Reproductive potential of Solanum mauritianum Scop. : implications for control.
Solanum mauritianum Scop. is rated the worst invader species in pine plantations throughout the Republic of South Africa. Control is costly and apparently ineffectual since the species is spreading in pine plantations at a rate of 16 % per annum. This is due to the high reproductive potential of the species. S. mauritianum produces fruits throughout the year. Fruit and seed yield is related to tree size. S. mauritianum produced approximately 7.2 million viable seeds per hectare during 20 months when growing under conditions unfavourable for growth. Seeds are efficiently dispersed by animals and birds. Although high seed or seedling mortality occurs, the initial prolific seed production and efficient dispersal ensures the rapid spread of this species in South Africa. Surviving seeds form the source for both further encroachment and reinfestation of areas in which S. mauritianum has been controlled. S. mauritianum seeds require the presence of both light and alternating temperatures for optimum germination. Transfer of seeds from unfavourable to optimum conditions or the application of gibberellic acid (GA ) can promote high germination percentages. However, the germination requirements of S. mauritianum are highly variable. Germination is influenced by site, season and year of seed shed. Seeds varied in terms of primary dormancy; conditional dormancy; the response to transfer from unfavourable to favourable conditions; the response to application of GA; and the occurrence of secondary dormancy. Germination requirements of seeds were also influenced by site, duration and depth of burial. All these factors contribute to a sporadic seedling emergence over a prolonged period, which results in current control operations being both costly and ineffective. Alternative control methods were therefore considered. These included the application of herbicides or heat to kill seeds, application of various gro~th regulators to stimulate germination, and the chemical extraction of alkaloids from fruits and seeds for use in the pharmaceutical industry. Two alkaloids (solasodine and a new molecule) were extracted from green bugweed fruits growing under unfavourable conditions. Although levels of solasodine extracted were very low compared with those from commercially grown species of this genus, extraction of the second alkaloid raised the potential of the species for utilization purposes. Utilization of the reproductive propagules could reduce the continual dispersal of seeds and thereby contribute to long-term control of this species.