Seed morphology and surface microstructure of the genus Plectranthus L. (Lamiaceae) in Arabian Peninsula highlighting on their systematic implications

Abstract

Seed coat morphology of 13 species of Plectranthus L. in Arabian Peninsula was comparatively examined using scanning and light microscopy methods. The diagnostic values for systematic were studies. Macro- and micro-morphological characteristics, including seed shape, colour, size, seed surface, epidermal cell shape, anticlinal boundaries, and periclinal cell wall are presented. Descriptions of seed size, shape, colour, surface, and seed coat types are summarised for the genus. Taxonomic phylogenetic implications o f the seed coat micromorphology are also discussed in comparison with the available gross morphological and molecular data. Results of the seed character analyses offer ed useful data for evaluating the taxonomy of plectranthus both on subgeneric and sectional levels. Monophyly of both subgenus Burnatastrum (Briq) Codd, and Germanea (Benth.) Briq. is also supported here. The remarkable result of this study was to identify P. arabicus as a separate group. The results suggested that it should be treated as a separate subgenus because of having reddish-brown colour seeds and flat-to-convex periclinal cell wall which is distinguishable from the rest of the species. Likewise, the endemic species of P. asirensis, P. ovatus, and P. hijazensis group and P. hyemalis and P. hadiensis group should be homogeneous groups. A key for the identification of the investigated taxa based on seed characters is also provided here.