TITLE:
A Systematic Revision of the Genus Plectranthus L. (Lamiaceae) in Saudi Arabia Based on Morphological, Palynological, and Micromorphological Characters of Trichomes
AUTHORS:
Kadry N. Abdel Khalik
KEYWORDS:
Morphological, Palynology, Seed, Taxonomic Revision, Trichomes
JOURNAL NAME:
American Journal of Plant Sciences,
Vol.7 No.10,
July
22,
2016
ABSTRACT: This study aims to investigate the morphology and ultrastructure
characters of pollen grains and trichomes in order to evaluate their systematic
value of these characters in specific and intraspecific separation of the Saudi
Arabian Plectranthus species. A critical systematic revision of 7 species
of Plectranthus (Lamiaceae) in Saudi Arabia was conducted by
means of numerical analyses based on thirty-one morphological characters,
including vegetative parts, seeds, pollen grains, and trichomes. Macro- and
micro-morphological characters, including seed and pollen shape, size, coat
sculpture, trichome structure, were studied. It reveals the presence of seven species,
including two endemic species. The pollen grains were zonocolpate, hexacolpate,
prolate to subprolate. Three types of exine ornamentation were recognized. Also,
two trichome types could be distinguished and classified into glandular and
non-glandular. The glandular trichomes could be distinguished as peltate,
capitate and digitiform. The eglandular trichomes were single, uniseriate,
multicellular. Pollen and trichome characters were found to be valuable, while
seed characters presented only minor taxonomic value. On the basis of UPGMA
clustering analysis four branches and clusters were distinguished. The results
offer useful data for evaluating the taxonomy of Plectranthus both at
subgeneric and sectional levels. Our results indicated some degree of
similarity among the species of subgenus Burnatastrum. Plectranthus
arabicus is considered as a separate group and may be treated as separate
subgenus. Furthermore, the endemic species Plectranthus asirensis and Plectranthus
hijazensis constitute a monophyletic group and there are close
relationships between this group and Plectranthus tenuiflorus. A key for
the identification of the investigated taxa based on studied characters is
provided.