Rutaceae : An important genus of Flowering plants having great economic importance

Sanjeet Kumar
sanjeet.biotech@gmail.com

Rutaceae, commonly known as citrus family, is a family of flowering plants, usually placed in the order Sapindales. These  are herbs, shrubs, and trees with glandular punctate, commonly strongly smelling herbage comprising about 150 genera and 1,500 species that are further characterized by the common occurrence of spines and winged petioles. The leaves are alternate or opposite, simple or palmately or pinnately compound, or sometimes heathlike or reduced to spines; stipules are absent. Flowers are bractless, solitary or in cyme, rarely in raceme, and mainly pollinated by insects. They are radially or (rarely) laterally symmetric, and generally hermaphroditic. They have four or five petals and sepals, sometimes three, mostly separate, eight to ten stamen (five in Skimmia, many in Citrus), usually separate or in several groups. Usually a single stigma with 2 to 5 united carpels, sometimes ovaries separate but styles combined. The flowers are also often sweet-scented, nearly always bisexual, and are actinomorphic or sometimes zygomorphic. The calyx consists of 3-5 distinct or basally connate sepals and the corolla consists of 3-5 distinct or sometimes connate petals or rarely the petals are lacking. The androecium consists of distinct or sometimes connate stamens that are commonly obdiplostemonous, that is in two whorls with the outer whorl opposite the petals. However, sometimes there may be (1)3-4 whorls or rarely up to 60 stamens. The gynoecium consists of a single compound pistil of commonly 2-5 or more, often incompletely connate carpels that may be united only basally or apically, either one or an equal number of styles, and a superior ovary with usually 2-5 or more locules, each bearing 1-several axile ovules. Generally, an intrastaminal nectary disk is situated between the stamens and the ovary. The fruit is variable. Species of the family generally have flowers that divide into four or five parts, usually with strong scents. They range in form and size from herbs to shrubs and small trees. The most economically important genus in the family is Citrus, which includes the orange (C. sinensis), lemon (C. × limon), grapefruit (C. paradisi), and lime (various, mostly C. aurantifolia, the key lime). Boronia is a large Australian genus, some members of which are plants with highly fragrant flowers and are used in commercial oil production. Other large genera include Zanthoxylum, Melicope and Agathosma. There are approximatively 160 genera in the family Rutaceae:List of Rutaceae genera Most species are trees or shrubs, a few are herbs (Boenninghausenia), frequently aromatic with glands on the leaves, sometimes with thorns. The leaves are usually opposed and compound, and without stipules. Pellucid glands, a type of oil containing cavities, are found on the leaves responsible for the aromatic smell of the family's members; traditionally they have been the primary synapomorphic characteristic to identify the Rutaceae. The fruit of Rutaceae are very variable: berries, drupes, hesperidiums, samaras, capsules and follicles all occur. Seed number also varies widely. The family is closely related to Sapindaceae, Simaroubaceae and Meliaceae, and all are usually placed into the same order, although some systems separate that order into Rutales and Sapindales. The families Flindersiaceae and Ptaeroxylaceae are sometimes kept separate, but nowadays generally placed in Rutaceae, as are the former Cneoraceae. The subfamilial organization has not been fully resolved, but the subfamilies Aurantioideae (=Citroideae) and Rutoideae are well supported; the placement of several genera remains unclear. The family is of great economic importance in warm temperate and sub-tropical climates for its numerous edible fruits of the Citrus genus, such as the orange, lemon, lime, kumquat, mandarin and grapefruit. Non-citrus fruits include the White sapote (Casimiroa edulis), Orangeberry (Glycosmis pentaphylla), Clymenia (Clymenia polyandra), Limeberry (Triphasia trifolia), and the Bael (Aegle marmelos). Other plants are grown in horticulture: Murraya species, for example. Ruta, Zanthoxylum and Casimiroa species are medicinals. Several plants are also used by the perfume industry, such as the Western Australian Boronia megastigma. The genus Pilocarpus has species (P. jaborandi, and P. microphyllus from Brazil, and P. pennatifolius from Paraguay) from which the medicine pilocarpine, used to treat glaucoma, is extracted. Spices are made from a number of species in the genus Zanthoxylum, notably Sichuan pepper. Rutaceae contain many members of economic importance. Most notable are the species of Citrus that produce both the citrus fruits of commerce (lemons, oranges, mandarins, tangerines, limes, kumquats, etc.) and the essential oils used in perfumery, the species of Pilocarpus that are the source of pilocarpine, a drug used to treat glaucoma, and the species of Boronia, Choisya, Poncirus, and Skimmia that are used as ornamentals. They are a large family comprising ∼155 genera with 1600 species, mostly tropical and subtropical in distribution. The family is usually placed in Rutales (Takhtajan, 1987⇓, 1997⇓; Dahlgren, 1989⇓; Thorne, 1992⇓) or in Sapindales (Cronquist, 1993⇓) because they, like other families in the order, are usually woody plants with typically compound, estipulate leaves, seldom more than two ovules per carpel, a nectary disk (sometimes modified into a gynophore), usually no more than twice as many stamens as sepals or petals, and a superior ovary.

Within the order, Rutaceae are grouped with Simaroubaceae, Meliaceae, Cneoraceae, and Ptaeroxylaceae (the latter included in Sapindaceae by Cronquist), primarily because of the absence of resin ducts in the bark, wood rays, and leaf veins and the presence of triterpenoid compounds (Cronquist, 1988⇓). Rutaceae are distinguished from these families, however, by glandular-punctate leaves, the production of limonoids, and the usual presence in parenchyma and pericarp of secretory cavities containing aromatic ethereal oils. A study of rbcL data from members of the Sapindales complex (Gadek et al., 1996⇓) supported the inclusion of Rutaceae within Sapindales and the family's distinction from, but close association with, Simaroubaceae, Meliaceae, Cneoraceae, and Ptaeroxylaceae.  In the only systematic treatment of Rutaceae, Engler (1896⇓, 1931)⇓ recognized seven subfamilies: Rhabdodendroideae, Aurantioideae (=Citroideae), Flindersioideae, Spathelioideae, Dictyolomatoideae, Rutoideae, and Toddalioideae. He defined these subfamilies primarily by characters of the gynoecium, especially fruit type. Although in modern classifications (Takhtajan, 1987⇓; Dahlgren, 1989⇓; Thorne, 1992⇓; Cronquist, 1993⇓), Rhabdodendroideae have been excluded from Rutaceae, the other six subfamilies (not always with the same circumscriptions) have been retained. Takhtajan (1987)⇓ recognized the six subfamilies. Thorne (1992)⇓ combined Toddalioideae with Rutoideae, making five subfamilies. Dahlgren (1989)⇓ and Cronquist (1993)⇓ did not mention subfamilies, but Cronquist did include Flindersiaceae as a synonym of Rutaceae. Hutchinson (1973)⇓ listed four subfamilies: Rutoideae (including Flindersia), Toddalioideae, Rhabdodendroideae, and Aurantioideae (=Citroideae) but did not mention Spathelia (Spathelioideae) or Dictyoloma (Dictyolomatoideae). He mentioned Chloroxylon (as Chloroxlum), the other genus of Flindersioideae, only in a paragraph of useful products of Meliaceae. A conspectus of the subfamilies and tribes is given in Appendix 1.

In recent classifications, Engler's monogeneric Rhabdodendroideae have been recognized as a family, Rhabdodendraceae, and placed within Rosales (Cronquist, 1988⇓, 1993⇓; Dahlgren, 1989⇓) or Rutales (Takhtajan, 1987⇓; Thorne, 1992⇓). Prance (1968⇓, 1972)⇓ surveyed morphological and anatomical characters and concluded that Rhabdodendron would be best recognized as a separate family within Centrospermae (=Caryophyllales). Analysis of rbcL sequence data by Gadek et al. (1996)⇓ described Rhabdodendron as sister to the caryophyllid clade (including Droseraceae and Nepenthaceae), as was also found by Fay et al. (1997)⇓. A chemical study of Rhabdodendron macrophyllum (Wolter-Filho et al., 1985⇓) revealed no metabolites typical of Rutaceae, such as alkaloids and limonoids, compounds characteristic of many members of Sapindales; however, the presence of ellagitannins indicated its placement in Rosales. Rhabdodendron is excluded from this analysis.

Engler defined Citroideae (∼33 genera and 210 species native to the Old World tropics) by the syncarpous ovary with one or two, sometimes several, ovules per carpel and by the indehiscent, fleshy fruit, often (in Citrinae) with pulp vescicles, but sometimes (in Aegle) with a hard shell. Citroideae have always been included in Rutaceae but divided into several of tribes; Engler (1931)⇓ recognized a single tribe, Tanaka (1932)⇓ eight, and Swingle and Reece (1967)⇓ two.

Engler assigned to Flindersioideae two genera, Flindersia and Chloroxylon, both native to the Old World tropics. They are characterized by a syncarpous ovary with two to several ovules per carpel and by a loculicidal (in Chloroxylon) or septifragal (in Flindersia) capsule with winged seeds. Affinities of these two genera have been uncertain, but because of the similarity of their fruits and seeds to those of Cedrela, early botanists (de Candolle, 1824⇓, 1878⇓; Bentham and Hooker, 1862⇓) included them in Meliaceae. Because of their glandular-punctate leaves, Engler (1877⇓, 1931)⇓ placed both in Rutaceae, in which Flindersia was recognized by its recent monographer (Hartley, 1969⇓). Characters of these two genera do not clearly indicate in which family, Meliaceae or Rutaceae, they should be placed, and they also have been recognized as a separate family, Flindersiaceae (Airy-Shaw, 1965⇓; Takhtajan, 1983⇓).

Dictyolomatoideae, consisting only of the South American genus, Dictyoloma, are characterized by an apocarpous ovary, several ovules per carpel, a fruit in which each carpel dehisces along its ventral suture with a separating endocarp, and winged seeds. Spathelioideae also consist of a single genus, Spathelia (native to the West Indies and South America), defined by a syncarpous ovary, one or two ovules per carpel, and an indehiscent winged fruit. Although originally described in Rutaceae, Dictyoloma was placed by Planchon (1846)⇓ with Spathelia in Simaroubaceae as tribe Spathelieae, characterized by appendaged staminal filaments. Bentham and Hooker (1862)⇓ agreed with their exclusion from Rutaceae and also placed them in Simaroubaceae. Engler (1874⇓, 1877⇓) maintained Dictyoloma in Simaroubaceae, but later (1896, 1931) treated it and Spathelia as members of their own monogeneric subfamilies of Rutaceae.

Rutoideae, largest of the subfamilies (∼100 genera and 1204 species of temperate and tropical regions worldwide), are defined by generally having two ovules per carpel in a dehiscent fruit (except in Pitavia and some Acronychia). In the characteristic fruit, each carpel dehisces along the ventral suture, leaving no persistent axis, with a separating, cartilaginous endocarp. A fruit with similar dehiscence and endocarp is otherwise found only in Dictyoloma.

Engler defined Toddalioideae (∼21 genera and 152 species also native to tropical and temperate regions worldwide) by an indehiscent drupe or samara developing from a syncarpous ovary with one or two ovules per carpel. Although always included in the family, Toddalioideae were submerged in Rutoideae by Thorne (1992)⇓.

Although the ordinal position of Rutaceae seems certain, recent studies have cast some doubt on Engler's subfamilial classification and indicated the inclusion in it of additional genera. On the basis of phytochemical data, Da Silva et al. (1988)⇓ suggested the elimination of Toddalioideae and assignment of the genera to several groups in the Rutoideae. More recently, rbcL data have indicated a relationship between Rutaceae and Harrisonia (Simaroubaceae), Cneoraceae, and Pteroxylaceae (Fernando, Gadek, and Quinn, 1995⇓; Gadek et al., 1996⇓).

The family contains economically important fruits. Citrus species include the lemon (Citrus limon), sour orange (C. aurantium), sweet orange (C. sinensis), lime (C. aurantifolia), tangerine and mandarin orange (C. reticulata), grapefruit (C. paradisi), and citron (C. medica). All of these are grown for their fruits. Other regionally important fruits are the kumquat (Fortunella), bael (Aegle), elephant apple (Limonia acidissima), and Japanese pepper (Zanthoxylum piperitum).

Source: From Literature