Diversity, biogeography, and photosynthetic pathways of Argusia and Heliotropium (Boraginaceae) in South-West Asia with an analysis of phytogeographical units

South-West and Central Asia are major centres of diversity in the genus Heliotropium . On the basis of detailed taxonomic studies and information gathered from the literature, a synopsis of 61 known species of Heliotropium and two species of Arguzia in the area is given. Iran, with 32 species and 14 (sub)endemic species, has the highest diversity. The photosynthetic pathways of 42 taxa were determined using the isotope composition method. Except for H. marifolium Retz s.l. and H. rariflorum Stocks, all remaining species analysed showed δ¹³C values characteristic for C₃ photosynthesis. Evidently, by contrast with the families Chenopodiaceae and Polygonaceae, the Irano-Turanian area is not an authochthonous region of developing C₄ species in Heliotropium . The distribution maps of 57 taxa are provided and their biogeographical importance is discussed in order to elucidate the distribution patterns in South-West Asia. In the Irano-Turanian region, the Irano-Anatolian province of Zohary, which extends from central Anatolia to the western Himalaya, is a very large and vaguely defined phytochorion that should be split into smaller units. The consideration of southern Iran and adjacent Pakistan as part of the Sudanian or Saharo-Sindian regions ( sensu either Zohary or Léonard) cannot be accepted, because most endemic species in this area are either typical Irano-Turanian or isolated relicts. Furthermore, it is concluded that the Saharo-Sindian flora is not an autochthonous flora; most species are of transgressive origin from the surrounding phytochoria. Finally, a new species, Heliotropium ziegleri Akhani, is described from Iran.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 155 , 401–425.