A) Sclf-pollinatcd Plants (Ricc, Wheat, Sorghum etc.)

In self-polliinated plants, introductions are important in setting a particular line with desired characters.

There arc two modes of selection for the sclf-pollinatcd plants. These arc Mass selection and Pure-line Selection.

Mass sclcction is the simplest and oldest method of crop improvement. In this process a group of plants, which are having desired characters, are selected from the field and their seeds are mixed. In the next year, the mixture of seeds is sown on mass and new crop is raised again and the same procedure is followed.

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The process continues for 8-9 years. The major drawback here is that it is not possible to know whelther the plants being grouped are homozygous or hetrozygous. Pure line selection is defined as a group of descendents selected among progeny of single genetically pure and self fertilized individuals. It is time consuming laborious process that demands much altention.

iii) Hybridisation:

It involves biparental crosses. The segregating population is subjected to selection.

B) Cross-pollinating Plants (Maize, Bajra ,Onion, Cabbage etc)

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The cross-pollinating plants are highly heterozygous and heterogenous. Inbreeding of these species disturbs the natural balance leading to inbreeding depression.

i) Selection: Various modes of selection are followed

Inbred line selection: Here selection is followed by selfing that may lead to segregation and cause progeny to deviate from its parents. Thus the vigour may be affected.

Mass selection; It is the most used method in the cross-pollinated plants. Here the selection is based om purification of unimproved or mixed varieties. Recurrent selection : It is defines as reselection generation after generation with interbreeding of selects to give a proper chance for genetic recombination.

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ii) Hybridisation:

It may be single cross (P X Q) double cross (P X Q) X (R X S) Or Top cross (Inbred X Variety)

C) Vegetatively Propagating Plants (Sugarcane, Potato, Roses etc.)

Since these plants reproduce asexually, the genotype remains same, although some superior phenotypes are selected in clonal selection. When an improvement in the genotype is sought sexual reproduction is resorted to. In fact, there are only very few cultivated plants that are not able to reproduce sexually.

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Suitable plants are selected from among the clones and are crossed to each other. This brings about new recombination of genes in the F. generation which is raised from a poltential stock plant for a new clone. Superior ones are selected and tested for desirable traits.

ii) Mutation is the only means of creating variability among the vcgctativcly propagating plants. There arc accassional somatic mutations in cells that may form a bud that develops. Such buds on isolation and multiplication would give a new- strain or variety.

Another method is to treat the cultured cells with mutagens,isolate individual cells and induce them to grow into plants with possible superior traits.

Intra-spccifie, Inter specific and Inter generic Crosses are often tried by breeders. Here hybrid vigour is observed in many characteristics such as height, size, productiveness, hardiness, viability in different plants. But the major difficulty experienced in crosses between two true species or genera is the dislocation of the gentic equilibrium .

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These are a number of modern methods to over come difficulties encountered and hence the results are spectacular.

There are thousands of examples of crop improvement. But of historical importance is that of sugarcane.

The very first interspecific cross in India was reported by Barber in 1914 when he crossed Saccharum officinarum and Sacharum spontaneum. The present day commerical cane are the results of complex interspecific hybrids between S.officinarum on one hand and S. spontaneum,S. barberi on the other.