SEED GERMINATION AND REPRODUCTIVE STRATEGIES OF TITHONIA DIVERSIFOLIA (HEMSL .) GRAY AND TITHONIA ROTUNDIFOLIA (P.M) BLAKE

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. Keywords. Tithonia species, reproductive strategies, seed dormancy.