Plant breeding: history, objectives and methods

Plant breeding: aims and methods

Plant breeding is the art and science of improving the genetic make-up of crops. It began with domestication, when early farmers chose seeds of the best plants. In India, the Green Revolution of the mid-1960s used semi-dwarf, high-yielding wheat (developed with Norman Borlaug's work in Mexico) and rice varieties; M. S. Swaminathan led the effort in India.

Objectives: higher yield; better quality (protein, oil, taste); resistance to diseases and insects; tolerance to drought, heat, salinity; early maturity; non-lodging dwarf plants; suitability for machines.

Methods:

• Introduction: bringing varieties from another region or country. Primary introduction: used directly; secondary: used as a parent.
• Selection: mass selection (seeds of many good-looking plants bulked; used in both self- and cross-pollinated crops), pure-line selection (progeny of a single self-pollinated plant; Johannsen's pure-line theory), and recurrent selection (cross-pollinated crops).
• Hybridisation: crossing genetically different parents and selecting in later generations by the pedigree, bulk or backcross method. The backcross method adds one gene (e.g. disease resistance) to a good variety. Heterosis (hybrid vigour) is the superiority of F₁ over its parents; exploited in hybrid maize, pearl millet, sorghum and rice.
• Mutation breeding: inducing sudden heritable changes using physical mutagens (X-rays, gamma rays) or chemical mutagens (e.g. ethyl methane sulphonate, EMS).
• Polyploidy: having more than two sets of chromosomes, often induced with colchicine. Autopolyploids have extra sets of the same genome (e.g. triploid seedless watermelon); allopolyploids combine genomes of different species (bread wheat; triticale, the first man-made cereal, from wheat × rye).
• Tissue and cell culture: based on totipotency (the ability of a single cell to grow into a whole plant). Uses: micropropagation, meristem culture for virus-free plants, anther culture for haploids, embryo rescue in wide crosses, and somaclonal variation.

Self-pollinated crops (wheat, rice, gram) suit pure-line and pedigree methods; cross-pollinated crops (maize, pearl millet) suit mass/recurrent selection and hybrid breeding.

Key points

• Heterosis = F₁ superiority; used in hybrid seed.
• Colchicine doubles chromosome number.
• Triticale = wheat × rye.
• Meristem culture → virus-free plants.

Exam tips

Match each method to crop type and example. Know the purpose of the backcross method.

Common mistakes

• Using pure-line selection for cross-pollinated crops.
• Calling heterosis a mutation.