SEED TRICHOME VARIATION IN HIBISCUS SECTION FURCARIA IN EAST AFRICAGeoffrey MwachalaEast African HerbariumNational Museums of KenyaP.O. Box 45166Nairobi, KenyaABSTRACTThe micromorphology of the seed trichomes in Hibiscus section Furcaria in Tropical East Africa was surveyed. The structure of the trichomes was found to be unique for each species examined, where such trichomes are present. Seed trichomes of specimens of H. altissimus Hornby from the Kenya and Tanzania coasts were found not to match those of the type for this species. It is considered that this is indicative of subspecific differentiation.INTRODUCTIONTribe Hibisceae (Family Malvaceae) has attracted great interest from biologists because of its potential as a source of fibre and its ornamental attraction. This tribe includes Gossypium and Hibiscus. Hibiscus species are utilised as food and as a source of cordage by many indigenous peoples in Africa. The wide variability of most species has offered annoying difficulties to botanists when trying to delineate these species or infraspecific taxa resulting in an alarming accumulation of names (Waalkes, 1966). There remains a great deal of work to be done to understand the taxonomy of Hibiscus in Africa, especially in East Africa where no taxonomic study of the genus has been undertaken since the work of Ulbrich (1921). Ulbrich divided the genus into 18 sections. Exell in Flora Zambesiaca (Exell & Wild, 1961) classified the genus into "series", most of which correspond roughly to Ulbrich's sections. This is clear indication that the taxonomy of Hibiscus in Africa is not fully understood. This led me to look for new taxonomic characters within the genus. In section Furcaria, I examined the seed coat lichomes. Seed structure has been found to be useful in the taxonomy of, among others, the Caryophyllaceae and the Portulacaceae (Stace, 1989). This feature has been examined for H. cannabinus by Khushk and Vaughan (1986). They found a trichome structure single and up to three in a group; bun shaped. The geographical origin of their material is not given. The value of seed coat characters in Hibiscus section Furcaria is unknown; this study aims to address this question.Section Furcaria DC. is one of the 18 sections of Hibiscus recognised by Ulbrich (1921). This section is distinguished by its five calyx lobes being leathery in flower and woody in fruit, with a prominent thickened midrib and two thickened marginal ribs. Many are handsome ornamentals possessing large, delicate, showy flowers ranging in colour from white to various shades of cream, yellow, pink, red or purple, and usually with a dark red or purple centre (petal spot). This group includes useful plants such as kenaf H. cannabinus L. (used as a fibre crop) and roselle H. sabdariffa L. (used for food and drink; Dalziel, 1948). The section occurs in the tropics and subtropics of Africa, Asia, Australia and South America. The name Furcaria is derived from the forked involucral bracteoles that characterise some of the species (Wilson, 1994). Tropical East Africa in this study refers to Kenya, Tanzania and Uganda.This study included twelve cultivated and wild species in the section, as follows. Hibiscus acetosella Welw. ex Hiem, H. altissimus Hornby, H. berberidifolius A.Rich., H. cannabinus L., H. diversifolius Jacq., H. greenwayi Bak. f., H. mastersianus Hiem, H. mechowii Garcke H. rostellatus Guill. & Perr., H. sabdariffa L. H. s⟨private-char name="pgrave"/⟩aceaculeatus A. Rich and H. surattensis L.MATERIALS AND METHODSThe work was carried out using seeds obtained from herbarium specimens in the Royal Botanic Gardens, Kew and from plants grown at Arizona State University kindly supplied by F.D. Wilson of the United States Department of Agriculture.Initially a few trichomes were removed from the seed surface under a dissecting microscope using a scalpel and placed in a drop of water containing safranin on a glass slide and examined under a light microscope to establish the cellular structure.The seeds of the various species were cleaned by putting them in 20 ml of water containing a drop of wetting agent and placing in a sonic bath for five minutes. This treatment was designed to remove dust and any fungal hyphae from the seed surfaces. The seeds were subsequently dried by placing in a warm oven (60°C) for half an hour. The cleaned seeds were later mounted on Scanning Electron Microscope stubs and coated with gold using a Polaron Scanning Electron Microscopy coating unit E5150. Five seeds from plants from widely separated localities were used for each species.The various specimens were loaded, three at a time, into a Jeol T20 scanning electron microscope.RESULTSThe trichomes were found to consist of unicellular hair-like units variously fused along their length to form scale-like structures as illustrated in figure 1. These hair-like units have a diameter ranging from 5-20 micrometres. No trichomes were seen in H. diversifolius Jacq.Three types of trichomes were unicellular hairs fused to recognised. Type 1 trichomes are J narrowly ovate to broadly ovate in outline, comprising 4-13 individual hairs 50-140 micrometres. long. They are either erect or appressed. H. acetosella, H. altissimus, H. mastersianus, H. sabdariffa, H. surattensis and H. berberidifolius illustrated in figure 2-7 have this kind of trichome. Type 2 trichomes are, linear in outline and erect,Figure 1. General structure of seed trichomes in Hibiscus section Furcaria.Figure 2. A seed trichome of Hibiscus acetosella from Jones 6962. Scale bar = 25 µmFigure 3. A seed trichome of H. altissimus from Luke & Robertson 1858. Scale bar = 25 µmFigure 4. Seed trichomes of H. berberidifolius from Osmaston 3677. Scale bar = 35 µmFigure 5. Seed trichomes of H. mastersianus from Burtt 3560. Scale bar= 25 µmFigure 6. Seed trichomes of H. sabdariffa from Greenway 2960. Scale bar = 25 µmFigure 7. Seed trichomes of H. surattensis from Toms 225. Scale bar= 25 µmFigure 8. Seed trichomes of H. greenwayi from Semsei 2121. Scale bar= 20 µmFigure 9. A seed trichome of H. cannabinus from a specimen cultivated at Arizona State University by F. O. Wilson. Scale bar = 25 µmFigure 10.A seed trichome of H. cannabinus from Tanner 5194. Scale bar= 25 µmFigure 11. Seed trichomes of H. mechowii from Kahurananga 2688.Scale bar = 25 µmFigure 12. Seed trichomes of H. rostellatus. From Maitland 166. Scale bar= 25 µmFigure 13. Seed trichomes of H. sparceaculeatus from Bally 11588. Scale bar =25 µmcomprising four or five fused hairs up to 110 micrometers long. These, shown in figure 8, were observed in H. greenwayi only. Type 3 trichomes have a fan-shaped or truncate outline and consist of up to 25 individual units. This type of trichome is found in H. cannabinus, H. mechowii, H. rostellatus and H. spaceaculeatus (figs. 9-13). These trichomes may be appressed or spreading, ranging from 36 micrometers long in H. rostellatus to 150 micrometres in H. mechowii.DISCUSSION AND CONCLUSIONSEach species in Hibiscus section Furcaria has distinctive trichomes on the seed coat. Species with similar trichome morphology could be distinguished by the size of the trichomes.Cultivated H. cannabinus (fig. 9) has very similar seed trichome structure to wild H. cannabinus. The structure elucidated here for H. cannabinus differs from reported by Khushk and Vaughan (1986). Since they do not give the origin of their material, I cannot comment on this apparent disparity. H. diversifolius is the only octoploid in this study and is the only one without seed coat trichomes.The study has established that the seed coat trichome structure is taxonomically significant at specific level within East Africa. The trichomes of each species studied are unique, enabling one to determine the species to which the seed belongs. Attempts to study the testa structure using scanning electron microscopy were not successful because the seeds were too old and shrunken.The three trichome types do not correspond to the subsectional classification of the section according to Ulbrich (1921), this being the latest such classification. This clearly shows the need for more studies utilising contemporary methods to work out the taxonomy of· the section.The seed trichome structure of H. altissimus from the Kenya/Tanzania coast does not match that of the type specimen (Hornby 4563) from Mozambique. I postulate that this is related to the regional variation, and that the Kenyan and Tanzanian material is of a different infra-specific taxon from the Mozambican material. Further work is needed to elucidate the infra-specific variation in seed trichomes in H. altissimus.ACKNOWLEDGEMENTSI am grateful to the following people woo have been of invaluable assistance during this study: Miss C.H.S. Kabuye and Miss B. Khayota of the East African Herbarium, National Museums of Kenya; Mrs C. Barker, Dr M. Cheek, Miss Y. Harvey, Dr R. Polhill and Dr K. Vollesen of the Royal Botanic Gardens, Kew, U.K.; Dr L. Bonner and Dr A. Culham of the University of Reading, U.K.; and Dr F.D. Wilson of the Western Cotton Research Laboratory of the United States Department of Agriculture, Phoenix, Az., U.S.A.This work represents a portion of an MSc thesis submitted to the University of Reading. Funding was kindly provided as part of the Global Environment Facility Project entitled "Institutional support for the protection of East African biodiversity".REFERENCESDalziel, J. M. 1948. The useful plants of West Tropical Africa. Crown Agents for. the Colonies, London.Exell, A. W. & H. Wild (Eds.) 1961. Flora Zambesiaca 1(2). Crown Agents for Overseas Governments and Administrations, London.Khushk, M.T. & J. G. Vaughan 1986. Seed structure in relation to the taxonomy of the Hibisceae (Hibiscus and Lagunaria). Pakistan Journal of Botany 18(2); 309-319.Stace, C.A. 1989. Plant taxonomy and biosystematics. Cambridge University Press.Ulbrich 1921. Hibiscus. In A. Engler (ed.), Die Pflanzenwelt Afrikas 3,2:391.Waalkes, J.B. 1966. Malesian Malvaceae revised._ Blumea 14(1) 1-213.Wilson, F.D. 1994. The genome biogeography of Hibiscus sect. Furcaria DC. Genetic Resources and Crop Evolution 41: 13-25.APPENDIXList of Hibiscus specimens sampled for seed trichomesH. acetosella Welw ex Hiem, Mwea, Embu, 21 Feb 1969; Jones 6962H. altissimus Hornby, Cha Simba, Kilifi, 14 Aug 1989, Luke & Robertson 1859; Vitengeni R., Kilifi, 17 Aug 1988, Luke 1323; Lusa R. Valley, Mozambique, Hornby 4563H. berberidifolius A. Rich, Kitale, 15 Aug. 1956, Bogdan 4214; Ujamba, Mahali Mts. 7 Aug 1958, Newbould & Jefford 151l;Toro, Osmaston 3677H. cannabinus L., Phoenix,Arizona, USA, Wilson sn (voucher in EA) Nairobi, 8 Jun 1948, Bogdan AB1640; Kungwe, Kigoma, 18 Jul 1959, Newbould & Harley 9005; Naivasha. 26 Dec 1931, Van Someren 1624; Moyale, 16 Aug 1952, Gillett 13 723; Malindi. Oct 1951, Tweedie 1032; Biharamulo, Tanner 5194.H. diversifolius Jacq., Kondoa, Burtt 812; Kisii, 1 Mar 1969, Jones 6981; Ruwenzori, Elliott 8234H. greenwayi Baker f., Same, Jul 1955, Semsei 2121; Taro, Oct 1954, Tweedie 1219H. mastersianus Hiem, Kazikazi, Manyoni, 14 May 1932, Burtt 3560; Ikowa, Dodoma, 29 Ju11970, Thulin & Mhoro 554H. mechowii Garcke, Kigoma,. 29 May 1975, Kahurananga et al. 2688H. noldeae Baker f., W. Usambaras, 11 Jul 1952, Greenway 8735H. rostellatus Guill. & Perr., Entebbe,. Oct 1922, Maitland 166H. sabdariffa L., Amani, 5 Apr 1932, Greenway 2960H. spaceaculeatus A. Rich., Mbulu, 23 Jul 1957, Bally B11588H. surattensis L., Zanzibar, 1927, Toms 225