Lukhoba C.W. 2001. Taxonomic revision of the genera Ocimum L. and Plectranthus L

Citation:
DR. LUKHOBA CATHERINEW. "Lukhoba C.W. 2001. Taxonomic revision of the genera Ocimum L. and Plectranthus L.". In: Ph. D. Thesis, University of Nairobi.; 2001.

Date Presented:

2001

Abstract:

Ocimum L. and Plectranthus L'Hér. 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 world- wide account of the two genera was by Briquet (1895-7), more than 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 the 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 (GLC/MS) on the essential oils from field samples were conducted in the chemotaxonomic study while the surface anatomical characters of Plectranthus 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, O. cufodontii and O. jamesii have n-shaped while O. spicatum have bowl-shaped anthers. Morphological features separating O. basilicum and O. 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 & J. Kokwaro as well as the varieties P. barbatus var. glabricalyx C. Lukhoba & J. Kokwaro and P. edulis var. longiflora C. Lukhoba & J. Kokwaro have been described. One name change, P. igniarius var. verdcourtii C. Lukhoba & J. Kokwaro formerly P. igniarius var. grandicalyx) 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 O. basilicum, O. kilimandscharicum and O. kenyense clustered together in the cluster analysis. These three species have traditionally been grouped together in the section Ocimum. O. gratissimum showed greater affinity to Plectranthus 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 stomata and glands 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 based on 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, O. basilicum, O. kilimandscharicum, O. 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.

Notes:

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