The reproductive growth of plants begins with flower bud differentiation, that is, the transition from vegetative growth to reproductive growth begins with the transition of the growth cone of meristem on stem or branch from differentiated leaf bud to differentiated flower bud.

Plants must reach a certain age when they enter flower bud differentiation, and the age of flowering varies greatly among different plants. If the peach lasts for three years and the plum lasts for five years, it will take decades for the bamboo to bloom. When plants reach the age of flowering, the differentiation of flower buds can only be limited to a certain season.

1. Flower bud type and differentiation stage

In a year, the period of flower bud differentiation, environmental conditions and plant growth at that time will affect the later flowering period, flowering number and flowering length; For plants that harvest fruits, it also affects the yield of fruits. Knowing the exact time of flower bud differentiation of various plants is of guiding significance in production, whether for viewing or harvesting fruits.

Plant flower buds are usually wrapped by scales or bracts. Flower buds can be divided into "top flower buds" and "side flower buds" according to different positions. Flower buds are transformed from terminal buds, which are called terminal buds, such as loquat, Daphne odora and monocotyledonous plants. Flower buds are transformed from lateral buds, called lateral flower buds, such as peaches, wintersweet and plums, which are mostly woody plants.

The period when flower buds begin to differentiate is generally related to the types of flower buds. The differentiation of terminal buds needs to stop after the growth of new shoots, or monocotyledonous plants stop pulling leaves, and terminal buds are formed instead of leaves at the top. At this time, scales are formed outside the terminal bud, and flower bud primordium and bracts are formed in the center of the bud. Usually, the formation of flower buds can be roughly judged from the external morphology. For perennial woody plants, it usually starts after the new shoots stop growing for 3-4 weeks; On the other hand, the interval between herbs during this period may be much shorter.

The process of lateral flower bud differentiation starts from the base of the branch first, and then proceeds in the order of the upper node of the branch. Before flower bud differentiation, branches often stop growing, and lateral buds often form soon, so it takes some time to start flower bud differentiation. For example, the flower buds of peaches and plums began to divide after 1 1, and the grapes began to differentiate after about 3 weeks. The situation is not absolute. On the vigorous branches, the top growth does not necessarily stop, and the lateral buds can still differentiate into flower buds. There are also some annual vegetable plants, such as eggplant and pepper, which have started flower bud differentiation one month after sowing, while tomatoes have started earlier.

The time of flower bud differentiation varies with plant species and climatic conditions. The same variety, in the same area, the annual flower bud differentiation period is roughly close. Only in this way can it appear at the same latitude and the flowering period of the same plant is similar. For example, in the south, peach blossoms in March, cherry blossoms in April and lotus flowers in July are basically unchanged.

Of course, in the same tree, the differentiation time of each flower will be in order because of the difference of branch type and bud position. 2. The process of flower bud differentiation. Flower bud differentiation is a series of physiological and morphological changes after meristem feels the signal of flower stimulation.

The process of flower bud differentiation can be divided into physiological differentiation stage, morphological differentiation stage and sexual cell formation stage.

(1) physiological differentiation

The physiological differentiation stage is carried out before morphological differentiation. At present, because the physiological indexes of physiological differentiation are not very clear, there is little research.

Most people think that the physiological differentiation stage occurs about 1-7 weeks before the morphological differentiation stage. Perennial woody plants, usually around 4 weeks. For example, in Japan, peach trees undergo physiological differentiation from mid-June to early August, and enter morphological differentiation in the middle and late August. The physiological differentiation period of walnut is from June to early July, and the morphological differentiation period begins in mid-July. The author observed that in Hangzhou, the physiological differentiation period of Osmanthus fragrans was in early July, about 20 days before morphological differentiation.

During the physiological differentiation of flower buds, the metabolic state of plants should be different from vegetative growth. For example, respiration, the activity of some enzymes, the supply of nutrients and the content of some hormones should be different. For example, the observation of plants such as wheat shows that the cells in the middle of the growth cone in the bud begin to accumulate starch, and the contents of protein and RNA in the surface cells increase. Some people used 5- fluorouracil, an anti-nucleic acid metabolite, to smear it on Xanthium sibiricum bud for 8 hours at the beginning of dark phase to inhibit RNA synthesis, thus inhibiting flower formation. It shows that the flower differentiation is related to the activation of DNA—RNA and protein system. (2) Morphological differentiation The morphological differentiation of the flower bud is immediately after physiological differentiation. When the vegetative growth cone in the bud changes from "Yuan Yeji" to "Flower Primordium", it distinguishes all parts of the flower organ according to a certain procedure, and the apical meristem changes from the top to the ball.

It is considered that in most dicotyledonous and monocotyledonous plants, the apical meristem of flowering stimulation, that is, the target tissue of the central part of flowering stimulation, can be divided into central area and marginal area, while the target tissue of herbs is in the position of axillary buds (Figure 2- 18).

In addition, it is considered that the stimulation of flowering changes the activity distribution of meristem, and the activity of apical meristem of most dicotyledonous plants is higher in Yuan Ye's basal stage. When it becomes the flower primordium, the activity of the central region is greatly enhanced, so that the activity of the whole growth cone becomes more uniform, and thus the tip of the leaf is deformed into a sphere.

From the microscopic examination, it can be seen that in the stage of flower bud differentiation, a certain number of tumor-like processes appear regularly on the surface of spherical meristem, which are arranged in a spiral wheel shape, and these tumors further develop into various parts of flower organs. Therefore, the process of flower bud morphological differentiation can be divided into flower primordia, sepals, petals, stamens and pistils. When the ovule and pollen are mature, the flowers can open (Figure 2- 19).

Fig. 2- 18 target tissue stimulated by flowering

The flowering of apical meristem on the right and dicotyledonous plants stimulates the target tissue (underlined area) and the target tissue on the left and the tip of grass stem (underlined area).

Cz: central area Pz: marginal area SP: spikelet position LP: Yuan Yeji PM: elliptic meristem S: apex (quoted from plant physiology of Beijing Forestry College).

Fig. 2- 19 Morphological Diagram of Peach Blossom Bud Differentiation

1, 2. The initial stage of differentiation. Sepals form 4. Petals form 5. Stamens form 6. Pistil formation.

Because of the diversity of flower shape and structure, the flower bud differentiation process of various plants has its own characteristics.

The flower bud differentiation of common woody flowers in South China was observed and the flower bud differentiation process of these woody flowers was divided into several types (Figure 2-20). Characteristics of flower bud differentiation of Rhododendron, Camellia, Chimonanthus praecox, Osmanthus fragrans, Magnolia grandiflora, Prunus mume, Prunus mume and Camellia japonica. The vegetative growth cones of these plants are sharp and small. Before flower bud differentiation, two short scales appeared on the periphery of the growth cone. When the growth cone became flat and round, it became a flower primordium, and then a tumor-like protrusion appeared on the spherical edge, while the whole flower primordium stretched in a "cup" shape (Camellia japonica and Camellia japonica did not stretch). Then the top of flower primordium produces tumor-like calyx, corolla and stamen primordium from the outside to the center, and finally the pistil primordium rises in the center.

Characteristics of flower bud differentiation of Chimonanthus praecox: The flower bud differentiation process of Chimonanthus praecox is basically the same as that of peach, cherry and other plants, except that the vegetative growth cone is small but not sharp. There are several types of flower bud differentiation of woody flowers before flower bud differentiation, as shown in Figure 2-20.

Fig. 2-20- 1 azalea bud differentiation

(1) Undifferentiated (2)-(3) Initial stage of differentiation (growing a cone-shaped process and extending into two florets) (4) Floret differentiation calyx, petals and stamens (5) Pistil differentiation stage.

① Yuan Ye Primordium ② Floret Primordium ③ Calyx Primordium ④ Petal Primordium ⑤ Stamen Primordium ⑤ Pistil Primordium.

Fig. 2-20-2 Differentiation of Camellia bud

(1) Undifferentiated (2) Initial stage of differentiation (growth cone flattened and widened to form calyx and petals) (3) Stamen formation stage (4) Pistil formation stage.

① Yuan Yequn ② Petal primordium ③ Stamen primordium ④ Pistil primordium.

Figure 2-20-3 Flower Bud Differentiation of Chimonanthus praecox

(1) Undifferentiated (2)-(3) Initial stage of differentiation (round-headed growth cone with perianth) (4) Stamen formation stage (5) Pistil formation stage.

① Yuan Ye primordium ② Flower primordium ③ Perianth primordium ④ Stamen primordium ⑥ Pistil primordium ⑥ Perianth ⑥ Stamen.

After the short and scaly leaves are transformed into round flower primordium, the perianth primordium appears to be rapidly elongated and slightly curled towards the center to surround the top of the flower primordium, but the whole flower primordium is not elongated.

Figure 2-20-4 Flower Bud Differentiation of Osmanthus fragrans

(1) Undifferentiated (2) Began to differentiate (the growth cone is spherical) (3) Produced five floret primordia (4) Floret differentiated calyx, petals, stamens and pistil (1) Yuan Ye primordium (2) Floret primordium (3) Floret (4) Petals (5) Stamens (5) Pistil.

Characteristics of flower bud differentiation of Osmanthus fragrans: The vegetative growth cone of Osmanthus fragrans is also very small. Only because osmanthus fragrans is a cluster of several small flowers, the spherical top of the flower primordium will be elongated and thickened after 3-5 tumor-like protrusions appear, becoming the primordium of the 3-5 small flowers. Then a tumor-like calyx, corolla, stamen and pistil primordium are produced at the top of each floret primordium from the outside to the center. In the future, the tumor-like primordium of the 4th-7th floret can also occur at the base of flower primordium.

Characteristics of flower bud differentiation of Magnolia grandiflora, Magnolia grandiflora, Michelia, Mubi: The flower bud differentiation process of these Magnoliaceae plants is very different from that of plum, peach and other plants. Except Michelia, these plants have broad bud axes and blunt vegetative growth cones. When the growth cone becomes thick and cylindrical, there are tumor-like protrusions on its periphery, which quickly extend into the perianth primordium. At the same time, the whole cylindrical flower primordium is further elongated and thickened, becoming a "receptacle extender". A large number of tumor-like stamen primordium appears in the middle and lower part of this extender, followed by a large number of tumor-like pistil primordium around the top of the extender. After that, the elongator is still gradually thickened and elongated, and the female and stamens continue to differentiate and mature.

Fig. 2-20-5 Flower Bud Differentiation of Magnolia Officinalis

(1) Undifferentiated (2) Early stage of differentiation (thickening and elongation of growth cone, formation of perianth and stamen) (3) Pistil formation period ① Yuan Yeji ② Flower primordium and receptacle extender ③ Perianth primordium ④ Stamen primordium ⑤ Pistil primordium.

Characteristics of flower bud differentiation: the cone of fragrant flower vegetative growth is pointed and small, but the bud axis is wide. Because Xiang Jie is a head inflorescence, flower bud differentiation begins, the growth cone flattens, and the surface increases to become the "inflorescence primordium". After that, some tumor-like processes (floret primordium) are produced on the surface of inflorescence primordium, and each floret primordium elongates and differentiates into various parts of the flower, but all of them are clustered on the disc-shaped receptacle developed at the base of the flower primordium.

Figure 2-20-6 Flower Bud Differentiation of Plum Blossom

(1) Undifferentiated (2)-(3) Initial stage of differentiation (growing and elongating like a cone) (4) Calyx formation (5) Petals formation (6) Stamens formation (7)-(8) Pistil formation.

① Yuan Yeji ② Flower primordium ③ Sepal primordium ④ Petal primordium ⑧ Stamen primordium ⑧ Pistil primordium ⑨ Petals ⑨ Stamens ⑩ Pistil? container

The flowers of Ziwei and Wisteria are panicles developed from mixed buds. The primordium of each floret has been formed in the axils of mixed buds, and the florets rapidly differentiate and develop with the appearance of buds.

The later stage of flower bud differentiation is the development of progressive cells, namely pollen and embryo sac. For plants that harvest fruits and seeds, the normal progress of this stage has a great relationship.

At present, the main research methods of flower bud differentiation are paraffin section or freehand section. It can also be determined indirectly by picking leaves, shading and girdling. As for the attempt to judge the flower bud differentiation period from physiological and biochemical indexes, there is no reliable result so far.