Structure of fruit
In the fruit developed from the ovary, the ovary wall forms a pericarp, which is divided into three layers: exocarp, mesocarp and endocarp.
epicarp
Traditionally called "pericarp", it is generally very thin and has obvious boundaries with mesocarp. It is composed of epidermis or some tissues adjacent to it, with pores and cuticle. Some are covered with wax and fruit powder, such as plums and grapes; Some are covered with fluff, such as peaches and almonds; Some form cystic, juicy and edible meat columns, such as bayberry; Some exocarp are thick and oily, and the boundary with mesocarp is not obvious, such as citrus. Exocarp contains chloroplasts in young fruits. When mature, anthocyanidins or colored bodies are produced, and various bright colors appear.
mesocarp
Structural changes are diverse. Some are composed of parenchyma cells, and the meat is edible, which is customarily called "pulp", such as peach, plum and apricot. Some parenchyma tissues contain thick-walled tissues, such as litchi; Some dry and shrink into film or leather when the fruit is ripe, such as bayberry; Others are loose and fibrous, with multiple vascular bundles, such as citrus sponge layer; Coconut mesocarp is developed, almond mesocarp has less juice and poor taste, so it is inedible. There is also no obvious boundary between mesocarp and endocarp, which is rich in pulp, such as grapes.
endocarp
Structural changes are also great. The epidermal cells of the endocarp specialize into large and juicy "juice sacs", such as citrus; A hard shell composed of stone cells, such as peaches, plums, apricots, coconuts, etc. There are also endocarp cells, such as grapes, that separate into fruit flesh when the fruit matures.
room
There are one or more ventricles in the center of the fruit, and each ventricle has one or more ovules, which develop into seeds and sometimes produce seedless fruits (see seedless fruits).
The origin, structure and edible parts of fake fruits are quite different (see nuts, berries and nuts).
Types of fruits
There are many ways to classify fruits, such as tables, and their interrelationships and edible parts are shown in figures 1 and 2.
Because there are many kinds of fruit trees, the origin, structure and edible parts are very different, so the problem of fruit ownership of some fruit trees has not been solved.
In addition, it can be divided into fruits and dried fruits according to the juice content of edible parts.
Fruit growth and development
Different species behave differently.
Fruit growing period
The time from flowering to fruit ripening varies. For example, strawberries only take more than 20 days; Rubus, cherry, gooseberry and gooseberry need 30-60 days. Apricot, fig, plum, jujube, litchi, annona, pomegranate, guava, olive, bayberry, etc. It takes 50 ~ 100 days. Longan, hawthorn, pineapple, kiwi, ginkgo, walnut, pecan, hazelnut, chestnut, pistachio, persimmon, etc. , it takes 100 ~ 200 days; Citrus, loquat, papaya, pineapple and avocado take about 200 days. Some special varieties take a long time, such as avocado in Guatemala system, which takes 300 ~ 500 days; Summer oranges last about 400 days. The longest growing period is Torreya grandis, which blooms in spring and matures in the following year, lasting more than 500 days. Most common fruit trees have great differences among different varieties within the same species, such as apples, pears and peaches. , early, middle and late maturing varieties range from 60 to 200 days; Plum, almond, grape, banana, busy fruit, carambola, blueberry, etc. , between 50 ~ 140 days. The same variety also has certain influence on the fruit growth period because of the differences in cultivation location, ecology and cultivation conditions.
Figure 1
Fig. 2 Fruit growth dynamics
During the development of fruit from fertilization to maturity, the growth rate is different at different stages. According to the volume, weight or diameter of fruit, there are two types of fruit growth curves. One type is S-type (Figure 3), which accelerates in the early stage and slows down before maturity, with only one rapid period, such as citrus, loquat, pear, apple, raspberry, pineapple, banana, avocado, date palm, chestnut and walnut. The other is the double-S type (Figure 4), which has two fast stages, with a slow stage in the middle. In the first stage, all parts of the ovary grow rapidly. In the second stage, the ovary wall grows slowly, the endocarp lignifies, and the embryo and endosperm grow rapidly; In the third stage, mesocarp grows rapidly again. Such as peach, plum, apricot, cherry, grape, jujube, persimmon, hawthorn, raspberry, blueberry, gooseberry, kiwi, fig, olive, pistachio and so on. The length of the second stage affects the fruit ripening sooner or later.
Fruit growth also has a certain circadian rhythm. For example, apples and young fruits generally increase at night, begin to shrink after dawn, and begin to recover in the afternoon. 16 recovers and begins to grow. Near maturity, it grows faster at night and in the morning.
Figure 3 Quantity increase
It depends on the number and volume of cells and the size of intercellular spaces. The cell division of fruit begins after the formation of flower primordium and ends at flowering. Post-anthesis division varies with tree species and varieties. Such as blackcurrant, after flowering, cells no longer divide, but increase in size. In strawberries and avocados, cells continue to divide after flowering until the fruit matures. Most tree species are in between, and the tree species with large fruit and late maturity have a longer division period under suitable temperature and soil moisture conditions. The division period of different tissues of the same fruit is also inconsistent. Generally, the placental tissue stops dividing first, and then the ovarian wall stops from the inside out in turn. For example, it takes 2 1 ~ 49 days for the cortex tissue of apple pulp to split, 25 ~ 45 days for pear, 42 days for fig, 20 ~ 28 days for the mesocarp of peach, plum, cherry and jujube, 15 days for apricot,/kloc-0 ~ 23 days for grape. The division period of epidermal tissue mostly lasts for a long time. The number of cells in pulp varies with tree species. For example, there are about 40 million ripe apples and 600,000 grapes. The cell length is generally 150 ~ 700 microns, and some of them exceed1mm. The edible epidermal cells of pomegranate seeds are 2 mm long. The cell volume is generally 106 ~ 107 cubic micron, and the maximum is 108 cubic micron. In the late development stage of many fruits, the intercellular space of parenchyma tissue increases rapidly, so the volume increases obviously. For example, the intercellular space of apple accounts for 20.6 ~ 35.7% of the total volume.
Fig. 4 shape of fruit
The fruits of most tree species are spherical, nearly spherical, oblate, rectangular, elliptical and conical. Some species or varieties have specific fruit shapes, such as olive spindle, lemon spindle, pomegranate vase with round bottom, litchi oval with tumor skin, pentagonal carambola, kidney-shaped cashew nuts, curved cylindrical bananas, cylindrical pineapples, flat flat peaches, oblong almonds, spike-shaped mulberries, fingered bergamot, rough-thorn durian, and sharp single-core chestnuts. There are often differences in fruit shape between different varieties of the same tree species, which can be used to identify varieties. For example, dates are round, long, cylindrical, and even oblate, pear-shaped, gourd-shaped and pot-shaped.
The color of fruit
There are different colors because of different pigments. The main types of pigments are chlorophyll, carotenoids and anthocyanidins.
chlorophyll
It exists in chloroplasts and rapidly transforms and disappears with fruit ripening, so chlorosis is usually a sign of maturity. The formation and transformation of chlorophyll are influenced by external conditions. Light is a necessary condition for the formation of chlorophyll, and low temperature destroys chlorophyll. Gibberellin and cytokinin help to keep green, while ethylene, abscisic acid and gibberellin have antagonistic effects, which can promote fading and coloring.
carotenoid
It exists in colored bodies and often coexists with chlorophyll, including carotene and lutein. The former includes α, β, γ carotene and lycopene, while the latter includes cryptoxanthin, zeaxanthin, anthracene lutein, purple lutein, orange lutein and lutein. Citrus fruits contain zeaxanthin, cryptoxanthin and lutein. Apricot contains three kinds of carotene, lycopene and lutein. Persimmon contains β carotene, lycopene, cryptoxanthin, zeaxanthin and anthracene. Pineapple contains purple yellow and orange yellow. The change of peel color of persimmon, yellow peach, apricot, citrus and loquat from yellow to orange-red is related to the variety and quantity of carotenoids contained. For example, apricot contains lycopene10%; Red persimmons contain 40% and yellow persimmons contain no lycopene. The optimum temperature for the formation of lycopene is 19 ~ 24℃, and the formation of lycopene is inhibited above 30℃. Therefore, when tomatoes are cultivated in high temperature areas, the fruit is only yellow without red, which affects the quality.
anthocyanidin
It is a water-soluble pigment, which exists in vacuoles. It is red when it is acidic, purple when it is neutral and blue when it is alkaline. When combined with metal ions, it presents various colors, with different pigment distribution and great color change. The main types of anthocyanidin are anthocyanin, methyl anthocyanin, anthocyanidin, 3'- Buddlequin, sunflower, primrose and so on. Apples, peaches, plums and cherries contain anthocyanins, while grapes contain anthocyanins, methyl anthocyanins, anthocyanidins, 3'- methyl anthocyanins and cherry elements. There are 2-3 red varieties and 4-5 black varieties. Figs contain anthocyanins and sunflower seeds. Pomegranate and strawberry contain sunflower seeds. Vaccinium contains anthocyanins, anthocyanins and 3'- methyl anthocyanins. The vast majority of fruit trees increase anthocyanins with fruit ripening, but only a few tree species, such as pineapple, decrease anthocyanins when fruits mature. The formation of anthocyanins requires a large accumulation of sugar, sufficient illumination, large temperature difference between day and night, coordination of mineral nutrition and appropriate water content. Satisfying these conditions and applying ethephon properly can promote fruit coloring.
Nutritional components of fruits
Mature fruit contains many nutrients. Fruit mainly contains sugar, acid and vitamin C, while dried fruit mainly contains starch, fat and protein (see the nutritional value of fruit).
The taste of fruit
The comprehensive influence of sugar, acid and aromatic substances on the senses is an important factor to determine the quality of fruit. Sweet and sour depends not only on the content of sugar and acid, but also on the ratio of sugar to acid. Sugar and acid are sweeter than big ones, and small ones have depth. The flavor with high sugar and acid is strong, and the taste with low sugar and acid is light. Sweetness is related to the composition of sugar, fructose is sweeter, sucrose is in the middle, and glucose is lighter. Spices are composed of volatile substances, mainly alcohols, esters and carboxyl compounds. Different species and varieties contain different kinds of aromatic substances, such as about 20 kinds of apples and strawberries. The composition, concentration, sensible quantity, interaction and coordination with sugar and acid of various aromas constitute different fruit flavors.
In addition, some fruits, such as some kinds of citrus, have special bitterness, which is caused by hesperidin and naringin. The astringency of immature persimmons is due to the content of soluble tannins, which decreases with the ripening of fruits. The soluble tannins contained in sweet persimmons can also be transformed into insoluble and naturally astringent in trees. The astringent persimmon can only be eaten after artificial astringency removal.