'n Biologiese ondersoek van Tylosema esculentum

Abstract

Tylosema esculentum (Schweinf.) Schreiber belongs to the family Caesalpiniaceae in the tribe Cercideae Bronn. The species is distributed throughout Southern Africa. The combination of climate and soiltype with the occurrence of lime and dolomite seem to be determinating factors in the distribution of the species. T esculentum is a perennial geophyte with an underground tuber bearing ephemeral climbing or creeping stems. Vegetative growth and development occurs from August to May. The main stems and 1st order-lateral stems are mainly responsible for the first high intensity flowering phase during November while the 2nd order-lateral stems are responsible for the second high intensity flowering phase during January. No significant difference (p<0.05) is to be found between the development of Pin and Thrum flowering plants. T esculentum has chasmogamic, entomophylous flowers which are zygomorphic and distylous. Nectar production in the hipantium is the primary attractant of pollinators and is supplemented by two secondary attractants, viz. the bright yellow flower colour and their mild sweet aroma. The stigmas are funnel-shaped, papillate and wet. Anthesis takes 18 to 21 hours and the flowers have an average life expectancy of four days. The peak of stigma receptivity is on days two and three after anthesis. An average of 29 088 pollen grains (from only two fertile anthers) are produced per flower and a pollen viability of 25.3% for pin flowers and 21 .3% for thrum flowers was obtained using the in vitro test method. A viscous, polisaccharide and lipid-like anther-mucilage is released simultaneously with the pollen during anther dehiscence. This anther-mucilage is produced within the anther-connective tissue and is analogous, in terms of pollination ecology, to pollenkitt and viscin threads. No pre-zygotic self-incompatibility system was found. A high level of outbreeding is maintained through reciprocal herkogamy and incomplete protogyny. A contributing factor may be the availability of pollen on the first day of anthesis only, as a result of the hardening of the anther-mucilage, while the peak of stigma receptivity is on days two and three after anthesis. The possibility of the presence of a post-zygotic late-acting self-incompatibility system is, however, not completely ruled out.