Palicourea is a Neotropical genus of about 650 species, which are found from the Antilles and northern Mexico to Bolivia and northern Argentina (Tayor, in prep.). These are shrubs and small trees with opposite or verticillate leaves, bilobed persistent stipules, generally terminal, thyrsiform inflorescences, small to showy, distylous, 4--5-merous flowers, corolla lobes that are valvate in bud, 2(--5)-locular ovaries with 1 basal ovule in each locule, and drupaceous blue, white, or purple-black fruits with 2(--5) pyrenes that each have 1 locule and one seed. Palicourea species grow in wet, lowland to montane areas, and generally have more species in mountains. The genus is well represented through most of its range, with generally a half dozen or more sympatric species in a given site. Palicourea has separate centers of diversity or radiation in southern Central America, the Andes, the Guayana Highlands, eastern Brazil, and the Amazon basin. The most widespread and commonly collected species include Palicourea triphylla, Palicourea crocea (perhaps a complex rather than a single species), Palicourea guianensis (also perhaps a complex), and Palicourea racemosa. The circumscription of this genus has changed over the last several decades, and it has been confused with Psychotria as outlined below.
Circumscription of Palicourea
Palicourea was long recognized "traditionally" to include about 250 species of shrubs and small trees that differ from Psychotria in their colorful flowers that have well developed, asymmetrically swollen corolla tubes and are pollinated by hummingbirds. The genus was diagnosed by these flower characters, which were contrasted with white, small, insect-pollinated flowers with corolla tubes that are not swollen. In this traditional circumscription Palicourea was studied regionally (Steyermark 1972, 1974; Bacigalupo 1952; Burger & Taylor 1993; Jung-Mendaçolli et al 2007; Taylor 1989, 1993, 2000, Taylor et al. 2007), and a synoptic overview was presented (Taylor, 1997).
However, morphological (Taylor, 1996) and molecular studies (Nepokroeff et al., 1999; Andersson, 2001; Andersson & Rova, 1999; Sedio et al., 2013, Razafimandimbison et al. 2014) have found that the flower characters that were used to separate traditional Palicourea from Psychotriaare much more variable than was realized, and that many of the individual lineages identified include species with both small, white, insect-pollinated flowers and larger, colorful flowers considered adapted for hummingbirdds. These studies found that "traditional" Palicourea included species that are more closely related to species that were separated in Psychotria than to other species classified in Palicourea, and specifically to species of Psychotria subg. Heteropsychotria (Steyermark, 1972). In fact, Psychotria subg. Hetereropsychotria and "traditional" Palicourea share a number of vegetative and fruit features, and they both differ from "true" Psychotria in these features. Molecular data confirm that Palicourea plus most of the species of Psychotria Subg. Heteropsychotria together comprise a single monophyletic group, which takes the name Palicourea and has sometimes been referred to as "Palicourea sensu lato". The majority of the species of Palicourea sensu lato were described and treated until recently in Psychotria, and this has generated extensive confusion over the difference between these genera. The group now called Palicourea sensu lato was actually recognized previously by one author, Mueller (1881), who treated it under the inaccurate name "Psychotria" and his work has also contributed to confusion between these genera.
Psychotria can be identified vegetatively in the Neotropics by its stipules that are bilobed or usually unlobed, so with an acute to aristate tip, and that are usually quickly deciduous leaving a line of persistent trichomes on the stem where they were attached; in contrast to generally persistent, bilobed stipules in Palicourea. Psychotria can be separated also by its fruits, which are orange to red at maturity; in contrast, the fruits of Palicourea are purple-black, blue, or white at maturity, and the fruits of some Palicourea species do pass through an orange or red immature stage but those fruits all become purple-black when mature. Psychotria can also generally be separated by its pyrenes that lack pre-formed germination slits and seed coats with an alcohol-soluble red pigment, while Palicourea has pyrenes with preformed germination slits in various arrangements and seed coats that lack an alcohol-soluble red pigment; however, pyrene form has not been surveyed in thorough detail in these and related genera, and the presence and form of preformed germination slits are apparently variable in Paleotropical Psychotria and may also vary in the Neotropical species.
Berger et al. (2017), Berger & Schinnerl (2019), and Berger et al. (2021, 2021) detailed some of the chemistry of secondary metabolites in this group, in Psychotria s. str., Palicourea s. lat., and other genera of Palicoureeae, and found chemical differences between Psychotria, which lacks alkaloids and contains tannins, and the genera of Palicoureeae, with some characteristic types of alkaloids and without tannins. Berger & Schinnerl found Palicourea characterized by strictosidine and chimonanthine metabolites, and Berger et al. (2017, 2021, 2021) newly reported a suite of compounds from six Costa Rican species.
Palicourea in its current, expanded circumscription includes many of the species that were once treated in Cephaelis, including the type of that genus. Cephaelis was also diagnosed by inflorescence characters related to pollination, specifically by having the flowers all grouped in a single inflorescence head that is enclosed by well developed bracts. Molecular data now show that this inflorescence arrangement has also arisen in parallel multiple times within Palicourea, as well as in other genera of the tribes Palicoureeeae and Psychotrieae (Nepokroeff et al., 1999; Andersson, 2001; Andersson & Rova, 1999).
Some species that were included in Psychotria subg. Heteropsychotria do not, however, belong to Palicourea, and have now been separated into other genera based on morphological and molecular characters. Those other genera are Carapichea, Coccochondra, Notopleura, Eumachia, and Ronabea. Transfer of the remaining species from Psychotria subg. Heteropsychotria to Palicourea is underway by Taylor and, independently, several other authors, with all the authors using different taxonomic concepts and species circumscriptions. The list of species of Palicourea below includes all the species that have been studied in detail by Taylor, and the placement in her infrageneric classification of Palicourea is noted on each species's page. Some species of Psychotria subg. Heteropsychotria have also been studied and transferred to Palicourea by Delprete and collaborators and others by Borhidi, with both of them working independently of each other (and of Taylor). Both of those authors use quite different principles for classification from each other and from Taylor, so the conclusions from these other authors are sometimes discordant with each other and the taxonomy of Taylor presented here. In particular, Delprete included in Palicourea several species that Taylor and collaborators classify in Rudgea, and he circumscribed a number of individual species much more broadly in morphology and range than done here. In contrast, Borhidi circumscribed species much more narrowly than any other Neotropical author, and his classification concept follows a different form. In particular in Borhidi's system, genera are circumscribed based on one or two distinctive or unusual characters that show a species's separation from others, rather than on shared characters that are proposed to indicate relationships. Thus, many of his genera that are diagnosed in this way include only one species that molecular data find deeply nested within a clade of other species that he places in a different, basal genus. Here genera are circumscribed primarily based on inferred relationships among the species, and these are inferred based on their shared characteristics, molecular systematic studies, and biogeography so that a genus is intended to include all the members of a lineage.
The Modern Genus Palicourea
Palicourea in its current, broader circumscription is currently estimated to include about 650 species, but known undescribed species are numerous and additional ones continue to be discovered so the final number is difficult to estimate. Wth this current number of known species, Palicourea is the largest genus of Rubiaceae in the Neotropics. This lineage has extensive parallel evolution in many reproductive characters that are correlated with pollination and dispersal, also many of the individual groups now identified within the genus also show variation in inflorescence form and color and flower size and color. The wide variation in inflorescence and flower features in Palicourea overall and many of its sections includes a range of floral display colors, which suggests that diversification in this group is related in good part to pollination adaptations as proposed for a diverse montane group of Solanaceae that grows together with Palicourea and has generally similar flowers, pollination, and variation (Muchhala et al., 2014). Characters related to dispersal, such as fruit size and color, also vary among species groups but generally show less variation within a group.
"Traditional" Palicourea was reviewed by Taylor (1997), who presented an infrageneric classification. Neotropical Psychotria subg. Heteropsychotria has never been reviewed as a whole, however, so the merging of these two groups into an expanded Palicourea requires study of the individual species of Psychotria. This study is ongoing, and is expanding the infrageneric classification of Palicourea (Taylor et al. 2010; Taylor 2014-2019, various articles; Delprete & Lachenaud, 2018).Taylor recognized two subgenera, Palicourea subg. Palicourea and Palicourea subg. Montanae, and molecular studies have found two large clades within Palicourea s. lat. that generally correspond to these subgenera. The morphological characterization or recognition of these subgenera is not yet clear, and both are found essentially throughout the range of the genus. The majority of the species in the genus appear to belong to Palicourea Subg. Montanae and Palicourea subg. Palicourea appears to include more of the species found at lower elevations, but the genus is not yet well studied with molecular data.
Palicourea is similar to Psychotria, and their separation is outlined in the previous section. Palicourea is also similar to and sympatric with Rudgea, and these can be subtle to separate. Palicouea has generally been separated from Rudgea by its stipules that are not glandular abaxially or marginally, in contrast to variously ornamented with well developed glands and colleters in Rudgea, but this is inaccurate: a number of species of Palicourea also have well developed glands and projections on their stipules (e.g., Palicourea sect. Nonatelia), which were not noted in taxonomic treatments apparently because they were not considered a character of the genus. Palicourea and Rudgea can be separated by their stipules, however, which are bilobed in Palicourea and overall, truncate to rounded or acute but not bilobed in Rudgea (Taylor et al., 2015). Palicourea is also similar to Carapichea, which can be recognzied by it generally unlobed stipules that become chartaceous and break with age, and to Eumachia, which has unlobed or sometimes bilobed, usually glandular stipules and fruits that are red at maturity.
Morphologically, as Palicourea is markedly variable in vegetative and reproductive features, and the genus grows in a wide range of habitats. Most of the species grow in evergreen, wet forest with relatively few found in seasonal areas, and those mostly in the notheastern Amazon basin and the Rio Negro basin. Sedio et al. (2013) found that on average, the habitat niche of Palicourea is wetter than that of Psychotria. Some species appear to be specialists on sandstone or white sand substrates, and a few perhaps are specialist on limestone substrates. Palicourea apparently has its highest species diversity from lowland to middle elevatations, ca. 0--1800 m, but a few, mostly blue-flowered species grow up to treeline in the Andes (e.g., Palicourea amethystina). A few species appear to be hyper-accumulators of aluminum, based on their deep turquoise drying color (Robbrecht, pers. comm.) or chemical analysis (Turner et al., 2021).
The stipules of Palicourea are characteristically united around the stem into a continuous sheath or have the intrapetiolar portion reduced, forming laminar or sub-interpetiolar stipules; in a few species the sheath is prolonged into a tube (Palicourea locellata, though this is not closed at the top to form a calyptrate stipule as in some species of Psychotria). The stipules range from emarginate or nearly truncate, to deeply lobed with the lobes short to very well developed; in some species the lobes are erose, fimbriate, or laciniate (e.g., Palicourea woronovii, Palicourea octocuspis). The stipules, and most other growing points, have colleters that secrete various amounts of exudate. The form of the colleters for several Palicourea species were detailed by Lopes-Mattos et al. (2015), who analyzed the exudate and found only mucilaginous, polysaccharide compounds. In another study, Tresmondi et al. (2015) found both mucilage and resin produced by the stipule colleters.
The leaves vary from small to quite well developed (e.g., Palicourea grandifolia), and are generally opposite but sometimes verticillate (e.g., Palicourea corymbifera). The habit of the plants varies from small unbranched subshrubs (e.g., Palicourea cuspidulata) to monopodial plants that accumulate detritus along the stems (Palicourea woronovii), or usually small to large shrubs or occasionally small trees; no annuals or climbers are yet known in this genus.
The inflorescences of Palicourea are terminal in position. However frequently the stem continues grown from one or both of the axillary buds that subtend the peduncle; when both buds develop the terminal position is clear, but when only one develops the position (or its terminology) has been confused. Here the inflorescences are considered terminal because they develop from the terminal bud of the stem. When the stem continues growth from one of the axillary buds, the inflorescences are subsequently displaced to an apparently axillary position, but in this case the inflorescence is solitary on the side and an undeveloped axillary bud can be located between the leaf and the inflorescence on that side, while the axillary bud on the other side has developed; these inflorescences are here considered pseudoaxillary (Robbrecht, 1988), and have been called by some authors also "lateral"; these inflorescences are considered developmentally terminal and distinct from axillary inflorecences that are formed by each of the axillary buds while the terminal bud continues to develop. The inflorescences of Palicourea vary widely in form, from lax and extensively branched with reduced bracts to subcapitate and densely congested ("capitate") with large showy bracts. Nearly all Palicourea species are distylous; this appears to be the ancestral condition for the genus, and it seems to have been lost in peripheral, isolated populations of some species (Sobrevila et al., 1983) and in some species on Caribbean islands (Taylor, 1993). The pollen is unusual in many if not most species in being inaperturate, with an incomplete or lacking exine layer.
Some species of Palicourea have relatively larger, fleshy fruits and are dispersed by monkeys (e.g., Palicourea allenii). No economically important species of the genus are known, although some Palicourea species are locally cultivated as ornamentals (e.g., Palicourea padifolia in Costa Rica).
The diversity of species and morphology, and in particular inflorescence arrangement, flower features, and floral display colors, is stunning in Palicourea and exploration of montane areas of Central and South America continues to discover new, morphologically unexpected species at a significant rate. Understanding of the component species and their features will be necessary to understand the factors that have stimulated this exceptional diversification in Palicourea. Molecular study now underway (Lagomarsino et al., in prep.) will contribute significantly to this understanding.
The content of this web page was last revised on 10 April 2020.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml