Citrus

Citrus . Set of species, which belong to the genus Citrus . They play a leading role in feeding many people around the world. A characteristic of the genus is the presence, in all the organs of the plant, of an essential oil that gives it its characteristic smell. The species that this group includes provide remarkable amounts of Vitamin C , Minerals ( calcium and phosphorous ). Citrus fruits belong to the class Angiospermae , to the subclass Dicotyledon , to the order Rutae , to the family Rutaceaeand to the citrus genus and has more than 145 species, among which stand out: orange Citrus sinensis , mandarin Citrus reticulata , lemon Citrus limon , lime Citrus aurantifolia , grapefruit Citrus paradisi . It is believed that the general area of origin of citrus is the Southwest of Asia including from Saudi East to the East to the Philippines and from the Himalava to the South to Indonesia or Australia, the dispersal movement of the different types of citrus occurred within the general area of ​​origin since before there was a historical record.

Summary

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  • 1 Taxonomy
    • 1 Scientific name
      • 1.1 Authors
    • 2 Common name
    • 3 Type species
    • 4 Species and varieties
      • 4.1 Classification of the main species
      • 4.2 Other species
      • 4.3 Main hybrids
    • 2 General of the crop
    • 3 Physiology
      • 1 Physiological drop of the fruits
    • 4 Flowering and fruiting
    • 5 Cultivation
      • 1 Climate
      • 2 Soil
    • 6 Patterns
      • 1 Other Patterns
    • 7 Citrus Cultivation Techniques
      • 1 Plantation
    • 8 Fertilization
      • 1 Annual fertilizer requirements
    • 9 Irrigation
    • 10 Growth Regulators
    • 11 Pruning of citrus
    • 12 Harvest and storage
    • 13 citrus plants
      • 1 General characteristics of the plant
        • 1.1 Root
        • 1.2 Stem
        • 1.3 Sheet
        • 1.4 Flower
        • 1.5 Fruit
        • 1.6 Seed
      • 14 References
      • 15 Bibliography
      • 16 Sources

Taxonomy

Scientific name

  • Citrus L. [1] [2]

Authors

  • Linnaeus, Carl von
  • Posted in: Species Plantarum 2: 782–783. 1753. (1 May 1753) [3]

Common name

Citrus, citrus.

Type species

  • Citrus medica[4]

Species and varieties

Classification of the main species

Species of the genus Citrus have a great ease of hybridization, so there has traditionally been much confusion in the taxonomic classification of many species.

In 1997 Mabberley proposed a pragmatic classification of the main citrus cultivated in order to clarify the chaos that often exists depending on whether sources are consulted. This classification simplifies and clarifies the genus and seems to have the consensus of the main botanists.

Mabberley postulates that there are only three main species Citrus maxima, Citrus medica and Citrus reticulata, all the remaining hybrids of these three being. This theory only refers to cultivated citrus, not the rest of the species of this genus.

This classification would be as follows:

  1. Citrus medica It is the citronor citron , includes the cultivar ‘Etrog’ used in the Jewish feast of tabernacles. With two hybrid taxa:
    • Citrus × limon(L.) Osbeck. What is the lemon tree ?
    • Citrus × jambhiri, Is the rough lemon.
  2. Citrus maxima(  ) Merr. it is the pampelmusa or pummelo . With two hybrid taxa:
    • Citrus × aurantifolia, are the limes , which are hybrids of C. medica or C. × limon with the small-flower muffin C. micrantha.
    • Citrus × aurantium(Citrus maxima x Citrus reticulata), which form the group of bitter oranges . Four other hybrids are considered within this taxon.
      • Citrus × sinensis, are also hybrids of Citrus maxima x Citrus reticulata but with a higher proportion of tangerine, they are orange trees , which produce the well-known sweet oranges.
      • Citrus × paradisi, is the grapefruit or grapefruit , which is a backcross between orange and C. maxima.
      • Citrus × nobilis, are the tangor and also the King mandarin. They are also backcrossed with C. maxima.
      • Citrus × tangelo, are the tangelos . Which are successive crosses of C. paradisi or C. maxima and C. reticulata.
  1. Citrus reticulataIt would include all tangerines, mandarins , satsumas and clementines. [5]
    • Citrus × tangerina, is the mandarin or tangerine . According to the authors, the taxonomic classification of this species varies: For some it is a hybrid between Citrus reticulata and Citrus aurantium . In this case, its scientific name is Citrus × tangerina Tanaka; for others it is a different species and it would be called Citrus tangerina Tanaka and for still others it would simply be a variety of Citrus reticulata.

Other species

  • Citrus assamensisRMDutta & Bhattacharya
  • Citrus aurantiacaSwingle
  • Citrus australasicaMuell.
  • Citrus australis(A.Cunn. Ex Mudie) Planch.
  • Citrus cavalerieiLév. ex Cavalerie
  • Citrus garrawayiFMBailey
  • Citrus glauca(Lindl.) Burkill
  • Citrus gracilis
  • Citrus halimiiBCStone
  • Citrus indicatesTanaka
  • Citrus inodoraFMBailey
  • Citrus japonica
  • Citrus junosSiebold ex Tanaka
  • Citrus khasyaMarkovitch
  • Citrus latipes(Swingle) Yu.Tanaka
  • Citrus polyandraTanaka
  • Citrus × polytrifoliaGovaerts
  • Citrus swingleiBurkill ex Harms
  • Citrus trifoliata
  • Citrus warburginaFMBailey
  • Citrus wintersii[6]

Main hybrids

  • Citrus × bergamia- Bergamot. It is a hybrid of limetta and bitter orange.
  • Citrus × depressa- Shikwasa
  • Citrus × latifolia- Persian lime
  • Citrus × limetta- Sweet lemon
  • Citrus × hystrix
  • Citrus × ichangensis- Papeda
  • Citrus × limodulcis D.Rivera, Obon , Méndez & S.Ríos
  • Citrus × LimoniaOsbeck – mandarin lime, Canton lime, rough lemon, rangpur lime, cravo lemon, hime lemon. [7]

Crop overview

The genus Citrus consists of several species of moderate to large evergreen trees. The shape of the trees varies from the erect crown of some mandarins to the extended one such as that of the Pomelos . The leaves are unifoliate with edges of various shapes and of very large, moderate or small size. Petiole size also varies with the species generally in a similar way to the size of the leaf. The flowers are born individually or grouped in the axils of the leaves and can be perfect or staminate.

Physiology

The germination of the seed is hipogea ie cotyledons remain underground. The temperature for the Radicule to begin to emerge ranges between 9 and 38ºC and varies with each cultivar. The number of days until the first emergence ranges from approximately 80 days at 15-20ºC, to only 14-30 days for most cultivars in the optimal range of 30-35ºC. Light intensity does not affect germination or emergence, but seedlings that thrive in the dark are pale and spindly.

Physiological drop of the fruits

It is a disorder probably related to competition between fruits for carbohydrates, water hormones and other metabolites. However, the problem is accentuated by stress, especially that caused by high temperatures and lack of water. Consequently, the physiological drop is usually more severe where leaf temperatures can reach 35-40ºC, and where water scarcity creates problems. One hypothesis is that high temperatures and a marked lack of water cause stomata to close, with a consequent decrease in net CO2 assimilation. So there is abscission in the fruits because they maintain a negative carbon balance.

Flowering and fruiting

The most likely factors for controlling flowering in citrus fruits are carbohydrates , hormones , nutrition and water relations , the first two of which have an important contribution to the development of the fruit. Striped branches: it produces a stimulus in fruit growth. In some varieties it is done during flowering or after the petals fall, to improve fruit set. This practice has a positive influence on the endogenous hormonal content, attributed to the changes caused in the transport and accumulation of carbohydrates. In this way the growth rate of the fruits is maintained, which consequently undergoes abscission to a lesser extent, thus improving fruit set and the final harvest. -Application ofSynthesis aids : increases the final size of the fruit with minimal or no thinning. The application period, regardless of the varieties, must be made after the physiological fall of the fruits, to increase the final size of the fruit; that is, for a diameter of the fruit between 25 and 30 mm for oranges or during the color change, to facilitate the maintenance of the fruit on the tree without any reduction in quality, in which case Gibberellic Acid is usually added . Fruit growth follows a Sigmoid curve, characterized by three well differentiated states: STATE I. The fruit presents an exponential growth, there is a maximum cell division that gives it a growth in the thickness of the pericarp. Juice bags are formed. STATE II. It lasts several months, it presents a linear growth in time with an increase in the size of the cells, there is differentiation of the cells, the fruit absorbs a large amount of water and reaches its final size. It ends with the color change of the surface layer of the shell. STATE III. There is a reduced growth rate, all the changes associated with its maturation occur, the soluble solids content increases.

Culture

Weather

Citrus is a subtropical genus where most species survive short periods of 0ºC. They endure the cold badly. The greater or lesser resistance to temperatures depends on the species, variety, graft on Poncirus, it is more resistant to cold, when it occurs, health status, nutritional status, etc.

At 2 ° C they do not cause appreciable damage (at most 4 or 5 hours at this temperature). At – 3 ºC damage to leaves and fruits is observed. In spring it is more harmful than in winter. At – 9 ºC we have damage to main branches. At – 11 ºC the tree dies. They prefer a sunny orientation and must be protected by windbreaks in windy areas.

From highest to lowest susceptibility to cold by citrus: Citron Lima Lemon Grapefruit Sweet orange Bitter orange Mandarin Fortunella (kumquat) (the most cold hardy on the list)

Ground

Citrus fruits are not too demanding in soils, an important condition is good aeration, which is not excessively clayey, and with 1-1.5 m of soil depth is sufficient, clayey soils give lower fruit quality than sandy ones: skin thicker, less juice and less sweet.

Citrus fruits show a preference for permeable soils. Excess water causes Gomosis disease (cracking of the bark at the neck level), rot and root suffocation, tolerates a wide range of soils, but thrives in fertile, well-drained and slightly acidic soils (pH 6-6.5) .

Patterns

  • Bitter Orangeis a good universal pattern, but susceptible to Sadness Virus, except with lemon, for which it is a good rootstock. – Resistant to phytophthora . – Well in limestone and for drought. – Average to acceptable production.
  • Citrange Troyer and Carrizo

– Both species are very difficult to distinguish. – Carrizo is resistant to limestone and salinity. – Both are sensitive to suffocation and drought. – High vigor, good and quality production. – Nurseries graft 95% on citrange and in recent years nothing in Carrizo.

Other Patterns

– Poncirus trifoliata : very susceptible to limestone. In Spain it is not used. – Cleopatra mandarin: less productive and better fruit size than citrange . – Citrumelo 4475: interesting pattern for grapefruit.

Citrus Cultivation Techniques

  • Pollination: Most citrus is self-fertile, so a pollinator is not required. You can plant an isolated citrus and it will bear fruit, you do not need another tree to provide pollen.

Plantation

The plantation frameworks depend on the area. For example, in the South of Spain they are:

Naranjo : 6×4, 7×5 – Pomelo: 7×6, 8×6. It develops quite a lot due to its great vigor.

Tangerine : Clementine are vigorous and satsumas less vigorous. Citrus fruits are susceptible to watering in a place where the soil is not sufficiently well drained. Plant each tree on a slight mound if the soil becomes waterlogged easily.

Fertilization

The tree takes up nitrogen (75% reserve and 25% soil). Therefore, we contribute 50% of the total nitrogen in spring and the remaining 50% in summer, so that it will form reserves.

In spring I can use urea, which is a cheaper fertilizer. In summer I use nutrients so that it has a faster absorption. Phosphorus and Potassium are normally supplied at once, together with nitrogen, in spring. No problem from washing the rain.

If fertigation is carried out (irrigation with dissolved fertilizer), the considerations are different. Nutrient needs increase over the years until he becomes an adult (over 8 years old).

Annual fertilizer requirements

– Nitrogen: 600 grams per tree – Phosphorus: 150 grams per tree – Potassium: 300 grams per tree

In citrus, fertilizer is widely used 15-15-15 Nitrogen is applied with ammonium sulfate, ammonium nitrate and urea, based on 0.6-1.2 kg of nitrogen per adult tree and year. 2 or 3 applications are usually made, preferring ammonia forms in spring and nitric forms in summer.

Phosphorus and Potassium are mainly supplied in spring, based on Calcium Superphosphate and Potassium Sulfate, and it depends on the amount of these nutrients in the soil. Average doses range from about 0.3 kg of P2O5 per tree and year of Phosphorus, and 0.15 kg of K2O per tree and year of Potassium.

Lately the use of complex fertilizers is very widespread. The application of nutritional deficiency correctors are also very common. Normally when a citrus presents visual symptoms of nutrient deficiencies, it is already quite serious. Retrieving that tree is slower and more costly.

Irrigation

The dose and frequency of irrigation depends on many factors. For example, in Seville (southern Spain), 120-140 liters per tree per day are needed in the month of maximum needs (July). Watering is absolutely necessary between spring and fall. Drip irrigation is widely used in new plantations. Removes weeds from the base of trees.

Growth Regulators

In the Navelate variety it is a common practice to spray with 2,4-D to keep the fruit on the tree and in the Washington Navel when it is collected late, it is also a frequent practice to spray with gibberellic acid at the fall of the petals to favor the curd of clementines.

Growth regulators can act:

– Increasing the size of the fruit in small tangerines. – Delaying maturation in Satsuma Owari and Clemenules. – Decreasing the fall of ripe fruit. Grupo Navel, Sanguina and grapefruits. – Advancing the coloration of the fruit in the field.

Citrus pruning

They have to be pruned as little as possible. Many reservations are removed from the tree. It is planted in autumn and stands about 80 centimeters above the ground. It is allowed to vegetate freely the first 2-3 years. If there is production, it is removed because the branch arches and does not grow.

We choose 3 or 4 branches inserted at different heights to form a very free glass. Pruning in citrus is not essential to do it every year, and if done lightly. However, it is not necessary to leave more than 3 years without pruning. It is frequent to prune every 2 or 3 years for economic reasons in producing farms. The mandarin, every year.

You put one inside and prune from the inside out, it looks better. Thinning of twigs on the compacted sides and maintaining a height from the ground. Parched, horizontal branches, branches facing inward, diseased, damaged branches, and all that touch the ground are removed. A misplaced pacifier should be removed.

Clear the center of the tree eliminating the too vigorous branches that go towards the center, the most recommended pruning season in spring, once the cold of winter has passed and we have harvested.

The cutting tools can transmit diseases from tree to tree if we have pruned a tree with a bad appearance, it can have virosis, and we must clean the tool well in large cuts and apply pruning mastic.

Harvest and storage

They are fruits that take a long time to mature from fruiting, between 6 and 8 months, or even more depending on the climate (colder, it takes longer to ripen).

  • The harvest should be done when the fruits have matured, cutting the stem of the fruit with pruning shears or with a knife or twisting the stem slightly.
  • The undamaged fruits can be stored for a few weeks at 4-6 ºC of temperature.

Citrus plants

Citrus plants

This is the name of the species of large perennial shrubs or saplings (between 5 and 15 m) whose fruits , from the Rutáceas family , have a high content of vitamin C and citric acid, which provides them with that typical acidic flavor so characteristic. This genus contains three species and numerous cultivated hybrids, including the most widely commercialized fruits, such as lemon, orange , lime, grapefruit and tangerine, with various varieties depending on the region in which each of them is grown. . Native to tropical and subtropical Asia

These species are included in the citrus genus to which oranges ( Citrus sinensis ), Chinese oranges ( Citrus japonica ), bitter oranges ( Citrus aurantium ), mandarins ( Citrus reticulata ), lemons ( Citrus limon ), grapefruits belong. ( Citrus paradisi ) limes ( Citrus aurantifolia ) or grapefruit ( Citrus medica ). Dwarf orange or kumquat (Citrus margarita = Fortunella margarita ) would belong to the genus fortunella.

General characteristics of the plant

Root

The root is the underground part of the plant, specialized as a support and absorption organ for substances . It arises from the radicle of the embryo .

The root is the organ of the plant that is typically below ground and can be both primary and secondary roots.

Citrus fruits are deep-rooted plants, depending on the type of soil and the pattern used, which need good oxygenation of the soil.

The citrus root system has a wide horizontal and vertical distribution, with the highest percentage of roots close to the surface. Its growth is cyclical, alternating with the growth of the aerial part, although as plants age, the periods of root growth lengthen and in adult trees it is considered that the growth of the root is almost continuous.

When the seed germinates, the first organ that appears is the primary root. The development of the root system includes the increase in thickness of the root, which depends on the functioning of the cambium, the longitudinal growth that takes place only at the tip, with growth cycles that alternate with those of the shoots and during which many of the root apices grow simultaneously, and the formation of secondary roots.

The primary root grows directly down and constitutes the main root. Two types of secondary lateral roots appear: thick roots and clusters of fibrous fine roots. The lateral roots are very numerous and their distribution is not regular. The primary root, the secondary roots, and the subsequent ramifications together form the root system. At each branch the roots are smaller, the diameter of the smallest being less than 0.5 cm.

From the apex to the base of the root, the cap or caliptra, the multiplication region, the elongation or maturation region and the differentiation region are presented.
The coif is made up of parenchymal cells that can suberify.

In a primary root cross section, the following can be distinguished: the epidermis, the hypodermis or exodermis below it and which begins the cortex; the endodermis which is the innermost cortical layer, on whose cell walls the characteristic suberized Caspary bands appear; the pericycle and the vascular cylinder in the central part of the root, where xylem and phloem cords alternate, and the center is occupied by xylem and parenchyma cells.

The number of protoxylem arches in the primary root is typically 8, which is reduced to 5 in the thick lateral roots and to 3 in the second and third branching level.
The secondary structure of the root is characterized by the formation and activity of the secondary cambium and the phylogen that originate from dedifferentiation of the pericycle. A cambium ring (the primary plus the secondary) is completed, producing a secondary phloem outward and a secondary xylem inward.
Abundant secondary vascular tissue forms in the main fibrous roots and little or none in the smaller ones.

The lateral roots are formed from the dedifferentiation of cells of the pericycle that give rise to the primordia of the lateral roots, which, when growing, break through the cortex to the outside.
After the onset of secondary growth, the peridermis forms in the pericycle or endodermis causing the cortex to shed.

Radical hairs of epidermal origin form on citrus roots, although they are rare and their number, size and duration basically depend on temperature, aeration and pH.
Under normal cultivation conditions it is very common for citrus roots to have mycorrhizae, constituting effective associations.
Roots play an important role in the general behavior of citrus fruits, which will be studied in the chapter on physiology.

Stem

Citrus fruits are characterized by being single-stem plants, which because they are trees are called trunks, with a more or less cylindrical shape and variable branching with the species and variety.

The stem is the vegetative organ of cormophyte plants that grows in the opposite direction to the root and serves as its tentacles for leaves, flowers and fruits: rhizomes are underground stems.

The young trunk is green in color and becomes more and more brown in color as its external tissues submerge.
The small branches that emerge from the young trunks are initially triangular and become more round with age.
The structure of the citrus stem does not show marked differences with the common type of dicotyledons.

In the stem two types of growth are distinguished, the longitudinal one due to the apical buds and meristems, and the growth in thickness, originated by the activity of the cambium, both promoted by auxins and growth hormones; The development of the shoots is rhythmic, in well marked cycles at a certain time of the year, all of which will be studied in the chapter on physiology.

The caulinar apex, unlike that of the root, is not covered by the coping, in addition to comprising the leaf primordia and they have knots and between nodes that do not appear at the root apex.
In a cross section of the stem the epidermis with stomata and covered by a thick waxy cuticle is observed. In the young cortex there are oil glands, spherical structures, apparently of lysigenic origin, which also appear just below the epidermis in the parenchyma in palisade of leaves, sepals, petals and other parts of the plant; At the completion of primary development, two regions can be seen: the outer cortex composed of small, thin-walled cells with chloroplasts, and the inner cortex composed of very vacuolated large cells with thick walls. The phloem, procambium, and xylem appear in that order between the cortex and the parenchymal medulla.
The phloem and xylem form open collateral bundles arranged around the medulla and the parenchyma that remains between them forms the medullary rays, a typical structure of dicotyledons.

Bark is commonly called the set of tissues outside the cambium, which contains considerable amounts of pectin.
The secondary structure of the stem is characterized by the appearance and activity of the cambium ring and the phylogen, this arises by differentiation of the outermost layer of the cortex and the secondary cambium by dedifferentiation of cells of medullary rays.
From secondary growth, the epidermis is gradually replaced by peridermis that forms in the external cortex and protects from drying out.

Between the root and the stem anatomical differences can be seen such as the absence of oil glands and stomata at the root, the surface of the epidermal cells covered with mucilage at the root and cutin on the stem, the presence of hypodermis and endodermis in the root and the different arrangement of the vascular tissue that in the root is alternate or radial and in the stem forming vascular bundles.

Leaf

Citrus leaves are perennial (abscission occurs all year and not all at once) and although apparently simple, they are odd-pinned compound leaves that retain only the terminal leaflet, as evidenced by the articulation between the limbus and petiole.

A leaf is a specialized plant structure or organ for photosynthesis.

The exception within this group is Poncirus trifoliata (L.) Raf. for being cadusifolio and for presenting trifoliate leaves, this dominant character that manifests itself in the hybrids resulting from the crossing in which it intervenes.

The leaves can have various shapes but are mostly elliptical, with a generally entire margin and in some species such as C. lemon and C. aurantifolia more or less toothed. With the exception of the lemon tree, they have the winged petiole with wings that vary in shape and size; petiole wings in sour orange, grapefruit and C. grandis are well developed.

The size of the leaves depends on the variety and the age of the tree. The leaves on the stem are arranged in a helix according to formula 3/8 (phyllotaxis) for Poncirus trifoliata, Fortunella margarita and most citrus species, except C. grandis whose phylactic pattern is 2/5. The direction of the spiral, to the right or to the left, is reversed with each sprouting.

In the axilla of the leaf there is a bud accompanied by a thorn of different size depending on the vigor of the branch or shoot and depending on the age of the clone, there are species that practically lack spines.
Young citrus leaves are pale green and change to dark green when the growth of the shoot ceases, except for the lemon and citron, in which the leaves at birth are reddish in color and become green with development.
Two abscission points are presented: between the petiole and the stem and between the limbus and the petiole.

The citrus leaf has reticulated venation that is frequent in dicotyledons and consists of a network formed by anastomosis of the vascular bundles, in which the smaller bundles diverge from the larger ones. In the vascular system of the citrus leaf, a prominent middle vein is distinguished and it is observed that most of the main lateral veins branch off near the edge forming a Y, and each branch anastomoses with that of another lateral vein.
In Rutaceae, leaf venation patterns are related to the oil glands present in the leaves. In citrus, the vascular bundles are not above or below these glands.
The leaf primordium is cylindrical and curved on the apical dome; as it lengthens, it becomes erect by gradually moving away from the axis.

In the formation of the leaf from the leaf primordium, the protodermis matures in the epidermis, the mesophyll is derived from the fundamental meristem and the vascular feces from the procambium.
In the leaf, as in the stem, the epidermis forms the outermost layer. Both the upper epidermis and the lower epidermis of the mature leaf are made up of tabular cells (wider than they are deep) covered by the cuticle, but stomata that do not appear on the upper one appear in the lower one.
It has been observed that the stomatal density generally decreases with leaf maturation and that in mature leaves, the stomatal density is inversely proportional to its dimensions.
In the mesophyll, the palisade parenchyma has two to three layers of cylindrical cells, and the spongy parenchyma comprises approximately eight layers and contains a large number of intercellular spaces. In palisade parenchyma, it is common to see cells containing calcium oxalate projecting into the epidermis.
It has been suggested that the ratio of the thickness of the palisade fabric to the total thickness of the leaf is constant for each species or variety and that it differs from one another; The thickness of the spongy parenchyma has been found to be correlated with the thickness of the leaf.

The leaves of almost all citrus species have typical subepidermal glands that contain essential oils and are translucent, visible by transparency and often give the leaf a stippled appearance; they are more frequent on the upper surface than on the lower one and the essential oils contained in them have a typical fragrant odor that makes it possible to distinguish the species by the smell given off by their leaves when crushed with fingers.
The vascular bundles in the blade are enclosed in the bundle sheaths and their arrangement corresponds to their venation.

Flower

The citrus flowers are generally hermaphrodite and of variable size, being larger those of the sour orange and the grapefruit, medium those of the sweet orange and the lemon tree, and small those of the mandarin and the lime tree, of ceráceous appearance and generally white color, although those of the citron and the lemon tree have a violet tint on the ventral side of the petals, making the color of the buttons stand out more.

The flower is the characteristic reproductive structure of plants called phanerogams.

The flowers of all cultivated species have a very characteristic pleasant aroma.
They appear isolated or grouped in clusters in the shape of a corymb, and sometimes a top, which can be terminal or develop in the axils of the leaves. Each flower has a short, naked, articulated and fleshy peduncle with its widened upper part, which constitutes the receptacle.
Depending on the latitude, the plants bloom in one month or another of the year. Flowering is regulated by external and internal processes. It is closely related to climatic conditions such as temperature and rainfall, with the presence of diseases and, above all, with water stress.

When a plant is going to flower, biochemical and physiological changes occur that lead to the transformation of the vegetative tips and buds into flower buds.
The calyx, crown, androceus and gyneceum are distinguished in the citrus blossom. Calyx: It is generally made up of five welded sepals (sinsépalos), it is persistent and light green in most cases. Together with the disc and part of the peduncle they are attached to the fruit when they are harvested and that set is the one commonly called the nipple.
The sepals are formed by two epidermis, abaxial and adaxial, among which is a parenchyma with parallel vascular bundles as well as oil glands and stomata both in abaxial position.
Crown: It is generally made up of five free petals, which alternate with sepals. The petals are much thicker, longer and leatherier than the sepals. The petals are sessile, imbricated and remarkably curved in the mature flower.

The waxy appearance of the flower is due to the cutinization of the surface of the petals.
The internal structure of the petals is similar to that of the sepals. The vascular bundles are almost parallel and the central bundle is longer than the rest; the stomata are not numerous and are slightly sunken. The oil glands, as in the sepals, are located just below the abaxial epidermis. Much of the cells of the adaxial epidermis are modified to give rise to unicellular glandular hairs that also appear in smaller numbers in the abaxial epidermis.

Androceo: It is the set of stamens consisting of filament and anthers.

Citrus flowers commonly have 20 to 40 stamens, sometimes free, generally polyadelfos forming groups of three or more, and together a kind of tube that surrounds the gyneceum.
The white filaments are more or less united at their base. The epidermal cells of the filament have thin, slightly cutaneous walls and few stomata. Surrounded by parenchymal cells, each filament has a main vascular bundle that ends in the anther.

The mature anthers are bright yellow due to the pollen they contain, when they are pale cream or white they do not contain pollen and their dehiscence does not usually occur. The anthers consist of four lobes, in each of which there is a relatively large cavity, in which haploid microspores or pollen grains are formed by meiosis.

The floral disc appears exactly above the point of union of the stamens, and until the petals fall, their external parts secrete an aqueous nectar through the stomata, which is why it is called a nectar.
The dehiscence of the normal anthers takes place in each half by a longitudinal slit in the zone of union of the lobes, when the petals have been separated.

The pollen grains are yellow, spherical or oval, sometimes with a polyhedral tendency as in the lemon tree, and with two to six grooves, mostly four, although they may not appear.
Gineceo or pistil: Set of carpels that includes stigma, style and ovary.
The stigma is the spherical structure, more or less flattened, that appears at the end of the style. Its epidermal cells are modified, giving rise to long single-celled hairs that secrete a sweet and viscous liquid that retains pollen and makes it germinate.
The style is cylindrical and somewhat smaller in diameter than the stigma.

In a cross-section of the style, as many canals are observed as there are cavities in the ovary.
These channels extend the entire length of the style and open at one end on the surface of the stigma, and at the other into the cavities of the ovary, thereby establishing communication between the ovary and the exterior, through which the tubes penetrate pollen to effect fertilization. The vascular bundles of the style are a continuation of the ovary, the spaces between the channels or bundles are filled with parenchymal cells where oil glands are found, and surrounding the set, an epidermis provided with stomata and with cutinized outer membranes.

The ovary is generally ellipsoidal, polycarpal, syncarpic, with a number of cavities generally between ten and fourteen, each with anthropoid seminal rudiments arranged in two collateral rows, in which the embryonic sacs are formed by meiosis.
In tropical countries fertilization in Citrus occurs three to five days after pollination.

Fruit

The citrus fruit is a hesperidium of variable size and color with the species and variety, as well as its shape, which may be oval, piriform or flattened or not flat. The crust is thick, indehiscent, with a more or less smooth or rough outer surface and whose color depends largely on the temperature at which it develops.

The fruit is the fertilized ovary of flowering plants.

The interior of the fruit is divided by membranous partitions into segments or segments with a pulp formed by juice vesicles of also variable color (green, yellow, orange or red) depending on the variety as well as the number of seeds that appear in these segments.

For your study, the fruit of the citrus fruit is divided into parts that are not related to its ontogeny or its phylogeny.
The pericarp, the part outside the segments, is divided into the epicarp or flavius, mesocarp or albedo, and endocarp.
The flavelo is made up of the outermost tissue layers of the fruit, it comprises the epidermis with stomata and a cuticle cover, and a parenchyma of adjacent compact cells. It is the colored part of the crust, in it are chromatophores and oil glands.
The albedo is generally white and its structure recalls the spongy mesophyll of the leaf.
The flavedo and albedo make up the rind of the fruit.
The endocarp is made up of the segments.
The vascular bundles that conduct water and food substances, which exist in the rind and in the central axis of the fruit, do not penetrate inside the segments, except for those that, starting from this axis, fed first to the seminal rudiments and then to the seeds.
Juice vesicles are composed of a thick body and a filamentous peduncle, the length of which depends on the position of the vesicle, which attaches it to the wall of the segment.
They contain the juice and very small chromatophores.

Seed

Citrus seeds are of variable shape and size, but there are common characteristics for the same species: those of lemon are small, rounded and pointed, while those of grapefruit are large, flattened and with projections of the head in the form of fins.

The seed is the structure through which the plants that are called spermatophytes (seed plants) carry out the propagation.

The most common forms are fusiform, ovoid, wedge-shaped and deltoid. The average number of seeds per fruit differs from one variety to another and is also related to the growing conditions.
The color of the seed varies between grayish white, cream or yellow and carmelite or greenish, with a difference between the varieties.
The characteristic polyembryonic seeds of most citrus species are derived from the ovules through a series of growth and development changes.

The mature ovum (seminal rudiment) consists of a very short phonicle, a central mass of cells that is the nucella with the embryo sac, and the two outer and inner tuguments. At the apex of the nape there is an opening through the tuguments, the necropolis and towards the opposite end the region of the chalaza.
The funicular is attached on one side to the outer tugument and the inner tugument at that point is much thicker and contains a vascular bundle that runs from the funicular to the chalaza.
The nucella is a fleshy tissue, one of whose cells transforms into the stem cell of the embryonic sac that undergoes meiosis to give rise to 4 haploid megasporas, 3 of which degenerate and the rest form the embryonic sac. This, in its mature state, contains the antipodal cells towards the chalazal end, the synergized ones and the ovocell towards the micro-pillar, and the polar nuclei towards the middle part.

When the pollen tube reaches the embryonic sac through the micropile, a spermatic nucleus penetrates and fuses with the ovocell, thus fertilizing; simultaneously, the other spermatic nucleus joins the two polar nuclei, which is the origin of the triploid endosperm.
The endosperm gradually increases with the growth of the seeds, exceeding the volume of the nucelus which, in turn, gradually becomes finer. In Citrus and related genera, the endosperm transports nutrients to developing embryos.
In the last stages of embryonic development, the endosperm and nucella almost disappear, leaving only traces that contribute to the formation of the inner seed coat.

In tropical conditions, there is a great acceleration of all Citrus seed development processes compared to other non-tropical regions, including fertilization, the beginning of zygotic division, zygotic embryogenesis, the evolution of the nucelus, the nucellar embryogenesis and the development of the endosperm as well as its degeneration.
In mature seeds, an internal covering or tegmen is distinguished that is thin, membranous in nature and with a characteristic coloration that is darker at the chalazal end, and an outer covering or testa of leathery, strong and woody texture, frequently with wrinkles or striae and whose surface is mucilaginous. Like other dicotyledons, the radicles of the embryos are at the end and the cotyledons towards the chalazal end.

In many varieties of Citrus as well as in Fortunella and Ponsirus, nucellar embryos derived from somatic cells of the nucella develop asexually by mitotic division, in addition to the zygotic embryo, so that the nucellar plants have the matema genetic constitution, except in cases of possible differences due to somatic variations.
This form of asexual reproduction has very important consequences for the improvement and propagation of citrus, as will be analyzed in these topics.

 

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