Author(s): Romina Vidal-Russell
Arjona Comm. ex Cav. is a South American genus with few species distributed in Chile, Argentina, Uruguay, Brazil, Bolivia, and Peru. Traditionally it has been placed in Santalaceae (Kuijt and Hansen 2015, p. 143) however, molecular studies (Nickrent et al. 2010) have shown a clear relationship with Quinchamalium Molina and Schoepfia Schr. in Schoepfiaceae, justifying its placement in that family. Arjona and Quinchamalium have long been associated with each other, but it was Van Tieghem (1896) who suggested including both genera in a separate family (Arjonaceae) closely related to Schoepfiaceae; but Van Tieghem's idea was not followed.
Arjona was first described by Commerson and validly published by Cavanilles in 1798. Since then there have been 17 species descriptions. The Catalogue of Southern Cone is a regional publication that covers most of the distributional area of the genus and recognizes only five species and two varieties (Zuloaga et al. 2008). This classification system is followed here. One species (A. megapotamica Malme) is endemic to Brazil and is restricted to high and cold regions (e.g. Sao Joaquim) in the states of Santa Catarina and Rio Grande do Sur (Rodriguez Mattos 1967; Caires and Dettke 2013). That species was categorized as rare and possibly extinct in Rio Grande do Sur (Dettke 2009). The other four species are distributed along the Andes from Peru (only one species) to Patagonia (Fig. 1). Arjona tuberosa var. tandilensis is found in Buenos Aires and Córdoba Province in Argentina and in Uruguay, but always associated with isolated hills around 400 to 500 m a. s. l. in the Pampa phytogeographic province and around 1500 m a. s. l. in Córdoba. All species in the genus (except A. pusilla ) are found in dry open areas. The only taxonomic revision of Arjona is a regional treatment of Argentina (Dawson 1944).
Fig. 1.: A. Overall distribution of the five recognized species of Arjona showing the different biomes (Olson et al. 2001). B. The different clades found with trnL-F. C. The different clades found with ITS.
Arjona are hemiparasitic rhizomatous herbs. In some species (such as A. tuberosa ) the rhizome swells into a fusiform tuber that is used by indigenous communities as a food source (Ladio and Lozada 2009). Morphologically, species within Arjona are similar and differentiated mainly by leaf shape and pubescence. Identification is not always straightforward because intermediate leaf shapes occur, such as between the varieties of A. tuberosa (Dawson 1944). Leaves are linear to ovate and can be stiff and acute in some species. Leaf pubescence is also variable. The flowers, grouped into a terminal spike, are purple to white, bisexual, and have four to five petals formed into a corolla tube. The staminal filaments are fused to petals and behind each anther on the petal there is a tuft of hair. The ovary is inferior and the fruit is a nutlet.
The objective of this study is to reconstruct the evolutionary history of the recognized species within Arjona . Since there are difficulties differentiating species...