The mode of pollination and reproduction play an important role in plant breeding. Based on this, crop plants are divided into three groups viz.

1. Self Pollinated 

2. Cross pollinated 

3. Vegetatively propagated

Self Pollinated Species: 

These are all self fertilizing species. In these species development of seed take place by self pollination (autogamy). Hence self pollinated species are also known as autogamous species or inbreeders.Various plant characters such as homogamy, cleistogamy, chasmogamy, bisexuality etc. favour self fertilization.

Important features of autogamous species

1. They have regular self pollination

2. They are homozygous & have advantage of homozygosity, means they are true breeding.

3. Inbreeders do not have recessive deleterious genes, because, deleterious genes are eliminated due to inbreeding by way of gene fixation.

4. Inbreeding does not have any adverse effects on inbreeders.

5. In autogamous species, new gene combinations are not possible due to regular self pollination.

6. Inbreeders are composed of several component (homozygous) lines. Hence variability is mostly among component lines.

7. Inbreeders have generally narrow adaptation and are less flexible.

Methods of Breeding in Autogamous Species

1. Plant introduction 

2. Pureline selection 

3. Mass selection 

4. Pedigree method 

5. Bulk method

6. Single seed descent method 

7. Backcross method 

8. Heterosis breeding 

9. Mutation breeding

10. Polyploidy breeding 

11. Distant hybridization 

12. Transgenic breeding

Cross pollinated species:

 This group refers to cross fertilizing species these species produce seed by cross pollination (allogamy) hence, referred to as allogamous species or out breeders. Various plant characters which promote cross pollination which include dichogamy, monoecy, dioecy, heterostylely, herkogamy, self incompatibility and male sterility.

Important features of out breeders

1. They have random mating. In such population, each genotype has equal chance of mating with all other genotypes

2. Individuals are heterozygous and have advantage of heterozygosity

3. Individuals have deleterious recessive gene which are concealed by masking effect of dominant genes.

4. Out breeders are intolerant to inbreeding. They exhibit high degree of inbreeding depression on selfing.

5. Cross pollination permits new gene combinations from different sources.

6. In these species, variability is distributed over entire population.

7. Wide adaptability and more flexibility to environmental changes due to heterozygosity and heterogenety.

Methods of Breeding Allogamous species

1. Plant introduction 

2. Mass and progeny selection 

3. Back cross method.

 4. Heterosis breeding

5. Synthetic breeding

 6. Composite breeding 

7. Polyploidy breeding 

8. Distant hybridization

9. Transgenic breeding 

10. Mutation breeding (rarely)

Three breeding approaches viz., recurrent selection, disruptive mating and biparental mating are used for population improvement.

Asexually propagated species: 

Some crop plants propagate by asexual means i.e. by stem or root cuttings or by other means. Such species are known as asexually propagated species or vegetatively propagated species. Such species are found in both self and cross pollinated groups. Generally asexually propagated species are highly heterozygous and have broad genetic base, wide adaptability and more flexibility.

Methods of breeding Asexually propagated species

1. Plant introduction 

2. Clonal selection 

3. Mass selection (rarely used) 

4. Heterosis breeding

5. Mutation breeding 

6. Polyploidy breeding 

7. Distant hybridization 

8. Transgenic breeding

Breeding Populations: 

The genetic constitution of plants is determined by mode of pollination. Self pollination leads to homozysity and cross pollination results in heterozygosity

Based on genetic constitution, breeding populations are of four types viz.,

1. Homogenous 

2. Heterogenous 

3. Homozygous 

4. Heterozygous

1.Homogenous population: 

     Genetically similar plants constitute homogenous populations. Ex. pure lines, inbred lines, F1 hybrid between two pure line or inbred lines and progeny of a clone. Pure lines and inbred lines generally have narrow adoption.

2.Heterogenous populations: 

    Genetically dissimilar plants constitute heterogenous populations. Ex. land races, mass selected populations, composites, synthetics and multilines. Heterogenous populations have wide adaptability and stable performance under different environments.

3.Homozygous populations: 

    Individuals with like (same) alleles at the corresponding loci are know as homozygous. Such individuals do not segregate on selfing. Thus non-segregating genotypes constitute homozygous populations. Ex. pure lines, inbred lines and mass selected populations in self pollinated plants. Thus pure lines and inbred lines are homozygous and homogeneous and mass selected varieties of self pollinated crops and multi lines are homozygous and heterogenous, because they are mixtures of several pure lines.

4. Heterozygous populations: 

    Individuals with unlike (different) alleles at the corresponding loci are referred to as heterozygous. Such individuals segregate into various types on selfing. Ex. F 1 hybrids, composites and synthetics. Thus F 1 hybrids are heterozygous but homogeneous and composites and synthetics are heterozygous and heterogenous population.