plant with large grain and many tillers?
Cross A × B (hybridisation): emasculate, bag, pollinate, collect seeds. Grow the offspring, select plants with the wanted mix of traits, self them for several generations to make a pure line, then test in many fields.
3. Inbred maize lines give 4 t/ha and 5 t/ha. Their F1 hybrid gives 9 t/ha. What is this called, and why must the farmer buy new seed next year?
Heterosis (hybrid vigour): the F1 beats both parents. In F2 the genes segregate again, so plants vary and yield drops; fresh F1 seed is needed each season.
Common mistakes
- Thinking breeding creates genes from nothing. Classical breeding reshuffles and selects existing variation (mutation and engineering add new variation).
- Saving seeds from F1 hybrids and expecting the same yield.
- Confusing inbreeding (relatives, pure lines) with outbreeding (unrelated plants).
- Thinking all hybrids between species are fertile. Many are sterile until the chromosomes are doubled.