Citrus

Citrus is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes.
The genus Citrus is native to South Asia, East Asia, Southeast Asia, Melanesia, and Australia. Various citrus species have been used and domesticated by indigenous cultures in these areas since ancient times. From there its cultivation spread into Micronesia and Polynesia by the Austronesian expansion (c. 3000–1500 BCE); and to the Middle East and the Mediterranean (c. 1200 BCE) via the incense trade route, and onwards to Europe.

Description

Tree

These plants are large shrubs or small to moderate-sized trees, reaching 5-15 m (16-49 ft) tall, with spiny shoots and alternately arranged evergreen leaves with an entire margin. The flowers are solitary or in small corymbs, each flower 2-4 cm (0.79-1.57 in) diameter, with five (rarely four) white petals and numerous stamens; they are often very strongly scented, due to the presence of essential oil glands.

Fruit

The fruit is a hesperidium, a specialised berry, globose to elongated, 4-30 cm (1.6-11.8 in) long and 4-20 cm (1.6-7.9 in) diameter, with a leathery rind or “peel” called a pericarp. The flavedo, also known as the zest, is the outermost layer of pericarp. The middle layer of the pericarp is the mesocarp, which in citrus fruits consists of the white, spongy “albedo”, or “pith”. The innermost layer of the pericarp is the endocarp. Each segment has a space that is filled with juice vesicles or “pulp”. From the endocarp, string-like “hairs” extend into the locules, which provide nourishment to the fruit as it develops. Many citrus cultivars have been developed to be seedless (see nucellar embryony and parthenocarpy) and easy to peel.

Citrus fruits are notable for their fragrance, partly due to flavonoids and limonoids (which in turn are terpenes) contained in the rind, and most are juice-laden. Citric acid and other organic acids give the juice its distinctive sharp taste. Commercially, the genus is important because many species are grown for their fruit. It can be eaten fresh or pressed for juice.

They are also rich in vitamin C.
The flavonoids include various flavanones and flavones.

Cultivation

Citrus trees hybridise very readily – depending on the pollen source, plants grown from a Persian lime’s seeds can produce fruit similar to grapefruit. Thus, all commercial citrus cultivation uses trees produced by grafting the desired fruiting cultivars onto rootstocks selected for disease resistance and hardiness.

Only climates with a cool (diurnal), winter colour can develop the colour of citrus fruits. In tropical regions with no winter at all, citrus fruits remain green until maturity, hence the tropical “green oranges”. The Persian lime in particular is extremely sensitive to cool conditions, thus it is not usually exposed to cool enough conditions to develop a mature colour.^{[citation needed]} If they are left in a cool place over winter, the fruits will change colour to yellow.

Although the terms “ripe” or “mature” are often used interchangeably, they can mean something completely different. A mature fruit is one that has completed its growth phase. The changes in the fruit’s structure that take place after it has reached maturity are called ripening. These changes usually involve starches converting to sugars, a decrease in acids, softening, and change in the fruit’s colour.

Citrus fruits are not climacteric, and their respiration slows down and the production and release ethylene is gradual. They do not “tree-ripe” in the sense that they go through a ripening stage. Cherry, for instance, matures physically and continues to ripen on its tree. Other fruits, such as pears, are picked when mature, but before they ripen, then continue to ripen off the tree. Citrus fruits go through stages of maturity, immaturity, and overmaturity while they are still on the tree. They stop ripening or increasing in sweetness once they have been removed from the tree. After being picked, the only way they can change is to eventually begin to decay.

With oranges, colour cannot be used as an indicator of ripeness because sometimes the rinds turn orange long before the oranges are ready to eat. Tasting them is the only way to know whether they are ready to eat.

Citrus trees are generally not frost-hardy. Mandarin oranges (C. reticulata) tend to be the hardiest of the common Citrus species and can withstand short periods down to as cold as -10 degC (14 degF), but realistically temperatures not falling below -2 degC (28 degF) are required for successful cultivation. Although tangerines, tangors, and yuzu can all be grown outdoors, they may not thrive in areas with subfreezing temperatures. However, this could affect the quality of the fruit. A few hardy hybrids can withstand temperatures well below freezing, but do not produce quality fruit. Lemons can be commercially grown in cooler-summer/moderate-winter, coastal Southern California, because sweetness is neither attained nor expected in retail lemon fruit. The trifoliate orange (C. trifoliata), can survive below -20 degrees Celsius (-4 degrees F). Its fruit are bitter and unpalatable unless they are cooked. However, there are some better-tasting hybrids and cultivars (see citranges).

Trees thrive in sunny, humid environments with adequate rainfall and irrigation. The dry summers of Central California’s Inner Coast Ranges can cause damage to abandoned trees in valleys, but they will still survive. Citrus can grow well at any age with only occasional irrigation in partial shade. However, the fruit crop is less. Being of tropical and subtropical origin, oranges, like all citrus, are broadleaved and evergreen. They do not drop leaves except when stressed. The stems of many varieties have large sharp thorns. The trees flower in the spring, and fruit is set shortly afterward. Fruit begins to ripen in fall or early winter, depending on cultivar, and develops increasing sweetness afterward. Some tangerines are ripe by winter. Some, like the grapefruit may take up 18 months to mature.

Production

According to the UN Food and Agriculture Organization, world production of all citrus fruits in 2016 was 124 million tons, with about half of this production as oranges. According to the United Nations Conference on Trade and Development (UNCTAD), citrus production grew during the early 21st century mainly by the increase in cultivation areas, improvements in transportation and packaging, rising incomes and consumer preference for healthy foods. In 2019-20, world production of oranges was estimated to be 47.5 million tons, led by Brazil, Mexico, the European Union, and China as the largest producers.

As ornamental plants

Citrus trees grown in tubs and wintered under cover were a feature of Renaissance gardens, once glass-making technology enabled sufficient expanses of clear glass to be produced. Through the 17th and 18th century, orangeries were a common feature in royal and aristocratic homes. The Orangerie at the Palace of the Louvre, 1617, inspired imitations that were not eclipsed until the development of the modern greenhouse in the 1840s. The Tayloe House in Mount Airy, Virginia is the oldest surviving American orangery. George Washington had an orangery at Mount Vernon.

Some modern hobbyists still grow dwarf citrus in containers or greenhouses in areas where the weather is too cold to grow it outdoors. If the trees are to flourish and bear fruit, they need good climate conditions, enough sunlight, and adequate watering. Citrus trees are more resilient to poor container care than many other “green” shrubs. For cooler winter areas, limes and lemons should not be grown, since they are more sensitive to winter cold than other citrus fruits. Hybrids with kumquats (x Citrofortunella) have good cold resistance. The roots of a citrus tree planted in a container might need to be removed every five years, as they can form a thick root-ball on the bottom.

Pests and diseases

Citrus plants are very liable to infestation by aphids, whitefly, and scale insects (e.g. California red scale). It is also important to note the viral infections that some of these parasites can transmit, such as the aphid transmitted Citrus tristezavirus. This virus is fatal to citrine plantations if it is not controlled. The newest threat to citrus groves in the United States is the Asian citrus psyllid.

The Asian citrus psyllid is an aphid-like insect that feeds on the leaves and stems of citrus trees and other citrus-like plants. The danger is that the psyllid may carry Huanglongbing (HLB), a fatal bacterial tree disease also known as citrus greening.

Citrus greening disease was first discovered in south Florida, around Homestead and Florida City, in August 2005. Since then, the disease has spread to all commercial citrus groves in Florida. In 2004-2005, USDA statistics reported the total Florida citrus production to be 169.1 million boxes of fruit. The estimate for all Florida citrus production in the 2015-2016 season is 94.2 million boxes, a 44.3% drop. Carolyn Slupsky, a professor of nutrition and food science at the University of California, Davis has said that “we could lose all fresh citrus within 10 to 15 years”.

The psyllid was found dangerously close to California in June 2008. It was located right across the border in Tijuana (Mexico). Only a few months later, it was detected in San Diego and Imperial Counties, and has since spread to Riverside, San Bernardino, Orange, Los Angeles and Ventura Counties, sparking quarantines in those areas. It has been detected in California as well as packages of citrus and ornamentals and bouquets of cut flower, sent from other countries and states.

The foliage is also used as a food plant by the larvae of Lepidoptera (butterfly and moth) species such as the Geometridae common emerald (Hemithea aestivaria) and double-striped pug (Gymnoscelis rufifasciata), the Arctiidae giant leopard moth (Hypercompe scribonia), H. eridanus, H. icasia and H. indecisa, many species in the family Papilionidae (swallowtail butterflies), and the black-lyre leafroller moth (“Cnephasia” jactatana), a tortrix moth.

The citrus leafminer ( Phyllocnistis citriella) is a pest in California. It bores meandering patterns through leaves since 2000.

In eastern Australia, the bronze-orange bug (Musgraveia sulciventris) can be a major pest of citrus trees, particularly grapefruit. In heavy infestations it can cause flower and fruit drop and general tree stress.

California’s European brown snails ( cornu aspersum), can be a problem. However, laying female Khaki Campbell or other mallard-related ducks may be an option to control them.

Deficiency diseases

Citrus plants can also develop a deficiency condition called chlorosis, characterized by yellowing leaves highlighted by contrasting leaf veins. If the plant loses too much, the shriveling leaves will eventually fall. This condition is often caused by an excessively high pH (alkaline soil), which prevents the plant from absorbing iron, magnesium, zinc, or other nutrients it needs to produce chlorophyll. You can treat this condition by using an acidic fertilizer that is specifically formulated for citrus. This can often revive the plant and allow it to produce new leaves or flower buds in a matter of weeks, provided the conditions are right. Too acidic soil can cause problems. Citrus prefers neutral soil (pH 6-8). Citrus plants are also sensitive to excessive salt in the soil. Soil testing may be necessary to properly diagnose nutrient-deficiency diseases.

Uses

Culinary

Many citrus fruits, such as oranges, tangerines, grapefruits, and clementines, are generally eaten fresh. They are typically peeled and can be easily split into segments. Grapefruit is more commonly halved and eaten out of the skin with a spoon. This is why special spoons, grapefruit spoons, are made with serrated tips. Orange and grapefruit juices are also popular breakfast beverages. More acidic citrus, such as lemons and limes, are generally not eaten on their own. Meyer lemons are sweet and tart, so they can be enjoyed with their fragrant skin. Lemonade or limeade are popular beverages prepared by diluting the juices of these fruits and adding sugar. Lemons and limes are also used in cooked dishes, or sliced and used as garnishes. Their juice can be used in many dishes. It is often found in salad dressings, squeezed over meats, fish, and vegetables, and then used as an ingredient.

Different parts and methods of treating citrus fruits can yield a variety of flavours. The rind and oil of the fruit is generally bitter, especially when cooked, so is often combined with sugar. The pulp of the fruit can be very sweet or extremely sour. Marmalade, a condiment derived from cooked orange and lemon, can be especially bitter, but is usually sweetened with sugar to cut the bitterness and produce a jam-like result. A garnish for water, soft drinks, and cocktails is lemon or lime. Mixing citrus juices, rinds and slices can be used to make a variety mixed drinks. The colourful outer skin of some citrus fruits, known as zest, is used as a flavouring in cooking; the white inner portion of the peel, the pith, is usually avoided due to its bitterness. The zest of a citrus fruit, typically lemon or an orange, can also be soaked in water in a coffee filter, and drunk.

Phytochemicals and research

Some Citrus species contain significant amounts of the phytochemical class called furanocoumarins, a diverse family of naturally occurring organic chemical compounds. In humans, some (not all) of these chemical compounds act as strong photosensitizers when applied topically to the skin, while other furanocoumarins interact with medications when taken orally. This is known as the “grapefruit-drug interaction” (common name for related grapefruit-drug interactions).

Due to the photosensitizing effects of certain furanocoumarins, some Citrus species are known to cause phytophotodermatitis, a potentially severe skin inflammation resulting from contact with a light-sensitizing botanical agent followed by exposure to ultraviolet light. In Citrus species, the primary photosensitizing agent appears to be bergapten, a linear furanocoumarin derived from psoralen. This claim has been confirmed for lime and bergamot. Particularly, bergamot essential oils has a higher level of bergapten (3000-3600mg/kg) than any other Citrus-based oil.

Three Citrus ancestral types (pomelos and citrons) produce high amounts of furanocoumarins. A fourth ancestral species, mandarins, is virtually devoid of these compounds. Since the production of furanocoumarins in plants is believed to be heritable, the descendants of mandarins (such as sweet oranges, tangerines, and other small mandarin hybrids) are expected to have low quantities of furanocoumarins, whereas other hybrids (such as limes, grapefruit, and sour oranges) are expected to have relatively high quantities of these compounds.

In most Citrus species, the peel contains a greater diversity and a higher concentration of furanocoumarins than the pulp of the same fruit. An exception is bergamottin, a furanocoumarin implicated in grapefruit-drug interactions, which is more concentrated in the pulp of certain varieties of pomelo, grapefruit, and sour orange.

One review of preliminary research on diets indicated that consuming citrus fruits was associated with a 10% reduction of risk for developing breast cancer.

Taxonomy

The taxonomy and systematics of the genus are complex and the precise number of natural species is unclear, as many of the named species are hybrids clonally propagated through seeds (by apomixis), and genetic evidence indicates that even some wild, true-breeding species are of hybrid origin.

Most of the Citrus spp. are cultivated. seem to be natural or artificial hybrids of a small number of core ancestral species, including the citron, pomelo, mandarin, and papeda (see image). Natural and cultivated citrus hybrids include commercially important fruit such as oranges, grapefruit, lemons, limes, and some tangerines.

These core citrus species are not the only ones that can be grown. Mangshanyegan and Australian limes, as well as newly discovered mangshanyegan, can also be grown. Kumquats, Clymenia and other spp. They are generally classified as belonging to the genus Citrus. Trifoliate orange, which is often used as commercial rootstock, is an outgroup and may or may not be categorized as a citrus.

Phylogenetic analysis suggests the species of Oxanthera from New Caledonia should be transferred to the genus Citrus.