Plant Breeding — Steps

NCERT grounding

The old NCERT Class 12 chapter Strategies for Enhancement in Food Production opens its plant-breeding section with a direct statement of scope: plant breeding is the purposeful manipulation of plant species in order to create desired plant types that are better suited for cultivation, give better yields and are disease resistant. Although this chapter was rationalised out of the current syllabus, it remains a recurring NEET source, and the five-step breeding sequence is the single most quoted passage from it. The NCERT lists the steps in a fixed order and stresses that breeding programmes are carried out systematically in government institutions and commercial companies.

Conventional plant breeding is ancient — recorded evidence dates back 9,000 to 11,000 years — but classical breeding as a technology means something precise: crossing or hybridisation of pure lines, followed by artificial selection, to produce plants with higher yield, better nutrition and disease resistance. Increased crop yield, improved quality, tolerance to environmental stresses, resistance to pathogens and tolerance to insect pests are the standard breeding objectives the NCERT names.

The five steps of plant breeding

The NCERT names exactly five sequential steps for breeding a new genetic variety of a crop. They form a pipeline: the output of each step is the input of the next, and a weakness early in the chain cannot be repaired later. A breeder cannot select for a trait that is absent from the starting collection, and cannot release a cultivar that was never made stable. Memorising the order is the fastest way to secure marks, because NEET frequently asks for the correct sequence or for the step that performs a particular function.

Step 1 — Collection of variability

Every breeding programme begins with genetic variability, which the NCERT calls the root of the entire process. Without diverse alleles to draw on, a breeder has nothing to recombine and nothing to select from. In many crops, useful pre-existing variability is locked up in the wild relatives of the cultivated species — landraces, weedy ancestors and related wild species that may carry valuable genes for disease resistance, stress tolerance or unusual quality even though they yield poorly themselves.

The deliverable of Step 1 is the germplasm collection — defined as the entire collection of plants or seeds having all the diverse alleles for all the genes in a given crop. Collection alone is not enough: the wild varieties, species and relatives of the cultivated crop must be both collected and preserved, and then evaluated for their characteristics. This evaluation is what allows the natural genes present in wild populations to be effectively exploited later. Germplasm is conserved in gene banks as seed stocks, living collections and in-situ reserves so that the alleles remain available to future breeders.

Step 2 — Evaluation and selection of parents

A germplasm collection may hold thousands of entries, but only a handful carry the exact combination of traits a breeder wants. Step 2 is the screening filter. The germplasm is evaluated to identify plants with a desirable combination of characters — for example, one accession with high protein and another with strong rust resistance. The plants chosen become the parents for the next step.

Two further actions belong to this step. First, the selected plants are multiplied so that enough material exists to make crosses. Second, pure lines are created wherever desirable and possible. A pure line is a homozygous, true-breeding population obtained by repeated self-pollination; using pure lines as parents means the breeder knows exactly which alleles each parent will contribute, making the outcome of a cross predictable. Selection at this stage does not create new traits — it only chooses among the variability that Step 1 supplied.

Step 3 — Cross-hybridisation among the selected parents

Step 3 is where the desired characters are actually combined. Often the traits a breeder wants are scattered across two different parents — the NCERT's example is high protein quality in one parent and disease resistance in another. The only way to bring both into a single plant is to cross-hybridise the two parents, producing hybrids that genetically combine the desired characters.

Mechanically, this is a careful artificial cross. Pollen grains from the plant chosen as the male parent are collected and placed on the stigma of flowers selected as the female parent. To prevent unwanted self-pollination, the female flowers are emasculated and bagged before they are dusted with the chosen pollen — the same procedure used in artificial hybridisation in flowering plants. The NCERT calls this a very time-consuming and tedious process, and it explains why: it is not guaranteed that the hybrids will combine the desirable characters. Because genes recombine independently, usually only one in a few hundred to a thousand crosses shows the desirable combination.

Step 4 — Selection and testing of superior recombinants

A successful Step 3 produces a large hybrid progeny, but most of those plants will not carry the wanted trait combination. Step 4 is the second great selection filter. It consists of selecting, among the progeny of the hybrids, those plants that have the desired character combination. The NCERT stresses that this selection process is crucial to the success of the breeding objective and requires careful scientific evaluation of the progeny — not a casual eyeballing of the field.

When done well, this step yields plants that are superior to both of the parents, and often more than one superior progeny plant becomes available. But a freshly selected hybrid is still genetically variable: if grown on, its characters would segregate in the next generation. To stop this, the selected superior recombinants are self-pollinated for several generations until they reach a state of uniformity, that is, homozygosity. Once homozygous, the line is true-breeding — the desired characters are fixed and will not segregate in the progeny. The output of Step 4 is therefore a stable, uniform line ready for performance trials.

Step 5 — Testing, release and commercialisation of new cultivars

The final step proves the new line on the ground. The newly selected lines are evaluated for their yield and other agronomic traits such as quality and disease resistance. This evaluation happens in two stages. First, the lines are grown in the breeder's own research fields, where their performance is recorded under ideal fertiliser application, irrigation and crop-management practices — a controlled measurement of potential.

Research-field performance is encouraging but not decisive, because farmers do not cultivate under ideal conditions. So the second stage tests the material in farmers' fields, for at least three growing seasons, at several locations representing all the agroclimatic zones where the crop is normally grown. Throughout, the new line is judged not in isolation but in comparison with the best available local crop cultivar — the check or reference cultivar. Only a line that consistently beats or matches the check across seasons and zones earns release as a new cultivar and moves into commercial cultivation.

Green Revolution cultivars and outcomes

The five steps are not an abstract scheme — they delivered India's Green Revolution. The Green Revolution depended heavily on plant breeding for high-yielding, disease-resistant varieties of wheat, rice and maize. Its signature output was the semi-dwarf plant: shorter, stiffer stems that do not lodge under the weight of a heavy grain head and respond well to fertiliser. Nobel laureate Norman E. Borlaug developed semi-dwarf wheat at the International Centre for Wheat and Maize Improvement in Mexico.

The numbers show the scale of the gain. Semi-dwarf breeding transformed Indian cereal output between 1960 and 2000, turning a food-deficit nation into one able to meet its national requirements and even export grain.

Worked examples

Common confusion & NEET traps

The five-step framework is simple, but NEET sets traps around the order of the steps, the precise definition of germplasm, and which step does what. The two callouts below target the errors that cost the most marks.