Plant Breeding: Essentials and Potential

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Plant breeding rises as an appealing option to set the supply-demand imbalance right (Source: Pexels)

As the year 2018 comes to an end, the population of Pakistan is expected to cross the figure of 202,000,00 and to meet a figure of 244,248,371 in the year 2030. The demand for food increases with the population however the supply through production remains pretty stagnant. According to the 2018 report of World Food Program, an estimate of 60% of Pakistan’s population faces food insecurity.

The problem is not just about the production technology but the area as well. A country can increase its population but the area can not be increased much. The total cultivated land of Pakistan is only 21.2 million hectares as opposed to 30.7 million hectares of Nigeria which has a population rank just under Pakistan. The total area for the production of the major crops of Pakistan in year 2013-14 was 17,135.2 hectares in total. The area can be increased with time but the wiser solution seems to call for an effort to increase the production per hectare.

Cereal yield (kg per hectare of harvested land) of different countries in 2016 (Source: https://data.worldbank.org) 
Agricultural land in different countries expressed as a percentage of total land area in 2015 (Source:  https://data.worldbank.org)

plant breeding is the science of changing the traits of plants in order to produce desired characteristics

The science of plant breeding emphasizes upon solving this problem. It is the science of changing the traits of plants in order to produce desired characteristics. It not only improves the quality of the production but its yield as well. Its objectives are to improve yield, quality, disease-resistance, drought, frost-tolerance, and other important characteristics of the crops.

A breeder employs various methods to increase the yield and improve its quality. However, the general practice can be divided into 4 steps.

1. The Collection of Variability

The difference among the individuals of the same population or species is called variation. Genetic variance is the difference between the DNA sequences of the individuals of a population. It is of utmost importance to a breeder as it is heritable and is inherited from one generation to the next. The collection of different varieties is performed randomly and a germplasm collection is formed. Only then it is possible to exploit the gene when the breeder has access to a wide range of diverse alleles of a gene.

2. Evaluation and Selection of Parents

It is the oldest breeding method and one of the most important of all the steps.

In the Pre-Mendelian era the selection was entirely based upon phenotype. The problem with this type of selection is that most of the phenotypes are heterozygous. Heterozygous phenotype does not provide the same result in the next generation. The phenotypic characters may arise from the environmental conditions and are not hereditary.

The parents are tested for the desirable characters. The chosen parents are then crossed to be used in the process of hybridization.

The selection process is never the same in self-pollinated and cross-pollinated crops. When a crop is repeatedly self-pollinated, eventually it becomes homozygous. It becomes easier to locate and isolate the best homozygous plant in the crop. The progeny of this selected homozygous plant is called pure line and is given a name. The seed of this plant can be used year after year. It does not undergo any change unless the environmental factors play a major role.

The selection of cross-pollinated crops is a continuous process as there are a lot of genes present in the population. Some varieties have a superior genotype while the others have inferior. The ones with a superior genotype are selected and the heterozygosity is reduced.

The selection process is divided into two parts:

a) Natural Selection

This type of selection occurs in nature and follows Darwin’s theory of ‘survival of the fittest’.

This process is ongoing since the dawn of man and our cultivated crops have originated through it.

b) Artificial Selection

The breeder selects specific plants from a bulked mix. The objective is to choose the best individuals from a random population.

3. Hybridization 

The selection process only isolates the best genes from the population; it does not make those genes superior. A breeder carries out hybridization for making the genes better and combining the best genotypic characters in a single individual plant.

Heterosis plays an important role in this step. It is defined as the superiority of the hybrid over its parents in one or more traits: in yield, pest resistance, etc. Heterosis is used commercially for crops as well as animals. To maintain heterosis, it is necessary to produce seeds by crossing the pure lines every year.

Hybridization is performed through hands and involves the following steps:

Selection and Isolation of parents

Two parents with desirable traits are selected. Both are grown separately so as to prevent cross-pollination. Parents undergo self-pollination and produce inbreds. The process goes through 6-8 generations until the plants become homozygous then these generations are further used for hybridization.

Emasculation

The anther of the plant is removed to refrain it from self-pollinating. It is done through hands, hot water, alcohol, or suction pressure method.

Bagging

The flowers are covered with polythene bags to make them safe from any unwanted pollen grain. The plants self-pollinate and produce pure progeny.

Collection of the pollen grains

Pollen grains from the other non-emasculated selected parent are collected.

Crossing

When the stigma of the emasculated plant matures, the bag is removed and fresh pollen is applied.

Tagging

A tag is attached to the plant with information such as date and details of the plants.

4. Testing and Evaluation and Releasing of the Superior Hybrids 

The breeder tests and evaluates the newly-formed variety. The yield, resistance, and quality is tested before the release. Breeder puts the varieties under scientific evaluation to bring out the best traits in the progeny. He aims to bring as much characteristics in a single individual as possible. These plants undergo self-pollination for many generations so they form homozygosity. This guarantees that no character shall segregate in the future progenies.

The best of the varieties are evaluated by government agencies in stressful conditions. The pure lines which pass the vigorous tests are approved by the government and released for the commercial use. The farmer gets a seed that gives better yield, disease and pest resistance, and quality.

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