Pollination and Fertilization

The main evolutionary advantage that seed plants gained through the development of pollination and water‑independent fertilization is overcoming dependence on a liquid medium. In spore plants (mosses, ferns), male gametes – spermatozoids – have flagella and can reach the egg only in the presence of a water film. This limited plant dispersal and made their reproductive success entirely dependent on environmental humidity (Becker et al., 2025).

In gymnosperms and angiosperms, the pollen tube solves this problem. It germinates from the pollen grain and delivers non‑motile male gametes (sperm) directly to the female gametophyte. Thanks to this, fertilization can occur even in arid climates, allowing flowering plants to colonize diverse ecological niches. As paleobotanists vividly put it, pollen became the “dry seed” for the transfer of male genetic material (Beck, 2010).

Pollination – especially cross‑pollination – creates the basis for maintaining high levels of heterozygosity in populations. When pollen is transferred from one plant to another (by insects, wind, birds), constant allele exchange occurs. This increases the adaptive potential of the species and reduces the risk of inbreeding depression (Hiscock & Dickinson, 1996).

Many flowering plants have evolved special mechanisms that prevent self‑pollination (self‑incompatibility), making cross‑pollination obligatory. Such a strategy is evolutionarily advantageous because it increases offspring variability and helps the species adapt successfully to changing environmental conditions (Kitashiba & Nasrallah, 2014).

Even in self‑pollinating species (wheat, barley, pea), occasional cross‑pollination maintains the necessary level of genetic diversity, preventing complete homozygosity of the population (Stern et al., 2021).

The outcome of successful pollination and double fertilization is the formation of two crucial structures – the seed and the fruit. The seed contains the embryo of a new plant and nutritive reserves (endosperm or storage tissues of cotyledons), ensuring high seedling survival. The fruit (the enlarged and often modified ovary wall) serves to protect and disperse seeds.

From an economic perspective, these processes underlie the yield of almost all agricultural crops:

Thus, understanding the biological essence of pollination and fertilization is not only a fundamental botanical issue but also a necessary condition for developing effective agronomic practices and breeding. As noted in modern agronomic literature, “plant reproduction is the foundation on which all human food security is built” (Lersten, 2004; Borghi et al., 2025).

The primary division of pollination is based on where the pollen that lands on the stigma originates.

Self‑pollination (autogamy, cleistogamy). In this case, pollen is transferred from an anther to the stigma of the same flower or another flower of the same plant (Stern et al., 2021). Self‑pollination is called a “fail‑safe” strategy. It ensures reproduction even in the absence of pollinators, under unfavorable weather conditions, or at low population density. However, the price of reliability is reduced genetic diversity of offspring, which in the long term can lower the fitness of the species (Hiscock & Dickinson, 1996). Typical self‑pollinators among agricultural crops are wheat, barley, pea, and bean. In many self‑pollinating species, flowers do not open fully, and anthers mature simultaneously with the stigma (homogamy).

Cross‑pollination (allogamy). This is the transfer of pollen from the flower of one plant to the stigma of a flower of another plant of the same species (Stern et al., 2021). From an evolutionary perspective, this is a more progressive strategy because it promotes gene exchange and maintains high heterozygosity in the population. Most wild and many cultivated plants (apple, pear, maize, rye, sunflower, many vegetable crops of the Cucurbitaceae family) are cross‑pollinated. They have evolved special mechanisms that prevent self‑pollination – dichogamy (asynchronous maturation of stamens and pistil), heterostyly, self‑incompatibility (Kitashiba & Nasrallah, 2014; Lersten, 2004).

Depending on what or who transfers pollen from the stamen to the stigma, two types of transfer are distinguished: abiotic (non‑living nature) and biotic (living organisms).

Differences in pollination have led to the formation of characteristic pollination syndromes – suites of floral traits that increase the likelihood of visitation by a particular group of pollinators (Johnson, 2025; Borghi et al., 2025).

Flower shape and size: Tubular, funnel‑shaped, or spurred corollas restrict access to nectar to insects with long proboscises (butterflies, bumblebees) or birds with slender beaks. Open, flat flowers are accessible to many insects. Small flowers are often clustered into inflorescences (capitula in Asteraceae, umbels) to increase overall visibility.

Corolla color: Blue, yellow, purple, ultraviolet patterns – for bees; white, pale yellow – for night‑flying moths; red, orange – for birds; dull, brown – for fly and beetle pollination (Stern et al., 2021).

Scent: Strong, pleasant, sweet – for diurnal pollinators; faint, intensifying at night – for nocturnal moths; unpleasant (putrid, meaty) – attracts flies and saprophagous beetles.

Nectaries and nectar: In entomophilous plants, nectaries are located deep within the flower, forcing the insect to touch anthers and stigma. Nectar composition (ratio of sucrose, glucose, fructose, amino acids) can be species‑specific and influence pollinator preferences (Borghi et al., 2025).

Pollen: In entomophilous plants, pollen is often large, sculptured, sticky, or equipped with special attachments (e.g., orchid pollinia). In wind‑pollinated plants, pollen is small, smooth, dry, light, and produced in large quantities.

It is important to note that these adaptations are not absolute: the same species may be visited by different pollinator groups (generalization), and in different parts of its range, a species may form “pollination ecotypes” adapted to the local pollinator community (Johnson, 2025).

Dichogamy is one of the most widespread and simple mechanisms preventing self‑pollination within a single flower. Its essence is that anthers and stigma of the same flower do not mature simultaneously (Stern et al., 2021).

Protandry (from Greek protos – first, andros – male): anthers mature first, and only after the pollen has been shed does the stigma become receptive. This type is widespread among Asteraceae (sunflower), Apiaceae (carrot, dill), Caryophyllaceae. For example, in sunflower, anthers mature several days before the stigma opens, virtually excluding self‑pollination within a single flower.

Protogyny (from Greek protos – first, gyne – female): the female structure matures first – the stigma becomes receptive before the anthers in the same flower open. Protogyny occurs in many Rosaceae (apple, pear, plum), Brassicaceae (cabbage, rapeseed), and grasses (rye). However, protogyny alone does not always completely exclude self‑pollination, because the stigma remains receptive after anther maturation in the same flower. But in combination with other mechanisms (e.g., self‑incompatibility), it effectively promotes cross‑pollination.

From an agronomic viewpoint, dichogamy is important when selecting pollinator varieties for orchards and for hybrid seed production.

Heterostyly is a more complex morphological adaptation in which within a single species there exist two (distyly) or three (tristyly) flower morphs differing in style length and stamen position (Lersten, 2004; Barrett et al., 2000, cited in Lersten).

A classic example is primrose (Primula): in some plants, flowers are “pin” (stigma positioned high, at the corolla throat, while anthers are attached low); in others, “thrum” (stigma positioned low, anthers high). Effective pollination occurs only when pollen from high anthers is transferred to a high stigma, and from low anthers to a low stigma. This requires pollen transfer between different flower morphs and almost completely eliminates self‑pollination. Bees collecting nectar inevitably touch both anthers and stigmas, and due to the different flower geometry, pollen from one morph lands on the stigma of the other morph.

Heterostyly is often accompanied by self‑incompatibility (i.e., even if pollen lands on a stigma of the same morph, fertilization will not occur). Among agricultural crops, heterostyly is characteristic of buckwheat (Fagopyrum esculentum), which must be taken into account in its cultivation and breeding.

Self‑incompatibility is the genetically controlled ability of a plant to prevent the germination of “self” pollen (or the growth of its tubes) on the stigma or in the style. This is the most effective and refined mechanism for preventing self‑pollination, and it is estimated to occur in about 50% of flowering plant species (Lersten, 2004; Hiscock & Dickinson, 1996; Kitashiba & Nasrallah, 2014).

Self‑incompatibility is based on the recognition of protein molecules on the surface of the pollen grain (or in its coat) and in the tissues of the pistil. If these molecules are encoded by the same alleles of the S‑locus, pollination is blocked.

Two main types are distinguished (Kitashiba & Nasrallah, 2014):

Understanding self‑incompatibility has enormous practical significance in breeding and seed production. For example, in Brassicaceae and many fruit crops, several pollinator varieties must be planted to ensure cross‑pollination. On the other hand, overcoming self‑incompatibility (e.g., by treating the stigma with a saline solution or the “mentor pollen” method) allows hybrid seeds to be obtained in intra‑ and even interspecific hybridization (Lersten, 2004).

The pollen grain is not merely a carrier of male genetic material but a highly organized structure that ensures its protection, delivery, and successful interaction with the female gametophyte.

Wall structure: The outer wall – exine – is composed of the very stable polymer sporopollenin. It often has a complex sculpture that can be species‑specific and serve for attachment to an insect’s body (entomophily) or for better aerodynamics (anemophily). The exine also contains proteins involved in the recognition response upon contact with the stigma (Lersten, 2004; Beck, 2010).

Two‑celled or three‑celled pollen status: At the time of shedding from the anther, the pollen grain may contain:

Reserve substances: Pollen contains nutrients (starch, lipids, proteins) used during the initial stages of tube growth (autotrophic phase). Interestingly, pollen of wind‑pollinated plants is generally richer in starch, while that of entomophilous plants is richer in lipids. This is related to the different energy efficiency of transport (Lersten, 2004; Borghi et al., 2025).

Thus, the mechanisms preventing self‑pollination and the structural features of pollen constitute an integrated system that ensures an optimal balance between reproductive reliability and offspring genetic diversity. Understanding these mechanisms is the key to managing pollination in agroecosystems and solving breeding tasks.

A pollen grain that lands on a receptive stigma of a compatible plant begins to absorb water (hydrate). This occurs especially quickly on wet‑type stigmas. On dry stigmas (e.g., Brassicaceae), the hydration process is regulated by protein interactions between the pollen coat and stigma cells (Lersten, 2004; Kanaoka, 2018).

Within minutes or hours (depending on the species), a pollen tube protrudes through one of the apertures (thickenings) in the pollen wall. The exine (outer wall) breaks or lifts, and the inner layer – the intine – gives rise to the tube wall. The tube grows exclusively at its tip (apical growth), allowing it to penetrate between style cells, secreting enzymes (e.g., pectinases and cutinases) to soften middle lamellae and cuticle (Beck, 2010; Hepler et al., 2006).

As the tube grows, the contents of the pollen grain flow into it. The vegetative nucleus (sometimes called the tube nucleus) moves ahead, followed by the generative cell or two already formed sperm (depending on pollen type). The growth rate of the pollen tube varies among species: in grasses, it can reach several centimeters per hour (e.g., in maize up to 1 cm/h); in pines, millimeters per day; in some palms, the tube may grow for a year before reaching the ovule (Lersten, 2004; Kanaoka, 2018).

Pollen tube growth is a complex, energy‑consuming process that depends on the availability of nutrients in the style tissues (especially carbohydrates), on ion balance (calcium ions Ca²⁺ play a key role, creating a concentration gradient at the tube tip), and on the cytoskeleton (actin microfilaments direct vesicle transport to the growing tip) (Hepler et al., 2006; Zheng et al., 2024).

Upon reaching the embryo sac, the pollen tube enters it through the micropyle. It penetrates one of the two synergid cells (usually the one that has begun to degenerate at that time). Synergids secrete attractants (e.g., LURE peptides) that guide the tube, and their destruction provides access to the egg and central cells (Kanaoka, 2018; Zheng et al., 2024).

Inside the synergid, the tip of the pollen tube ruptures (or a pore forms), and two non‑motile sperm cells are released. Then follows a unique event for angiosperms – double fertilization (Beck, 2010; Stern et al., 2021):

Thus, both sperm participate in double fertilization, and two female cells are fertilized: the egg cell and the central cell of the embryo sac. The two sperm may differ in size, organelle content (plastids, mitochondria), and may be functionally predetermined to fuse with a particular female cell (e.g., in some species, one sperm carries more plastids and is destined for the central cell) (Beck, 2010; Zheng et al., 2024).

After double fertilization, rapid seed development begins:

Gymnosperms (pine, spruce, juniper, ginkgo) lack double fertilization and endosperm as found in angiosperms (Beck, 2010; Zheng et al., 2024).

Thus, double fertilization and the formation of triploid endosperm are unique features of angiosperms that provide efficient and rapid nourishment of the embryo, likely contributing to their dominance in the terrestrial flora (Becker et al., 2025).

Abiotic factors include temperature, humidity, wind, precipitation, light, and other climatic elements.

Temperature: Each species has its own temperature optimum for pollen germination and pollen tube growth. Very low temperatures slow generative cell division, reduce tube growth rate, and can cause damage to stigma cells. High temperatures (especially short‑term heat waves) are extremely dangerous:

Air and soil humidity (drought): Water deficit is one of the main stress factors limiting reproduction.

Low air humidity causes the stigma to dry out (especially in species with “dry” stigmas, e.g., Brassicaceae), preventing pollen hydration and germination. Pollen tubes become brittle and their growth slows.

Soil drought reduces overall plant turgor, decreases nectar secretion, reduces flower size, and the number of flowers per inflorescence (Brunet et al., 2025). Additionally, drought reduces pollen fertility (increases the percentage of aborted pollen grains) and can cause retardation in embryo sac development (Yao et al., 2024). Studies show that even moderate water deficit during flowering can reduce seed set by 20–50% depending on the crop.

Wind and rain: For wind‑pollinated plants, lack of wind during flowering reduces pollination efficiency. Excessive wind can dry out stigmas. Heavy rain washes pollen from the air, removes it from stigmas (especially in entomophilous species with open flowers), and hinders the flight of insect pollinators (Lersten, 2004).

Light: Light intensity and spectral composition affect flower opening, nectar secretion, and the synthesis of pigments that attract pollinators. In many plants, pollination occurs only at specific times of the day (e.g., in nocturnal species – by moonlight or twilight).

Biotic factors include all living organisms that directly or indirectly affect pollination and fertilization.

Presence and activity of pollinators: For entomophilous and ornithophilous plants, this is a decisive factor. Low numbers of bees, bumblebees, butterflies, or birds during flowering lead to incomplete pollination, reduced fruit and seed set, and the formation of deformed, asymmetric fruits (Borghi et al., 2025; Johnson, 2025). This is especially critical in monoculture agroecosystems, where the food base for pollinators is limited and insecticide use reduces their numbers.

Coincidence of flowering phases with pollinator flight: For successful pollination, the flowering period of plants must be synchronized with the period of mass emergence and activity of pollinators. Under global warming, a phenological mismatch (mismatch) is increasingly occurring: for example, apple flowering may begin earlier than bumblebees emerge from hibernation (Brunet et al., 2025).

Competition for pollinators: In communities with a high density of flowering species, plants may compete for a limited resource – pollinator visits. Less attractive species (with small flowers, scarce nectar) may remain unpollinated.

Pathogens and herbivores: Infection of flowers by fungal diseases (e.g., Monilinia in fruit trees) or damage to reproductive organs by insect pests directly reduces pollination efficiency.

A special group comprises abnormalities that occur under extreme abiotic stresses during critical phases of reproductive organ development. These effects are often underestimated, but they can lead to catastrophic yield losses (Yao et al., 2024; Brunet et al., 2025).

Heat shock during microsporogenesis: A brief (only a few hours) temperature increase to 35–40 °C during the tetrad stage in cereals or during pollen grain formation in tomato causes pollen sterilization. Pollen grains may be empty, and when germinating, tubes burst quickly (Yao et al., 2024).

Frost during flowering: Spring frosts damage not only petals but also critical structures: watery liquid in nectaries can freeze, rupturing cells; ovaries and ovules lose turgor and die; pollen loses viability. The result – complete crop failure in stone and pome fruits.

Drought combined with high temperature: This is the most damaging combination. It causes complex damage: inhibition of photosynthesis → carbohydrate deficiency for pollen and nectar formation; direct damage to tapetum cell membranes and microspores; disruption of Ca²⁺ signaling in the growing pollen tube. The outcome – high pollen sterility, poor pollination, massive ovary abortion (“blossom drop”).

Mineral nutrient deficiency: Boron (B) deficiency is especially critical, as boron is involved in the synthesis of pectin substances necessary for pollen tube growth and affects carbohydrate metabolism. Calcium (Ca²⁺) deficiency disrupts the formation of the ion gradient at the tube tip and its directional growth. Potassium (K^\+) deficiency reduces turgor and overall drought tolerance. Signs of stress: poor pollen germination, twisted or bursting tubes, poor seed set (Yao et al., 2024; Borghi et al., 2025).

Consequences: The ultimate result of stress is the formation of empty, shriveled seeds, non‑viable embryos, ovary abscission (physiological drop), and consequently, a sharp reduction in yield.

Humans can significantly influence pollination and fertilization conditions through targeted agronomic practices.

Plant density: Overly dense cereal stands reduce ventilation, hampering wind pollination. For entomophilous crops, excessive density can limit insect access to flowers.

Cultivar composition and arrangement on the site: For cross‑pollinated varieties (apple, pear, cherry, many plum varieties, sea buckthorn), planting of pollinator varieties (usually at least 10–20% of total trees) is mandatory. Pollinator varieties must be compatible and flower simultaneously. Arrangement pattern (row intercropping) affects the efficiency of pollen transfer by bees or wind.

Fertilization: Balanced nutrition (especially phosphorus‑potassium fertilizers) increases plant stress resistance and improves pollen quality. Avoid excess nitrogen, which stimulates vegetative growth at the expense of reproductive development.

Irrigation: Maintaining optimal soil moisture during budding and flowering is a key factor for stable fertilization, especially in regions with risky agriculture.

Protection from pesticides: Any insecticide treatments (and to a lesser extent fungicides) during flowering are harmful to bees and other pollinators. Strict application regulations must be followed: spray only in the evening when bees have ceased flying, and choose products with low bee toxicity (Borghi et al., 2025).

Thus, to ensure high yields, integrated management of all factor groups is required – from selecting tolerant varieties and optimizing mineral nutrition to creating conditions for active pollinator activity and protection against stressful weather events.

In agricultural crop production, the main task is to ensure the most complete and timely pollination.

Placement of pollinator varieties. For cross‑pollinated crops (apple, pear, cherry, plum, many sea buckthorn and honeysuckle varieties), one or more pollinator varieties are planted in the orchard. Key requirements: compatibility (absence of self‑incompatibility), simultaneous flowering, and regular (annual) flowering. Typically, pollinators occupy 10–20% of total trees. Planting pattern – staggered or in rows to ensure cross‑pollination even in the center of the canopy. In commercial horticulture, special variety placement maps are used (Kitashiba & Nasrallah, 2014; Borghi et al., 2025).

Bringing in beehives during flowering. For entomophilous crops (sunflower, buckwheat, rapeseed, fruit orchards, cucurbits, open‑field cucumbers), renting honeybee (Apis mellifera) or bumblebee (Bombus spp.) colonies is widely practiced (Borghi et al., 2025). Density calculation: 2–4 honeybee colonies per hectare of orchard, 1–2 colonies per hectare of sunflower or buckwheat. Hives are placed at the beginning of flowering (usually 1–2 days before mass bloom). To improve bee orientation, sugar syrup scented with the crop’s flowers is used (the “training” method). Bee pollination efficiency increases sunflower seed yield by 30–50% and apple yield by 40–60% (Borghi et al., 2025).

Creation of windbreaks. For wind‑pollinated crops (maize, rye, millet, rice), reducing wind speed in the field can impair pollination. However, excessive wind is also harmful. The optimal wind speed for maize is 2–5 m/s. Field shelterbelts should be porous (permeable) to reduce wind speed but not block it completely. In breeding nurseries, windbreak screens made of burlap or polymer mesh are used for plot isolation (Lersten, 2004).

Artificial pollination. In protected cultivation (greenhouses) where natural pollinators are absent, hand pollination (using a brush or mechanical shaking) or bumblebee hives are used. In field conditions under unfavorable weather (rain, cold), hand pollination may also be applied in small nurseries or seed production plots (see section 6.2).

Pollination management is the basis of hybrid breeding and elite seed production.

Artificial pollination (cross‑cross). When creating hybrids, the following techniques are used:

Use of cytoplasmic male sterility (CMS). This is a fundamental technique in commercial F1 hybrid seed production. CMS is the heritable inability of a plant to produce fertile pollen, caused by a mutation in mitochondrial DNA (cytoplasmic factor). CMS plants have normal female organs but are male‑sterile. For hybrid seed production in maize, sorghum, sunflower, rice, onion, and many other crops, three lines are created (A‑B‑C scheme):

This method allows hybrid seeds to be produced on an industrial scale without labor‑intensive emasculation (Stern et al., 2021; Kitashiba & Nasrallah, 2014).

“Mentor pollen” method. Used to overcome self‑incompatibility in intra‑ or interspecific hybridization (Lersten, 2004). A mixture of “self” (incompatible but live) pollen and “foreign” (compatible but killed (e.g., by gamma radiation or alcohol)) pollen from another species is applied to the stigma. The killed “mentor” pollen releases substances that “trick” the stigma, suppressing the rejection response, allowing the incompatible pollen to germinate. The method is used in fruit breeding (apple, pear), Brassicaceae, and other crops.

Pollen storage. To overcome flowering asynchrony and preserve valuable genotypes, pollen is dried and frozen at –80 °C or in liquid nitrogen (–196 °C). In cryobanks, pollen of many crops (e.g., maize, pine) remains viable for years.

Pollination management is inseparable from protecting pollinators and ensuring their active work.

Limiting insecticide applications during flowering. This is a cardinal rule. Insecticides (especially systemic and pyrethroids) are highly toxic to bees and bumblebees. Applications during flowering are prohibited or carried out only in the evening (after sunset) when bees have returned to their hives. It is recommended to choose products with a short waiting period (12 to 24 hours) and use tank mixes with minimal toxicity. In beekeeping, hives are isolated during the treatment of adjacent fields (Borghi et al., 2025).

Biological protection of pollinators. The use of biological products (e.g., based on Bacillus thuringiensis) is less dangerous to bees, but still requires caution. It is also important to prevent bee poisoning by pesticides accumulated in pollen and nectar from previous treatments (systemic neonicotinoid insecticides cause chronic toxicity).

Creating conditions for pollinators. Sowing nectar‑producing plants along field edges, preserving natural biotopes (windbreaks, meadows), providing water sources for bees – these measures increase the abundance and activity of both wild and managed pollinators (Borghi et al., 2025; Johnson, 2025).

Protection against frost and drought. Smoking, sprinkler irrigation, row covers, drip irrigation – these techniques are not directly related to pollinator protection, but they preserve pollen and stigma viability, enabling fertilization under adverse weather conditions (Yao et al., 2024; Brunet et al., 2025).

Thus, pollination management is an integrated task involving agronomy, breeding, seed production, and plant protection. Solving it allows not only to increase yield but also to improve fruit and seed quality, and to preserve the biological diversity of pollinators – a key component of sustainable agroecosystems (Borghi et al., 2025).