The role of the cultivar and the fundamentals of the breeding process in modern crop production

A cultivar is a collection of plants of one crop that are similar in their economic and biological properties and morphological characteristics, related by origin, and selected and propagated for cultivation under specific natural and production conditions with the aim of increasing yield and product quality.

Existing natural cultivars and plant forms are used as initial material, for example, wild-growing forms, local cultivars of cultivated plants, cultivars of foreign breeding, and also forms

Theoretical foundations 52 of plant productivity and crop formation of plants created during the breeding process itself. The special value of time-tested local cultivars as initial material lies in their adaptation to growth conditions. However, among the local cultivars of agricultural crops, there may be a lack of forms with high technological and taste qualities of products, resistance to pests and diseases, etc.

The exceptional value of wild-growing forms consists in their resistance to adverse conditions — drought, frost, soil salinity, pests, diseases, and others. It is very important to have not just high-yield cultivars, but cultivars with wide ecological plasticity, resistant to adverse weather conditions and particularly dangerous diseases and pests. New cultivars must ensure the production of high-quality products and contain the highest amount of those substances (protein, fat, starch, vitamins, fiber, etc.) for the sake of which a particular crop is cultivated. Furthermore, they must be well adapted to mechanized cultivation. Such cultivars already exist, and breeders are continuing the breeding process in these most important directions.

The role of a cultivar in increasing yield and improving the quality of agricultural products is universally recognized. A cultivar is the biological foundation of the harvest. According to recent estimates, the contribution of breeding to yield increases in world agriculture reaches 70%, and this will continue to grow, which is associated both with the general tendency towards the biologization and ecologization of modern agriculture and with the increasing possibilities of breeding.

An important place in the breeding process is occupied by the hybridization method. In plant breeding, intraspecific hybridization is most common, in which the crossing individuals belong to the same species. The technique of crossing various agricultural plants is not uniform. It depends on the structure of the flowers (bisexual or dioecious), the method of pollination and the nature of flowering (self-pollination or cross-pollination, wind pollination or insect pollination, closed or open flowering), the duration of flowering, the duration of viability of pollen and the stigma of the pistil, and several other reasons. For example, to obtain maize hybrids, the plants intended for hybridization are sown in alternating rows (2 rows of the paternal form are sown after every 4 rows of the maternal form) and the tassels on the maternal plants are removed by hand a few days before flowering. Subsequently, a less labor-intensive chemical castration method appeared, based on the CMS effect — cytoplasmic male sterility, created as a result of the selective damage of male gametes by a toxin of the phytopathogenic fungus Helminthosporium maydis.

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With artificial forced hybridization of two lines or cultivars, heterosis is observed — the phenomenon of "hybrid vigor," or an increase in viability, growth power, and productivity of first-generation F1 hybrids compared to the parental forms. In subsequent generations (F2, F3), this effect fades. Only specific pairs of parental forms produce heterotic hybrids when crossed. The effect of heterosis is especially frequent when crossing pure lines, one of which is homozygous for dominant genes and the other for recessive genes. Such lines are created to produce heterotic hybrids, which, unlike cultivars, must be reproduced annually.

For practical use in production, methods for obtaining hybrid seeds of the following crops have been developed and are widely used at present:

• maize;
• rye;
• cucumber;
• tomato;
• onion;
• sugar beet;
• some other crops.

A special position is occupied by the group of vegetatively propagated plants, in which it is possible to fix heterosis in the offspring (some potato cultivars are obtained from hybrid seeds).

For cross-pollinated plants, an important new source of initial material is self-pollinated pure lines, or inbred lines. They are obtained through repeated forced self-pollination. Various hybrid populations (intraspecific, interspecific, and even intergeneric) can also be used as initial material in the selection process.

Interesting results are obtained using distant hybridization. It often allows for the production of plant forms with valuable qualities that are resistant to diseases and pests. Many modern cultivars of sunflower, wheat, and sugar beet have been obtained through interspecific hybridization.

Intergeneric hybrids are also known:

• wheat-rye (triticale);
• rye-wheat (secalotriticum);
• wheat-wheatgrass, etc.

One of the significant obstacles to distant hybridization is incompatibility or difficulties in crossing distant species. Sterility of the resulting hybrids is also frequently observed. To overcome the incompatibility of distant forms, methods such as the bridge species technique, pollination with a mixture of pollen, and the application of physiologically active substances are used. Furthermore, polyploidy is widely practiced to overcome incompatibility and the sterility of the resulting hybrids.

Polyploidy is a special type of genomic mutation in which there is a multiple increase in the number of chromosomes in plant cells compared to the normal haploid set. This mechanism makes it possible to create organisms with increased vitality. It is important for an agronomist to consider that polyploid forms of crops have a wider reaction norm.

Polyploid plants adapt more easily to unfavorable environmental conditions and are distinguished by high vitality and ecological plasticity. Breeders obtain polyploids not only from natural sources but also artificially by treating plants with special chemical compounds. Such exposure directly stimulates plant productivity and accelerates the formation of the harvest.

Based on polyploidy, scientists have developed numerous cultivars and hybrids that provide a significant increase in the harvest. They are distinguished by improved adaptability to stress factors. The following crops are of practical interest:

• wheat;
• maize;
• beet;
• buckwheat;
• potato;
• fruit crops.

Breeding centers, experimental stations, and specialized institutes are engaged in the creation of promising cultivars and hybrids of wheat, rye, triticale, and other crops. Their specialists conduct continuous breeding work to improve genetic material. New developments undergo rigorous selection before reaching farm fields.

How new cultivars undergo testing and why they require high-level agricultural practices

Each new cultivar must undergo verification at special cultivar testing sites before being introduced into production. Experts evaluate the economic value of the plants and their degree of suitability for specific climatic zones. Based on the results of these tests, the most promising cultivars and hybrids are included in the state register of approved achievements.

The main theoretical basis of the breeding process remains genetics — the science that studies the patterns of heredity and variation. Modern methods of molecular genetics, genetic engineering, and biotechnology allow for the purposeful design of new gene combinations. To achieve this, specialists can use and transfer useful genomes of completely different organisms.

Transgenic potato cultivars with targeted protection against threats are already actively used in global agriculture. Either the gene of a microorganism responsible for producing a toxin against the Colorado potato beetle or the gene for the antiviral protein interferon is inserted into its genome. Similar technologies are used to create high-yielding transgenic cultivars of maize, soybean, and other crops.