PLANT REPRODUCTIVE BIOLOGY - N. L. Kolyasnikova - 2017

SECTION 3. TYPES OF POLLINATION

Pollination is The process of transferring pollen to the stigma of a pistil. There are two MAIN TYPES OF pollination: self-pollination and cross-pollination. Cross-pollination is predominant in angiosperms, whereas self-pollination is less common. Continuous self-pollination is considered an evolutionary dead end that leads to degradation.

3.1 Self-Pollination

During self-pollination, pollen from a flower lands on the stigma of the same flower or another flower on the same plant. This type of pollination is characteristic only of bisexual flowers.

Various adaptations promote self-pollination, including autogamy, geitonogamy, and cleistogamy.

Autogamy can occur in several ways: through direct contact (contact autogamy), via pollen shedding and settling under its own weight (gravitational autogamy), wind-mediated pollination (wind autogamy), or through small insects (thrips autogamy). Contact autogamy is typical of chickweed wintergreen, maianthemum, and wild ginger; gravitational autogamy has been described in one-flower wintergreen; and wind autogamy occurs in blueberries.

Geitonogamy is neighborhood pollination, where pollen from one flower in an inflorescence germinates on the stigma of another flower within the same inflorescence. It often takes place toward the end of the flowering season, facilitated by wind, animals, and insects. This pollination method is found in species of the Asteraceae and Brassicaceae families. Plants utilizing this strategy typically feature small flowers clustered in dense inflorescences.

Cleistogamy is pollination that occurs within closed flowers. Both obligate and facultative cleistogamy are recognized. Cleistogamous flowers are smaller, and their floral parts are reduced: petals are rudimentary or entirely absent, the number of stamens is decreased, and pollen grains are smaller. Examples of such plants include the wonder violet and wood sorrel (Figs. 20, 21).

Class="center">Fig. 20. Water/152.html">Double Fertilization in cleistogamous flowers of wood sorrel: 1 - emptied anther of a cleistogamous flower after pollen tube germination, showing remaining pollen grain walls, pollen tubes, and degenerated pollen grains; 2 - penetration of the pollen tube between The Cell walls

Fig. 21. Pollen tubes inside the anther of a cleistogamous flower in Viola rupestris - 1; pollen tubes emerging through a rupture in the anther wall of a cleistogamous flower in Viola rupestris - 2

In addition to cleistogamous flowers, these plants also produce chasmogamous flowers with open pollination. Facultative cleistogamy is not constant and develops under specific environmental conditions, showing no signs of floral reduction. It is typically triggered by unfavorable environmental factors and is found in barley, oats, wheat, feather grass, and others.

3.2 Cross-Pollination

During cross-pollination, pollen from the flowers of one plant is transferred to the stigmas of flowers on another plant of the same species.

Adaptations for cross-pollination include dioecy, dichogamy, self-incompatibility, and daily flowering rhythms.

Dioecy is the Separation of male and female flowers onto different plants of the same species. Dioecious species include sea buckthorn, willow, poplar, aspen, sorrel, nettle, asparagus, and others. The adaptive significance of sex separation lies in ensuring cross-pollination and differentiating sexual forms across ecological niches. Phenologically, male plants bloom first, while female plants have a longer flowering duration. Staminate flowers are larger than pistillate ones, and the number of male flowers per inflorescence and per plant overall exceeds that of female flowers.

Dichogamy is the asynchronous maturation of anthers and stigmas, representing a functional separation of sexes where a flower Functions in either a male or female phase. Different species exhibit various types: a male-First stage known as protandry, or a female-first stage known as protogyny.

Protandry is quite common and widespread among dicotyledonous plants. In some cases, the staminate and pistillate phases are clearly separated in time, with stigmas becoming receptive to pollen only after all stamens in the flower have withered.

Protogyny occurs in monocots and primitive dicots (such as the Ranunculaceae family).

Self-incompatibility is the inability of pollen tubes to grow the full length of the style and effect fertilization. The genes responsible for self-incompatibility are designated as S. If the pollen and the stigma share identical alleles of the S-Gene, fertilization fails to occur. Self-incompatibility is classified into heteromorphic and homomorphic types, with heteromorphic further divided into distyly and tristyly. In homomorphic self-incompatibility, flowers are structurally identical. Homomorphic self-incompatibility is more common and operates under two Genetic control systems: sporophytic and gametophytic. Under sporophytic incompatibility, the germination capacity of pollen is determined by the genotype of the sporophyte—that is, the plant on which microspores are formed. With sporophytic control, pollen germination is inhibited early, right on the stigma surface. The stigma surface is dry and covered with a pellicle, the style is closed, and pollen grains are tricellular. This type of self-incompatibility is found in the Asteraceae, Brassicaceae, Caryophyllaceae, and other families.

Under gametophytic incompatibility, the pollen's ability to germinate is determined by its own genotype (i.e., the genotype of the gametophyte itself) (Fig. 22).

Fig. 22. Pollen tube growth and fertilization under gametophytic incompatibility

In gametophytic control, pollen tubes initially grow rapidly, after which their growth slows down and eventually stops. The stigmas are wet, and the style is open. Pollen grains are binucleate. Gametophytic self-incompatibility has been reported in plants of families such as Fabaceae, Liliaceae, Rosaceae, Solanaceae, and others.

Distyly is the condition of having styles of two different lengths. It was first described in primroses. Two Types of flowers occur: long-styled and short-styled. They exhibit pollen dimorphism: in short-styled plants, the pollen is larger and the stigma papillae are shorter. Legitimate (successful) pollination involves The transfer of pollen from short stamens to the stigma of a short style and, conversely, from long stamens to the stigma of a long pistil. Distyly has also been studied in detail in buckwheat (Fig. 23).

Fig. 23. Distyly in buckwheat: (a) short-styled form, (b) long-styled form

In tristyly (e.g., purple loosestrife), there are Three types of flowers characterized by short styles with long and medium stamens, medium styles with long and short stamens, and long pistils with medium and short stamens.

Distyly is controlled by a single supergene: the genotype of plants with short-styled flowers is SS or Ss, while that of plants with long-styled flowers is ss. Neither self-fertilization nor cross-fertilization occurs between flowers of the same morphotype (an illegitimate pollination pathway), but it is possible between plants of different morphotypes (a legitimate pollination pathway). In tristyly, the genetic architecture of the various forms is based on two loci, M and S: long-styled flowers are mmss; mid-styled flowers are Mmss or MMss; and short-styled flowers are MmSs, mmSs, MMSs, MMSS, or mmSS.

Pseudocompatibility. In many plant species, the appearance of pseudocompatibility is associated with artificial pollination. The level of pseudocompatibility is influenced by light, Temperature, and humidity. Examples include "end-of-season" pseudocompatibility, or pseudocompatibility achieved through bud pollination, removal of the style apex, or the APPLICATION OF PLANT Hormones.

Circadian flowering rhythm. Most flowers respond to the alternation of day and night. Some flowers open and close multiple times, whereas others bloom only once. Depending on the timing of flower opening relative to specific periods of the day, plants are classified into those with morning (dandelion), daytime (alfalfa), evening (matthiola), nighttime (tropical plants), round-the-clock (lingonberry), and portioned explosive (Poaceae family) flowering patterns. The circadian rhythm of flower opening is considered by A number of researchers to be a manifestation of biological rhythms.

A methodology for observing the circadian dynamics of plant flowering was proposed by A.N. Ponomarev. The daily dynamics of flower opening are determined in 50 inflorescences of the studied species over the course of 24 hours in triplicate. Every two hours, the number of newly opened flowers is counted. Concurrently, it is necessary to record the air temperature and relative humidity (Fig. 24).

Fig. 24. Circadian dynamics of flower opening in Astragalus onobrychis on June 30, 1992: temperature, °C, – humidity, – number of flowers

Cross-pollination occurs via Two main mechanisms: biotic and abiotic.

Biotic pollination. Pollination was originally carried out by beetles, which fed on pollen. Later, with the appearance of nectaries, Hymenoptera, Diptera, and Lepidoptera became the primary pollinators. Thus, pollen and nectar became the primary attractants. Pollen contains Proteins, Lipids, CARBOHYDRATES, Enzymes, and Vitamins. In entomophilous plants, pollen grains contain more lipids and proteins, whereas in anemophilous plants, they are richer in carbohydrates. Pollen is relatively long-lived. Nectar is an aqueous solution of sugars, primarily fructose and sucrose, along with Amino Acids, proteins, organic acids, vitamins, and Other Compounds. Sugar concentrations can reach up to 74% (in horse chestnut). Nectar secretion depends on the time of day, as well as the age and developmental stage of the flower. Nectaries can be found on various floral Organs, including sepals, petals, receptacles, and staminal filaments.

In addition to nutritional rewards, secondary attractants such as color and scent serve to attract pollinators. Flower color is determined by pigments, including Flavonoids, carotenoids, anthocyanins, and anthophaeins. Often, coloration is heterogeneous, featuring patterns or markings that act as nectar guides. In some plants, color changes occur during the flower's lifespan, which is linked to Changes in the pH of the cell vacuolar sap in petal Cells following fertilization. For example, in lungwort, the flowers are initially pink-purple and later turn blue. This signals to pollinators that nectar and pollen are no longer available.

Pollinators show specific color preferences. Bees cannot perceive the color red, but they are sensitive to ultraviolet light. When plants with blue and yellow corollas bloom simultaneously, bees preferentially pollinate the blue flowers. Geographical variations in floral coloration also exist: red flowers predominate in the tropics, white flowers in high-altitude regions, yellow flowers in deserts and polar regions, and blue flowers in temperate lowland plains.

The scent of entomophilous flowers is highly diverse, with over 500 distinct odors identified. On rare occasions, this is a putrid scent of decay, characteristic of aroids and Rafflesia. Sometimes the scent is due to amines (as in hawthorn or rowan), but more commonly it is associated with Essential Oils. The source of the scent may include petals, stamens, staminodes, nectaries, or pollen. Odor intensity fluctuates and peaks at specific times of the day—for instance, at night in petunias and during the day in clovers.

The characteristics of biotic pollination modes are summarized in Table 4.

Table 4. Modes of biotic pollination

Abiotic pollination. There are two main modes of abiotic pollination: wind pollination (anemophily) and water pollination (hydrophily). The characteristics of abiotic pollination modes are presented in Table 5.

Table 5. Modes of abiotic pollination

3.3 Floral Polymorphism

The Diversity of sexual forms is an evolutionary adaptation promoting cross-pollination.

E.I. Demyanova [13] proposes the following Classification of sexual forms in plants:

1. Hermaphroditic. The population consists of plants with bisexual flowers only.

2. Monoecious:

- Strictly monoecious. Male and female flowers develop on the same individual (e.g., birch, oak, corn);

- Andromonoecious. Both bisexual and male flowers occur on the same plant (fam. Apiaceae, Rosaceae, etc.);

- Gynomonoecious. Bisexual and female flowers occur on the same plant (fam. Asteraceae, etc.);

- Trimonoecious. Bisexual, female, and male flowers occur on the same plant (e.g., Norway maple).

3. Dioecious:

- Strictly dioecious. The population consists of plants with female flowers only and plants with male flowers only (e.g., aspen, poplar, willow, hemp);

- Androdioecious. The population includes plants with bisexual flowers and plants with male flowers only (e.g., hellebore, buttercup, marsh marigold);

- Gynodioecious. The population consists of plants with bisexual flowers and plants with female flowers (fam. Lamiaceae, Caryophyllaceae, etc.).

4. Trioecious:

- Trioecious (subthree-homogamous). The population contains plants with bisexual flowers, plants with male flowers, and plants with female flowers (e.g., common ash, juzgon, spinach, pedate buttercup);

- Polyecious. A variety of combinations of bisexual, male, and female flowers is observed within three or more individuals (maple species).

Monoecious plants. Most researchers believe that monoecious plants evolved from hermaphroditic ones. They are more characteristic of the class Liliopsida (Monocotyledons). A correlation between monoecy and anemophily is also observed (Fig. 25).

Fig. 25. Birch SHOOT with male and female inflorescences

Certain regularities have been identified regarding the arrangement of male and female flowers on the plant. Typically, female flowers are located at the lower part of the inflorescence and open first. They are also smaller in size.

In terms of flowering duration, female flowers surpass male ones; sometimes the difference in flowering time between female and male flowers is several days, in which case they function as dioecious.

Environmental conditions such as temperature, light intensity, and mineral Nutrition can enhance the traits of one sex. The ratio of flowers also changes with age. For instance, in castor bean plants, predominantly female flowers are formed at a young age, shifting to male ones as they age. In some species, this ratio changes in the reverse direction.

In andromonoecious plants, the ratio between bisexual and male flowers varies. For example, male flowers predominate in chestnuts; furthermore, they are usually smaller than bisexual flowers and located at the periphery of the inflorescence. Male and bisexual flowers do not differ in pollen quantity or the growth rate of pollen tubes.

In gynomonoecious plants (mainly in the family Asteraceae — yarrow, wormwood, aster, inula), the ratio of bisexual and female flowers in the inflorescence is stable and genetically fixed. Female flowers are located along the margin of the flower HEAD, while bisexual ones are in the center.

In trimonoecious plants (family Poaceae), male flowers are located in the lower part of the inflorescence, bisexual ones in the middle, and female flowers in the upper part.

Dioecious plants. Sex ratios have been studied in detail specifically in woody plants. In most cases, male plants predominate over female ones (Fig. 26). The ratio can also be 1:1 (aspen, sea buckthorn). Less frequently, female plants predominate over male ones.

Fig. 26. Mountain everlasting (Antennaria dioica): 1 - female plant; 2 - male plant

The sex ratio is influenced by numerous factors:

- in pure stands of annual plants, female plants predominate due to the mortality of male plants;

- under arid conditions, the proportion of female plants decreases;

- female plants exhibit greater resistance to toxic substances and disease infection;

- on saline soils, male plants predominate (such as in poplar and willow);

- at the edge of the distribution range, only a single sex form is found, and plants shift to Vegetative Reproduction.

Phenologically, male plants are the first to bloom. The flowering period of female plants is longer. The number of male flowers in an inflorescence and on the plant as a whole is greater than that of female flowers.

From an evolutionary perspective, scientists believe that bisexual flowers are primitive. Occasionally, bisexual flowers appear in dioecious plants and are regarded as atavisms.

The adaptive significance of sex segregation lies in ensuring cross-pollination and in the ecological niche differentiation of sex forms.

Androdioecious plants are quite rare, as forming male flowers is inefficient (they do not produce seeds).

Gynodioecious plants occur frequently. This condition is primarily characteristic of dicotyledonous plants and has been identified in 613 species. A correlation between gynodioecy and entomophily is also observed.

Morphologically, female flowers are smaller in size than bisexual flowers. In female flowers, the stigma and style are more developed, and there are frequently more ovules. The stigma matures earlier, at the bud stage. However, female flowers sometimes yield less nectar, occasionally lacking nectaries altogether.

Trioecious plants. Trioecious plants are rare. The ratio of sex forms can vary. Populations are most commonly dominated by female and male plants, with fewer bisexual plants. Female plants bloom first, followed by male plants, and subsequently bisexual plants.