The art of Plant Breeding is perhaps older than the science of Genetics. Ever since men took to agriculture they came to find that better seed means better harvest. There was, therefore, a natural tendency to select the best ear of grains or the best fruit for seed. With the improvement of communication facilities, the importance of introduction of good and novel varieties from foreign countries was realised and finally, some pioneer plant breeders like Burbank and Michurin showed what miracles hybridisation might perform in the direction of getting new and better varieties.
It is an experience of farmers that if only a part of the harvest is used as seed year after year, the quality and the quantity of the harvest deteriorates. Production of seed is a technique more specialised than ordinary agriculture and that is why seed farms with capitals running into crores of rupees have sprung up in progressive countries like the U.S.A. Here, trained plant breeders supervise the production of better seed.
Moreover, it is often found that the local varieties grown are poor in certain qualities. For example, it may be susceptible to some disease, the grains may be too coarse or the yield may be too low. It is then the work of the plant breeder to do away with these defects. Even if the local variety is good there is always scone to make it better.
The pioneer plant breeders were often vigorous workers with no scientific training. But an automobile is always better in the hand of a driver when the driver is also a mechanic and knows the ins and outs of the machine in his charge. Similarly, a modern plant breeder must have a thorough knowledge of genetics in all its aspects. Moreover, he must be sufficiently conversant in the statistical methods of designing and laying out his experiments as well as those of interpreting his data (field plot technique and biometry).
He should be well aware of special techniques adapted to meet special ends. With this training he must tackle the particular problem. For this, he must know all about the characteristics of the crop (Crop Botany) he wants to improve and then must know what exactly are the improvements needed by the growers and the country. The improved seed is now to be distributed. Here comes the need of seed farms like that at the Panth Agricultural Institute.
The geneticist must have a clear idea as to what is a Pure Line. A pure line is a strain of plants which is homozygous and will not segregate any further. Ordinarily, cultivated crops become contaminated by natural crossing, etc. So, a plant breeder has to segregate pure lines. But, it should be borne in mind that the term pure line is only relative. With innumerable insignificant genes, it is impossible to get perfect homozygosity. But a pure line is one which is homozygous for the characters usually studied in economic plants.
In studying the characters of crops the plant breeder has to study the modes of propagation of plants.
Cultivated plants fall under the following groups according to the mode or propagation:
A. The Asexual Group:
Many horticultural crops and some agricultural crops are propagated vegetatively. Most fruit trees and many flower plants are propagated by grafting’s, cuttings, layering’s, etc. Sugarcane is cultivated by nodes of stems (sets) and potato by seed tubers. A group of plants propagated from a single original stock is called a clone.
Ordinarily, there can be no change in the genotypic constitution of plants propagated through clones and there is no segregation. Heterozygotes, may be propagated in this way. It may, however, be necessary to get the seed of these plants for experimental purposes like obtaining new hybrids, etc.
B. The Sexual Group:
Most agricultural crops and some horticultural crops are propagated sexually.
(a) Naturally self-pollinated, group:
Some plants are found to be naturally self-pollinated. Cross-pollination is rare (less than 4%) even if no precaution is taken to stop it. Under this group come rice, wheat, barley, oats, potato, peas, beans, tobacco, flax, tomato, etc. Most jute varieties, specially Corchorus capsularis, also come under this group.
(b) Often cross-pollinated group:
Though self-pollination is the rule, cross-pollination takes place very often unless one is very careful about it. Cotton and sorghum come under this group.
(c) Naturally cross-pollinated group:
Cross-pollination is the rule among these plants so that strict measures are to be taken if selfing is desired. Some plants in this group have unisexual flowers which simplify the technique to be followed in hybridisation. In this group are maize, rye, many grasses, cucurbits, Brassica species, sugar beets, clover, many fruits and root vegetables.
(d) Dioecious plants:
The male and female plants being separate, there can be nothing but cross-pollination. Here come hemp, dates and palms, spinach, etc. Naturally, under this group breeding must be undertaken both for male and female lines.
The most important aim of field experimentation technique is the avoidance of soil heterogeneity. However careful one may be a plot of land is never uniform in the fertility of the soil. As a result, plants at one end may vary from plants at another end because of the difference in soil and not because of any genetic cause. Statistical methods are employed to minimise this serious error and also other errors due to competition, etc.