Citruses . Set of species, belonging to the genus Citrus . They play a prominent 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 encompasses provide notable amounts of Vitamin C , Minerals ( calcium and phosphorus ). Citrus fruits belong to the class Angiospermae , to the subclass Dicotyledonous , to the order Rutae , to the family Rutaceaeand the citrus genus and has more than 145 species, among which are: Citrus sinensis orange , Citrus reticulata mandarin , Citrus limon lemon , Citrus aurantifolia lime , Citrus paradisi grapefruit . 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 any historical record.
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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 Generalities of the crop
- 3 Physiology
- 1 Physiological drop of fruits
- 4 Flowering and fruiting
- 5 Cultivation
- 1 Climate
- 2 Soil
- 6 Patterns
- 1 Other Patterns
- 7 Citrus Growing Techniques
- 1 Plantation
- 8 Fertilization
- 1 Annual fertilizer requirements
- 9 Irrigation
- 10 Growth Regulators
- 11 Citrus pruning
- 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
- 1 General characteristics of the plant
- 1 Scientific name
- Citrus L.  
- Linnaeus , Carl von
- Published in: Species Plantarum 2: 782–783. 1753. (1 May 1753 ) 
- Citrus medica L. 
Species and varieties
Classification of the main species
Species of the genus Citrus have a great facility for hybridization, so traditionally there has 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 one source or another is consulted. This classification simplifies and clarifies the genus and appears 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 being hybrids of these three. This theory only refers to cultivated citrus fruits, not to the rest of the species of this genus.
This classification would be as follows:
- Citrus medica L. It is the citronor citron , it includes the cultivar ‘Etrog’ used in the Jewish festival of tabernacles. With two hybrid taxa:
- Citrus × limon (L.) Osbeck. What is the lemon tree.
- Citrus × jambhiri Lush., Is the rough lemon.
- Citrus maxima ( Burm. ) Merr. It is the pampelmusaor pummelo . With two hybrid taxa:
- Citrus × aurantifolia , are the limes, which are hybrids of C. medica or C. × limon with the small-flowered C. micrantha.
- Citrus × aurantium (Citrus maxima x Citrus reticulata), which form the group of bitter oranges. Within this taxon four more hybrids are considered.
- Citrus × sinensis , are also hybrids of Citrus maxima x Citrus reticulata but with a higher proportion of mandarin, are the orange trees, which produce the well-known sweet oranges.
- Citrus × paradisi , is the grapefruitor grapefruit , which is a backcross between orange and C. maxima.
- Citrus × nobilis , are the tangorand also the King mandarin. They are also backcrosses with C. maxima.
- Citrus × tangelo , they are the tangelos. That they are successive crosses of C. paradisi or C. maxima and C. reticulata.
- Citrus reticulata Blanco. It would encompass all tangerines, mandarins, satsumas and clementines. 
- Citrus × tangerina , is the mandarinor tangerine . Depending on the authors, the taxonomic classification of this species varies: For some it is a hybrid between Citrus reticulata and Citrus aurantium . Being in this case its scientific name Citrus × tangerina Tanaka; for others it is a different species and it would be called Citrus tangerina Tanaka and for others it would simply be a variety of Citrus reticulata.
- Citrus assamensis RMDutta & Bhattacharya
- Citrus aurantiaca Swingle
- Citrus australasica F.Muell.
- Citrus australis (A.Cunn. Ex Mudie) Planch.
- Citrus cavaleriei H.Lév. ex Cavalerie
- Citrus garrawayi FMBailey
- Citrus glauca (Lindl.) Burkill
- Citrus gracilis Mabb.
- Citrus halimii BCStone
- Citrus indica Yu.Tanaka
- Citrus inodora FMBailey
- Citrus japonica Thunb.
- Citrus junos Siebold ex Tanaka
- Citrus khasya Markovitch
- Citrus latipes (Swingle) Yu.Tanaka
- Citrus polyandra Yu.Tanaka
- Citrus × polytrifolia Govaerts
- Citrus swinglei Burkill ex Harms
- Citrus trifoliata L.
- Citrus warburgina FMBailey
- Citrus wintersii Mabb. 
- 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 , F.Méndez & S.Ríos
- Citrus × limonia Osbeck – Mandarin Lime, Canton Lime, Rough Lemon, Rangpur Lime, Cravo Lemon, Hime Lemon. 
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 Pomelos . The leaves are uniform with edges of various shapes and of very large, moderate or small size. Petiole size also varies with species generally similar to leaf size. The flowers are born individually or grouped in the axils of the leaves and can be perfect or staminate.
The germination of the seed is hipogea ie cotyledons remain underground. The temperature for the radicle 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. The intensity of the light does not affect germination or emergence but Seedlings that develop in the dark are pale and spiny.
Physiological fruit drop
It is a disorder probably related to the competition between fruits for carbohydrates, water, hormones and other metabolites. The problem, however, is greatly accentuated by stress, especially that caused by high temperatures and lack of water. Consequently the physiological fall is usually more severe where the temperatures of the leaves can reach 35-40ºC, and where the lack of water creates problems. One hypothesis is that the high temperatures and the marked lack of water cause the stomata to close with the consequent decrease in the net assimilation of CO2. Then there is abscission in the fruits because they maintain a negative carbon balance.
Flowering and fruiting
The most probable control factors of flowering in citrus fruits are carbohydrates , hormones , nutrition and hydric relations , the first two have an important contribution to the development of the fruits. Striped of branches: it produces a stimulus in the growth of the fruit. In some varieties it is carried out 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 suffer from abscission to a lesser extent, thus improving the fruit set and the final harvest. -Application ofSynthetic auxins : increases the final size of the fruit with minimal or no thinning. The time of application, regardless of the varieties, must be carried out after the physiological fall of the fruits, to increase the final size of the fruit; in other words, 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 loss of 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 shows exponential growth, there is a maximum cell division that gives it growth in the thickness of the pericarp. The juice sacs are formed. STATE II. It lasts several months, shows 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 change of color of the superficial layer of the shell STATUS III. There is a reduced rate of growth, all the changes associated with its maturation occur, the content of soluble solids increases.
Citrus fruits are a subtropical genus where most of the species survive short periods of 0ºC. They bear the cold badly. The greater or lesser resistance to temperatures depends on the species, variety, the graft on Poncirus is more resistant to cold, when it occurs, health status, nutritional status, etc.
At – 2 ºC they do not cause appreciable damage (at most 4 or 5 hours at this temperature). Damage to leaves and fruits is observed at – 3 ºC. In spring it is more harmful than in winter. At – 9 ºC we have damage to main branches. At – 11 ºC the death of the tree occurs. They prefer a sunny orientation and must be protected by means of windbreaks in windy areas.
From highest to lowest susceptibility to cold by citrus fruit: Citrus Lima Lemon Grapefruit Sweet orange Bitter orange Mandarino Fortunella (kumquat) (the most resistant to cold on the list)
Citrus fruits are not too demanding on soils, an important condition is good aeration, that it is not excessively clayey, and with 1-1.5 m of soil depth is enough, 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 the disease Gomosis (cracking of the bark at the neck level), rot and root suffocation, tolerates a wide range of soils, but thrives in those fertile, well drained and slightly acidic (pH 6-6.5) .
- Bitter Orangeis a good universal rootstock, but susceptible to the Sadness Virus, except with lemon tree, for which it is a good rootstock. – Resistant to phytophthora . – Good 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 quality production. – The nurseries graft 95% on citrange and in recent years nothing in Carrizo.
– Poncirus trifoliata : very susceptible to limestone. In Spain it is not used. – Cleopatra mandarin: less productive and fruit size better than citrange . – Citrumelo 4475: interesting pattern for grapefruit.
Citrus Growing Techniques
- Pollination: Most citrus fruits are self-fertile, so a pollinator is not required. You can plant an isolated citrus and it will bear fruit, it does not need another tree to provide pollen.
The plantation frames depend on the area. For example, in the South of Spain they are:
Orange : 6×4, 7×5 – Grapefruit: 7×6, 8×6. It develops a lot due to its great vigor.
Mandarin : clementines are vigorous and satsumas less vigorous. Citrus fruits are susceptible to waterlogging in a place where the soil is not sufficiently well drained. Plant each tree on a slight mound if the soil puddles easily.
The tree takes nitrogen (75% from reserve and 25% from the soil). Therefore, we contribute 50% of the total nitrogen in spring and the remaining 50% in summer, so that it can 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 added at once, along with nitrogen, in spring. There is no problem for washing from the rain.
If fertigation is done (irrigation with the dissolved compost), the considerations are different. Nutrient needs increase over the years until adulthood (more than 8 years).
Annual Fertilizer Needs
– Nitrogen: 600 grams per tree – Phosphorus: 150 grams per tree – Potassium: 300 grams per tree
In citrus, the fertilizer 15-15-15 is widely used. 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 provided 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 has visual symptoms of nutrient deficiencies, it is already quite serious. Recovering that tree is slower and more expensive.
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. Remove weeds from the base of the trees.
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 harvested late, it is also a frequent practice to spray with gibberellic acid when the petals fall to favor the set of clementines.
Growth regulators can act:
– Increasing the fruit size in small mandarins. – Delaying maturation in Satsuma Owari and Clemenules. – Decreasing the fall of ripe fruit. Navel, Sanguina and grapefruit group. – Advancing the coloring of the fruit in the field.
They are pruned as little as possible. Many reserves are removed from the tree. It is planted in the autumn and stands out about 80 centimeters from the ground. It is allowed to vegetate freely for 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 vase. Citrus pruning is not essential to do it every year, and if it is done, light. However, it is not necessary to leave more than 3 years without pruning. It is common 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 by compacted sides and maintaining a height from the ground. Dry, horizontal branches, branches that look inward, diseased and damaged branches and all those that touch the ground are removed. A poorly placed pacifier must be removed.
Lighten the center of the tree by eliminating the overly vigorous branches that are directed towards the center, the most recommended pruning time in spring, once the cold of winter has passed and that we have harvested.
Cutting tools can transmit diseases from tree to tree if we have pruned a tree with a bad appearance, it may have viruses, and the tool must be well cleaned in large cuts, apply pruning mastic.
Harvest and storage
They are fruits that take a long time to ripen after fruiting, between 6 and 8 months, or even longer depending on the climate (colder, the longer it takes to ripen).
- The harvest should be done when the fruits have matured, cutting the stem of the fruit with pruning shears or a knife or twisting the stem slightly.
- Undamaged fruits can be stored for a few weeks at 4-6 ºC.
This is how the species of large shrubs or perennial trees (between 5 and 15 m) are designated whose fruits , from the Rutaceae family , have a high content of vitamin C and citric acid, which gives them that typical acidic flavor so characteristic. This genus contains three species and numerous cultivated hybrids, including the most widely traded fruits, such as lemon, orange , lime, grapefruit, and mandarin, with various varieties depending on the region in which each is grown. . Native to tropical and subtropical Asia
These species that are included in the citrus genus to which belong oranges ( Citrus sinensis ), Chinese oranges ( Citrus japonica ), bitter oranges ( Citrus aurantium ), mandarins ( Citrus reticulata ), lemons ( Citrus limon ), grapefruits ( Citrus paradisi ) limes ( Citrus aurantifolia ) or grapefruits ( Citrus medica ). The dwarf orange or kumquat (Citrus margarita = Fortunella margarita ) would belong to the fortunella genus.
General characteristics of the plant
The root is the underground part of the plant, specialized as an organ of support and absorption of substances . It arises from the radicle of the embryo .
The root is the organ of the plant that is typically underground and can be primary roots and secondary roots.
Citrus fruits are plants with deep roots, 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 near the surface. Its growth is cyclical, alternating with the growth of the aerial part, although as the plants age, the periods of root growth lengthen and in adult trees it is suggested that the growth of the root is almost continuous.
When the seed germinates, the first organ to appear 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 tips grow simultaneously, and the formation of secondary roots.
The taproot grows directly downward and constitutes the taproot. Two types of secondary lateral roots appear: thick roots and clusters of fine fibrous roots. The lateral roots are very numerous and their distribution is not regular. The primary root, the secondaries and the subsequent branches together form the root system. In 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 calyptra, the multiplication region, the elongation or maturation region and the differentiation region are presented.
The cap is made up of parenchymal cells that can suberify.
In a transverse section of the primary root, 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, in whose cell walls the characteristic suberized Caspary bands are present; the pericycle and the vascular cylinder in the central part of the root, in which cords of xylem and phloem alternate, and the center is occupied by xylem cells and parenchyma.
The number of protoxylem arches in the primary root is typically 8, which is reduced to 5 in the lateral thick roots and 3 in the second and third levels of branching.
The secondary structure of the root is characterized by the formation and activity of the secondary cambium and the phellogen that originate by dedifferentiation of the pericycle. A cambium ring is completed (primary plus secondary) producing secondary phloem outward and secondary xylem inward.
Abundant secondary vascular tissue is formed 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 burst through the cortex to the outside.
After the beginning of secondary growth, the peridermis is formed in the pericycle or in the endodermis which causes the shedding of the cortex.
Root hairs of epidermal origin are formed on the roots of citrus fruits, 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.
The roots play an important role in the general behavior of citrus, which will be studied in the chapter on physiology.
Citrus fruits are characterized by being plants with a single stem, which because they are trees is called a trunk, with a more or less cylindrical shape and variable branching with the species and variety.
The stem is the vegetative organ of cormophytic plants that grows in the opposite direction to that of the root and serves as its tentacles to the leaves, flowers and fruits: the rhizomes are underground stems.
The young trunk is green in color and becomes increasingly brown in color as its external tissues suberify.
The small branches that are born from the young trunks are initially triangular and become rounded with age.
Citrus stem structure does not show marked differences from the common dicot type.
Two types of growth are distinguished in the stem, the longitudinal one due to the buds and apical meristems, and the growth in thickness, caused by the activity of the cambium, both promoted by auxins and growth hormones; shoot development is rhythmic, in well-marked cycles at a certain time of year, all of which will be studied in the chapter on physiology.
The stem apex, unlike that of the root, is not covered by the cap, in addition to comprising the foliar primordia and have nodes and between nodes that do not appear at the root apex.
A cross section of the stem shows the epidermis with stomata and covered by a thick waxy cuticle, in the young cortex there are oil glands, spherical structures, apparently of lysigenic origin, which are also present exactly below the epidermis in the palisade parenchyma of leaves, sepals, petals and other parts of the plant; At the completion of primary development, two regions can be observed: the outer cortex composed of small cells with thin walls and chloroplasts, and the inner cortex composed of large, highly vacuolated cells with thick walls. Between the cortex and the parenchymal medulla, the phloem, procambium and xylem appear in that order.
The phloem and xylem form open collateral bundles arranged around the medulla and the parenchyma between them forms the medullary rays, a typical structure of dicotyledons.
The set of tissues outside the cambium is commonly called cortex, 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 phellogen, 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 outer cortex and protects it from desiccation.
Between the root and the stem, anatomical differences can be appreciated such as the absence of oil glands and stoma in the root, the surface of the epidermal cells covered with mucilage in the root and cutin in the stem, the presence of hypodermis and endodermis in the root and the different arrangement of the vascular tissue that in the root is alternately or radially and in the stem forming vascular bundles.
Citrus leaves are evergreen (abscission occurs throughout the year and not all at once) and although apparently simple, they are compound odd-pinnate leaves that retain only the terminal leaflet, as evidenced by the joint between the blade and the petiole.
A leaf is a structure or an organ of plants specialized for photosynthesis.
The exception within this group is Poncirus trifoliata (L.) Raf. for being cadusifolio and for presenting trifoliate leaves, this dominant character that is manifested in the hybrids resulting from the cross 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 winged petioles with wings that vary in shape and size; petiole wings on 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 the formula 3/8 (phyllotaxis) for Poncirus trifoliata, Fortunella margarita and most citrus species, except for C. grandis whose phylactic pattern is 2/5. The direction of the spirality, to the right or to the left, is reversed at each bud.
In the axil 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 thorns.
Young citrus leaves are pale green in color and change to dark green when the growth of the shoot ceases, except for the lemon tree and citrus, which the leaves at birth are reddish in color and turn green with their development.
There are two abscission points: between the petiole and the stem and between the blade and the petiole.
The citrus leaf presents reticulated venation that is frequent in dicots 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 near the edge forming a Y, with each branch anastomosing 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 neither above nor below these glands.
The foliar primordium is cylindrical and curved on the apical dome; as it lengthens, it becomes erect gradually moving away from the axis.
In the formation of the leaf from the foliar 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 composed of tabular cells (wider than deep) covered by cuticle, but stomata are present in the lower one that do not appear in the upper one.
It has been observed that stomatal density generally decreases with leaf maturation and that in mature leaves, 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, cells containing calcium oxalate projecting toward the epidermis are common.
It has been suggested that the ratio of the thickness of the palisade tissue to the total thickness of the leaf is constant for each species or variety and that it differs from one to another; The thickness of the spongy parenchyma has been found to correlate 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 allows the species to be distinguished by the smell that their leaves give off when crushed with the fingers.
The vascular bundles in the leaf are enclosed in the bundle sheaths and their arrangement corresponds to the venation of the leaf.
Citrus flowers are generally hermaphroditic and of variable size, those of the sour orange tree and grapefruit being larger, those of the sweet orange tree and the lemon tree medium, and those of the mandarin and lime tree small, with a waxy appearance and generally white color, although those of the citron and the lemon tree have a purplish tint on the underside of the petals, which is why the color on the buttons stands 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 corymb-shaped clusters, and sometimes tops, which can be terminal or develop in the axils of the leaves. Each flower has a short, naked, articulated and fleshy peduncle with its upper part widened, 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 about to flower, biochemical and physiological changes occur that lead to the transformation of vegetative tips and buds into flower buds.
In the flower of citrus fruits, calyx, crown, androceous and gynoecium are distinguished. Calyx: It is generally made up of five welded sepals (sinépalos), 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 it is collected and that set is the one commonly called the nipple.
The sepals are formed by two epidermis, abaxial and adaxial, between 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, alternating with sepals. The petals are much thicker, longer and leathery than the sepals. The petals are sessile, overlapping and noticeably 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. A large part of the cells of the adaxial epidermis is modified to give rise to unicellular glandular hairs that also occur in smaller numbers in the abaxial epidermis.
Androecium: It is the set of stamens consisting of filament and anthers.
Citrus flowers commonly have 20 to 40 stamens, sometimes free, generally polyadelphs forming groups of three or more, and together a kind of tube that surrounds the gynoecium.
The white filaments are more or less united at their base. The epidermal cells of the filament have thin, slightly cutinized walls and few stomata are present. Surrounded by parenchymal cells, each filament has a main vascular bundle that ends in the anther.
Ripe anthers are bright yellow due to the pollen they contain, when they are pale cream or white they do not contain pollen and 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 occurs exactly above the point of union of the stamens, and until the petals fall, its external parts secrete a watery nectar through the stomata, which is why it is called nectary.
The dehiscence of the normal anthers takes place in each half by a longitudinal slit in the area of union of the lobes, when the petals have 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.
Gyneceous 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 unicellular hairs that secrete a sweet and viscous liquid that retains the pollen and makes it germinate.
The style is cylindrical and somewhat smaller in diameter than the stigma.
In a cross section of the style, there are as many style canals 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 in the cavities of the ovary, thus 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 channels or bundles are filled with parenchymal cells where oil glands are found, and surrounding the whole, an epidermis provided with stomata and with cutinized outer membranes.
The ovary is generally ellipsoidal, polycarpelar, syncarpic, with a number of cavities generally between ten and fourteen, each with ananthropic seminal rudiments arranged in two collateral rows, in which embryonic sacs are formed by meiosis.
In tropical countries fertilization in Citrus occurs three to five days after pollination.
The citrus fruit is a hesperidium of variable size and color depending on the species and variety, as well as its shape, which can be oval, pyriform or spherical, flattened or not. The bark is thick, indehiscent, with the outer surface more or less smooth or rough 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 vesicles of juice of also variable color (green, yellow, orange or red) depending on the variety as well as the number of seeds that occur in said segments.
For its 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 epicarp or flavelo, 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 cuticle covering, and a parenchyma of adjacent compact cells. It is the colored part of the cortex, in it are chromatophores and oil glands.
The albedo is generally white and its structure resembles the spongy mesophyll of the leaf.
The flavedo and the 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 into the interior of the segments, except for those that, starting from said axis, first fed the seminal rudiments and then the 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 very small juice and chromatophores.
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 testa in the form of fins.
The seed is the structure through which the plants propagate, which is why they are called spermatophytes (plants with seeds).
The most common forms are spindle-shaped, 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 greyish white, cream or yellow and brownish 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 ovule (seminal rudiment) consists of a very short phoniculus, a central mass of cells that is the nucelle with the embryo sac, and the two outer and inner tuguments. At the apex of the nucela there is an opening through the huts, the necropolis and towards the opposite end the region of the chalaza.
The funiculus is attached to the outer tube on one side, and the inner tube at that point is much thicker and contains a vascular bundle that runs from the funiculus to the chalaza.
The nucela is a fleshy tissue, one of whose cells becomes the stem cell of the embryo sac that undergoes meiosis to give rise to 4 haploid megaspores, 3 of which degenerate and the rest form the embryo sac. This, in its mature state, contains the antipode cells towards the chalazal end, the synergists and the ovocell towards the micro pillar, and the polar nuclei towards the middle part.
When the pollen tube reaches the embryo sac through the micropyle, a sperm nucleus penetrates and fuses with the egg, thus carrying out fertilization; simultaneously, the other sperm nucleus unites with 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 nucela, which in turn gradually becomes finer. In Citrus and allied genera the endosperm carries nutrients to developing embryos.
In the last stages of embryonic development, the endosperm and nucela almost disappear, leaving only traces that contribute to the formation of the inner seed coat.
Under tropical conditions there is a great acceleration of all Citrus seed development processes compared to other non-tropical regions, including fertilization, initiation of zygotic division, zygotic embryogenesis, nucela evolution, nucellar embryogenesis and endosperm development as well as its degeneration.
In mature seeds, an internal cover or tegmen is distinguished that is thin, membranous in nature and with a characteristic coloration that is darker at the chalazal end, and an external cover or testa of leathery, strong and woody texture, frequently with wrinkles or stretch marks and whose surface is mucilaginous. Like other dicots, 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, in addition to the zygotic embryo, nucellar embryos derived from somatic cells of the nucela that develop asexually by mitotic division develop in the ovule, for which reason the nucellar plants have the genetic maternal 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 fruits, as will be analyzed in these topics.