33 PHYTOLOGIA BALCANICA 20 (1): 33 – 48, Sofia, 2014 Micromorphology of leaf trichomes in Onosma (Boraginaceae) and their systematic relevance in Iran Ahmad Reza Mehrabian 1 , Masoud Sheidai 1 & Valeyollah Mozaffarian 2 1 Department of Plant Sciences, Faculty of biological sciences, Shahid Beheshti University, GC, Tehran, Iran, e-mail: a_mehrabian@sbu.ac.ir (corresponding author) 2 Department of Botany, Research Institute of Forests and Rangelands, Tehran, Iran Reveived: Jully 3, 2013 ▷ Accepted: March 31, 2014 Abstract. This study provides the first detailed description of leaves and nectar trichome characteristics in Onosma. The indumentum characteristics of 32 Onosma taxa collected from Iran are given in it, as well as the results obtained from LM and SEM investigations. Three groups have been recognized within the genus, relatively in agreement with the infrageneric taxa of Onosma that Boissier had originally described. The obtained results were confirmed by many molecular, palynological and morphological studies. Furthermore, a diagnostic key has been provided according to which the indumentum, accompanied by other morphological evidences, shows distinct boundaries between sections, subsections and species in Onosma. Although there are taxa which cannot be determined solely using trichome morphology, these evidences can be a very useful tool in distinguishing between many of the currently recognized Onosma taxa. Key words: Boraginaceae, Iran, Onosma, morphology, systematics, trichome Introduction Onosma L. is a genus of the tribe Lithospermeae Du- mort., belonging to the large family of Boraginaceae which comprises ca. 150 species (Weigend & al. 2009; Cecchi & Selvi 2009; Kolarčik & al. 2010) distributed mainly in West and Central Asia and in the Mediter- ranean area, and growing in dry, sunny, rocky, sandy, and steppe habitats (Javorka 1906; Meusel & al. 1978). Stems and leaves of the Boraginaceae are cov- ered with hairs which may be glandular or eglandu- lar (Metcalf & Chalk 1950). Density of these hairs varies with different habitats. Due to the presence of dense trichomes, the Boraginaceae family is occa- sionally referred to as the “shagy coat family” (Simp- son 2006). The family Boraginaceae is characterized by a great diversity of setae forms. Many authors (Al- Nowaihi & al. 1987; Selvi & Bigazzi 2001; Diane & al. 2003; Taia 2006;Ventrella & Marinho 2008; Per- veen 2009) have proved that the setae characters sup- port isolation of many genera and species of Borag- inaceae. Trichomes are widely distributed on the reproductive and vegetative parts of Boraginaceae. Most descriptions have been published in the con- text of the general studies of Boraginales, and many of them were published in the last decades (e.g. Met- calf & Chalk 1950; Johnston 1952, 1953a,b, 1954a,b; Solereder 1908). Published are also some new stud- ies on the trichome morphology of Boraginaceae (Al- Nowaihi & al. 1987, Selvi & Bigazzi 2001, Diane & al. 2003, Taia 2006, Ventrella & Marinho 2008, Perveen 2009). Non-glandular trichomes associated with cys- tolith bodies in the basal portion and with partially calcified walls, called cystolith-hairs, are widely dis- tributed in Boraginaceae and are responsible for the wrinkled leaf surface (Solereder 1908). The taxonomic value of indumentums and their involvement in systematics and phylogenetics are well known in Boraginaceae and in the closest to it fami- lies (Metcalf & Chalk 1950, El-Gazzar & Watson 1968,