Generative stage of plant ontogenesis

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Generative stage of plant ontogenesis (from Latin genero — I give birth, I produce; also known as the reproductive or generative period) is the stage of individual plant development during which it first acquires the ability for sexual reproduction and forms specialised reproductive organs: in flowering plants – flowers, fruits and seeds (Gatsuk et al., 1980; Yakovlev et al., 2008). Unlike the preceding virginile (vegetative) stage, when the plant merely accumulates vegetative mass and does not form generative structures, the generative stage marks the transition to fulfilling the main biological task – producing offspring. In perennial polycarpic plants (e.g., most trees, shrubs and many herbs) this stage can be repeated many times throughout life: after the first flowering and fruiting, the plant returns to vegetative growth and then enters the generative phase again. In annual and biennial monocarpic plants (e.g., wheat, carrot) the generative stage occurs once at the end of life and ends with the death of the individual (Gatsuk et al., 1980; Yakovlev et al., 2008).

From a biological point of view, the generative stage is central to the plant’s life cycle, because it is during this stage that the sexual process takes place: male and female gametes (pollen grains and egg cells) are formed in flowers, pollination and fertilisation occur, and a zygote is formed, from which the embryo of a new sporophyte develops. The entire set of these processes ensures genetic diversity of offspring and dispersal of the species (Pandey et al., 2022).

For agronomy and crop production, the generative stage is of paramount practical importance, because it is during this time that the crop – seeds, fruits, infructescences – is set and formed. Understanding the phases of generative development (budding, flowering, ripening) allows the agronomist to:

choose optimal sowing dates and plant care regimes;
apply irrigation and fertilisation during critical periods (e.g., the “flowering – grain filling” phase);
carry out protection measures against diseases and pests that attack reproductive organs;
determine harvest dates taking into account the attainment of full seed maturity.

Thus, the generative stage of ontogenesis is not just a biological milestone in a plant’s life, but also a key object of management in agricultural practice, on the correct recognition and regulation of which the productivity of crops directly depends. In the following sections of the article we will examine the stages of the generative stage, its characteristics in different life forms, factors affecting reproductive success, and the connection with the preceding (virginile) and subsequent (senile) stages of ontogenesis.

1. External and internal signs of the onset of the generative stage

The transition of a plant from vegetative growth to generative development is not an instantaneous act but a gradual process that affects all levels of organisation: from organ morphology to cell physiology. For an agronomist, it is extremely important to recognise this transition in a timely manner, because it signals the beginning of a critical period when the plant is most sensitive to stress and when the future harvest is being laid down. The signs of the onset of the generative stage can be divided into external (visually observable) and internal (physiological).

1.1 External (morphological) signs

The most obvious and reliable signs of a plant’s transition to the generative stage are associated with the appearance and development of reproductive organs. In the Russian school of ontogenetic morphology (Gatsuk et al., 1980), the following key changes are distinguished that separate generative plants (g₁, g₂, g₃) from the preceding age states (virginile – v):

Transformation of the apical (terminal) shoot meristem. In a vegetating plant, the shoot apex (growing tip) forms leaf primordia. Upon transition to flowering, the apex elongates, widens and begins to initiate not leaves but flower primordia – the beginnings of flowers or inflorescences (Serebryakova et al., 2006). In most annual herbs, after this, terminal shoot growth ceases (monocarpic shoot), whereas in perennial polycarpics the apical meristem may later revert to a vegetative state.
Appearance of flower buds and floral buds. The earliest visual sign is the swelling and change in shape of terminal or axillary buds. The buds become more rounded, their scales (if present) open slightly, and green or coloured buds emerge from them. In many woody species (ash, birch, oak) at the end of winter – beginning of spring, one can observe how generative buds are noticeably different from vegetative ones in size and shape even before the leaves unfold.
Formation of inflorescences and opening of the first flower. The successive development of inflorescences and finally the opening of the first flower serve as an unequivocal confirmation that the plant has entered the generative stage. For field crops such as wheat or sunflower, the flowering phase is determined by the appearance of anthers on the flowers (Stern & Jansky, 2021).
Cessation or slowing of vegetative growth. In annual monocarpic plants, after the beginning of flowering, the formation of new leaves and stem elongation almost completely stop. All resources are redirected to the development of reproductive organs. In perennial plants (fruit trees, shrubs), a seasonal weakening of shoot growth is also observed during mass flowering.
Change in leaf colour and structure. This is often the most noticeable sign of metabolic “switching”. Leaves may become a paler green, and sometimes turn yellow – this is associated with the outflow of nitrogen and other nutrients from vegetative organs to developing flowers and seeds (Raven et al., 2013).

1.2 Internal (physiological) signs

Internal changes, although not visible to the naked eye, precede external ones and determine them. It is important to note that within this course we will describe them at a descriptive level, without going deep into biochemistry and molecular genetics.

Induction of flowering. The transition to the generative stage is triggered by a complex interaction of internal (age, hormonal status) and external signals – primarily photoperiod (day length) and temperature (vernalisation). In long-day plants (spinach, radish, many cereals), flowering is stimulated when the light period exceeds a certain critical value. In short-day plants (chrysanthemum, soybean, proso millet), flowering occurs when the day becomes shorter than the critical length. In day-neutral plants (tomato, cucumber, sunflower), flowering does not depend on day length (Raven et al., 2013; Bidlack & Jansky, 2021). Vernalisation is the stimulation of flowering by prolonged exposure to low positive temperatures (usually from 1 to 7 °C), which is necessary for many winter crops and biennials (e.g., carrot, beet, cabbage).

Redistribution of assimilates. The internal physiological “backbone” of the generative stage is the change in source–sink relationships within the plant. While during the vegetative stage the main sinks (consumers) of photosynthesis products are growing leaves and roots, with the onset of flowering the main sinks become the generative organs – flowers, and then developing fruits and seeds. In leaves, processes of breakdown and outflow are activated: proteins are broken down into amino acids, starch into sugars, and these compounds are transported via phloem to the reproductive organs (Raven et al., 2013). This is what causes the visual signs described above – yellowing of leaves and slowing of their growth.

Hormonal reorganisation. A change in the balance of phytohormones is a key mechanism for switching developmental programmes. The content of gibberellins (stimulating the development of flower stalks and flowers) increases, especially in annual plants, as does ethylene (promoting leaf senescence and redirecting resources to seed ripening). At the same time, levels of auxins and cytokinins, which support vegetative growth, decrease (Raven et al., 2013; Bidlack & Jansky, 2021).

1.3 Comparison of signs with adjacent stages of ontogenesis

For a clearer understanding of the signs of the generative stage, it is useful to compare them with the manifestations of the preceding (virginile) and subsequent (senile) stages (Table). Note that a detailed description of the virginile and senile stages is provided in the corresponding articles of our block.

Table. Comparative characteristics of signs of ontogenetic stages (after Gatsuk et al., 1980; Yakovlev et al., 2008)

Thus, diagnosis of a plant’s transition to the generative stage is based on a set of external morphological signs that reflect deep physiological changes in the organism. Early recognition of these signs allows the farmer to apply necessary agronomic practices (fertilisation, irrigation, treatments) precisely during the period when the plant is most responsive to them and when the basis of the future harvest is being formed.

2. Stages of the generative stage

The generative stage of ontogenesis, despite its apparent brevity in many crops, is a complex sequence of interconnected processes. Each stage has its own morphological, physiological and ecological characteristics and is critical for the formation of a full-fledged crop. In agronomic practice, several key phases are distinguished, reflecting the dynamics of reproductive organ development from the moment the plant receives the signal to flower until full seed maturation and dispersal.

The basis of this division is the concept of alternation of generations (sporophyte and gametophyte) and ontogenetic switches, which are regulated by a complex network of endogenous and exogenous signals (Pandey et al., 2022). For flowering plants, the sequence of stages of the generative stage in a generalised form is as follows:

Flowering induction – perception of external signals (photoperiod, temperature) by the plant and triggering of the molecular-genetic programme for transition to reproductive development.
Formation of generative organs – transformation of the shoot apical meristem into a floral (or inflorescence) meristem and initiation of flower structures (sepals, petals, stamens, pistil).
Flowering – the period during which flowers open, allowing pollination.
Pollination – transfer of pollen grains from the anthers to the stigma (in angiosperms).
Fertilisation – fusion of male and female gametes, leading to the formation of a zygote (embryo) and the central cell (endosperm) in the embryo sac.
Seed and fruit formation – successive development of the seed from the fertilised ovule, and of the fruit from the ovary (or other parts of the flower).
Fruiting and dissemination – the final stage, including ripening of fruits and seeds, their separation from the mother plant, and dispersal.

It is important to understand that these stages do not necessarily follow a strict linear order: in some plants, pollination may occur before full flower opening (cleistogamy), and fruit formation is sometimes initiated without fertilisation (parthenocarpy). However, the described sequence is typical for the vast majority of flowering plants and serves as the basis for phenological observations in agronomy (Lersten, 2004).

Below we examine each of these stages in detail, starting with the very first and perhaps least obvious stage – flowering induction.

2.1. Flowering induction – triggers of the transition

Flowering induction is the process by which a vegetating plant, having reached a certain age and being in suitable environmental conditions, “decides” to switch its apical meristem from the leaf‑formation programme to the flower‑formation programme. This process has enormous adaptive significance: it synchronises plant reproduction with the most favourable season, ensuring the highest probability of successful pollination, seed maturation and offspring survival (Raven et al., 2013; Pandey et al., 2022).

The main triggers (factors that initiate induction) are three groups of signals: photoperiod, temperature (vernalisation) and internal age status (the plant reaching a certain stage of development). In agronomy, knowledge of these triggers underlies the zoning of varieties, determination of sowing dates and the development of technologies for managing flowering (e.g., in forcing or greenhouse production).

Photoperiodism

Photoperiodism is the response of an organism to the ratio of the duration of light and dark periods (the photoperiod). It is the length of the continuous night period, rather than the day, that is the critical factor for most photoperiodic plants (Raven et al., 2013; Bidlack & Jansky, 2021).

Depending on their response to day length, three main groups of plants are distinguished:

Short‑day plants (SDP). They flower when the light period becomes shorter than some critical value (i.e., the night becomes long enough). Typical examples: soybean, proso millet, rice, chrysanthemum, tobacco (some varieties), sunflower (many varieties). For them, short day acts as a trigger signal, usually corresponding to the autumn period.
Long‑day plants (LDP). They flower when the light period exceeds the critical value (i.e., the night becomes too short). Examples: spinach, radish, lettuce, potato, wheat (spring forms), barley. Long day is characteristic of spring and early summer in temperate latitudes.
Day‑neutral plants. They flower independently of day length, as soon as they reach a certain age and accumulate sufficient vegetative mass. Examples: tomato, cucumber, maize, buckwheat, pea.

The critical day length for different species and even varieties can vary significantly. For example, for some soybean varieties the critical photoperiod is 14 hours, while for others it is 12 hours. This determines their adaptability to cultivation at different latitudes (Bidlack & Jansky, 2021).

Perception of photoperiod occurs in leaves, which contain photoreceptor proteins (primarily phytochromes and cryptochromes). The signal from the leaf, in the form of a floral stimulus (the protein FT – FLOWERING LOCUS T), is transported via phloem to the shoot apical meristem, where it triggers the flowering programme (Raven et al., 2013). For the agronomist, knowledge of the photoperiodic sensitivity of a variety is crucial: sowing SDP in northern latitudes with long summer days can severely delay flowering, while sowing LDP in southern latitudes with short days can cause premature flowering and reduce yield.

Vernalisation

Vernalisation is the ability of some plants to only transition to flowering after prolonged exposure to low positive temperatures (usually from 0 to +10 °C, on average about +1…+7 °C) during early development (Raven et al., 2013; Bidlack & Jansky, 2021). The site of cold signal perception is the shoot apical meristems and growing leaves.

This mechanism prevents flowering before winter and synchronises reproduction with the spring‑summer period. Vernalisation is characteristic of:

Winter crops (winter wheat, rye, barley). If sown in spring, they will vegetate all summer but will not flower or yield.
Biennial plants (carrot, beet, cabbage, celery, onion). In the first year they form a rosette of leaves and a storage organ (taproot, head, bulb), and after overwintering and undergoing vernalisation they produce a flowering stem and seeds in the second year.
Some perennial grasses (clover, alfalfa), in which vernalisation may influence flowering intensity in subsequent years.

Artificial vernalisation (stratification of seeds or cold treatment of seedlings) is used in agronomy to accelerate flowering and fruiting, and also to obtain seeds from biennials in a single year (e.g., in breeding or when growing seeds in southern regions). It is important to note that in some species the effect of vernalisation can be replaced by treatment with gibberellins, indicating a link between this process and hormonal regulation (Bidlack & Jansky, 2021).

Age and hormonal factors

Even under optimal photoperiod and temperature, a young plant will not flower until it has reached a certain age status. This stage, which lasts throughout the virginile period, is associated with the accumulation of the necessary vegetative mass and the transition of meristems to a “competent” state, in which they can adequately respond to floral signals. In different species this minimum age varies from a few weeks (in many annual herbs) to several decades (in woody species, e.g., ash first flowers at 30–50 years – Gatsuk et al., 1980).

At the regulatory level, phytohormones are key. It has been shown that:

Gibberellins (especially GA₃) can induce flowering in many plants, especially LDP and biennials, replacing the effect of long day or vernalisation (Raven et al., 2013).
Auxins have an indirect effect, stimulating ethylene synthesis and participating in flower formation.
Abscisic acid (ABA) and ethylene may inhibit or stimulate flowering depending on the species and conditions.
It has been established that in rice and Arabidopsis there are specific genes (e.g., FT, SOC1, LFY) whose products integrate signals from the photoperiodic, temperature and hormonal pathways and directly activate the flowering programme in the apical meristem (Pandey et al., 2022).

Thus, flowering induction is a multifactorial process in which external signals (day length, cold) through a chain of perception and signal transduction (Raven et al., 2013) trigger the expression of developmental genes, switching the shoot apex from a vegetative to a generative programme. The consequence of this switch is the formation of generative organs – flowers and inflorescences – which will be discussed in the next section.

2.2. Formation of generative organs

After the plant has received and integrated the signals to flower (photoperiod, vernalisation, internal hormonal changes), the next key event is the switching of the developmental programme of the shoot apical meristem. Instead of continuing to initiate vegetative metamers (leaves with axillary buds), the growing point is transformed into a generative (floral) meristem, which forms flowers or inflorescences (Serebryakova et al., 2006; Yakovlev et al., 2008).

This transition is morphologically well visible: the shoot apex becomes broader, more rounded, ceases to form leaf primordia and begins to form bumps – the primordia of flowers (Raven et al., 2013). In annual plants, this process often leads to the cessation of shoot apical growth (monopodial growth is replaced by sympodial, or the shoot ends in an inflorescence). In perennial trees and shrubs, the initiation of generative organs may occur in buds a year before flowering (e.g., in apple, pear, cherry), and such buds are called generative (flower) or mixed (vegetative‑generative) if, along with flowers, they also contain leaf primordia (Gatsuk et al., 1980).

Flower development

The flower is a shortened, modified and growth‑limited shoot bearing sporophylls – modified leaves involved in sexual reproduction (Lersten, 2004; Stern & Jansky, 2021). In a typical flower, four whorls of organs are distinguished, located on the receptacle:

Sepals (calyx) – the outer whorl, usually green, protect the bud. They are formed first from the generative meristem.
Petals (corolla) – the inner whorl of the perianth, often brightly coloured, serves to attract pollinators. In wind‑pollinated plants, petals are often reduced or absent.
Stamens (androecium) – male reproductive organs. They consist of a filament and an anther, in which microspores are formed by meiosis, developing into pollen grains (male gametophyte).
Pistil (gynoecium) – female reproductive organ, formed by one or more carpels. The pistil consists of an ovary, style and stigma. The ovary contains ovules, in which the embryo sac (female gametophyte) with the egg cell is formed.

Depending on the presence and development of male and female organs, flowers are divided into:

Bisexual (hermaphroditic) – contain both stamens and a pistil (most crops: wheat, tomato, apple, cherry, pea, bean).
Unisexual – contain only stamens (staminate, or male flowers) or only pistils (pistillate, or female flowers). Unisexual flowers can be located on the same plant (monoecious species: maize, cucumber, pumpkin, hazel) or on different plants (dioecious species: sea buckthorn, hemp, willow, poplar) (Stern & Jansky, 2021; Raven et al., 2013). Understanding the type of sexuality is important for the agronomist: when growing dioecious crops (e.g., sea buckthorn), both male and female plants must be planted in a certain proportion to obtain a fruit harvest.

Inflorescences as an adaptation

In many plants, flowers are grouped into inflorescences – clusters of flowers arranged on a common peduncle in a certain order. Inflorescences perform several functions: they increase the visibility of flowers to pollinators, ensure sequential flower opening (which lengthens the pollination period and reduces the risk of damage from frost or rain), and may improve aerodynamics for wind‑pollinated species (Serebryakova et al., 2006).

The main types of inflorescences important for agronomy:

Simple inflorescences – flowers sit on the main axis (raceme – in currant, lupine; spike – in wheat, barley; spadix – in maize; head – in sunflower, chamomile).
Compound inflorescences – the main axis branches, and the lateral branches bear partial inflorescences (compound spike – in rye; panicle – in oat, millet, lilac; compound umbel – in carrot, dill).

Knowledge of the type of inflorescence helps in breeding (e.g., for creating non‑shattering grain varieties) and in agronomic practices (e.g., for fungicide application during flowering).

2.3. Flowering as a phenological phase

Flowering is the period during which a plant undergoes anthesis (flower opening), making pollination possible. In agronomy and phenology, flowering is considered one of the key developmental phases, which determines the timing of many field operations (fertilisation, irrigation, plant protection, artificial pollination, and in some cases – harvest, e.g., for leafy greens).

Flower opening process and its regulation

Flower opening (anthesis) is a coordinated process involving petal growth, their unfolding (often due to uneven growth or changes in cell turgor), and the extension of stamens and stigma. In different species, the time of day when flowers open is strictly genetically determined and often synchronised with pollinator activity or the diurnal course of meteorological factors (e.g., in cereals, flowering often occurs in the morning, while in many insect‑pollinated plants it occurs during the day) (Raven et al., 2013).

Regulation of flower opening involves hormonal and hydraulic mechanisms. Gibberellins, auxins and especially ethylene play important roles; ethylene promotes petal senescence and wilting after pollination. In some plants (tulip, crocus), flower opening and closing depend on temperature and light.

Duration of flowering

The duration of flowering varies widely depending on the species, variety and weather conditions:

For a single flower – from a few hours (in some cereals, e.g., wheat flower remains open for 15–30 minutes) to several days (in legumes, Rosaceae).
For the whole plant or crop – the flowering period may last from a few days (in field crops with synchronous flowering, such as flax, buckwheat) to several weeks or even months (in perennial grasses, fruit trees, tomatoes in protected cultivation).

Phenologists usually distinguish the following flowering sub‑phases (applied to crops and plantations):

Beginning of flowering – when the first 5–10% of the expected total number of flowers (or inflorescences) open.
Full (mass) flowering – when 50–75% of flowers are open. This is the most critical period for pollination and fertilisation.
End of flowering – when over 90% of flowers have finished blooming, with only a few late flowers remaining.

Adaptations for pollination

The appearance of the flower (size, shape, corolla colour), the presence and type of scent, and nectar production – all these are evolutionary adaptations to attract pollinators (entomophily – pollination by insects; ornithophily – by birds; chiropterophily – by bats) or for efficient use of wind (anemophily) (Stern & Jansky, 2021).

Insect‑pollinated flowers are usually brightly coloured, scented, and often contain nectaries (specialised glands that secrete a sweet liquid). For example, apple, cherry, sunflower, clover, buckwheat. Bees, bumblebees, butterflies, flies and other insects, visiting flowers in search of nectar and pollen, transfer pollen from one flower to another.
Wind‑pollinated flowers – inconspicuous, without a perianth or with a reduced perianth (in cereals – floral scales), do not produce nectar, have no scent. Their pollen is dry, fine, produced in huge quantities, and the stamens are on long filaments, the stigmas are often feathery to capture pollen from the air. Examples: all cereals (wheat, rye, maize), birch, alder, poplar.

For the agronomist, the type of pollination is crucial. When growing cross‑pollinated crops (rye, maize, sunflower), it is necessary to ensure spatial isolation of varieties, plant pollinator varieties (e.g., for apples – pollinator varieties), or place beehives near the fields during flowering (for buckwheat, clover, alfalfa). For self‑pollinated crops (wheat, barley, pea, tomato), pollination occurs before flower opening (cleistogamy), and they are less dependent on weather conditions and insects, making them more reliable for cultivation in unfavourable conditions (Stern & Jansky, 2021).

Thus, flowering is not only an aesthetically important but also a economically critical phase, determining fruit and seed set. In the following sections we will examine the stages of pollination and fertilisation, as well as seed and fruit formation.

2.4. Pollination

Pollination is the process of transferring pollen grains from the anthers to the stigma (in angiosperms) or to the micropyle of the ovule (in gymnosperms). Pollination is a necessary precursor to fertilisation, but is not identical to it: pollination merely delivers the male gametophytes (pollen grains) into close proximity with the female gametophytes (embryo sacs) (Raven et al., 2013; Stern & Jansky, 2021).

Pollination success is a critical factor determining the future harvest. In agronomy, two main types of pollination are distinguished: self‑pollination and cross‑pollination, each with its own characteristics and significance for seed production and breeding.

Self‑pollination (autogamy)

Self‑pollination is the transfer of pollen from anthers to the stigma of the same flower or (less often) between different flowers of the same plant.

Mechanisms of self‑pollination. In many self‑pollinating plants (e.g., wheat, barley, pea, bean, tomato), pollination occurs before flower opening (cleistogamy). The stamens are positioned directly next to the stigma, and pollen germinates while still in the bud. This guarantees pollination even under unfavourable weather conditions and in the absence of pollinators (Stern & Jansky, 2021).

Agronomic significance. Self‑pollination ensures genetic uniformity of the offspring, which is important for preserving varietal traits. When growing self‑pollinating crops (wheat, barley, oat, pea, tomato, eggplant), no special pollination measures are needed, and isolation between varieties is required only to prevent mechanical admixture, not to prevent cross‑pollination (although a small amount of cross‑pollination may occur).

Cross‑pollination (allogamy)

Cross‑pollination is the transfer of pollen from the anthers of a flower of one plant to the stigma of a flower of another plant of the same species. It promotes genetic diversity of offspring, which increases the adaptive potential of the population, but creates dependence on pollinator agents.

Agents of cross‑pollination. The main vectors of pollen are wind (anemophily), insects (entomophily), and less often birds (ornithophily), bats (chiropterophily) or water (hydrophily) (Raven et al., 2013). For agronomists, the first two types are the most important:

Wind‑pollinated (anemophilous) crops: rye, maize, proso millet, sunflower (although sunflower is partially insect‑pollinated), beet, hemp, all cereal grasses. Their flowers are usually inconspicuous, without a perianth, with long filaments and large, often feathery stigmas. Pollen is dry, fine, produced in huge quantities. Successful pollination requires a certain humidity and wind speed. When growing wind‑pollinated crops for seed production, it is necessary to position the fields taking into account the wind rose and to ensure spatial isolation between different varieties (e.g., for maize – at least 300–500 m) to avoid cross‑pollination and loss of varietal purity.
Insect‑pollinated (entomophilous) crops: buckwheat, clover, alfalfa, sunflower, apple, cherry, plum, pear, cucurbits (cucumber, pumpkin, squash), flax (partially), mustard, coriander. Their flowers are bright, fragrant, often with nectaries. The main pollinators are bees, bumblebees, butterflies, flies. The yield of these crops directly depends on the activity and abundance of insect pollinators. In agronomic practice, migration of beehives to fields of buckwheat, sunflower, and entomophilous legumes is widely used. For example, the increase in buckwheat yield from bee pollination can reach 30–40% or more.

Adaptations preventing self‑pollination

In many cross‑pollinating plants, evolutionary mechanisms have evolved to prevent self‑pollination (self‑sterility):

Dioecy – male and female flowers are on different plants (sea buckthorn, willow, poplar, hemp). This completely excludes self‑pollination, but requires planting both male and female specimens to obtain a fruit harvest (Stern & Jansky, 2021).
Monoecy – male and female flowers are on the same plant but separate (maize – tassel and ears; cucumber, pumpkin). Self‑pollination between flowers of the same plant is possible but often limited.
Dichogamy – temporal separation of anther and stigma maturation within the same flower.
Protandry – anthers mature before the stigma (many Apiaceae, Caryophyllaceae, Asteraceae).
Protogyny – stigma matures before the anthers (Brassicaceae, Rosaceae, Plantaginaceae).
Heterostyly – different style lengths: flowers of one plant have long styles and short stamens, while those of another have short styles and long stamens (e.g., in buckwheat, primrose). Effective pollination occurs only between different forms.
Cytological (genetic) self‑sterility – the plant’s own pollen fails to germinate on the stigma or the pollen tube cannot reach the embryo sac due to genetic incompatibility. Widespread in fruit crops (cherry, plum, apple, pear), many legume grasses (clover, alfalfa), and rye.

The agronomist must take self‑sterility into account: for fruit orchards, several mutually pollinating varieties are planted; for cross‑pollinated grasses (clover, alfalfa), insect pollinators and isolation from other varieties are needed.

2.5. Fertilisation

Fertilisation is the process of fusion of male and female gametes, leading to the formation of a zygote – the first cell of a new diploid sporophyte. In angiosperms, fertilisation is preceded by pollen grain germination and pollen tube growth (Lersten, 2004; Raven et al., 2013).

The path of the pollen tube

After landing on the stigma, the pollen grain (male gametophyte) swells and germinates under the influence of stigma secretions (often containing sugars and specific proteins). A pollen tube emerges through one of the apertures of the pollen wall. It grows through the style tissue (through the stylar canal or between cells of the conducting tissue) and is directed into the ovary cavity, and then to the ovule. Pollen tube growth is a chemotropic process: it is guided by substances secreted by the synergids (cells of the embryo sac). As the pollen tube grows, two sperm cells (male gametes), formed from the vegetative and generative cells of the pollen grain, move into it. In gymnosperms, spermatozoa are motile (have flagella) and reach the egg cell with the help of water, while in angiosperms the sperm lack flagella and are passively transported by the pollen tube (Pandey et al., 2022).

Double fertilisation in flowering plants

A unique feature of angiosperms is double fertilisation, discovered by S.G. Navashin in 1898 (Lersten, 2004). The essence of the process:

The pollen tube enters the embryo sac through the micropyle (the opening of the ovule) and ruptures.
One of the two sperm fuses with the egg cell (haploid female gamete), forming a zygote (2n).
The second sperm fuses with the central cell of the embryo sac, which contains two haploid polar nuclei (sometimes already fused into a secondary nucleus). This results in a triploid (3n) cell, which then develops into the endosperm – the seed storage tissue rich in starch, oils and proteins.

Thus, double fertilisation produces two products: the zygote (giving rise to the embryo) and the triploid endosperm (providing nutrition for the embryo). This is one of the key evolutionary advantages of flowering plants, ensuring their dominance in the modern flora (Lersten, 2004).

Significance for seed production

Fertilisation success depends on many factors:

Compatibility of pollen and pistil (genetic self‑incompatibility system).
Physiological state of the plant (supply of nutrients, especially boron, potassium, phosphorus).
Weather conditions during flowering and pollen tube growth (optimal temperature +15…+25 °C, relative air humidity around 70% for many crops). Heat, frost, high humidity (washing away pollen by rain) or drought (drying of the stigma) sharply reduce seed set.

Understanding this stage allows the agronomist to adjust the technology: apply boron fertilisation (boron stimulates pollen tube growth), irrigation (to maintain optimal humidity), supplementary pollination (in greenhouses, using bees or artificial), and correctly select pollinator varieties for self‑sterile crops.

After fertilisation, the next stage of the generative period begins – seed and fruit formation, which will be discussed in the following sections.

2.6. Seed and fruit formation

Fruit formation diagram — ©Chiswick Chap commons.wikimedia.org CC BY-SA 4.0
Fruit and seed development from a flower
The diagram shows the path from a flower with ovary and ovules, through pollination and fertilisation, to the formation of a mature fruit (e.g., peach) and seed.

After fertilisation, processes begin in the flower that lead to the formation of seeds and fruits – the end products of the generative stage, which are of paramount agronomic importance. The seed is a mature ovule containing the embryo of a new sporophyte and a store of nutrients. The fruit is an enlarged and modified ovary (often with the participation of other parts of the flower) that protects the seeds and aids their dispersal (Lersten, 2004; Stern & Jansky, 2021).

Embryo development

The zygote (fertilised egg cell) is the first cell of the new sporophyte. It usually does not divide immediately but enters a resting period, which may last from a few hours to several weeks (longer in woody plants). During this time, the zygote becomes polarised and prepares for its first division. The endosperm (the triploid tissue formed during double fertilisation) begins to form by the time the zygote starts dividing, providing the future embryo with nutrients (Lersten, 2004; Yakovlev et al., 2008).

The process of embryo development from the zygote is called embryogenesis (Lersten, 2004). It includes the following main stages (using the example of dicotyledonous plants such as bean or cabbage):

First division of the zygote – asymmetric. Two cells are formed: a large basal cell (facing the micropyle) and a small apical cell (facing the central part of the embryo sac).
Formation of the suspensor. The basal cell divides repeatedly, forming a cellular strand – the suspensor, which pushes the embryo deeper into the endosperm and probably participates in its nutrition. The suspensor is a temporary structure that degenerates at later stages.
Division of the apical cell and formation of a spherical proembryo. Subsequent divisions lead to the formation of the globular embryo (globular stage).
Transition to the heart stage. The embryo flattens, and two bumps – cotyledon primordia – are initiated at its apex. In monocots, a single terminal cotyledon (scutellum) is formed.
Heart and torpedo stages. The cotyledons grow, the embryo elongates, becoming torpedo‑shaped. Future tissues differentiate: protoderm (will give epidermis), procambium (vascular tissues), ground meristem (parenchyma). At the base of the embryo, the root apex (radicle) with root cap is initiated, and between the cotyledons, the shoot apex (plumule) with the primordia of the first true leaves.
Mature embryo – a fully formed miniature sporophyte, usually curved inside the seed. In legumes, cucurbits, and Brassicaceae, the cotyledons may be large, fleshy and occupy almost the entire volume of the seed (exalbuminous seeds). In many other plants (e.g., cereals, Solanaceae, Apiaceae), the mature embryo is relatively small, and the bulk of the seed is occupied by endosperm (Lersten, 2004; Serebryakova et al., 2006).

In different plant groups, the details of embryogenesis vary, but the general scheme remains. In cereals (wheat, maize), the embryo has specialised structures: the scutellum (modified cotyledon), coleoptile (protective sheath of the shoot) and coleorhiza (protective sheath of the root) (Serebryakova et al., 2006; Stern & Jansky, 2021).

Formation of endosperm and perisperm

Endosperm is the storage tissue of the seed, formed as a result of double fertilisation from the central cell of the embryo sac after fusion with one of the sperm (Lersten, 2004). In most angiosperms, the endosperm is triploid (3n), which is a unique feature. Endosperm development can proceed in three types:

Nuclear type (nuclear). Initially, the endosperm is a multinucleate cytoplasm (free‑nuclear stage); then cell walls form between the nuclei, and it becomes cellular. Characteristic of many dicots (tomato, tobacco, peanut) and most monocots (cereals, palms).
Cellular type. Each nuclear division is accompanied by the formation of a cell wall from the beginning. Found in many dicots (e.g., Solanaceae, Asteraceae, Fabaceae – in pea, bean).
Helobial type (intermediate). The first division is accompanied by the formation of a wall dividing the endosperm into two parts, one of which develops by the nuclear type, the other by the cellular type. Rare (e.g., in some Campanulaceae).

The mature endosperm may be homogeneous (in cereals) or differentiated into an outer aleurone layer (rich in proteins) and an inner starchy part (in wheat, maize). In many legumes, cucurbits, and Brassicaceae, the endosperm is completely or almost completely absorbed by the growing embryo during seed maturation, and in the mature seed only the large fleshy cotyledons remain, serving a storage function (Lersten, 2004; Stern & Jansky, 2021). Such seeds are called exalbuminous.

Perisperm is a storage tissue formed from the nucellus (the central part of the ovule, of sporophytic origin). It occurs less frequently, usually in plants with a massive nucellus (e.g., Nymphaeaceae, Piperaceae, Caryophyllaceae). Perisperm can be haploid or diploid, unlike endosperm. In some seeds (e.g., beet, corncockle), both endosperm and perisperm may be present (Yakovlev et al., 2008; Serebryakova et al., 2006).

Fruit formation

The fruit develops from the ovary (and often from other parts of the flower – receptacle, hypanthium, sepals) under the influence of hormones produced by the developing seeds (primarily auxins and gibberellins). If fertilisation does not occur, the ovary usually withers and falls off. However, in some crops (banana, pineapple, cucumber, certain tomato varieties) parthenocarpic fruits – seedless, without fertilisation – can form (Stern & Jansky, 2021). Such fruits are valued in vegetable growing, but they do not produce seeds.

Fruits are divided into simple, aggregate and multiple (many‑seeded) fruits; a detailed classification is given in the plant systematics course. For agronomy, the division of fruits into:

Dry fruits – the pericarp becomes dry, leathery or woody upon ripening. Examples: caryopsis (cereals), achene (sunflower, buckwheat), legume (pea, bean, soybean), silique (cabbage, rapeseed), capsule (flax, poppy, cotton), nut (hazel).
Fleshy fruits – the pericarp is fleshy, juicy. Examples: berry (tomato, potato, grape, currant), drupe (cherry, plum, peach, olive), pome (apple, pear, quince), pepo (cucumber, pumpkin, watermelon), hesperidium (citrus).

Understanding the fruit type is important for determining the timing and methods of harvest, transportability and shelf life.

2.7. Fruiting and dissemination

Fruiting is the final stage of the generative stage, covering the period from the beginning of fruit formation to their full ripening and, in many cases, to the release of seeds. In agronomy, this stage is often subdivided into the phases of ovary set, fruit growth, ripening (technical and biological maturity) and senescence.

Seed ripening phases (using cereal crops as an example)

For most crops grown for seeds or fruits, determining the optimal maturity phase is key. The ripening phases have been studied in most detail in cereal grains (wheat, rye, barley, oat, rice, maize), but the general patterns apply to other plant groups as well (Serebryakova et al., 2006; Stern & Jansky, 2021).

Milk ripeness. The grain contains a milky‑white liquid with high sugar and water content. The grain is soft, easily crushed. Dry matter content is 30–40% of the final value. Grain moisture about 65–70%. The beginning of milk ripeness is an important guide for determining the timing of irrigation and fertilisation, especially with nitrogen.
Wax ripeness. The grain loses moisture, its contents acquire the consistency of wax, becoming plastic. When pressed with a fingernail, an indentation remains on the grain. Dry matter content – 50–65%. Grain moisture decreases to 35–45%. At the end of wax ripeness, the grain no longer loses viability and can be harvested by the swathing method (cutting into windrows), which accelerates ripening and reduces losses.
Full ripeness. The grain is hard, easily threshed from the ear. Moisture is almost eliminated from the grain (moisture 14–18% for wheat, 12–14% for rice). Dry matter content reaches 80–85% or more. The grain has the colour and size characteristic of the variety. This is the optimal phase for direct combining of most cereals. Delaying harvest leads to yield loss due to shattering (especially in shattering wheat) and damage by pests.

In leguminous crops (pea, bean, soybean), the ripening phases are determined by the change in colour of the pods and seeds (from green through yellow to brown), as well as by seed moisture. In oilseed crops (sunflower, rapeseed, flax), the phase of technical ripeness (accumulation of the maximum amount of oil at optimal seed moisture) is important.

Dissemination

Dissemination is the process of spreading seeds and fruits away from the mother plant. It has enormous biological significance: it allows avoidance of competition between the mother plant and its offspring, promotes species dispersal and colonisation of new habitats. In cultivated plants, dissemination mechanisms are often weakened or modified by breeding towards reduced shattering and improved harvestability (e.g., non‑shattering ears in cereals; pods that do not split open at maturity in legumes). However, in nature, and in weedy plants, dissemination is well developed (Stern & Jansky, 2021).

The main modes of dissemination:

Anemochory (by wind). Seeds and fruits have adaptations: wings (maple, ash, birch, pine), pappi of hairs (dandelion, thistle, fireweed), cottony hairs (poplar, willow). Tiny seeds (orchids, heathers, poppies) are dispersed by wind like dust.
Zoochory (by animals):
- Endozoochory – passage of seeds through the digestive tract of animals (many fleshy fruits: rowan, viburnum, bird cherry, raspberry; as well as apples, pears, cherries). The seeds are not damaged, and sometimes even stratified (e.g., in juniper).
- Epizoochory – attachment of fruits and seeds to animal fur, bird feathers or human clothing by means of hooks, bristles, sticky outgrowths (beggarticks, burdock, cleavers, stickseed). Also dispersal by ants (myrmecochory): seeds of many plants (violet, greater celandine, asarum) are equipped with fleshy appendages (elaiosomes) that ants eat, while the seeds are carried away.
Hydrochory (by water). Fruits and seeds have air‑filled tissues, are non‑wettable, and float. Characteristic of coastal and aquatic plants (sedge, cattail, water lily, black alder, coconut palm).
Autochory (self‑dispersal). Fruits open with force upon ripening, scattering seeds some distance. Characteristic of many legumes (pea, acacia, caragana – when the pod valves split), of “squirting cucumber” (Ecballium elaterium), of touch‑me‑not, of geranium (seeds are thrown out when the valves curl). In grasses with awns (feather grass, fescue), the seeds “drill” into the soil with changes in humidity.

Agronomic aspect: managing fruiting and harvest

For agricultural production, the following points are most important:

Determination of optimal harvest dates. Based on ripening phases (milk, wax, full) taking into account the intended use of the product (for grain, for oil, for feed, for seeds). For cereals, harvesting at full ripeness gives the maximum grain yield with minimal moisture, but requires well‑organised work to prevent shattering. For maize for silage, harvesting is done at the milk‑wax ripeness stage.
Prevention of shattering. Breeding for non‑shattering varieties, as well as timely and proper harvesting (direct combining or swathing followed by threshing). For some crops (flax, clover), desiccation (drying on the stem) is carried out to accelerate ripening and reduce moisture.
Weed control. Understanding the dissemination methods of weeds (especially zoochory and autochory) allows the development of control measures: timely mowing before seed ripening, cleaning of seed material, quarantine measures.
Seed production. Collecting seeds from plants at the stage of full (biological) ripeness, their proper drying, cleaning and storage ensure high germination and vigour. For some crops (beet, carrot, cabbage), seeds are collected selectively as they mature (two‑ or three‑time harvesting).

Thus, the stages of seed and fruit formation, as well as dissemination, represent a complex, genetically programmed process controlled by hormones and environmental factors. The completion of the generative stage in annual plants leads to ageing and death of the individual, while in perennials it leads to a state of dormancy (winter or summer), after which a new virginile stage begins (Gatsuk et al., 1980; Yakovlev et al., 2008). The connection with other stages of ontogenesis will be discussed in the following sections.

3. Features of the generative stage in different life forms

Plants of the same systematic group can differ significantly in the duration and nature of the generative stage depending on their life form (biomorph). A life form is the external appearance of a plant that has evolved as an adaptation to a set of environmental conditions (Serebryakova et al., 2006; Yakovlev et al., 2008). In agronomy, the most important division is that of plants based on lifespan and fruiting frequency into monocarpic and polycarpic.

Monocarpic plants (from Greek monos – one, karpos – fruit) flower and fruit once in their lifetime, after which they die. These include all annuals, most biennials, and some perennials (e.g., bamboo, agave).

Polycarpic plants (from Greek poly – many, karpos – fruit) flower and fruit repeatedly throughout their life. These are most perennial herbs, shrubs and trees.

Understanding these features underlies the development of crop rotation systems, soil tillage, care of perennial plantings and seed production.

3.1. Annual monocarpics

Annual monocarpic plants (true annuals) complete their full life cycle from seed to seed within one growing season. These include spring wheat, barley, oats, buckwheat, pea, bean, flax, sunflower (cultivated as an annual), tomato, cucumber (in cultivation) and many others.

Features of the generative stage: In annuals, the generative stage generally occurs after a certain time after emergence, when the plant has accumulated critical vegetative mass. After initiation of generative organs, terminal shoot growth ceases (monocarpic shoot). All plant resources are redirected to flowering, fertilisation and seed maturation. After seed maturation (biological ripeness), the mother plant completely dies.

Agronomic significance: The seed crop is the main product. It is important to provide optimal conditions for rapid progression through the generative stage (moisture, temperature, nutrients) and to prevent lodging or sprouting of grain on the stem. Annual varieties often have different maturity groups (early‑, mid‑, late‑season), which allows a conveyor‑belt production system.

3.2. Biennial monocarpics

Biennials are plants that require two growing seasons to complete their life cycle. In the first year, they form a rosette of leaves and a storage organ (taproot – in carrot, beet, turnip; bulb – in onion, garlic; head – in white cabbage), and in the second year, after overwintering and undergoing vernalisation, they form a flowering stem, flower, fruit and die (Gatsuk et al., 1980; Raven et al., 2013).

Features of the generative stage: The generative stage in biennials occurs only in the second year of life and only if low positive temperatures (vernalisation) act on dormant buds or seeds. If a biennial does not overwinter (or is not artificially vernalised), it will remain in a vegetative state and will not flower. The flowering shoot elongates (this phenomenon is called bolting or going to seed), often reaching a considerable height.

Agronomic significance: The crop (taproots, bulbs, heads) is formed in the first growing season. To obtain seeds, the mother plants must be left for the second year (or seed plants transplanted). In breeding and seed production, artificial vernalisation (stratification) is used to accelerate flowering and obtain seeds from biennials in one year (e.g., when growing beet seeds in southern regions).

3.3. Perennial monocarpics

This is a relatively small group of perennial plants that flower and fruit once at the end of their life, then die. They have limited importance in agronomy but are of biological and evolutionary interest (Gatsuk et al., 1980; Yakovlev et al., 2008).

Examples: Some bamboo species (flower once every 30–100 years and then die), American agave (flowers at 10–15 years of age), some palms (e.g., the talipot palm, flowering once every 40–80 years), and some tropical and subtropical herbs.

Features of the generative stage: The plant vegetates for a long time, accumulating biomass, and then, upon reaching a certain age (sometimes very considerable), initiates a huge generative inflorescence (in agave the flower stalk reaches 10 m). Flowering and fruiting exhaust all the plant’s resources, and it dies.

3.4. Perennial polycarpics

This is the most diverse and ecologically significant group, including most perennial herbs, shrubs and trees. They are capable of flowering and fruiting many times over their lifetime (Gatsuk et al., 1980; Serebryakova et al., 2006).

Features of the generative stage: After the first flowering, the plant does not die. In its above‑ground and below‑ground spheres, perennial skeletal axes and renewal buds are preserved, from which generative shoots grow again in the next season (or after several seasons). In polycarpics, the generative stage is subdivided into three age states (according to Gatsuk et al., 1980):

Young generative (g₁): plants begin to fruit, but the number of generative shoots is small, growth processes predominate over death. The crown shape or habit has not yet reached the typical dimensions for the species.
Middle‑aged (mature) generative (g₂): period of maximum fruiting. Formation of new organs and death of old ones are in equilibrium. The plant reaches the typical size and shape for the species (in trees – maximum height and crown diameter, in herbs – maximum number of generative shoots). This is the most productive phase for economic use.
Old generative (g₃): fruiting decreases, death processes prevail over formation. In trees, wood quality deteriorates, growth decreases, the crown becomes sparse. In perennial herbs, the structure simplifies, the number of generative shoots decreases, they become smaller. In many species, senile disintegration begins (the individual breaks up into separate particles).

Return to the vegetative stage: After the end of fruiting, polycarpics enter a dormant period (winter or summer), after which vegetative growth resumes. In herbaceous polycarpics, complete dieback of above‑ground shoots often occurs at the end of the growing season, and renewal occurs from perennial underground organs (rhizomes, bulbs, tubers, caudices). In woody plants, vegetative growth (shoot elongation, new leaf initiation) and preparation for flowering occur annually.

Below is a summary table summarising the features of the generative stage in the main life forms in an agronomic context.

Table. Features of the generative stage in different life forms

[cols="1,1,1,1"] |=== | Life form | Examples of agricultural crops | Typical lifespan | Features of the generative stage | Annual monocarpics | Spring wheat, barley, oats, buckwheat, pea, tomato (in cultivation), cucumber | 2–6 months | Flower and fruit once, then die. The generative stage is the only one in life, ends with complete death. Entire crop is seeds or fruits. | Biennial monocarpics | Carrot, beet, white cabbage, onion, parsley, celery | 12–16 months (two seasons) | In the first year they vegetate, accumulate reserves. In the second year, after vernalisation, they flower, fruit and die. Seeds are obtained in the second year. | Perennial monocarpics | Agave, some bamboos, some palms | From 5–10 to 100 years or more | Flower once at the end of life, then die. Rare in agronomy (ornamental, industrial crops). | Perennial polycarpics (herbs) | Clover, alfalfa, timothy, fescue, lawn grasses, strawberry, rhubarb | From 3–5 to 20–30 years or more | Flower and fruit many times, annually forming new generative shoots. Above‑ground part often dies back in winter. Crop (green mass, hay, seeds) obtained for many years. | Perennial polycarpics (shrubs) | Currant, gooseberry, raspberry, honeysuckle, sea buckthorn, hazel | From 10–15 to 30–50 years | Flower and fruit annually, starting from a certain age (usually at 2–4 years). Have perennial skeletal branches and bark. Crop of fruits and berries. | Perennial polycarpics (trees) | Apple, pear, cherry, plum, peach, apricot, walnut, chestnut | From 20–30 to 100–200 years or more | Begin fruiting at 3–10 years (pome fruits) or 3–6 years (stone fruits). Peak fruiting at 10–30 years. Repeated annual flowering and fruiting. Require special pruning for rejuvenation. |===

Thus, the same stages of the generative stage (flowering induction, organ formation, pollination, fertilisation, seed and fruit ripening) occur in different life forms with substantial differences in duration, rhythmicity and age dynamics. In the next section, we will consider the factors influencing the success of the generative stage and the relationship of this stage with the virginile and senile stages of ontogenesis.

4. Factors affecting the success of the generative stage

The success of the generative stage – from flowering induction to seed maturation – depends on a complex of external and internal environmental factors. Even with a genetically fixed development programme, its realisation may be disrupted or substantially modified by abiotic (light, temperature, humidity, mineral nutrition), biotic (pollinators, competitors, pathogens) and anthropogenic (agronomic) factors. Understanding these influences underlies the development of technologies that ensure stable high yields.

4.1. Abiotic factors

Light

Light acts on the generative stage in two main ways: as an energy source for photosynthesis and as a signalling factor (photoperiod).

Light intensity. Lack of light (crop density, cloudy weather) leads to reduced photosynthesis, limiting the supply of assimilates to the generative organs. Consequences: reduced number of flowers, their abortion (fall), reduced fruit set, formation of shrivelled, poorly filled seeds. Many cereals and legumes are particularly sensitive to shading during flowering and grain filling (Raven et al., 2013; Stern & Jansky, 2021).

Photoperiod. As already noted in section 2.1.1, day length serves as a trigger for flowering induction in photoperiodic plants. Disturbance of the natural photoperiod (e.g., when using artificial lighting or when moving a variety to a different latitudinal zone) can completely suppress flowering or delay it indefinitely. This has enormous significance for variety zoning (Bidlack & Jansky, 2021).

Temperature

Temperature affects all stages of the generative stage, but the most critical periods are flowering and fertilisation.

Low temperatures (frosts) during flowering lead to flower damage: death of pollen (especially sensitive), necrosis of the stigma, damage to the ovary. In fruit crops, this can completely destroy the current year’s crop. In cereals, even short‑term frosts during heading‑flowering cause empty spikelets (“blanking”). The critical temperature for flowering plants of most crops is close to 0…-2 °C, but there are relatively cold‑resistant species (e.g., apple, cherry) and very sensitive ones (peach, apricot, early tomato varieties) (Raven et al., 2013).

High temperatures (heat, above 30…35 °C) during flowering are also dangerous. They cause pollen sterility (disruption of its formation and germination), accelerated stigma wilting, and suppression of pollen tube growth. In many crops (tomato, cucumber, pepper, bean, wheat), high temperatures sharply reduce fruit set, even with normal flowering (Bidlack & Jansky, 2021).

Temperature and vernalisation. As described in section 2.1.2, for winter and biennial crops, low positive temperatures (1…7 °C) are a necessary condition for the transition to flowering. Absence of vernalisation or its insufficiency (mild winters) causes plants to remain in a vegetative state and not bear fruit (Raven et al., 2013).

Optimal temperatures for pollen tube growth and fertilisation lie within 18…25 °C for most temperate crops and 25…30 °C for thermophilic ones (maize, rice, soybean, cotton). Deviations from the optimum slow or completely block the fertilisation process.

Humidity

Water status is important at all stages of the generative stage, but is especially critical during flowering, fertilisation and grain filling.

Water deficit (drought) during flowering causes drying of the stigma, reduction of nectar secretion (in entomophilous crops), decreased turgor in perianth cells, which may disrupt the flower opening process. The most serious consequence is disruption of pollen germination and pollen tube growth, leading to mass non‑fertilisation and empty grains (Raven et al., 2013; Stern & Jansky, 2021). In maize, drought during flowering (“dry wind”) causes “tipped” ears (the upper part of the ear without kernels).

Excessive humidity (rainy weather) during flowering is also unfavourable: pollen is washed off stigmas, swells and bursts on anthers, pollen tubes cannot grow properly. In wind‑pollinated crops (rye, maize), rain mechanically impedes pollen transfer. In entomophilous crops (buckwheat, clover, sunflower), rainy and cold weather reduces the flight activity of insect pollinators.

Humidity during seed formation and filling. During the filling phase (milk and wax ripeness), plants have a particularly high water requirement. Water deficit during this period sharply reduces the 1000‑seed weight (grain test weight, plumpness). However, at the end of ripening (full ripeness), dry weather is desirable for uniform drying and prevention of seed sprouting on the stem (especially in wheat, barley, rapeseed).

Mineral nutrition

Nitrogen. Excess nitrogen prolongs vegetative growth and delays the onset of the generative stage, and also leads to crop lodging (especially in cereals), which disrupts pollination and worsens seed formation conditions. Nitrogen deficiency reduces photosynthetic productivity, limiting seed formation. Optimal nitrogen supply (especially at the beginning of the stem elongation phase in cereals) promotes good development of generative organs (Bidlack & Jansky, 2021).

Phosphorus. Phosphorus accelerates the transition to the reproductive phase, stimulates flowering, improves fruit set and seed ripening. Phosphorus deficiency (especially at an early age) sharply reduces yield and seed quality. Phosphorus is especially important for legumes, as it participates in the functioning of nodule bacteria and protein accumulation in seeds (Raven et al., 2013).

Potassium. Potassium promotes the outflow of carbohydrates from leaves to generative organs, increases resistance to lodging and drought, improves seed quality (test weight, 1000‑grain weight). Potassium deficiency leads to the formation of shrivelled, wrinkled seeds with low germination.

Trace elements.

Boron (B) – one of the most critical elements during flowering and fertilisation. Boron stimulates pollen germination and pollen tube growth. Its deficiency leads to massive flower and ovary drop, empty ears in cereals, “heart rot” in beet, empty pods in pea and bean (Stern & Jansky, 2021).
Manganese (Mn) and zinc (Zn) – participate in the hormonal regulation of flowering and formation of reproductive organs. Their deficiency can also cause sterility and reduced yield.
Copper (Cu) – important for seed protein synthesis, especially in cereals (affects the quality of wheat gluten).

Thus, balanced mineral nutrition, especially during the critical “flowering – grain filling” phases, is a necessary condition for realising potential yield.

4.2. Biotic factors

Biotic factors include the influence of pollinators, competitors (weeds), as well as pathogens and phytophages.

Pollinators. For entomophilous crops (buckwheat, clover, alfalfa, apple, cherry, sunflower, rapeseed), the presence and activity of insect pollinators (bees, bumblebees) is a critical factor. The absence of pollinators or unfavourable weather conditions limiting their flight leads to massive non‑pollination and crop failure (Bidlack & Jansky, 2021). For wind‑pollinated crops, the wind regime and spatial structure of the crop (overcrowding or thinness) are important for effective pollen transfer.

Weeds. Competition with weeds for light, water and nutrients during the generative stage particularly strongly affects the harvest. Weeds that flower simultaneously with the crop can become sources of pollen (for wind‑pollinated species) or attract insect pollinators away from the crop plants (Stern & Jansky, 2021). In addition, weed seeds contaminate the harvest and reduce its quality.

Diseases and pests. Infection of generative organs (flowers, ovaries, fruits, seeds) by diseases (e.g., smut and ergot of cereals, brown rot of fruit, grey mould of strawberry) and pests (codling moth, sunn pest on cereals, apple blossom weevil) can completely destroy the crop. Integrated plant protection during the generative stage is an essential component of agronomic practice.

4.3. Anthropogenic (agronomic) factors

Humans can purposefully influence the success of the generative stage by optimising other factors and using special techniques:

Sowing dates. Correct choice of sowing date allows the critical flowering phase to be “steered away” from unfavourable weather conditions (frost, heat, drought) and synchronised with the optimal photoperiod.
Seeding rate and planting pattern. Optimal plant density ensures good illumination and aeration of generative organs, which improves pollination and reduces the risk of disease infection (especially in cereals and industrial crops).
Fertiliser application. Timely fertilisation during the stem elongation (in cereals) and budding‑flowering (in row and vegetable crops) phases with nitrogen, phosphorus, potassium and trace elements (especially boron) significantly increases fruit set and seed quality.
Irrigation. Carrying out growing‑season irrigation during critical phases (budding‑flowering, seed filling) avoids water stress and ensures normal fertilisation and filling.
Artificial pollination and use of pollinators. In protected cultivation (greenhouses), artificial pollination (shaking plants, using specialised pollinators) or introduction of bees and bumblebees is used. In open ground, for entomophilous crops, migration of beehives to the fields during flowering is practiced (Bidlack & Jansky, 2021).
Treatment with growth regulators. The use of gibberellins (to enhance flowering and fruit set), retardants (to prevent lodging and improve seed filling), desiccants (to dry plants before harvest) allows management of the generative stage on an industrial scale (Raven et al., 2013).
Plant protection (fungicides, insecticides). Treatments against diseases and pests during the budding‑flowering and fruit‑setting phases are necessary to preserve the harvest.

Thus, the success of the generative stage is an integral result of the interaction of the plant’s genetic programme with a complex of abiotic, biotic and anthropogenic factors. By managing these factors, the agronomist can minimise risks and ensure maximum realisation of the productive potential of agricultural crops. In the next section, we will consider the importance of the generative stage in agronomy and breeding, as well as the connection with the virginile and senile stages of ontogenesis.

5. Significance of the generative stage in agronomy and breeding

The generative stage of ontogenesis is central to agricultural production because it is during this stage that the economically valuable crop – seeds, fruits, infructescences – is formed. Understanding the patterns of this stage, the factors that influence it, and the possibilities for managing it is the foundation of modern crop production, seed production and breeding (Gatsuk et al., 1980; Raven et al., 2013; Bidlack & Jansky, 2021).

5.1. Formation of the seed and fruit crop

In most field, vegetable and fruit crops, it is precisely the generative organs (grain, legume, capsule, berry, drupe, pome) that are the marketable product. The quantity and quality of this product are determined by the success of all stages of the generative stage: from flower initiation to full seed maturation.

In cereals and pseudocereals (wheat, rice, maize, buckwheat) the crop is seeds (caryopses). The main elements of productivity: number of productive stems (tiller number), number of spikelets (or grains) per inflorescence, 1000‑grain weight (test weight). All these parameters are formed precisely during the generative period.
In oilseed crops (sunflower, rapeseed, flax, soybean) the crop is seeds rich in oil. The oil content and quality are determined during the filling and ripening stages.
In fruit and berry crops (apple, cherry, currant, strawberry) the crop is fleshy fruits that develop from the ovary and often from other parts of the flower (receptacle, hypanthium). Fruit set, their size, taste, aroma, shelf life – all depend on successful pollination, fertilisation and subsequent development.
In vegetable crops (tomato, cucumber, pepper, aubergine, pumpkin) the generative stage is also productive: fruits are harvested at technical or biological maturity. In root crops (carrot, beet) and leafy crops (cabbage, lettuce), the crop is formed during the virginile stage, but to obtain seeds (seed production) it is necessary to ensure the generative stage.

5.2. Reproductive capacity of a variety and quality of seed material

Reproductive capacity (seed productivity) is the property of a variety to form a certain number of viable, high‑quality seeds. It includes:

Potential productivity – the maximum number of seeds that can be formed under optimal conditions.
Actual (economic) productivity – the number of seeds actually formed, taking into account losses (abortion of flowers and ovaries, damage by pests, diseases, unfavourable factors).

The most important indicators of seed quality:

Germination – the ability of seeds to produce normal seedlings under standard conditions. It is formed during the ripening process and depends on the completeness of the endosperm and embryo (Lersten, 2004; Stern & Jansky, 2021).
Germination energy – the speed and uniformity of germination. It is related to physiological maturity and the absence of deep dormancy.
1000‑seed weight (test weight) – an integral indicator of seed plumpness. It is determined by filling conditions (moisture availability, temperature, mineral nutrition) and the genetic characteristics of the variety.
Seed quality standards – seeds must be clean, healthy, free of impurities, with high germination. This is achieved by proper organisation of seed production during the generative stage (timely harvesting, threshing, cleaning, grading).

Seed production is a special branch of crop production, the aim of which is to multiply seeds of elite and regionalised varieties while preserving their genetic and economically valuable traits. In seed production, the following are critically important:

Spatial isolation of varieties for cross‑pollinated crops (rye, maize, sunflower, beet) to avoid cross‑pollination and loss of varietal purity (Stern & Jansky, 2021).
Pollination control (artificial pollination, use of bees).
Timely harvesting at full ripeness to obtain seeds with maximum germination and minimal moisture.
Post‑harvest drying and cleaning to standard parameters.

5.3. Managing the generative stage in crop production

The agronomist can actively influence the generative stage using the following agronomic practices:

Selection of sowing dates. Allows “steering” flowering away from frost (for early spring crops), from summer drought (for late crops), or from an unfavourable photoperiod. For example, winter crops are sown in autumn so that they undergo vernalisation in winter and flower in optimal spring‑summer periods (Raven et al., 2013).
Use of growth regulators.
- Retardants (chlormequat chloride, tebuconazole, mancozeb‑containing) inhibit gibberellin synthesis, shorten internodes, prevent lodging of cereals and flax, thereby improving pollination and grain filling conditions (Bidlack & Jansky, 2021).
- Gibberellins are used to enhance flowering and fruit set in some crops (grape, citrus, apple), as well as to induce flowering in biennials without vernalisation.
- Auxins are used to prevent abscission of ovaries and fruits (pre‑harvest drop in apple, pear).
Irrigation during critical phases. Irrigation during the “flowering – grain filling” period (for cereals) or “budding – flowering – fruit set” (for vegetables and fruits) is critical for obtaining a yield. Drip irrigation allows precise dosing of water and mineral fertilisers (fertigation) (Stern & Jansky, 2021).
Foliar fertilisation with trace elements. Foliar boron fertilisation during the budding‑flowering phase is particularly effective, significantly increasing seed and fruit set.
Plant training (pruning, topping). Removal of terminal inflorescences (topping) in cotton, tomatoes, cucurbits redirects assimilates to the remaining fruits, increasing their size and accelerating ripening. In fruit trees, pruning improves crown illumination and fruit quality.
Use of pollinators. Migration of beehives to fields of buckwheat, sunflower, entomophilous legume grasses, alfalfa, clover, as well as to orchards (apple, cherry, plum) – an essential agronomic practice that increases yield by 30–50% or more (Bidlack & Jansky, 2021).
Disease and pest control. Integrated plant protection during the budding‑flowering and fruit‑setting phases (e.g., against sunn pest on wheat, codling moth on apple, grey mould on strawberry) prevents direct yield losses.

5.4. Significance of the generative stage in breeding

The generative stage is a key object of breeding programmes. Breeders work with the following traits:

Productivity and its components (number of productive inflorescences, number of seeds per fruit, 1000‑seed weight, oil content, protein content).
Earliness – duration of the generative stage. Creation of early‑maturing varieties allows avoiding frost, drought and fitting into a short growing season (northern regions).
Product quality: fruit size, taste, aroma, shelf life, technological properties (gluten content in wheat, oil content in sunflower, sugar content in beet).
Resistance to abiotic and biotic stresses during the generative period: cold tolerance of flowers, heat tolerance of pollen, drought tolerance during filling, resistance to diseases (smut, ergot, brown rot) and pests.
Type of pollination and self‑sterility/self‑compatibility. Creation of self‑compatible varieties for cross‑pollinating crops simplifies seed production (e.g., in rye, beet).
Suitability for mechanised harvesting: non‑shattering of seeds, uniformity of ripening, strength of peduncles (in cereals – lodging resistance).

Breeding methods such as hybridisation and selection, as well as the use of heterosis (production of F1 hybrid seeds), are directly related to the management of the generative process (controlled pollination, flower emasculation, isolation).

5.5. Relation to product quality and harvest timing

The quality of agricultural products is often determined by the stage of ripeness at which harvesting is carried out:

Cereals: harvesting at full ripeness gives grain with minimal moisture, maximum 1000‑grain weight and best germination. Delay leads to shattering and reduced quality. For certain purposes (production of groats, malting barley), specific grain conditions are required.
Oilseeds: harvesting is carried out when the maximum amount of oil has accumulated (technical ripeness) and seed moisture has decreased to a safe level (sunflower – to 12–14%, rapeseed – to 8–10%).
Grain legumes: harvesting at full ripeness, when the pods become yellow or brown and the seeds become hard. Delay leads to pod shattering (especially in soybean, pea) and seed loss.
Fruit and vegetable crops: the harvest date determines taste quality, shelf life and transportability. For apples, pears, tomatoes, cucumbers, there are varieties differing in ripening times (early, mid‑, late), which allows a conveyor‑belt supply of products.

Thus, the generative stage is not only a biological but also an economic axis of all agricultural production. Knowledge of its stages, influencing factors and management methods is necessary for sustainable provision of the population with food and high‑quality seeds. In the next, concluding section (6) we will examine the connection of the generative stage with other stages of ontogenesis – the virginile and senile stages.

6. Relationship with other stages of ontogenesis

The generative stage is not an isolated period in the life of a plant. It is naturally prepared by the preceding virginile (vegetative) stage and, in turn, determines the onset of the subsequent senile stage. The nature of the relationships between these stages differs between monocarpic (annuals, biennials, some perennials) and polycarpic (perennial herbs, shrubs, trees) plants (Gatsuk et al., 1980; Yakovlev et al., 2008).

6.1. Transition from the virginile stage to the generative stage

The virginile (vegetative) stage (virginile, v according to the classification of Gatsuk et al., 1980) is the period when the plant has attained the typical vegetative structures (leaves, stem, root system) for the species but is not yet capable of flowering and fruiting. The transition to the generative stage (first flowering) occurs when two sets of conditions are met:

Attainment of a certain age and size (ontogenetic competence). The plant must accumulate a critical vegetative mass, form a sufficient number of leaves and meristems to “afford” the cost of reproduction. In annual plants this period is short (a few weeks), in perennials it can last for years (in ash – 30–50 years, in apple – 3–5 years) (Gatsuk et al., 1980; Raven et al., 2013).
Action of inducing environmental factors (photoperiod, vernalisation) or internal hormonal signals. As described in section 2.1, these factors trigger the switch of the apical meristem from a vegetative to a generative programme.

In annual monocarpics, the transition from the virginile to the generative stage is generally irreversible. After initiation of flowers, vegetative shoot growth ceases, and all resources are directed to seed formation. In perennial polycarpics, the transition is reversible: after the end of fruiting and the onset of a dormant period (winter or summer), the apical meristems again begin to form vegetative organs (leaves, shoots), and the plant returns to a virginile state until the next generative cycle (Gatsuk et al., 1980; Serebryakova et al., 2006).

6.2. Succession of generative cycles in polycarpic plants

In perennial polycarpics (herbs, shrubs, trees), after the first flowering, repeated (secondary, tertiary, etc.) generative cycles occur. Between cycles, the plant is in a virginile (or near‑virginile) state and actively vegetates.

It is important to distinguish between senile stage and dormancy:

Dormancy is a reversible physiological state in which growth and metabolism are sharply slowed down. It is necessary to survive an unfavourable season (winter, drought). After dormancy ends, the plant resumes vegetative growth and can flower again (Raven et al., 2013; Bidlack & Jansky, 2021).
Senile stage is an irreversible phase of ageing leading to the death of the individual. At this stage, reproductive capacity is sharply reduced or completely lost.

In polycarpics, several age states are distinguished within the generative period (according to Gatsuk et al., 1980):

Young generative plants (g₁): first flowering, the number of generative shoots is small, but already noticeable.
Middle‑aged (mature) generative plants (g₂): maximum fruiting, equilibrium between formation and death of organs.
Old generative plants (g₃): fruiting decreases, death processes begin to predominate (dieback of skeletal axes, reduction in flower number, reduced seed quality).

After the old generative plants, the subsenile (ss) and then senile (s) stages follow, in which flowering and fruiting completely cease, and vegetative structures simplify (leaves become smaller, sometimes acquiring juvenile features) (Gatsuk et al., 1980; Yakovlev et al., 2008).

6.3. Onset of the senile stage and completion of ontogenesis

The senile stage is characterised by:

Cessation or sharp reduction of reproductive capacity. The plant no longer produces flowers, or produces a few that do not yield viable seeds.
Simplification of morphological structure: in trees and shrubs, large skeletal branches die off, the crown becomes sparse, “stag‑headedness” appears; in perennial herbs, the number of shoots decreases, they become shortened, leaves become smaller.
Return of juvenile traits: in some species, leaves similar to juvenile ones (in shape, pubescence) reappear at the senile stage (Gatsuk et al., 1980).
Breakdown of the individual into particles (vegetative propagation). In many herbaceous polycarpics (e.g., tussock grasses, long‑rhizomatous plants) and some shrubs, natural particulation occurs at the senile stage – the disintegration of the mother individual into several younger particulates (clones) that may continue vegetative existence (Serebryakova et al., 2006). This phenomenon should not be confused with generative renewal.

In annual monocarpics, the senile stage occurs immediately after the completion of the generative stage: the plant completely dies, leaving seeds. In biennials – after fruiting in the second year of life.

In some tree species (e.g., ash, beech), the senile stage may be absent or very short: old trees continue to bear fruit until death (Gatsuk et al., 1980).

6.4. Generalised scheme of the relationship between ontogenetic stages

Below is a scheme summarising the sequence and interrelation of age states (according to Gatsuk et al., 1980 with additions).

Table. Scheme of age state succession (ontogenetic stages) in flowering plants

In perennial polycarpic plants, multiple transitions from the generative state to the virginile state (after a dormant period) and back are possible. In annual monocarpics, the generative stage is immediately followed by the senile stage and death.

Thus, the generative stage occupies a central place in ontogenesis, being a logical continuation of the virginile stage and preceding the senile stage. Understanding these relationships is necessary for managing the productive process (especially in perennial crops such as fruit trees, berries, perennial grasses) and for developing seed production systems. In the next, concluding section (7) we will summarise and suggest review questions for self‑assessment.

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