Rubiaceae

Description

Rubiaceae are morphologically coherent, characterized by opposite or whorled simple entire leaves, interpetiolar stipules, tubular sympetalous actinomorphic corollas, and an inferior ovary. Growth forms include shrubs (e.g., Coffea, Psychotria), trees (e.g., Cinchona, Nauclea), lianas (e.g., Psychotria samoritourei), herbs (e.g., Galium, Spermacoce), and some epiphytes (e.g., Myrmecodia). Plants typically contain iridoids, alkaloids, and raphide crystals. Leaves are simple and entire, except for one species with pinnately compound leaves (Pentagonia osapinnata).

Phyllotaxis is usually decussate, rarely whorled or seemingly alternate. Stipules are fused into an interpetiolar structure often bearing colleters that produce mucilage. Inflorescences are cymes, rarely solitary flowers, terminal or axillary. Flowers are 4-5-merous, usually bisexual and epigynous, with a biseriate perianth.

The calyx lobes are fused at the base, sometimes unequal, and one lobe may be enlarged and colored (semaphyl). The corolla is sympetalous, mostly actinomorphic and tubular, typically white or creamy, but also yellow, rarely blue or red. Stamens are alternipetalous and epipetalous; anthers dehisce longitudinally or are poricidal. The gynoecium is syncarpous with an inferior ovary, rarely secondarily superior.

Ecology

Most Rubiaceae are zoophilous, pollinated mainly by insects attracted to nectar from an epigynous disk. Ornithophily occurs in red-flowered species of Alberta, Bouvardia, and Burchellia. Anemophilous species are found in tribes Anthospermeae and Theligoneae, with features such as sexual dimorphism, increased stigma surface, and pendulous anthers. Although most species are hermaphroditic, outbreeding is promoted through sequential hermaphroditism, spatial isolation of reproductive organs, secondary pollen presentation, heterostyly, and unisexual flowers.

Secondary pollen presentation (ixoroid pollen mechanism) occurs in Gardenieae and related tribes: pollen is shed early onto the style or stigma, which acts as a receptacle; after style elongation, animals transport pollen to receptive flowers. A pollen catapult mechanism in Molopanthera and Posoqueria projects a pollen mass onto hawk moths. Heterostyly is widespread, especially in Spermacoceae and Psychotrieae, but absent in groups with secondary pollen presentation. Unisexual flowers occur in dioecious taxa, with male flowers having rudimentary pistils and female flowers having sterile stamens; functionally dioecious flowers occur in Pyrostria.

Systematics

The name Rubiaceae was published in 1789 by Antoine Laurent de Jussieu, though mentioned earlier in 1782. Several historically accepted families are now included in Rubiaceae, such as Cinchonaceae, Coffeaceae, Gardeniaceae, and Psychotriaceae. Classical classification distinguished two subfamilies: Cinchonoideae (more than one ovule per locule) and Coffeoideae (one ovule per locule), but this was criticized due to the placement of related tribes like Gardenieae and Ixoreae, and variable ovule numbers in Tarenna.

Later morphological characters led to recognition of three to eight subfamilies. The last morphological classification divided Rubiaceae into four subfamilies: Cinchonoideae, Ixoroideae, Antirheoideae, and Rubioideae. Problems in subfamily delimitation are linked to the family's extreme naturalness and low divergence among members.

Uses

No staple foods come from Rubiaceae, but some species are consumed locally or as famine food, including African medlar fruits, African peach (Nauclea latifolia), and noni (Morinda citrifolia). The most economically important genus is Coffea, used for coffee production; of its 124 species, only three are cultivated: C. arabica, C. canephora, and C. liberica. Cinchona bark yields quinine, an antimalarial alkaloid.

Woodruff (Galium odoratum) contains coumarin, a warfarin precursor, and Carapichea ipecacuanha provides the emetic ipecac. Psychotria viridis is used as a source of dimethyltryptamine in ayahuasca. Breonadia salicina bark is used in traditional African medicine.

Kratom (Mitragyna speciosa) leaves contain psychoactive alkaloids and are used in Southeast Asia as a painkiller and stimulant, acting as a μ-opioid receptor agonist. Ornamental plants include Gardenia jasminoides, Ixora coccinea, Mussaenda cultivars, Coprosma repens, Rothmannia globosa, Nertera granadensis, Mitchella, Morinda, Pentas, and Rubia. Rose madder (Rubia tinctorum) root yields a red dye, and Morinda citrifolia yields a yellow dye.

Did You Know?

• The highest number of Rubiaceae species is found in Colombia, Venezuela, and New Guinea; when adjusted for area, Venezuela is the most diverse.
• The name Rubiaceae is derived from Rubia, a name used by Pliny the Elder for madder, from the Latin ruber meaning red.
• Only a few Rubiaceae genera are pantropical; many are paleotropical, and Afro-American distributions are rare.