Function of bud
Branches, leaves and flowers of fruit trees are formed by buds. Bud plays an important role in crown expansion and branch rejuvenation. Bud has the function of adapting to bad environment such as low temperature and drought. Before the low temperature and dry season comes, the branches stop growing and form buds to avoid injury. Buds are also reproductive organs. Buds or branches with buds can propagate new plants with the same genetic characteristics as the mother through grafting and cutting.
Species of buds
There are many kinds of fruit tree buds, which have different names because of different classification basis. The bud at the top of the branch is called the terminal bud, and the bud between the axils of the lateral leaves is called the lateral bud or axillary bud. The branches of apricot, persimmon, chestnut, orange and other fruit trees are called self-cutting, and the buds at the top of the branches are actually side buds, so they are called false terminal buds. The terminal bud and lateral bud occur in a certain position on the branch, which is called fixed bud; Buds without a certain position and buds from roots are called adventitious buds.
After germination, the bud is called a leaf bud that only grows branches and leaves, and can grow flower buds of inflorescences or flowers. The flower buds of stone fruit trees and bayberry only grow flowers without branches and leaves, which are called pure flower buds; Stone fruit trees and flower buds of grapes, persimmons, chestnuts, oranges, etc. There are both flowers and branches and leaves, which are called mixed buds. The flower buds of walnut, hickory and hazelnut have both pure flower buds and mixed flower buds. The terminal bud is called the terminal flower bud and the lateral bud is called the axillary flower bud. Fruit trees such as drupe and grape only have lateral flower buds; Some varieties of loquat, apple and pear only have terminal buds; Some fruit trees have both terminal buds and axillary buds. Flower bud type and occurrence position are the main manifestations of fruit tree species' fruiting habits.
A leaf axil (node) is called a single bud and a compound bud of more than two buds. Most fruit trees have a single lateral bud, but walnuts, grapes and jujubes often have two buds on a node, and drupe fruit trees often have three or four buds on a leaf axil. Generally, the one located in the middle of leaf axils is called the main bud, and the one located above or around the main bud is called the secondary bud. Small buds on the scales on the left and right sides of branches of fruit trees such as nuts, persimmons, chestnuts and oranges are also called secondary buds. The secondary buds of these fruit trees often become hidden buds, which is very important for the regeneration of branches.
Buds that can germinate in the second year after formation are called active buds, and those that do not germinate are called dormant buds (see dormant buds). Flower buds and most leaf buds are active buds. The number and life span of dormant buds vary with tree species and varieties.
The big and full buds of the same tree species are called full buds, and the small and not full buds are called flat buds. Full buds are easy to germinate, sprouted branches are stout, flower buds are full, and fruit is easy to set; Shrinking buds are not easy to germinate, and the sprouted branches are thin. Full buds mostly occur in the middle and upper parts of strong branches, and shriveled buds mostly occur in the lower parts of branches and underdeveloped branches, but apple shriveled buds often occur at the junction of spring shoots and autumn shoots of developing branches.
Buds wrapped by bud scales are called scale buds, and buds without bud scales, tender leaves or bare organs in buds are called naked buds. The buds of deciduous fruit trees are mostly scale buds, and most tropical fruit trees and some subtropical evergreen fruit trees are naked buds. A few deciduous fruit trees, such as summer buds of grapes, male flower buds of walnuts and flower buds of figs, are also naked buds.
Some fruit tree buds have special names. The secondary buds of grapes germinate in the summer of that year, often called summer buds; The main bud usually germinates after overwintering, which is often called winter bud. The winter buds of grapes and bananas are also called bud eyes. Buds in different parts of pineapple plants are called sucking buds, crown buds and Asian buds respectively.
Structure of bud
Buds develop from shortened stems. The basic structure of a bud consists of a bud axis (shortened stem), scales or young leaves, a top growth point (male) or flowers differentiated from the growth point (Figure 1, 2). Leaf buds, pure flower buds and mixed buds can represent different structural types of buds. Leaf buds belonging to scale buds are composed of bud axes, scales, young leaves and growing points, while bare buds lack scales. Bud axis with young leaves, young leaves and growing points constitutes young branches (embryoid branches), which are the parts that form branches after bud germination. The pure flower bud of drupe consists of bud axis, scales and flowers. The structure of mixed buds is different from that of leaf buds. Flowers or inflorescences (nuts, oranges, lychees, walnuts) are formed by the top growth points of young branches, or flowers or inflorescences (persimmons, chestnuts, grapes, etc.). ) formed in the axils or nodes of young branches.
Figure 1 The number of bud scales varies with the fruit tree species. Chestnut, persimmon and grape have smaller bud scales, but the number of bud scales is the same. 4 chestnuts; Persimmon has 4 pieces and 2 transitional scales; Grape has 2 and 2 ~ 3 transitional scales. There are many buds and scales in stone fruit trees and stone fruit trees, and the number of buds and scales in different parts of different trees, varieties and branches varies greatly. Sweet cherries with the largest number of drupe bud scales can reach 16 ~ 24, and peaches generally have 8 ~ 12. Pear bud scales are generally between 9 ~ 18, and apples are between 5 ~ 12. The number of bud scales of drupe fruit trees and drupe fruit trees of the same variety varies with the type of bud, planting place and the strength of branches. Compared with leaf buds, flower buds have 1 ~ 2 or equal scales; The neck bud has more scales than the lateral bud; The lateral buds of different parts vary with the size of their leaves. The bigger the leaf, the more scales there are in the lateral buds of leaf axils. There are many scales in the bud, and the quality of the bud is good and the germination is strong. There are often accessory buds between the axils of the outermost bud scales of lateral buds.
Fig. 2 When the buds formed in that year are dormant in winter, except pure flower buds, the number of nodes or young leaves in the buds depends on the richness of tree species and the development of buds. Apples, pears and peaches are about 3-5 knots, hawthorn and grapes are about 5-9 knots, chestnuts and persimmons are about 5- 12 knots. The more nodes, the fuller the bud development. After winter dormancy and before germination, the number of nodes in some buds will increase, which determines the length of nodes in buds of different tree species or different types of branches.
The process of bud formation and differentiation of deciduous fruit trees generally goes through the generation period of bud primordium, the differentiation period of scales and the differentiation period of young branches (Figures 3 and 4).
Fig. 3 Emergence period of bud primordium
Apple, pear, peach, apricot and other buds with few shoots in winter dormancy period, in the spring of the second year after the formation of the bud (mother bud), the primordium of a new generation of lateral buds appeared between the leaf axils of the shoots. The primordia of lateral buds are composed of protoepidermis and the first and second layers of cells inside it. For chestnuts, persimmons, walnuts, hawthorn and other buds with more tender branches in winter dormancy, the primordia of lateral buds appear in the axils of tender branches in the year when the mother buds were formed. After the mother bud germinated, with the continuous differentiation of the number of nodes in the new shoot (outside the bud), the lateral bud primordium appeared in the axil of the new shoot in turn.
Fig. 4 The terminal bud has no obvious bud primordium, and the terminal bud is differentiated from the growing point at the top of the young branch into scales.
Scale differentiation stage
After the bud primordium appears, it immediately differentiates into scales. In the short branches of apple, pear, peach and other fruit trees, the scale differentiation of the terminal bud began from the mother bud, and by the time the mother bud germinated, the number of scales had already differentiated. After the primordium of lateral buds appeared, the scales were differentiated immediately until the leaf area to which they belonged stopped growing and the number of scales of lateral buds had differentiated. Buds with few scales, such as chestnut, persimmon and grape, have differentiated when they emerge from leaf axils.
Bud differentiation stage
After the stage of scale differentiation, the buds of pure flower bud fruit trees turn to flower bud differentiation if they have the internal and external conditions to form flower buds. Otherwise, it will enter the bud differentiation stage and become a leaf bud. The buds of chestnuts, persimmons, grapes, etc. After the scale differentiation stage, it continues to differentiate into young branches. After the young branches differentiate into a certain number of nodes, the buds have the basic structure of leaf buds. On this basis, if there are internal and external conditions to form flower buds, flower buds will be transformed into flower buds (see flower bud formation), otherwise they will still be leaf buds.
The branch differentiation process of deciduous fruit trees can be roughly divided into two stages: branch differentiation before winter and branch differentiation after winter. Bud scale differentiation is followed by bud differentiation, that is, the stage of bud differentiation before winter. During the dormancy period of deciduous leaves, the differentiation of young branches stops. After dormancy, some buds continue to differentiate into new shoots and increase the number of nodes; Some buds no longer increase the number of nodes of new shoots or the top of new shoots turns into terminal buds.
Characteristics of bud
The location and occurrence period of buds show the heterogeneity of different characteristic buds.
There are differences between buds of different branches of the same tree species and buds of different parts of the same branch. It shows that the different number of bud scales, the number of new shoots and the size of buds lead to different difficulty in germination and branch strength. The heterogeneity of buds is caused by different nutrition, environmental conditions and bud development stages. Fruit tree pruning and bud grafting make use of the heterogeneity of buds.
Maturation of bud
The buds formed on the new shoots of some fruit trees germinate in the same year, forming secondary branches, tertiary branches or higher branches. This feature is called premature bud, and this kind of bud is called premature bud. Grape, peach, jujube, orange, litchi and other fruit trees with early buds branch many times a year, and their crowns form quickly and bear fruit early. Other fruit trees, such as apples, pears, chestnuts and walnuts, generally do not germinate in the year when the buds are formed. This characteristic is called late-maturing bud, and this kind of bud is called late-maturing bud. Removing branches and cores can promote the germination of late-maturing buds in the same year, accelerate branching and bear fruit early.
bud mutation
The meristem cells of buds undergo genetic material mutation. Bud mutation includes chromosome number, structural variation, gene mutation and other genetic material variation. When the mutation only occurs in the meristem of the bud, it is not easy to be detected because its variation characteristics have not yet been displayed. Only when the mutant buds germinate into branches, even the flowering and fruiting show the characteristics of mutation, are they discovered by people. Therefore, the change of buds always appears in the form of branch change or variation when branches and buds propagate into new plants, that is, plant change.
Fruit trees are highly heterozygous perennial woody plants, which often germinate, and their variation characters can be fixed, preserved or utilized through asexual reproduction. Bud mutation is the source of new plant variation, which can not only provide new material resources, but also directly select excellent new strains and varieties, so bud mutation plays an important role in fruit tree breeding (see bud mutation selection).
The characteristics of bud mutation are: ① diversity. Bud mutation has both morphological and physiological characteristics. The former includes the morphological variation of leaves, fruits, branches and plants. For example, the narrow-leaf seedless bud grafted with Liu Mei orange produced by Qu orange, the polyploid big rose fragrance produced by rose grape, and the short-branched new red star appeared in red star apple. The latter includes variations in fruit maturity, fruit quality, resistance and fertility. Baigua pear in Jinchuan Sydney can ripen 10 ~ 15 days in advance, and soluble solids in bud brocade of Hongyu apple increase by 3%, with obvious sweetness. ② Repeatability. The same type of bud mutation can occur repeatedly in different periods, different places and different plants. The plant type of Yuan Shuai apple changed from ordinary type to short branch type, and the color of peel changed from stripe red to flake red, which happened at different time and place. The essence of this phenomenon is the variability of some genes. ③ Stability. The variation characteristics of budding varieties are relatively stable, which can be maintained by asexual propagation. However, some buds may lose their mutant characteristics and return to their original types in the process of growth and development, which is called regression mutation. Some dark-colored apple buds tend to return to their original colors. The essence of this phenomenon is the reversibility of gene mutation, which is mostly due to the chimerism of bud mutation, that is, some buds are unstable peripheral chimeras or chimeras. ④ Limitation and pleiotropy. Bud mutation is not the recombination of genetic material, but the mutation of the original type of genetic material, so it is generally limited to a few traits. For example, many color buds found in various fruit trees are mainly different in peel color variation, among which red variation is the most common. There are also some changes in buds, and their changes are varied. For example, the apple bud mutation is compact, the branches are short and thick, the crown is short and the fruit is early, and the yield is high, which is the reason of gene pleiotropy.
Bud selection
A method of growing branches or plants from mutated buds and obtaining new varieties through identification and selection. This method is based on the excellent comprehensive characters of the original variety, and selects the type with better characters from the mutation of individual or a few characters. There are many famous and excellent fruit tree varieties that come from bud mutation. At the beginning of the 20th century, the United States bred bud varieties of Red Crown and Red Star from Marshal varieties. In the early 1950s, a new red star was selected from the bud mutation of the red star. Since 1970s, many new varieties of red and short branched marshal lines have been selected. Bud selection of mandarin oranges is also a successful example. Before 1909, several citrus varieties were cultivated in Japan. Later, a budding branch was found on a tree with its tail stretched out, named Zaoshukou. Early-maturing varieties such as Songshan and Miyagawa were bred from Weizhang in 1960s.
The selection of buds is carried out in two steps. The first step is to select primary superior lines from the production garden, including branch variation and single plant variation; The second step is to screen the asexual offspring of primary superior lines, including two stages: re-selection and final selection.
Primary selection is to observe and select at all stages of the whole growth and development stage, especially before the fruit harvest period 1 ~ 2 weeks, focusing on the variation of fruit economic characters; In addition, after serious natural disasters, seize the opportunity to choose mutants with strong resistance. The advantage lines obtained in the primary election should be numbered, clearly marked and recorded in the investigation. And compared with the suitable control tree species in the local ecological environment.
In the process of selection, decorative materials should be screened out first, and bud mutation and decorative mutation can be identified by referring to the following aspects: ① the nature of variation traits. Observe and analyze the variation of quantitative or qualitative traits. Quality traits generally do not show decorative changes due to the influence of environmental conditions. If the skin is colorless and colored, flaky red and striped red can be judged as bud change. ② the range of variants. There are three kinds of variation: branch variation, single plant variation and multi-plant variation. If it is a branch change, it is necessary to observe whether it is a chimera. If it is identified as a fan chimera, it can definitely be a bud change; If it is a single plant variation, it is difficult to determine the nature of the variation. There are three possibilities: bud variation, decorative variation, or seedling variation; If it is a multi-strain variation, the site conditions are different. The influence of environment can be ruled out, and it is determined to be bud change. ③ Variation direction. Decoration changes are consistent with environmental conditions, but bud changes have no obvious relationship with environmental conditions. For example, the color of fruit changes with the change of lighting conditions. The fruit color of the outer branch of the crown is darker, while the fruit color of the inner branch is lighter. Therefore, the crimson variation found in the canopy cavity may be bud variation, while the type found in the sunny area may not be bud variation. ④ Stability of variation traits. Decorative changes are phenotypic changes caused by changes in general environmental conditions, so phenotypes only show this variation when this environmental condition exists, and without this condition, the variation will disappear. Therefore, it can be judged to understand the performance of variation traits over the years and analyze the changes of environmental factors. After the decorative materials can be excluded according to the above points. If it is found that the variation character is excellent, but it is not sure whether it is budding, the selected materials can be put into the high grafting appraisal garden, and the future work can be decided according to their performance; There is sufficient evidence to determine that it is bud mutation, with excellent characters, but some characters are not clear, so it can directly enter the seed selection nursery; There is sufficient evidence to prove that the mutation is a very good bud mutation, and there is no related deterioration. It can directly participate in the re-election without going through the high-grafting identification nursery and seed selection nursery.
Naturally occurring bud mutations often exist in the form of chimeras, such as peripheral chimeras, which are easy to be stable, while some sector chimeras or peripheral distinguishing chimeras are often unstable. In order to purify and stabilize mutants, pruning or vegetative propagation can usually be used to transform them into peripheral chimeras or homogeneous mutants for production and application.
bud grafting
Grafting propagation method with bud piece as scion. Grafted seedlings are called bud seedlings or semi-mature seedlings, and mature seedlings are called bud grafted seedlings. The method is simple and rapid; Preserve breeding materials; The interface has the advantages of small wound, easy dressing protection, high survival rate and long transplantation time. When grafting, the rootstock is not truncated, and those that are not grafted can be replenished. You can also graft a bud on the rootstock at regular intervals, which provides favorable conditions for the rapid cultivation of fruit seedlings by using the intermediate rootstock (see intermediate rootstock). It is the most commonly used seedling raising method in modern fruit tree production. In the middle of 5th century AD, budding began to be used in Mediterranean area, and it was widely used in Europe after17th century. China's Introduction to Agriculture and Mulberry (1286) records "bad grafting", which is similar to modern bud grafting, but the modern bud grafting method was introduced from Japan.
Type and grafting period
According to whether the scion bud has xylem or not, it can be divided into wood and wood-free. Buds without wood buds only have cortex, and buds with wood buds have a thin xylem layer under the cortex. When the cortex can be peeled off, the grafting speed of xylem with bud is fast and the survival rate is high, which is the best time for fruit tree grafting and seedling raising. When grafting, if the cortex is not easy to peel off, you can cut a little xylem bud for grafting; When the cortex on both sides of rootstock and ear can't be peeled off, only bud grafting can be carried out. Therefore, before and after germination in spring and autumn, insert buds or graft "T" buds with xylem buds; In the season of vigorous growth, the "T" type bud grafting without wood buds is adopted.
Bud grafting
There are mainly ①T-bud grafting. China called it T-shaped bud grafting, which cut the bud into a shield shape and inserted it into the T-shaped interface of the rootstock, hence the name T-shaped bud grafting. The operation step are as follows: firstly, cut that bud piece to the depth of wood above the bud tip, then cut a small amount of wood from the lower part of the bud, and peel the bud piece to a horizontal knife edge to make the inner surface of the cortex smooth; Under the condition that the cortex is not easy to peel off, fine wood can also be cut into buds for grafting. Secondly, the rootstock is cut in a T-shape horizontally and vertically 1 knife, which goes deep into the wood and peels off the cortex. After inserting the grafted bud, make it close to the rootstock, the incision on the bud piece should be connected with the transverse incision of the rootstock, and it should be tightly bound, only the petiole and bud tip are exposed (Figure1); Some change the incision of the rootstock into a horizontal incision, peel off the bud, cut the rootstock into an "I" or "426374"-shaped interface with the size corresponding to the scion bud, or peel off the rootstock with the same size as the bud. It is best to cut with a double-edged bud grafting knife with adjustable spacing, peel off the cortex of the rootstock incision, and paste the scion buds with the same size; Or peel off the incision of the rootstock, insert buds and tie them tightly (Figure 2). This method is complicated to operate, and requires the thickness of rootstock and panicle to be close to the same, so it is rarely used except persimmon trees. ③ tubular budding. Buds are tubular. The key points of the operation steps are that no vertical incision is made under the horizontal knife edge, and the cortex is broken with the knife tip and inclined to both sides to facilitate the insertion of bud pieces; The stems of orange seedlings are rhombic, and the rootstock incision can also be cut into a "T" shape, and the buds are inserted from one side. ② Skin grafting. The bud is square, which is also called square bud grafting in China, commonly known as hot sticking skin. First, cut a square with xylem depth on the scion as an annular cut with xylem depth above and below the scion, then cut it longitudinally between two annular cuts in the bud back direction, peel off the tubular bud, cut a cut with the same size on the rootstock according to the scion bud stripping method, peel off the cortex, replace it with the scion bud, and tie it tightly. There is also a sleeve budding method, which unscrews the bud by hand, takes it out from the top and puts it on the cut of the rootstock (Figure 3). Tubular bud grafting is a primitive method and easy to survive because of the large contact surface between rootstock and panicle. Because of its complicated operation, it requires the same growth thickness of rootstock and panicle, so it has no application in production. 4 budding. The long shield-shaped base of the bud slice is flat. When cutting, the first knife is above the bud, and then it is cut obliquely into the xylem, and then it is cut vertically downwards. The second knife is cut at a 45-degree angle below the bud and intersects with the first knife to take off the scion bud slice with wood. Cut off the rootstocks with the same shape and size, remove the buds, embed the scion buds, align them with the cambium on both sides (at least to one side), bind them tightly and firmly, pay attention to moisturizing, and improve the survival rate (Figure 4).
Figure 1
Figure 2
Figure 3
Figure 4