| dc.description.abstract | Ocimum L. and Plectranthus L'Her. are cosmopolitan genera in the tribe Nepeteoideae in the family
Labiatae Juss. The accumulation of essential oils in their flower and foliage has contributed to their
prominence and popularity in traditional and conventional medicine, in cosmetology and
perfumery, in the food industry as preservatives and spices, and more recently in the manufacturing
industry as ingredients for pesticides, plastics, paints, etc.
The taxonomic aspect of the group has lagged well behind the economic one. The latest worldwide
account of the two genera was by Briquet (1895-7), more than a 100 years ago. In both these
taxa, the taxonomic delimitation has been inadequate, the major problem being the continuous
nature of the variation of characters particularly the morphological ones, which results in
difficulties in circumscription of species. To further complicate the situation, Ocimum species tend
to hybridize readily and undergo polymorphism. Morphological characters have proved to be
inadequate in delimitation of the genera It is therefore imminent that other criteria be investigated
to substitute or corroborate the morphological ones in the delimitation of these two genera.
The major aim of this project was therefore to find novel morphological characters or novel
combinations of already known characters and-She value of new criteria from phytochemical data
and anatomy to augment the morphological data in the delimitation of Ocimum and Plectranthus.
Field and herbarium collections were used in this study. Gas-liquid chromatography/mass
spectrometry (GLCIMS) on the essential oils from field samples were conducted in the
chemotaxonomic study while the surface anatomical characters oj Elf!!!..tranthus were analyzed
using the scanning electron microscope (SEM).
In the revision of Ocimum, new morphological characters have been identified that can differentiate
between sections and even between species, for instance, in the dendrite-haired species of
subsection Gratissima, 0. cufodontii and 0. jamesii have n-shaped while 0. spicatum have bowlshaped
anthers. Morphological features separating 0. basilicum and 0. americanum were
established. In Plectranthus, three new species namely, P. agnewii C. Lukhoba & A. Paton, P.
xylopodus C. Lukhoba & A. Paton sp. nov. and P. kwalensis C. Lukhoba & 1. Kokwaro as well as
the varieties P. barbatus var. glabricalyx C. Lukhoba & 1. Kokwaro and P. edulis var. longiflora C.
Lukhoba & J. Kokwaro have been described. One name change, P. igniarius vaL verdcourtii C.
Lukhoba & 1. Kokwaro formerly P. igniarius var. grandiealyx) has been made.
The chemical analysis conducted on four Ocimum and eleven Plectranthus species revealed the
presence of numerous essential oil constituents. Some compounds were species-specific, some
genera-specific while others were common to both genera thus can be of taxonomic value. The
species 0. basilicum, 0. kilimandscharicum and 0. kenyense clustered together in the cluster
analysis. These three species have traditionally been grouped together in the section Ocimum. 0.
gtatissimum showed greater affinity to Pleetranthus than to Ocimum. The Plectranthus species
came out as one homogenous group with two subgroups.
The anatomical analysis of Plectranthus showed the indumentum to have a large number of sessile
glands (peltate, gland dots), some capitate glands and numerous multi-cellular eglandular hairs. The
viscid species P. kamerunensis and P. agnewii had the highest number of capitate glands. The
presence, structure and nature of the st~mata and"~lands were of taxonomic importance. Seven
species had amphistomatic stomata and another seven species had stomata on the stems. The
stratification of the leaf and stem surfaces was also found to be taxonomically significant. Cluster
analysis showed that members of subgenus Calceolanthus clustered together into one group. They
were also distinct from subgenus Plectranthus. A key basedon these anatomical characters was
drawn up to identify species in Plectranthus.
Ethnomedicinal data revealed that Ocimum and Plectranthus species are popularly used to cure or
alleviate gastro-intestinal, febrile, respiratory and skin conditions. Four species namely,
basilicum, O. kilimandscharicum, 0. gratissimum and P. barbatus showed high potential for
further pharmacological evaluation, and for incorporation into local health care systems. This data
also revealed that the medicinal species in the two genera are greatly affected by the current
degradation of the environment. Thus propagation of the potentially medicinal ones was
recommended. It is recommended that future taxonomic revisions of the two genera should include
chemotaxonomic and anatomical data as they have proved to be promising criteria. Members of
the two genera that have substantial amounts of essential oils can be commercially utilized in
industry.The promising medicinal Ocimum and Plectranthus species can be incorporated into the
primary health care systems. It is hoped that the new criteria used in this study will be relevant for
the on-going revision of the family Labiatae for the Flora of Tropical East Africa. | en |
