Seed Germination and Reproductive Strategies of Tithonia Diversifolia (Hemsl .) Gray and Tithonia Rotundifolia (P.M) Blake

Abstract

Abstract. Seed germination and reproductive strategies of Tithonia diversifolia and Tithonia rotundifolia, two invasive species introduced into Africa from North and Central America, were studied. The aim was to determine the characteristics that make them invasive species in the continent. Tithonia diversifolia is a perennial and polycarpic plant reproducing both sexually and asexually while Tithonia rotundifolia is an annual monocarpic plant reproducing only sexually. The seeds of these species exhibit a period of dormancy before germinating. Tithonia diversifolia produces small sized light and numerous seeds while Tithonia rotundifolia produces larger sized, heavier and fewer seeds. Tithornia rotundifolia allocates a high proportion of of dry matter (41.6 %) and Tithonia diversifolia low proportion (10.5 %) to reproduction. It is concluded that small sized light and numerous seeds produced by Tithonia diversifolia accounts for its wide dispersal and rapid spread in colonized areas. Also its perennial habit and ability to reproduce sexually and vegetatively accounts for the species colonizing and stabilizing fast in new habitats. While the larger heavier seeds and high reproductive effort of Tithonia rotundifolia ensures its early vigorous start in seedling growth, quick establishment, survival for longer and to grow to more aggressive size in an environment that is starved of resources. Seed dormancy exhibited by the two species ensures they survive adverse conditions in their environment as seeds and germinate only when the environmental conditions favour the survival of their seedlings.

Growth, reproduction and resource allocation of Tithonia diversifolia and Tithonia rotundifolia

“…In view of the low plant densities and the differential growth periods of the species, it is likely that there was little competition in mixed plantings, with perhaps more intraspecific effects in the monoculture plantings. The higher number of branches in T. rotundifolia than in T. diversifolia observed during this study is in contrast to the observation of Muoghalu and Chuba (2005) on natural populations of these species. This difference is possibly due to the fact that plants in the natural populations were of different ages, with T. diversifolia having increased its number of branches through coppicing.…”

“…The seeds of T. rotundifolia were larger and heavier than those of T. diversifolia. This is in agreement with the results of Muoghalu and Chuba (2005). The larger and heavier seeds of T. rotundifolia imply that the embryo carries large food reserves.…”

“…In contrast, T. rotundifolia reproduces only sexually as evidenced by the failure of the slashed stem or stem cuttings to sprout. The perennial habit and ability to reproduce sexually and vegetatively probably accounts for T. diversifolia colonising new habitats and rapid stabilisation of populations in new sites (Muoghalu & Chuba, 2005). Vegetative reproduction can aid survival where competition is intense and may allow recruitment where a species is limited by a narrow range of germination microsites (Daubenmire, 1968).…”

Temperature Effects on the Seed Germination of Some Perennial and Annual Species of Asteraceae Family

“…Furthermore, results of Wu et al (2007) on Flaxleaf fleabane (Conyza bonariensis L. Cronquist) (an annual or short-lived perennial plant) indicated that there was no germination activity at 5 and 35°C for mentioned species. Muoghalu & Chuba (2005) conducted a survey on seed germination of Tithonia diversifolia as a perennial plant reproducing both sexually and asexually and Tithonia rotundifolia as an annual plant reproducing only sexually. They proved that the seeds of these species had a period of dormancy before germination.…”

Effects of Aqueous Shoot Extract of Tithonia diversifolia on the Growth of Seedlings of Monodora tenuifolia (Benth.), Dialium guineense (Willd.) and Hildegardia barteri (Mast.) Kosterm

“…They have become naturalized in Southern Africa, while Tithonia diversifolia was naturalized in West Africa (Moughalu and Chuba, 2005). In these areas, the species have established themselves as serious weeds of arable crop, plantations, abandoned lawns and roadsides.…”