Chapter 2 An introduction to the Plectranthus in South Africa
2.1 Phylogeny, occurrence and description
The genus Plectranthus is one of twenty-five genera in the subfamily Nepetoideae of the family, Lamiaceae. Plectranthus is an “Old World” genus belonging to the Mint/Sage family. It contains about three-hundred species found in tropical Africa, Asia, India, Madagascar, Australia and a few Pacific islands (Rijo et al., 2007; Lukhoba et al., 2006;
van Jaarsveld, 2006). The name Plectranthus literally means ‘spur flower’ due to the characteristic spurred corolla tube present in the first Plectranthus species that was discovered. This physical attribute however, is not consistent throughout the genus and may easily be confused for belonging to the Solenostemon or Thorncroftia species.
Within Africa, Plectranthus can be found in the southern part for example, Namibia, Swaziland, Lesotho and South Africa. It grows abundantly in eight out of the nine provinces within South Africa, with the largest concentration being found in the north- eastern part of the Eastern Cape and southern KwaZulu-Natal which has as much as thirty-six species. Plectranthus is desirable as a garden plant because it suppresses weed growth and prevents erosion (van Jaarsveld, 1988) and is simple and inexpensive to maintain. It is also of ornamental interest and is cultivated for their attractive foliage.
They are however susceptible to attack by the eel-worm which, if not treated appropriately can lead to complete deterioration of the plant (van Jaarsveld, 1988).
They are usually found growing in shade under trees where the soil is rich in humus and well drained. However, not all Plectranthus species thrive under these conditions.
Species with succulent leaves prefer growth in drier regions such as the dry bushveld or in rockeries (Figure 13). Even though Plectranthus species perish or wilt under extreme weather conditions such as frost and heat, they recover relatively quickly after a shower of rain or sprout again in spring (van Jaarsveld, 1988).
16 The flower colour varies among the species in Plectranthus, either being white, blue or mauve to pink. At the end of February, the plants begin to develop flowerbuds and reach full bloom between March and April. While some species of Plectranthus grow as upright shrubs to a height of approximately 1.5 meters, for example, ecklonii, fruticosus and hadiensis, others occur as groundcover plants, varying between 10 to 30 centimetres in height, for example madagascariensis and saccatus. Plectranthus hadiensis can be easily identified because of its large, hairy leaves.
Figure 13: Plectranthus hadiensis (picture courtesy of Prof. N. Crouch) Species of Plectranthus are used as ornamental, economic and medicinal plants.
The phylogeny of Plectranthus was well documented by Paton et al. (2004) and Lukhoba et al. (2006) based on its DNA sequence and augmented morphological data of the genus.
This information was presented in the form of a cladogram (figure 14) which divided the Plectranthus genus into two clades or groups. Clade 1 contains Plectranthus species formally known as Coleus and are grouped together based on their ethnobotanical/medicinal uses with Clade 1b containing a greater number of medicinally active species than Clade 1a. The groups in bold in figure 14 all have reported medicinal uses. Of the three subclades, subclade 1b seems to be the most widely used medicinally,
17 followed by subclade 2 and lastly subclade 1a where only groups 2 and 8 are used medicinally. Species within Clade 1 are also sources of food and are used as food flavourants, fodder for domestic animals, ornamental displays in homes and gardens as well as other uses such as building material (Lukhoba et al., 2006). For example, the wood from Plectranthus insignis can be used to build huts or temporary houses (Cheek et al., 2000; Lukhoba et al., 2006).
There are further species that fit into clades 1 and 2 but could not be placed into any of the subclades as they were morphologically different species within these subclades.
These are listed as unplaced groups A-E in each clade.
Plectranthus is a synonym for Coleus (Lukhoba et al., 2006) and therefore in searching the literature for phytochemical reviews of Plectranthus, one has to also consider the Coleus species. Plectranthus amboinicus itself has synonymns of Plectranthus aromaticus Roxb., Coleus aromaticus Benth. and Coleus amboinicus Lour. (Lukhoba et al., 2006).
Beside morphological characteristics and DNA sequencing, it would also be useful to have chemotaxonomic data linking the different species together and hence, phytochemical studies on the different species of Plectranthus is important to build up a database of the secondary metabolites linking these species together.
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Group A: Plectranthus melleri (P. luteus) P. esculentus
Group B: P. edulis, P. punctatus Group C: P. lactiflorus, P. stachyoides
Group D: P. gracillimus Group E: P. mollis, P beddomei
Clade 1 unplaced groups
Group 1: Pycnostachys
Group 2: Plectranthus sylvestris, P. alpinus, P. defoliatus, P. insignis, P. decurrens Group 3: P. robustus, P. baumii, P. foliatus,
P. katangensis, P. insolitus Group 4: P. thysoideus, P. daviesii, P. sereti Group 5: P. glabratus, P. parishii Group 6: Anisochilus, Leocus africanum Group 7: P. helfleri, P. albicalyx Group 8: P. hadiensis, P. grandidentatus,
P. madagascariensis, P. argentatus, P. parvifolius, P. congestus P. graveolens, P. asirensis Group 9: P. calycinus, P. rehmanii
Group 1: Plectranthus igniarus
Group 2: P. tetensis, P. barbatus, P. kivuensis, P. lanuginosus, P. caninus Group 3: P. coeruleus
Group 4: P. fredericii, P. hereroensis
Group 5: P. bojeri, P. rotundifolius, P. vettiveroides,
Group 6: P. buchananii
Group 7: P. montanus, P. prostratus, P. occidentalis, Plectranthus sp. aff occidentalis, P. variifolius
P. pseudomarruboides, P. lanceolatus Group 8: P. aegyptiacus, P amboinicus
Group A: Plectranthus adenophorus, P. modestum
Group B: P. longipes Group C: P. gracilis Group D: P. pulcherissima Group E: P. pubescens, P. radiatus
P. viphyensis
Clade 2 unplaced groups Group 1: Tetradenia
Group 2: Thomcroftia
P.ambiguus, P. zuluensis, P. parvus Group 3: Plectranthus elegans, P. fruticosus, P. ciliatus, P. oertendahlii, P. saccatus
Group 4: Aeollanthus
Group 5: Capitanopsis, Dauphinea, Madlabium Group 6: Plectranthus glandulosus, P. longipes
P. laxiflorus, P. kamerunensis, P. stolzii P. mandalensis, P. eckonliii, P. verticillatus, P. grallatus
1
1a
1b
2
Key
represents groups cited as being useful represents groups with no/few cited uses Groups shown in bold have recorded medicinal uses
Figure 14: Illustration of the 2 clades within the Plectranthus species, categorized according to their phylogeny (reproduced from Lukhoba et al., 2006)
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