Factors Influencing the Formation, Development of Buds, and Flowering of Temperate Fruit Trees

Abstract

The condition for the formation of fruit on fruit plants is the presence of flower buds, flowering and proper pollination/fertilisation of flowers. Fruit trees and shrubs are perennial plants, and the processes of flower bud formation and flowering are distant in time. The formation of flower buds occurs in the year preceding flowering and fruiting. The number and quality of flowers are the basic factors that determine the potential yield of fruit trees. Therefore, the review focuses on a thorough review of the latest research on the various stages in the development of trees, in which the processes that determine their flowering take place. The greatest emphasis was placed on the influence of factors that determine the yield of trees after the juvenile stage. Climate change leading to global warming will undoubtedly affect the formation of flower buds, which determine the size of crops. To avoid the unforeseen effects of abiotic factors on the availability of raw materials, such as fruits, it is good to diversify the structure of cultivated plants. Most fruit plants come from the Rosaceae family, so they have many pathogens and pests in common. To increase crop, economic, and habitat biodiversity, it is necessary to look for other, more genetically distant, sometimes even less known fruit-bearing species.

1. Genetic Diversity Among Temperate Fruit Plants

Fruit plants are found on all continents except Antarctica. Among the fruits of a temperate climate, there are perennial plants that differ in morphology [1]. These can be trees, shrubs, and perennials. There are also differences between them in physiological processes related to vegetative growth, flowering, and fruiting. In the 18th century, the Swedish naturalist, Carl Linnaeus, created the first system of plant classification, introducing the two-part name of the species, which is still used today. It was based on the similarity of the morphological features of the flower. Modern classification systems also place emphasis on phylogenetic relationships, i.e., the order in which species arise (their origin) [2].

2. Systematics of Fruit Plants

Most fruit plants grown in temperate climates belong to the Rosaceae family (Figure 1). Plants with very economically important fruits also occur in another family of the order Rosales—Grossulariaceae. Ribes is generally considered the only genus in the family Grossulariaceae [3]. The application of RAPD analysis to the study of genetic polymorphism and phylogenetic associations proved that gooseberries and currants are closely related to each other [4]. Grapevines (of the order Rhamnales) are also grown on a very large scale for the purpose of fruit, mainly for processing, but also as a dessert fruit (

Table 1. Description of generative parts of fruit plants and place of their formation [own work based on 5–9].

Species	Place of Flower Bud Formation	Description of Buds	Description of Flowers	Description of Fruit
Monoecious species
Corylus avellana.	Male buds are single in the axils of the scaly leaflets. Female buds on the flanks or at the end of last year’s shoot.	Male buds in the form of long, roller-shaped catkins. Female buds are ovoid and flattened.	Male in inflorescences (♂K0C0A4G0). Female as small, red-centred clusters (♀K0C0A0G2).	Globular-ovoid achene composed of a woody pericarp containing the kernel, an edible nut [5].
Juglans regia	Male flowers at the end of the shoot and on the axils of the leaves. Female flowers at the top of the shoots from this year.	Male flower buds, single, less often several, in the form of shortened catkins. Female flower buds are gathered in spikes of 2–24, depending on the cultivar.	Male inflorescences, 5–10 cm long with many stamens (♂K0C0A~G0). The female flower has a pistil with two stigmas (♀K0C0A0G2).	A pseudo-drupe, it is formed from a lower pistil, covered with the remains of the perianth. The soft covering is the exocarp, whereas the hard shell is the endocarp.
Hermaphoriditic species
Cornus mas	The flowers are on short stems, opposite each other.	The flower buds are round and larger. The leaf buds are narrowly conical and pointed [6].	Small yellow flowers are clustered in umbels (* K4C4A4G1).	A juicy drupe weighing about 2–8 g.
Vaccinium corymbosum	The flowers are at the ends of one-year-old shoots or on short shoots [7].	Flower buds (inflorescence) are clearly different from leaf buds.	The petals of the corolla of the flowers (4–5) and the sepals of the calyx are fused, white or pinkish, and bell-shaped. The calyx is fused to the ovary.	A pseudoberry that develops not only from the lower ovary but also from the sepals and the floral receptacle [8].
Vitis vinifera	Flowers are formed on fruiting shoots, growing on 3–4 nodes of the stem.	Buds are formed on a one-year-old cane.	The flowers are inconspicuous, with yellow-green petals of the corolla, gathered in panicle-type inflorescences.	Berry.
Amelanchier canadensis	The flowers are on one-year-old, well-lit shoots.	Flower buds are thicker and rounder than vegetative ones.	White, gathered in several or a dozen or so clusters of inflorescences, 12 cm long	False fruits, developed from the ovary and the expanded floral receptacle, are spherical, and have a mass of 0.5–0.7 g.
Aronia melanocarpa	Flowers at the ends of the branches [9].	Flower and leaf buds are similar. Leaf buds are smaller, pointed and attached to the shoot.	Flowers in corymbs, white or pale pink, with stamens with purple anthers.	False fruits, about 1 g, dark blue, almost black, with a waxy coating.
Chaenomeles japonica	The flowers are grouped on short shoots.	Flower heads are spherical, arranged in several. Leaves in threes, central conical, lateral ones smaller and almost spherical.	Brick red, sitting on branches in bunches of 2–6, up to 5 cm in diameter.	Pseudo-fruit, yellow, apple-shaped, sometimes with a pink-red blush. The flesh is hard, sour, and very aromatic.
Crataegus monogyna, C. laevigata	Flowers set on short shoots.	The terminal buds are wide and conical; the lateral buds are ovate or almost spherical, protruding. The flower buds are slightly larger.	White or pink, five-petalled, forming terminal corymbs. C. monogyna has one pistil (* K5C5A~G-1) and C. laevigata has two (* K5C5A~G-2).	Pseudo-fruit, apple-like, spherical, brownish-red. The fruit of C. monogyna has one seed with a brittle hull, and C. laevigata has two or three seeds without a hull.
Cydonia oblonga	The flowers are formed on short shoots from the previous year and on short shoots.	The buds are conical, rounded at the top, adhering to the shoot.	White-pink, five petalled (K5C5A~G-(5)). The flowers are on short, hairy pedicels.	A pseudofruit. Shape variable, depending on cultivar. It has large, often green calyx sepals. Lemon yellow, covered with hairs.
Malus domestica	The location of flower buds, depending on the cultivar, on short shoots or long shoots.	The flower buds are always larger, rounded and more hairy. The leaf buds are pointed.	Inflorescences with 5–7 flowers. Flowers on 1–2 cm pedicels, white or pink, five-petalled, 5 sepals, 15–50 stamens, and 1 lower pistil (K5C5A~G-(5)).	Pseudofruit, formed from the ovary, the floral receptacle, the sepals, the petals of the corolla and the stamens. Fruit weight from 25 to 300 g.
Mespilus germanica	The flowers develop at the ends of short branches.	The winter buds are pointed, ovoid and up to 5 mm.	The flowers are single, five-petalled, white (K5C5AG-(5)).	Pseudofruit, spherical or pear-shaped, with 5 calyx sepals at the top. Weight from 3 to 35 g.
Pyrus communis	Flowers on the tops of shoots, 2–3-year-old wood and on short shoots.	Mixed buds are larger and rounder than leaf buds.	Flowers are 5–9 in number, collected in umbels of shaped inflorescences (K5C5A~G-(5)).	Pseudofruit. It has stone cells that form around the sieve-vascular bundles. Fruit weight from 25 to over 300 g.
Sorbus aucuparia	The flowers are formed at the ends of the shoots and on short shoots.	Mixed buds: the apical ones are large, slightly bent at the top and indistinctly three-scaled; the lateral ones are apparently single-scaled.	Fivefold (* K5C5A~G-(3–5)), numerous, white-yellow, small, gathered in terminal corymbs.	A pseudo-fruit in the shape of a berry. Initially orange, ripe scarlet red.
Prunus armeniaca	Flowers are formed on one-year-old shoots and spurs.	The leaves are smaller and broadly conical; the flowers are larger and spherical.	The flower is almost sedentary. The petals are white or slightly pink.	A drupe weighing from 9 to 90 g [9].
Prunus avium	The flowers are most often found at the base of long shoots and on short shoots.	The buds are not very differentiated, the flower buds are larger, wider and rounder.	Flowers (* K5C5A10G1) are gathered in umbels, usually 2–3, white.	Drupe.
Prunus cerasifera	The flowers are on spurs of perennial shoots.	Single or triple leaf buds, central leaf bud conical, lateral flower buds ovate.	Flowers (* K5C5A10G1) in buds are usually single, diameter 2–2.5 cm.	A spherical drupe weighing about 20–30 g, yellow or red in colour.
Prunus cerasus	The flowers are on one-year-old shoots or at the base of two-year-old shoots.	The buds are not very differentiated. Ovoid, pointed at the top, deviating from the shoot.	Flowers (* K5C5A10G1) are gathered in several in the inflorescence, white.	A spherical, single-seeded drupe of red colour.
Prunus domestica	Flower buds are formed on long and short shoots, often several in number.	The leaf bud is pointed; the flower bud is smaller and more rounded.	Flowers (* K5C5A10G1), gathered in groups of 2–3 on short pedicels, 1.5–3 cm in diameter.	A drupe weighing 10–50 g.
Prunus persica	Flower buds are formed on one-year-old shoots.	Flower buds broadly ovate, leaf buds smaller, narrowly conical, often in threes: central leaf bud and lateral flower buds.	Two types of flowers, depending on the cultivar. With wide petals—rosaceous—and with narrow petals—bell-shaped (* K5C5A10G1).	Spherical drupe weighing 60 to 300 g.
Prunus serotina	The flowers are formed on the tops of the shoots and young shoots.	Buds narrowly egg-shaped, pointed, adhering to or slightly protruding from the shoot.	White flowers (* K5C5A10G1) are gathered in clusters.	A single-seeded drupe, spherical, black, and shiny.
Prunus spinosa	The flowers are formed mainly at the end of young shoots.	The buds are small, single or in pairs or threes. Leaf buds are wider and pointed; flower buds are spherical.	Flowers singly (* K5C5A10G1) or in clusters of 2 (−5), up to 20 mm in diameter.	A spherical drupe, navy blue in colour, covered with a waxy coating.
Fragaria ananassa, F. vesca	The flowers arise from leaf buds on the shoot crown.	Flower buds of 5–25 flowers in the inflorescence at the top in the rosette.	White corolla, occurring singly or in inflorescence (* K5C5A~G~).	Aggregate fruits, developed from multiple ovaries with the fleshy part being the swollen receptacle and the “seeds” being the actual fruits (achenes).
Rosa canina	Flowers are formed on woody shoots that are two years old or older.	Apical buds are wide, rounded, lateral protruding, and variable in shape.	The flowers are pink or whitish, growing singly or in groups of several on long pedicels.	Rose hip—an actual fruits—achenes are embedded inside a fleshy and colourful flower receptacle, the so-called hypanthium.
Rubus idaeus	The flowers grow at nodes on annual shoots or at the tips of shoots and in the axils of the upper leaves.	The terminal buds are slightly larger than the axillary buds. The lateral buds are conical and andhered to the shoot.	Large, fivefold (* K5C5A~G~) flowers with a white corolla are gathered in clusters.	Aggregate fruit, composed of numerous small fruits, called drupelets, that are fused together on a single receptacle.
Ribes glossularia	Flower buds on short shoots, 3–4-year-old branches.	Narrowly conical buds, slightly deviated from the shoot.	Flowers (* K5C5A5G-(2)) small, bell-shaped, pendulous, single or 2–3 in short clusters, largely self-pollinating.	A spherical or oval berry, yellow, green or red, covered with glandular hairs.
Ribes nigrum	Flowers are most numerous on one- or two-year-old shoots.	Buds are ovoid, set on short stems, deviating from the shoot.	Small, cup-shaped flowers (* K5C5A5G-(2)), with reddish petals, gathered in hanging clusters of 4 to 40 flowers.	A black, spherical berry with a dried perianth at the top.

(*) flower is radial symmetry, (K) calyx, (C) corolla, (A) stamens, (G) pistils; the number in the index next to the letter symbol indicates the number of given elements, i.e., the number of sepals, petals, stamens or pistils. Numerous elements (>10) are indicated by the infinity sign (~). The carpels that form the pistils can be free or fused together into one pistil; in this case, the number of carpels is given in round brackets. The lower pistil (with the ovary recessed in the bottom of the flower) is described by a line above the number. In the case of plants with dioecious flowers, the following symbols are given before the flower pattern: ♂—male flower or ♀—female flower.

). Dessert fruits have Vaccinium corymbosum of the order Bicornes. Commonly grown nuts from Corylus avellana and Juglans regia belong to separate orders, Fagales and Juglandales, respectively. The assortment of fruit crops, due to its very health-promoting fruits, can be expanded by Cornus mas of the Cornales order.

3. Types of Flowers and Fruits

Most fruit-bearing plants are hermaphrodite species, producing flowers with male and female organs (Table 1). In the above collection of species, only Corylus avellana [10] and Juglans regia [11] are examples of dioecious but monoecious plants. Thus, there are male and female flower buds in one plant. Fruit plants differ in the structure of the buds. Some have homogeneous leaf buds or only with generative (flower) parts; others produce flowers and vegetative parts, shoots, and leaves in a single bud. In general, “mature” flower buds differ from leaf buds because they are larger and rounder. The flowers of fruit plants are different in size and are located individually or form inflorescences of different shapes (Table 1). Plants in the Rosaceae family have a unified flower structure. A common characteristic of this family is five-fold flowers consisting of five sepals, five petals of the corolla, and five or multiples of five stamens. There are 1–5 (sometimes ~) upper or lower “1–5-~ chambered” pistils. The base of the flower is clearly flared and flat, convex, or concave [12].

Most fruit plants have fleshy fruits, among which are the proper ones, i.e., those that arise from the ovary, such as drupes of the subfamily Prunoideae (Table 1). Some plants produce pseudofruits because, in addition to the ovary, other parts of the flower (e.g., bottom, floral) are involved in their formation, and these are apple-type fruits (from the subfamily Pomoideae) or hypanthium in rosehips. Quasidrupes are edible nut fruits because they are formed from the lower pistil overgrown with the remains of the perianth, but the edible part is formed by the seed. Berries are often formed on the shrubs, e.g., currants, gooseberries, and blueberries. Collective fruits produce strawberries and wild strawberries, where the proper fruits, nuts, raspberries and blackberries are gathered at the bottom of the overgrown flower, where drupes fused together are gathered on the bottom of the flower. Among fruit plants, only hazel has dry fruits, which are single-seeded nuts [9].

Place of Flower Bud Formation

Individual fruit plants differ in the place of flower bud formation (Table 1). Flower buds form on the axils of the leaves and on the tops of this year’s long shoots (e.g., in peaches, blackcurrants, grapevines, young apricots, some plums, and apple trees) or on short shoots (most cultivars of apple, plum, pear, red currants, and gooseberries). Flower buds occur on long shoots, usually one or two next to the leaf bud, while on short shoots they can occur in large numbers, and then they are called bouquet buds. Sweet cherries bear fruit in short spurs or near the base of longer shoots, mostly in short shoots 2 years and older [8]. Strawberries have cyme-type inflorescences at the tops of short shoots, usually several on one plant.

Within the same species, there may be differences in terms of where flower buds form. According to Lespinass [13], apple trees are distinguished by cultivars that set flower buds only in short shoots on two-year-old and older wood. Most of the strongest flower buds are found on 2–4-year-old wood. The second group includes cultivars that bear fruit mainly on short shoots on 2–5-year-old wood. The flower buds of these apple trees can also occasionally appear at the ends of annual shoots. The third group is the most numerous. These are apple trees that set flower buds on the tops of small 1–3-year-old twigs. Some flowers also tie in short shoots. In the least numerous fourth group, there are apple trees fruiting on the youngest, 1–2-year-old shoots. The flowers, and thus the fruits, are formed in the apical position of the shoots. Short shoots are present in the crown, but there are very few.

4. The Juvenile Phase

The growth and development of fruit plants consist of three phases: the juvenile phase, the mature vegetative phase and the mature generative phase. Each phase is distinguished by several morphological and physiological features [14]. Even within the same species, the transition from the mature vegetative stage to the generative stage, which manifests itself in flowering, is different. This is influenced by genetic variability and various environmental factors. The duration of the juvenile phase depends, among other things, on temperature, light, sugar content, and endogenous hormones. Many researchers have tried to determine the transition from the vegetative phase to the generative phase by observing the morphogenic changes that precede the initiation of floral organs.

4.1. Effects of Light

The amount of light energy reaching the leaves is of great importance in the initiation of flowering. The ability of the plant to adapt to the intensity of light varies in different stages of development. Among fruit species, there are many perennial plants that are characterised by a long juvenile phase. Most experiments on the effect of light on the initiation of flowering of young plants refer to strawberries or shrubs (raspberries, blueberries). However, unlike perennial fruit plants, they respond not only to the intensity or type of light but also to the length of exposure to light, the so-called photoperiod. Tsuruyama and Shibuya [15] compared the response of seedlings grown from seeds of two strawberry cultivars, ‘Yothsuboshi’ and ‘Elan’, to far-red light (FR) in different photoperiods (16 or 24 h). They observed that the cultivar ‘Elan’ produced many flower buds in FR and longer photoperiods. Under controlled conditions [16], highbush blueberry (Vaccinium corymbosum) was induced to bloom less than a year after seed sowing (after 300 days) using a combination of techniques to promote germination and control the conditions of the growing environment. The blueberry seedlings bloomed earlier at high light intensity (400 μmol·m⁻²·s⁻¹) on an 8 h short day than on a 12 h long day. Articles on the influence of light on flowering most often concern the induction and initiation of flowering of mature fruit trees. The influence of light on the end of the juvenile state and the beginning of the maturity period is discussed primarily for herbaceous plants. In the literature, we found only work on a tropical plant such as coffee. Young coffee leaves (Coffea arabica), at the beginning of their development, have been shown not to adapt to increased light intensity because their protective mechanism is not yet fully developed. Mechanisms that contribute to maintaining good levels of photosynthesis have been discovered. Under increased light, these leaves maintained a high photosynthetic response due to rapid nitrogen remobilisation, as indicated by the SPAD values and photorespiration rate. Effective photoprotection was accompanied by a high export ability of sucrose, which prevented excessive inhibition of the Calvin cycle by accumulation of hexose [17].

4.2. Method of Reproduction

The method of its reproduction and the conditions under which this process takes place have a very large impact on the fruiting period of fruit plants. Trees propagated from seeds begin to bloom and bear fruit earlier if they are grown in conditions that ensure their rapid growth. Aldwinckle [18] grew apple seedlings in a greenhouse and started flowering after 16 months, whereas under field conditions this can only happen after 8 years. Way [19], increasing the spacing of the apple seedlings, observed an increase in the number of fruiting trees. Mechlenbacher and Smith [20] removed suckers from hazel seedlings and shortened the juvenile period three months earlier. They explained this by the stronger growth of hazel after the removal of suckers.

Due to genetic variability, generative reproduction of fruit plants is not used. Vegetative propagation with shoots and buds from mature specimens eliminates the juvenile stage, in which the tree cannot flower. However, even with vegetative reproduction, individual species and even cultivars of the same species begin to fruit at different ages. Cornelian cherry propagated by seeds begins to bloom only after 5–8 years [21], while propagated by vegetative cuttings—after 4 years, and by grafting or budding—after 2 years [22].

To shorten the juvenile period, planting in dwarf or semi-dwarf rootstocks is used, especially in the cultivation of apple and pear trees [23]. The rapid transition from the juvenile phase to the mature phase may be due to various mechanisms, and this is most often explained by hormonal control of the distribution of assimilates in the apical meristem zone.

Research on the genetic control of flowering is carried out on model plants, e.g., Arabidopsis. In these studies, homologous genes associated with flowering are searched for among fruit species, i.e., genes with a common evolutionary origin that arise during the specialisation of species and usually have a similar function [24]. The basis of flowering induction is the cooperation of a group of genes characterised by high activity in conductive bundles. Katoda et al. [25] showed that by manipulating FT genes, it is possible to shorten the juvenile period in apple trees. Overexpression of the MdTFL gene shortens the juvenile phase. The MdTFL gene is homologous to the Arabidopsis TERMINAL FLOWER 1 (TFL1) gene. It suppresses the identity genes of the LEAFY (LFY) and APETALA1 (AP1). As part of the investigation, transgenic apple shoots overexpressing the MdTFL gene were obtained and then grafted onto rootstocks. These plants bloomed after a period of 8 to 15 months after inoculation. The control plants, without transformation, did not bloom for five years. The flowers grown on the transgenic shoots of the apple tree were properly developed and fertile. Song [26] isolated a gene homologous to the Arabidopsis (TF) gene of the ‘Bluecrop’ cultivar Vaccinium corymbosum (VcTF). After performing a functional analysis, they showed that this overexpression of the VcFt gene also significantly accelerated blueberry flowering.

5. Mature Phase

The transition from vegetative to generative period depends on the physiological conditions in individual buds. When this happens, that is, when the process of inducing the bud takes place, its further development toward the production of flowers is irreversible [27]. Most fruit plants are perennial polycarpic plants that bear fruit many times during their lifetime, and the ageing process is slow. Depending on environmental (external) and internal (regarding the condition of the tree) factors, they usually yield every year (Figure 2). The quality of these fruits depends, among other things, on the quality of the flower buds [28]. The stages related to the flowering of fruit plants can be illustrated by the diagram below.

5.1. Structure of the Flower Bud

Mature perennial plants can form three types of buds: flower, leaf, or mixed. A flower bud is a miniaturised shoot whose apical leaf rudiments have been transformed into flower parts. The flower buds of apple trees are mixed buds from which flowers and leaves grow. The structure of the flower bud was first described by Bijhouwer [29], who stated that it consists of a helical arrangement of nine bud scales, three transition leaves, six true leaves and three bracts on the axis.

Flower induction, initiation, and differentiation are developmental stages that vegetative buds need to undergo on their way to becoming floral.

5.1.1. Induction and Initiation of Flower Buds

The first changes in physiology that initiate flowering in the following year occur just after the end of flowering. During this period, the processes of induction and then initiation of flower primordia occur, which end when the growth is inhibited [30]. The first step towards the transition from the vegetative stage to the generative stage is flowering induction. The buds receive various types of stimuli, which are the supply of assimilates, the length of the day, and phytohormones. Under their influence, induction of flowering occurs in the apical meristems, which consists of activating genes that condition the development of the flower [31]. The possibility of transforming a leaf bud into a flower bud is determined by the stage of maturity of the bud. The bud meristem goes through a juvenile period, similar to the juvenile phase of the entire tree, in which a sufficient number of leaf buds must be formed before it becomes capable of transforming into a flower. This stage of physiological and morphological development of the bud is measured by the number of nodes. The number of nodes must reach a certain critical value, depending on the genotype. Observations of the intensity of development and the rate of formation of individual bud elements were used to determine the critical number of appendages for the bud to achieve the ability to initiate flowers. The number of appendages necessary to form a flower bud of ‘Early McIntosh’ is 22 [32]; ‘Cox’s Orange Pippin’ [33] and ‘Starkspur Supreme Delicious’—20 [34]; ‘Baldwin’—18 [32]; and ‘Golden Delicious’, only 16 [35]. In sweet cherries, the apical meristem of the buds initially produces only bud scales [36], and when its number in June–July reaches 22–25, the primordia of bracts (bracts) and flowers appear. In the next stage, the flower is initiated, histological changes are underway, but morphological differences are not yet visible. The initiation of flowering consists exclusively in the qualitative transformation of the bud meristem and occurs, according to Luckwill [37], as a result of changes in the hormonal balance, leading to the unblocking of genes responsible for flowering. It is difficult to distinguish the time of induction and initiation of the flower, which is why these two stages are treated together as a critical period in which the buds are sensitive to the factors under the influence of which their further development takes place.

The induction of individual buds is very short, but within the whole tree, the induction and initiation can take about 5 weeks. The initiation of cherry flowering varies depending on the cultivar, climate, and cutting method. In the ‘Bing’ cherry cultivar in central Washington, initiation has been observed to occur from mid-to-late July; in Japan, around 30 June; and in British Columbia, Canada, from late June to early July [38].

Factors influencing flowering initiation

For the initiation of the flower bud, not only is the number of nodes in the bud important, but also the duration of the plastochrone, i.e., the period between the formation of two successive leaf buds. The duration of the plastochron depends on the vegetation conditions and the influence of organs in the immediate vicinity of the bud (fruits, leaves). Therefore, light, temperature, water, and crop load have a significant impact on this [39].

Genetic features

The transition from the vegetative to the generative period depends on genetic characteristics. Individual species of fruit plants set flower buds at different times of the growing season, from mid-May to mid-October [40]. The earliest flower buds are initiated by raspberries that bear fruit in autumn, and the latest flower buds are male by walnut (

Table 2. Initiation of flower buds of individual fruit species in the conditions of the Central European Lowlands (between 49°00′ and 54°50′ north latitude and between 14°07′ and 24°09′ east longitude).

Genus (in English)	May			June			July			August			September			October
	I	II	III	I	II	III	I	II	III	I	II	III	I	II	III	I	II	III
Raspbery primocane
Walnut			♂													♀
Hazelnut					♂								♀
Sour cherry Sweet cherry
Red curant
Vine
Pear
Apricot
Apple Plum Rapsberry floricane Blackberry Gooseberry
Black currant
Peach Highbush blueberry
Strawberry

♂—male flower or ♀—female flower. —Initiation period.

). The initiation of flower buds of apple, plum, summer raspberry, blackberry and gooseberry occurs at the same time, i.e., from the beginning of the second decade of July to the end of the first decade of August [41].

Light

Depending on the latitude, tropical, subtropical and temperate climate plants require different periods of light and darkness for full flowering. The relative length of the daily periods of light and darkness is conducive to the meristem of the transformation of the vegetative shoot into a flower bud [31]. The induction of flowering in response to the relative length of the diurnal period of light and darkness is called photoperiodism. The discovery of the relationship between the duration of light during the day and the flowering of a number of plants divided plants into three groups: long-day and short-day plants, as well as photoperiodically neutral plants. Among monocarpic plants (annual or biennial), there are also plants that require a certain period of time to bloom first with a long day, then with a short day, and vice versa. The photoperiodic reaction is shown mainly by monocarpic plants and some shrubs. Some plants, such as blackcurrants and many strawberries, initiate flowering during short days, such as in autumn, before the leaves fall. The flowering of fruit trees is less affected by the photoperiod. Of the fruit plants, only some are sensitive to the length of the day [42].

However, the amount of light reaching the leaves is important in the initiation of flowering. This is evidenced by the arrangement of flower buds within the crown. They develop best on the outskirts of the crown, in well-lit places. A reduction in the inflow of light, whether due to excessive density of trees or other obstacles, results in a decrease in the number of flower buds that are formed. Shade (reduced access to light) has long been known to significantly reduce apple tree flowering [43,44]. Shading at 37% full sunlight limited flowering to 44% relative to control, and a reduction of up to 30% full sunlight was considered a threshold level for the formation of apple flower bud formation. Jackson [45] observed that weaker illumination of apricot shoots of ‘Moorpark’ decreased the initiation of flower buds at each node and promoted internode elongation. He conjectured that the initiation of flower buds depends on the supply of metabolites in quantities that exceed the demand of shoots, fruits, and roots, and this process is regulated by the plant’s endocrine system. Therefore, the negative effect of shading on the initiation of flower buds is not only the result of impaired photosynthesis. There are many more auxins in shady leaves than in fully lit ones. Under the influence of sunlight, auxins decompose in the leaves.

The quality of light energy also affects the induction of flowering of fruit plants. This was proven by an example of an experiment carried out on a strawberry that was neutral in terms of the length of the day. Strawberry can produce flowers and bear fruit several times per season. Lower wavelengths have been observed to accelerate flowering and higher wavelengths to help direct morphological trait changes toward more flowers. The results suggest that the combination of far red and blue LEDs in a 1:5 ratio may be a potential light source to improve flower bud induction and flower development, which consequently increases fruit production [46].

The correlations between the photoperiod, the temperature, and the age of the shoots are also very important. In warmer regions of Argentina, the buds of the highbush blueberry flower can set twice. Pescle et al. [7], studying the initiation of flower buds in one of the highbush blueberries, observed that flower buds in 2-year-old shoots formed in an increasing photoperiod (up to 15 h) and an average temperature of 22.5 °C, while in 1-year-old shoots—at a short photoperiod (8 h). Initiation of flower buds occurred in both types of shoots after their growth had stopped. The flower bud development was higher on 2-year-old wood than on one-year-old wood.

Air temperature

High temperatures generally stimulate the vegetative growth of fruit trees and can inhibit the formation of flower buds [47]. However, in grapes, induction is better if the temperature at the time of induction is high, regardless of the lighting regime [48]. The mechanism of temperature action in the initiation of flowering is difficult to elucidate because other physiological processes that run in parallel in the plant, such as photosynthesis, water and mineral uptake, and hormone levels, are strictly dependent on temperature [47].

Water

Tropical tree flowering generally occurs under the influence of environmental factors, whereas in temperate deciduous trees, flowering initiation often occurs autonomously [49]. In this regard, in the scientific literature, there are articles on the beneficial effect of water stress on the development of flower buds in olive [50,51], mango [52,53], and citrus [54]. In plants in temperate climates, the stimulation of generative processes by limiting water is the result of stopping the growth of trees. Jackson [45], comparing the effect of irrigation and fertilisation on the initiation of flower buds, found a greater effect of fertilisation than water in apricots.

Theories of flower bud formation

Fruit plants differ in the regularity of yielding. Some apple cultivars are characterised by a two-year fruiting cycle, which means that they produce abundantly (ON) in one year and very poorly or not at all (OFF) in the next. During the 4 weeks before the bud initiation, they investigated physiological mechanisms and molecular changes within the bud tissues. Using histological analysis, they precisely determined the time of the initiation of flower buds of both cultivars [30]. Buds collected from the fruiting ‘Gala’ apple tree in the year (ON) showed a higher content of thiamine, chlorogenic acid and adenine derivative compared to the buds of ‘Fuji’ apple trees (OFF), which were rich in tryptophan. This indicates that flowering induction depends on the activation of the genes responsible for flowering [41].

There are two theories about the formation of flower buds: hormonal and nutritional. [24]. Abscisic acid from leaves stimulates the formation of flower buds, and cytokinin from the root apex and ethylene from ripe fruits similarly promote flower induction. Plant hormones such as auxins, cytokinins, ethylene, and abscisic acid interact with the formation of flower buds. The concentration of individual hormones is fully responsible for the metabolism of various nutrients and therefore affects the differentiation of plant flower buds and leads to the activation or suppression of the genes that form flowers. Among plant hormones, gibberellins (GA) have the clearest and best-proven effect on the induction of flower buds. Gibberellins are produced primarily in fruit seeds; hence, the excess of fruit on the tree inhibits the formation of flower buds for the next year. This causes alternating fruiting in some cultivars [55].

The theory of nutrition assumes that the formation of flower buds depends on the degree of nutrition of the plant, i.e., the concentration of the cytosol at the site of the bud. The leaf bud transforms into a floral bud when the accumulation of carbohydrates in the plant exceeds the nitrogen content. That is why this theory is also called the theory of the C/N ratio [56].

The beneficial effect of curricula thinning treatment on increasing flower buds confirms both theories. Many experiments [57,58] indicate that early removal of excess buds promotes good flower bud formation, which results from a decrease in the level of gibberellins, as well as competition for carbohydrates between fruits growing. The influence of crop load on the quality of the yield in a given year and on the formation of flower buds for the following year has been studied very extensively for trees such as apple [59,60], pear [61,62], sweet cherry [63], plum [64], peach [65] and apricot [66]. A reduction in crop size in a given year affects the higher quality of flower buds that determine the crop in the following year [67].

5.1.2. Differentiation and Maturation of Flower Buds

After induction and initiation of flower buds, the stage of flower primordia begins in apical meristems. The transition from vegetative to reproductive development is characterised by an increase in the frequency of cell division in the midzone of the apical meristems of shoots. This leads to an increase in the size of the meristem and then to the induction of the development of individual parts of the flower instead of the leaves [42]. This occurs only some time after initiation; e.g., for grapevines, this period lasts 3 weeks. Intensive cell division and the formation of individual elements of the flower take place in the buds. The first symptoms of differentiation of a flower or inflorescence bud can be observed by a change in the shape of the apex from flattened to domed through the organisation of growth centres forming the flower parts [33,35]. The differences between the cultivars can be up to more than a month [68]. Flower differentiation and maturation of flower buds of temperate fruit plants are processes that last about 5–9 months. It is characterised by visible morphological changes in the bud. After some time, the intensively dividing apical cells of the meristem begin to form squamous and proper leaves and then individual flowers and their parts in the apical (acropetinal) direction, i.e., first the sepals, then the petals, stamens and pistil. Georgiev [69] reports that in cherries, from the beginning of flower bud formation to the formation of some pistils (in September—October), depending on habitat and temperature conditions in a given year, 67 to 90 days pass, while according to Faust [70]—their number varies from 86 to 112. Georgiev [69] observed that the degree of development of the sweet cherry flower bud can affect its resistance to frost in winter. He emphasised that less clearly expanded stigmas and smaller sepals, petals, stamens, and pistils determine the greater resistance of flowers to frost.

Factors influencing flower bud differentiation

Differentiation of flower buds, like previous processes, depends on external conditions, for example, temperature and water availability, as well as those related to the tree: age, cultivar [71], rootstock, and place of bud formation.

Temperature

Zhu et al. [72] studied the effect of temperature on the formation of flower buds of three-year-old apple trees of the ‘Summerresd’ cultivar. The trees were subjected 6–7 and 12 weeks after full flowering (FB) to the following temperatures: 13 and 13 °C, 13 and 20 °C, 20 and 20 °C, 20 and 27 °C, 27 and 27 °C and 27 and 13 °C. The growth of the shoots was clearly stronger at 20 °C, and then at 13 °C, it tended to decrease as the temperature increased. The temperature of 13 °C 6–7 weeks after full flowering and 20 °C 12 weeks after FB turned out to be the most suitable. Intermediate temperatures of 18–21 °C appear to meet the requirements for the vegetative and generative growth of apple trees [56]. The time of exposure of sweet cherries to high temperatures during the flower differentiation period influenced the occurrence of double pistils. Beppu et al. [73], conducting research on sweet cherries of the ‘Satohnishiki’ cultivar, found that high temperatures (35 °C/25 °C day/night) for 15 days in the period from the end of June to the beginning of September cause the formation of flower buds with a double pistil.

Water availability

An important role in the differentiation process is also played by the appropriate availability of water, which determines the development of leaves, responsible for the proper nutrition of plants. Bartollini et al. [74] compared the differentiation rate of irrigated apricot (Prunus armeniaca L.) flower buds to those subjected to drought stress in June, July or October. The lack of water in each period caused a delay in flower buds. However, after the drought period, when the trees began to water regularly, the flower buds resumed differentiation, unlike the subject trees. drought stress in July. This was explained by a disturbance in the xylogenesis process, which resulted in weaker development of the conductive vessels until the exit from the forced dormancy. As a consequence, abnormal development of the flower organs led to the formation of damaged flowers, resulting in a poor set of fruits.

Sometimes, disturbances in flower buds contribute to the formation of incomplete flower buds that wither and fall off the branches in spring, limiting yield. Liu et al. [75] used transcriptome profiling to reveal differentially expressed genes associated with the formation of wizened flower buds in Chinese pear (Pyrus bretschneideri Rehd). They found that the formation of wizened buds in pear trees was caused by environmental stress during the differentiation period of the buds. Under abiotic stress, hormone-related genes were abnormally expressed, resulting in a large accumulation of ABA and a small accumulation of IAA, GA, and CTK in flower buds. Changes in the content of these hormones affected the development of flower buds in spring, and these not fully developed flower buds became limp. The activity of the stress response enzymes, POD and SOD, was enhanced and weakened in the wizened buds, respectively.

Effect of the rootstock

Taking into account that fruit plants usually consist of rootstock and cultivar, attempts were made to determine the effect of rootstock on flower buds. Hirst and Ferree [34] showed that the rootstock had no effect on the number of critical appendages in the bud of an apple cultivar of ‘Starkspur Supreme Delicious.’ Visible symptoms of flower bud differentiation occurred every year after 70 days of full flowering. The rootstock had an effect on the proportion of buds in which flowers developed only in one year of the study, when there were high temperatures and little rainfall at the time of flower bud formation. Papachnatzis et al. [76] found that under conditions favourable to the winter dormancy of sweet cherries on different rootstocks, the proportion of flowers developed in one fruit bud was the same. However, fluctuations in air temperature in winter meant that in spring the number of flowers in trees growing on dwarf rootstocks (Gisela 5, Gisela 4 and Weiroot 72) was lower than on vigorous trees (P1 and Weiroot 13). In the study by Soysal et al. [77], the number of sweet cherry flowers was studied in flower buds on shoots of different ages of the same cultivar in different rootstocks. Rootstocks were found to have an effect on the number of flowers in flower buds. PHLC and Gisela 6 rootstocks, relatively more dwarf compared to M × M 14 and CAB 6P rootstocks, exhibited similar behaviour in terms of the number of flowers in the floral buds. In the study by Jacyna et al. [78], it was found that the flower buds of sweet cherry cultivars ‘Kordia’ and ‘Sylvia’ were weaker in vigorously growing rootstock F12/1 compared to those growing on dwarf rootstocks. Similarly, Dziedzic et al. [79], comparing the number of flower buds in trees of sweet cherries cvs. ‘Burlat’, ‘Kordia’ and ‘Regina’, growing on different rootstocks, found that Gisela 5 stimulated an increase in the number of flower buds. The differences in flower buds between sweet cherries on dwarf and vigorous rootstocks are explained by the different inflow of light into individual parts of the crowns. The crowns of trees that grow on poorly growing rootstocks are better lit throughout their volume than on stronger rootstocks. Bartolini et al. [80] did not demonstrate the effect of Myrobalan 29/C and apricot seedling rootstocks on the flowering and fruit set of apricots from the ‘Pisana’ cultivar. Although the flower buds of the ‘Pisana’ cultivar on the rootstock apricot seedling were characterised by slower xylem development, this did not interfere with their development.

Place of flower bud formation

In the initial phase of development, the apical bud in the inflorescence bud of apple trees differentiates faster than that of the side flowers. Already 2–3 weeks after the first noticeable changes in the apical bud, almost all elements of the flower appear. Gradually, the rate of differentiation of the top flower slows down (September), and the side flower buds begin to develop intensively. Throughout the summer to late autumn, numerous cell divisions continue; from mid-October to November, the volume of buds increases by 23–36%. The formation and development of flowers in inflorescences in other species of fruit species may proceed in a different way than in apple trees [30]. Many pear tree cultivars have a different direction of bud formation, the so-called centrifugal, where the lowest bud in the inflorescence develops first [81]. It has been observed that in the case of the inflorescence of Amelanchier alnifolia, the flower develops first in the flower bud at the base and then in the terminal flower [82]. The final flower, although it is formed last, is equivalent to the development of the inflorescence at the base of the inflorescence at all subsequent stages of development. Buds in the axils of three to five bracts directly below the terminal flower stop at an early stage, and differences in inflorescence size may be partly due to variability in their development [83].

Changes in flower buds in all species during the late autumn dormancy period are inhibited, and only when the tree enters the relative (imposed) winter dormancy, the continuation of the differentiation of the generative elements of the flower begins.

Chilling hours

Fruit plants show different rates of flower bud development during the growing season due to the different lengths of their dormancy. In the dormancy of woody plant buds, three stages are distinguished: summer, deep, and forced dormancy. Dormancy deepens from summer to winter and decreases with the onset of cold weather. Summer dormancy is mainly not deep and is controlled by correlative inhibition. At this stage, the buds can be forced to develop by decapitation or defoliation of the tree [84]. Buds can also develop spontaneously after a period of high temperature or as a result of tree damage. During deep winter dormancy, the buds do not develop even at positive air temperatures. On the other hand, in the forced dormancy stage, buds do not develop only due to inappropriate external conditions, mainly due to too low temperature [85].

In studies of dormancy, there is no information on the mechanism by which the plant receives a thermal stimulus. It is assumed that there is a thermoreceptor system in the buds. The thermoreceptor, after receiving a certain number of cold units, leads to the activation of the biochemical and hormonal system, which in turn causes the transformation of food compounds, as well as anatomical and cytological changes. The final effect of these changes is to change the developing bud [86]. The depth of dormancy is genetically determined (

Table 3. Chilling hours of temperate fruit plants.

Genus (in English)	Species (in Latin)	Chilling Hours	References
Almond	Prunus dulcis	8–713	[87,88]
Apple	Malus domestica	200–2900	[89,90,91]
Apricot (European)	Prunus armeniaca	171–1812	[84,92]
Apricot (Japanese)	Prunus mume	239–1148	[93]
Blackberry	Rubus sp.	200–800	[94]
Blueberry	Vaccinium sp	150–1100	[95]
Currant	Ribes sp.	800–1600	[96,97]
Goosberry	Ribes grossularia		[98]
Grapes	Vitis vinifera	100–500	[99]
	Vitis rotundifolia	500–1000	[100]
	Vitis labrusta	1000–1400	[101]
Hazelnut	Corylus avellana	800–1600	[102]
Mulbery	Morus nigra	400–1400	[103]
Peach	Flat peach (Prunus persica)	239–536	[104]
	Nectarine (Prunus persica)	93–426	[105]
	Peach (Prunus persica)	79–1390	[106,107]
Pear	Pyrus communis	400–1050	[108,109]
	Pyrus pyrifolia	150–720	[110,111]
Plum (European)	Prunus domestica	579–1323	[112]
Plum (Japanese)	Prunus saliciana	118–685	[112,113]
Quince	Cydonia oblonga	100–400	[114]
Sour cherry	Prunus cerasus	700–1200	[115]
Sweet cherry	Prunus avium	176–1100	[112,116]
Strawberry	Fragaria x ananassa	200–800	[117]
Walnut	Juglans sp.	400–1500	[118]

).

As a result of breeding, it is possible to obtain cultivars with an extended dormancy period, which is expected in countries where springs are late and with frost periods [119]. On the other hand, for cultivation in countries with warm winters, cultivars with a very shallow winter dormancy are sought. The depth of dormancy also depends on the type of bud. In general, vegetative buds show deeper dormancy than generative buds [7]. The buds located in the lower part of the crown and in the lower part of the shoot begin dormancy earlier than the buds located in other parts of the tree. Bud dormancy in two-year-old and older wood is shallower than on this year [120]. However, winter dormancy is essential for the full development of the buds.

Due to the large effect of dormancy temperature on the flowering of temperate fruit trees, various empirical models (

Table 4. Examples of models used to estimate the temperature requirements needed to complete the dormancy of flower buds in winter.

Model	Description	References
Chilling Hours (CH)	Temperature 0–7.2 °C	[121,124,125]
Chilling Units (CU), syn. Utah model	1 h below 1.4 °C—0.0 chill units 1 h between 1.5 and 2.4 °C = 0.5 chill units 1 h between 2.5 and 9.1 °C = 1.0 chill units 1 h between 9.2 and 12.4 °C—0.5 chill units 1 h between 12.5 and 15.9 °C = 0.0 chill units 1 h between 19 and 18 °C = −0.5 chill units 1 h above 18.1 °C = −1.0 chill units	[70,126]
Chilling Portions (CP), syn. dynamic model	The dynamic model assumes that winter cold accumulates in a two-step process. Initially, low temperatures lead to the formation of an intermediate product. Once a certain amount of this intermediate product has been accumulated, it can be converted into what is known as a chill portion in a process requiring relatively high temperatures.	[127]

) have been developed to estimate dormant cooling requirements [70,121,122,123].

In the first trials, which began in 1937 [121], very simple methods were used, for example, counting hours with a temperature between 0° and 7.2 °C. Subsequent methods: the Utah Method [126] and the North Carolina Method [122] assumed that different “cool” temperatures produced different effects. When trees have accumulated a sufficient number of hours of cold and their requirements are satisfied, the buds need a sufficient period of warmth to develop. Developed by Fishman et al. [127] in Israel, the dynamic model appears to be more accurate under warming conditions [128]. This model was successfully applied by Erez et al. [129] to determine the dormant state of a peach bud. It was also adopted by cherry growers in California and proved to be superior to current models in Spain. It is also recommended for commercial fruit cultivation in Chile.

Fadón et al. [106] studied the development of cherry blossom buds during their dormancy period. At the beginning of winter dormancy (mid-October), within each bud, there were 3–4 flower buds covered by external scales. In each flower primordium, the following whorls are distinguished: sepals, petals, stamens, and pistil. The pistil was visible in the form of a suture, the anther heads had reached their characteristic shape, but the threads had not yet developed. Histopathological analysis of the ovary revealed irregularly shaped cells and vacuoles without starch. No anatomical changes were observed during the dormant period, but cytochemical analysis showed that there were clear changes in the pistil. After being out of absolute dormancy, a high starch content was found in the ovary cells, the content of which gradually decreased until the flower buds burst. In cherries, as in other species of the genus Prunus, flowering occurs before the appearance of leaves. Therefore, the development and growth of flower buds are supported by the carbohydrate reserves accumulated in the previous season.

Marafon et al. [110] attempted to explain flower buds, which is one of the main problems that limits the commercial production of pear trees (Pyrus pyrifolia) in the southern region of Brazil. The main reason for this is inadequate cooling during the winter. Cold deficiencies interfere with the mobilisation of carbohydrates in pear trees, reducing the ability to synthesise sucrose in wood tissues and the import of sucrose through flower buds. However, sufficient cooling during the dormant period increases acid invertase of the cell wall and sucrose-6-phosphate synthase activities, ensuring an increase in the content of reducing sugars and starch in bud tissues, which are used for the break and flowering in spring.

Bud dormancy is influenced not only by chilling hours but also by photoperiod, rainfall, fog, dew, wind, and light intensity. In an Italian study of several apple cultivars, Finetto [130] tried to determine how rainfall in autumn and winter can alter the dormancy of the buds. He found that the completion of endodormancy and rainfall data were moderately negatively correlated, indicating that wetting the buds reduced their need for cooling. Cumulative rainfall in autumn, in particular, was clearly negatively correlated with the cooling days necessary to complete the endodormancy. Rainfall can reduce the need for cooling by removing plant growth inhibitors found on bud scales or by accumulating useful cooling temperatures that occur during rainfall.

6. Flowering

The development of flower buds continues throughout spring and ends when the flower opens [131]. Then, within the short life of the flower, pollination and fertilisation of the flowers must take place. Fertilised ovules transform into seeds that determine the growth of fruits. At the same time, the production begins in the seeds, which are transported to short shoots and inhibit the formation of flower buds for the next year. In this way, abundant flowering and then excessive fruit set in a given year affect the fruit yield in the next year, which can lead to alternate fruiting [132].

After the dormant period, external factors, such as among others temperature, also have a very important impact on the pace of development of individual stages of development of fruit plants [123]. Proper plant development depends on the systematic accumulation of “heat portions” needed to initiate growth and transition to the next stages of development, i.e., the so-called Growing Degree Days (GDD). The GDD variable begins to be calculated from the so-called physiological zero-temperature (thermal threshold) individual for each plant, i.e., from the moment its growing season begins. Once this is reached, the summation of heat units (the so-called degree days) begins, in which the temperature is higher than the minimum temperature necessary for the plant’s development [133]. Mathematically, the sum of heat needed for a given plant to enter the next stage of development (GDD) is calculated as the difference between the average daily air temperature and the thermal threshold and then added on the following days. The sum of effective temperatures, as well as the life zero, is strictly defined for a given species and its individual stages of development [123]. For example, for grapevines, the GDD ranges from 945 to 1164 °C to reach full maturity [134]. In many studies, the heat requirements were investigated using m.in. GDD from the beginning of the growing season to flowering (Table 5). A key part of calculating the sum of temperatures is choosing the right base temperatures. Most often, they are obtained statistically because a determination based on a physiological basis requires carefully controlled laboratory and field experiments. Ruml et al. [135] compared several different methods to calculate the baseline temperatures for flowering and ripening and concluded that the RMSE method (least mean square error between observed and predicted days) is the most accurate.

Two aspects of climate are determinants of the seasonal pattern of bud burst: heat accumulation in spring and low temperatures in the preceding autumn–winter period. Climate change is causing differences in the distribution of chilling and heat accumulation, which are necessary for the flowering and yield of temperate fruit crops. In apples, there are cultivars with a wide range of chilling requirements. Cultivars with low chilling requirements have been bred and can be grown in warmer climates [136]. Guo et al. [137] conducted long-term observations to correlate the factors that determine the start date of Castanea mollissima Blume and Zizyphus jujube Mill. under Beijing (China) conditions. Based on regression analysis, they identified chilling and forcing periods for chestnut and jujube. They observed that the forcing period began when half of the chilling requirements were reached. They found that heat accumulation during dormancy increased significantly over 50 years, while chilling accumulation remained relatively stable for both species. It was found that the reduction in cold should not have a negative impact on the course of the deep dormancy period of species typical of temperate climates. However, the increased accumulation of heat that results in accelerated flowering is a bigger problem. This carries the risk of damaging flowers in periods exposed to frost.

Sarisu [138] conducted research on sweet cherries and found that temperature changes that occur over many years in temperate climate regions can have a significant effect on the phenology of species and even the cultivar. It is believed that a 1 °C change in the average air temperature before flowering can cause a 4-day advance in flowering and an 8-day advance in harvesting, which can negatively affect the yield and quality. Therefore, delaying flowering is an important strategy to avoid spring frost damage. The mechanism of flowering delay in perennial plants is based, among others, on the modification of amino acid profiles. Park et al. [1] treated peaches with arginine, glutamate, and proline during the phenological development of flower buds. Using the computational metabolomics method, they investigated proline biosynthesis and its traits, gene anthology, gene synteny, cis-regulatory elements and gene organisation to decipher proline metabolism in flowering delay. Furthermore, the presence or absence of endogenous hormones, as well as the application of exogenous hormones, can influence the flowering pattern of fruiting plants. Hassankhah et al. [11] showed that the application of GA₃, 2 and 4 weeks after flowering, on the walnut cv. ‘Chandler’ significantly increased the number of male flowers, the total number of flowers and the ratio of male to female flowers. It was found that the number of female flowers increased with the increase in the diameter of the trunk. The highest number of female flowers was found after applying 100 mg GA₃/L with the largest diameter of 14–16 cm. The highest levels of flowering inducers functioning in the photoperiodic and autonomous pathways were found in long-day conditions at high ambient temperatures. At the same time, active gibberellins accumulate, which determine the appropriate expression of genes related to meristematic identity and decide, together with other hormones, about the proper development of flowers [139].

Table 5. GDD and base temperature for individual fruit species.

Fruit Plant	GDD to Full Bloom (°C)	Base Temperature (°C)	References
Apple	598–655	4 or 0	[140]
Apricot	332–425	−1	[141]
Blueberry	376–409	0	[142]
Grapes	222–414	10	[143]
Hazelnut	Catkins: 217–387, Female flowers: 128–276	0	[102]
Peach	217–333	4.4	[144]
Pear	202–260	4.4; 8.2	[145]
Plum (European)	462–548	0	[142]
Plum (Japanese)	381–428	0	[142]
Sour cherry	123	4	[146]
Sweet cherry	109–130	4	[147]
Strawberry	1892–2039	3	[148]
Walnut	721–827	0	[149]

Table 5. GDD and base temperature for individual fruit species.

Fruit Plant	GDD to Full Bloom (°C)	Base Temperature (°C)	References
Apple	598–655	4 or 0	[140]
Apricot	332–425	−1	[141]
Blueberry	376–409	0	[142]
Grapes	222–414	10	[143]
Hazelnut	Catkins: 217–387, Female flowers: 128–276	0	[102]
Peach	217–333	4.4	[144]
Pear	202–260	4.4; 8.2	[145]
Plum (European)	462–548	0	[142]
Plum (Japanese)	381–428	0	[142]
Sour cherry	123	4	[146]
Sweet cherry	109–130	4	[147]
Strawberry	1892–2039	3	[148]
Walnut	721–827	0	[149]

The flowering period of individual fruit plants in the conditions of the Central European Lowlands is extended from February to the end of July (

Table 6. Flowering of individual species of fruit plants in the conditions of the Central European Lowlands 52004′ N, 19°28′ E.

Species (in Latin)	February			March			April			May			June			July
	I	II	III	I	II	III	I	II	III	I	II	III	I	II	III	I	II	III
Corylus avellana		♂	♂	♀	♀
Cornus mas
Prunus cersifera
Prunus armeniaca
Ribes grossularia
Ribes nigrum
Chaenomeles japonica
Prunus persica
Prunus cerasus
Pyrus communis
Prunus domestica
Malus domestica
Amelanchier alnifolia
Fragaria x ananassa
Juglans regia									♂	♂	♂	♀	♀
Rubus idaeus										flII prI	flII, prI	flII, prI	flI, prI	prI	prI	prI	prI	prI
Aronia melanocarpa
Vaccinium corymbosum
Crataegus monogyna, C. laevigata
Cydonia oblonga
Mespilus germanica
Sorbus aucuparia
Rosa canina
Vitis vinifera

♂—male flower or ♀—female flower; —flowering period; fl—floricane raspberries, pr—primocane raspberries.

). Corylus avellana and Cornus mas bloom the earliest (February, March), and Vitis vinifera (early June) the last, for about 2 weeks. Some early flowering species, for example, Cornus mas and Prunus armeniaca, develop flowers even before the development of leaves (

Table 7. Flowering and pollination of fruit plant flowers ranked from the earliest flowering in the growing season.

Species	Flowering	How Flowers Are Pollinated
Corylus avellana	There is a phenomenon of nonsimultaneous development of male and female flowers. Male flowers bloom even before the leaves develop.	Anemophilous, bees collect only pollen from hazel trees (there are no honey and nectaries) and do not participate in the pollination of flowers. Almost all hazel are open-pollinated.
Cornus mas	For about 30 days, before leaf development.	Open-pollinated, insect-pollinated.
Chaenomeles japonica	Mostly before the leaves appear.	The flowers are eagerly visited by insects, especially bumblebees.
Prunus cersifera	With leaf development.	Most cultivars are self-pollinating.
Prunus armeniaca	before leaf development for about 8 days.	Most cultivars are self-fertile.
Ribes grossularia	For about 2 weeks.	The flowers secrete nectar abundantly on the entire surface of the flower base and are eagerly flown to by insects.
Ribes nigrum	For about 2 weeks.	It provides bees with nectar and pollen, and if there are no other flowering plants in the area, it is quite eagerly visited by them.
Prunus avium	For 10–12 days.	All are open-pollinated; intersteriality occurs between some pairs of cultivars.
Prunus persica	Before leaves appear.	Most cultivars are self-pollinating.
Prunus cerasus	The flowers develop almost simultaneously with the leaves.	Most cultivars are self-pollinating.
Pyrus communis	With leaf development.	It requires cross-pollination. Not very eagerly visited by bees. They are prone to parthenocarpy.
Prunus domestica	With the development of leaves or shortly before them.	Self- and open-pollinated, generally pollinated by insects
Malus domestica	With leaf development.	Most cultivars are open-pollinated by insects.
Amelanchier alnifolia	Slightly earlier than leaves develop.	Self-pollinating, pollinated by insects or wind.
Fragaria x ananassa	The flowers develop consecutively, depending on their position at the apex, for 3–4 weeks. They are the first to bloom in the middle of the inflorescence.	The vast majority of cultivars are self-pollinating; only dioecious cultivars need a pollinator.
Juglans regia	The male flowers begin right after the development of leaves and can last for 30 days. Female later, when the leaves are already developed.	Anemophilous, pollen is carried by the wind only up to 100 m. In exceptional conditions, the fruits can develop without fertilisation. Rain makes dust very difficult.
Rubus idaeus	About 3 weeks.	Most cultivars are self-pollinating or insect-pollinated.
Aronia melanocarpa	About 2 weeks.	It is insect-pollinated, but in unfavourable conditions it can self-pollinate. Pollination with foreign pollen increases fruit set.
Vaccinium corymbosum	About 3 weeks.	Most cultivars are self-pollinating, but cross-pollination guarantees a higher yield.
Crataegus monogyna, C. laevigata	The lower flowers open first, and the axis of a corymb continues to produce flowers (indeterminate growth).	Open-pollinated, insect-pollinated.
Cydonia oblonga	The flowers develop later than the leaves.	Most cultivars need cross-pollination.
Mespilus germanica	Flowering 6—11 days.	Most varieties are self-pollinating.
Sorbus aucuparia	After leaf development.	Most cultivars are self-pollinating.
Rosa canina	Flowering for about 30 days.	Flowers do not give nectar, only pollen.
Vitis vinifera	Flowering for 10–14 days.	Mostly self-pollinating.

). Species that bloom before mid-May are exposed to damage to their flowers by spring frosts. Among raspberries (Rubus idaeus), there are cultivars that bloom on two-year-old shoots (floricane) and those that repeat fruiting (primocane), i.e., in the first year they bloom in the upper part of this year’s shoots and in the next year on two-year-old shoots [150].

Most fruit species require pollination with foreign pollen, i.e., its transfer not only from another flower but also from a flower of another cultivar. The most important role in pollen transport is played by insects or the wind (Table 7).

Analysing data on the flowering dates of fruit plants, the expansion of commercial crops to include species outside the Rosaceae family is also important from an economic point of view. Crops lost due to frosts during the period of mass flowering of fruit species can be replaced by species flowering at an earlier (Cornus mas, Corylus avellana) or later (Vitis vinifera) date.

7. Conclusions

Temperate fruit plants are diverse in terms of their morphology and physiological processes. However, the structure of the flowers allowed us to create their systematics. The formation of flower buds of perennial plants is possible only after the juvenile phase is completed, which is largely dependent on light but also on the rootstock. Fruit plants for large commercial crops must be propagated vegetatively because only then do they duplicate the characteristics of the mother plants. Budding or grafting is often used, and this shortens the juvenile period. Mature plants generally flower annually, unless there are factors that inhibit flower bud formation, e.g., excessive crop load. Flowering is a key physiological process that determines the yield of fruit plants. Most fruit species in temperate climate conditions bloom once a growing season, and this process is preceded by many months of flower bud development. First, there is a summer weakening and completion of shoot growth, followed by induction, initiation, differentiation, and further development of some inflorescences and flowers, and finally a period of dormancy and the phase of spring bud break and flower development. During the summer and fall of the previous year, individual elements of flowers are formed in the developing bud. The entire period from the moment of induction of the flower bud to flowering determines its quality, so environmental factors such as temperature, water availability and those related to the tree, rootstock and place of flower bud formation play a very important role here. Surviving unfavourable conditions in winter is ensured by its dormancy, during which the processes taking place in the flower bud cease for some time. It can only be interrupted after a sufficiently long cooling period. Insufficient cold can result in the fall of flowers because the substances necessary to nourish them will not develop. From the beginning of the growing season, changes in the flower buds resume, allowing the proper development of elements necessary for proper pollination and fertilisation of flowers. Ovules and pollen grains ripen. At this time, temperature again plays a very important role because the flowering date depends on the accumulation of appropriate portions of heat.

Climate change leading to global warming will undoubtedly affect the formation of flower buds, which determine the size of crops. Individual fruit plants have very different resistance to frost. To avoid the unforeseen effects of abiotic factors on the availability of raw materials, such as fruits, it is good to diversify the structure of cultivated plants. Moreover, most fruit plants come from the Rosaceae family, so they have many pathogens and pests in common. To increase crop, economic, and habitat biodiversity, it is necessary to look for other, more genetically distant, sometimes even less known fruit-bearing species.