When choosing planting material in the description for a particular plant variety, many summer residents find mention of various methods of pollination or self-pollination. These are concepts that we all studied at school in botany lessons. But not many already remember what they mean. Let's refresh our memory and remember the types of pollination in plants and their biological significance. And at the same time we will find out why some of our plants planted in the country or on the windowsill do not bear fruit.
Generative organ of higher plants
A flower is a modified shoot where spores and gametes are formed. Higher plants (angiosperms) have complexly arranged flowers with many adaptations to various types of pollination. Diverse in details, the flower combines the processes of both sexual and asexual reproduction. The main components of a flower are its reproductive parts - the male androecium (stamens) and the female gynoecium (pistil with ovary, style and stigma). Flowers can be bisexual (there is both a pistil and stamens) andsame-sex (there is either a pistil or stamens). The other parts of the flower are very varied and have specific functions.
Meeting of stamens and pistil
Pollination is the process of transferring pollen from the stamens to the stigma of the pistil. Without this, reproduction of plants, the formation of fruits and seeds is impossible. In the process of evolutionary development, plants have developed several ways to carry out this transfer using biotic and abiotic factors of nature. In ecology, two types of pollination are distinguished:
- The transfer of pollen from one flower to the pistil of another. This process is called cross-pollination, or xenogamy. It is carried out through biotic (insects, birds, bats) and abiotic (wind, water) factors.
- Autogamy (self-pollination). This is the transfer of pollen on the stigma from one flower. Autogamy is not as common in wild forms.
These are types of pollination that can alternate between some plants.
Self-pollination conditions
But a mandatory condition for the implementation of self-pollination is the bisexuality of the flower. Accidental self-pollination of flowers is not uncommon. But it can only occur when the pollen grain and pestle are physiologically compatible. In many plants, pollen does not germinate into the pollen tube, which is a limiting factor for cross-pollinated plants. There are quite a few factors that contribute to random autogamy. Regular self-pollination of plants (for example, peas, beans) can have a gravitational mechanism. In this case, the pollenfalls on the stigma under the action of gravity. In other cases, self-pollination occurs as contact autogamy - the stamen is in contact with the stigma of the pistil. Pollen carriers in the middle of the flower can be dew drops and small insects (thrips) that live in the flower. In some plants, the process occurs in the bud and completely eliminates the possibility of cross-pollination.
Optional self-pollination
A feature of this type of autogamy is the presence of unstable conditions that do not favor cross-pollination. This type of self-pollination is found in cereals, sundew, and feather grass. In these plants, in drought or at low temperatures, unisexual flowers are formed, and in warm and humid weather - bisexual. Cross-pollination of these plants is carried out with the help of the wind, and in conditions of difficulty in the implementation of such pollination, it is biologically advisable to resort to self-pollination.
Evolutionary value
Self-pollination in evolutionary terms has a negative meaning. In accordance with modern concepts, evolution requires free crossing, which is provided by cross-pollination. It is this that increases the diversity of alleles (the degree of expression of a gene) in populations. And self-pollination, on the contrary, leads to homozygosity (uniformity) of alleles. But under certain circumstances, self-pollination can lead to the isolation of new forms, isolation and fixation in the population of alleles that give favorable signs to the plant. Exactly atthis is the positive evolutionary significance of the alternation of autogamy and xenogamy.
Self-pollinating plants
In such plants, the transfer of pollen is more often carried out in an unopened bud (for example, in beans and peas) or in the period of an unopened leaf tube (barley). Peas, beans, barley, wheat, oats, tomatoes, eggplants and many others are considered self-pollinators from agricultural crops. Why do they count? Because self-pollination cannot be absolute, there is always the possibility of introducing pollen from other plants. Even closed buds are sometimes gnawed by insects and carry pollen from other plants! What are the characteristics of self-pollinators? These are definitely plants with bisexual flowers, large feathery stigmas and a lot of pollen. In addition, their flowers do not have bright petals, nectaries and a pleasant smell.
Self-pollination in violets
In nature, violets are cross-pollinated and autogamous. Our indoor violets are the product of the painstaking work of breeders. They have such a structure of stamens and pistil that cross-pollination without human intervention is almost impossible. Pollination occurs even in an unopened bud, and only a patient amateur, using special techniques, can pollinate violets of different colors to breed new varieties. Thanks to the enthusiasts for the variety of these flowers decorating our window sills!
Parthenocarpic cucumbers
Modern breeding offers many varietiescucumbers, both self-pollinated (parthenocarpic) and pollinated by insects. These plants are bred specifically for early cultivation in greenhouses where there are no natural pollinators. When buying seeds, you need to stop at reading the qualities of the variety, since both self-pollinated and cross-pollinated varieties have their advantages and disadvantages.
Pollination in cereals
Oats, rye, wheat, millet, barley are representatives of agricultural cereals. The flowers have 2 lemmas, 2 pellicles, three stamens and one pistil. They self-pollinate in unopened flowers. Once the flower has opened, cross-pollination is virtually impossible.
Self-pollination in fruit trees
Although most fruit varieties have flowers that contain both pistils and stamens, self-fertilization is excluded in most. The reason is the time-separated maturation of the stamens and pistil. That is why you can increase the yield, for example, cherries, by planting several trees nearby. But in artificially bred varieties, self-pollination is welcome. An example is nectarines. But do not expect to grow a productive plant from a seed. In such hybrid forms, subsequent generations experience hybrid depression - a decrease in viability and yield.
Selection and self-pollination
This phenomenon is widely used in plant breeding. We know that self-fertilization and crossing of closely related organisms leads to the transitiongenes into a homozygous state and leads to a decrease in viability and productivity, and subsequently to degeneration. The continuous process of mutations that accumulate, most of which are recessive and unfavorable, is the cause of this oppression. In plants with cross-pollination, these mutations are in a heterozygous state and do not manifest themselves in any way. With self-pollination, the probability of their transition to a homozygote increases many times over, but they do not remain in the population due to natural elimination. Self-pollination in breeding is used as a tool for creating pure (homozygous) lines with fixed traits. Despite the decrease in productivity, after hybridization, the phenomenon of heterosis often appears - the strength of hybrids from varieties with self-pollination. This phenomenon is called interline hybridization, and in stores we can see just such hybrid seeds (they are marked with the F1 symbol). In the first generation, hybrids outperform pure lines in terms of yield, but in subsequent generations, the effect of the strength of hybrids disappears.
