Black wattle (Acacia mearnsii) is a leading commercially grown forestry tree species in South Africa. However, it is still considered an exotic invader species and is listed as a Category 2 invasive species. This invasive status adds unwanted pressure to the industry, making future afforestation to black wattle difficult, particularly for small growers. The production of a sterile or seedless variety of black wattle would be a solution to help reduce the contribution of seed to the existing seedbank and simultaneously remove the invasive status of black wattle. Sterility in black wattle would therefore satisfy the needs and concerns of the environmentalists and allow the industry to continue as a thriving contributor to the South African economy whilst being viewed in a better light.
This document summarises the work that has been conducted over a period of 13 years from 2000 until 2012 in the Sterility Project, within the Acacia Breeding Programme at the Institute for Commercial Forestry Research (ICFR). Two approaches were followed to produce a sterile or seedless tree, namely (i) the production of a triploid variety, and (ii) the use of gamma irradiation techniques to sterilise the tree. The ultimate aim of this project was to guarantee that all seed sold or distributed to growers would produce sterile or seedless trees. A number of advances were achieved in trying to produce a sterile or seedless variety of black wattle, namely (i) tetraploids were successfully induced and confirmed using a suite of tools developed to assess ploidy, (ii) an in-depth reproductive biology assessment was conducted which revealed that the production of triploid black wattle is possible provided that barriers within the ovary can be overcome, and (iii) gamma irradiation offers a means of reducing the seed load to some extent.