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Taking the invader out of wattle

Published: 28th Apr 2017
Author: Dr. Sascha Beck-Pay; Senior Research Scientist: Acacia Tree Improvement; Institute For Commercial Forestry Research (ICFR)

Production of a sterile black wattle as a means of controlling the spread of a commercially important forestry species in South Africa.

The black wattle, a native of Australia, is farmed in SA, Zimbabwe, Kenya and elsewhere, and is one of the principal sources of the tannin extract used in vegetable tanning and re-tanning worldwide. In SA, it is also a Category 2 invasive species, and is a serious problem plant in many areas. The Institute for Commercial Forestry Research conducted studies into the feasibility of producing sterile black wattles – a project which closed at the end of 2012. The abstract and final conclusions of the project follow.
 
The original aim of the project
Although Acacia mearnsii is a leading commercially grown tree species in South Africa, it is also an alien invasive species.
The objective of this project was to develop sterile or seedless black wattle (Acacia mearnsii) for commercial deployment by 2015, and to guarantee that all seed sold or distributed to growers would produce sterile trees. This would limit the spread of wattle seed and trees outside plantation boundaries and reduce the contribution of seed to the existing seed bank.
  Two approaches were assessed:
The production of triploid trees (sterile) through chemical treatment, and
Gamma irradiation of seeds to induce sterility.
 
Triploid Production
The production of a triploid variety entailed crossing diploids with chemically-induced tetraploids in an attempt to produce viable triploid seed which could be grown commercially. Due to the uneven number of chromosomes present in the triploid variety, the trees would potentially be unable to undergo successful sexual reproduction, resulting in reduced seed set and/or sterile trees. 
  This was a long-term approach, which aimed to produce true triploids which were sterile. The triploids would need to be propagated vegetatively, through cutting, grafting or tissue culture.
 
Gamma Irradiation
Gamma irradiation disrupts the chromosomes responsible for sexual reproduction, resulting in non-viable gametes and infertile pollen. Flowering, seed set and viability studies are early phenotypic indicators of whether or not irradiation has been successful. 
  This approach is quick and inexpensive. However, it may not completely prevent seed production, but rather significantly reduce it.
 
Abstract
Black wattle (Acacia mearnsii) is a leading commercially grown forestry tree species in South Africa. However, black wattle 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. 
 
Final conclusions 
From the research that has been conducted over the past 13 years, a number of advances have been made towards developing a sterile variety of black wattle. In the research focused on producing a triploid variety, stable tetraploids have been successfully induced and confirmed using a suite of tools developed to assess ploidy.
An extensive reproductive biology assessment has been conducted and has confirmed that triploidy is possible in black wattle, provided that identified barriers within the ovary can be overcome.
In the research aimed at inducing seedlessness or sterility through the use of gamma irradiation, a range of doses have been identified that have shown to reduce flowering and seed set without damaging the growth of the trees. However, viable seed is still being produced by the irradiated trees and thus it still needs to be determined whether the reduction noted in flowering and seed production is due to male sterility and whether these reductions are consistent across a range of growing areas. If successful, male sterile or semi-seedless black wattle could be made available to the Wattle Industry with minimal cost implications. In addition, the developed technology of gamma-induced male sterility can be applied across genera, not only for controlling invasive species, but also to add value to breeding programmes e.g. male sterility could be used to facilitate natural hybridisation between species where one species must specifically be the mother. Gamma irradiation techniques can also be used as a means of introducing variability into the main breeding population. 
Although this has been a long-term project, it should be noted that new fruit cultivars take between 15 to 20 years to be developed and released, and thus the progress achieved in this project is favourably comparable. The importance and value of pursuing the production of a triploid variety of black wattle lies in a number of reported benefits in addition to that of sterility to control invasiveness. Polyploid plants, in general, have reduced fertility and thus their resources are channeled into increased vegetative growth which could play a role in future breeding programmes (Wright 1962). The production of a sterile black wattle variety could therefore, not only result in a reduction in flowering and seed load, but also in improved growth and wood properties. 
With further research being supported financially, the production of a sterile variety of black wattle can be achieved. This, together with bio-control agents, currently being tested, could limit the spread of black wattle outside of plantation boundaries. In addition, potential additional growth benefits from a triploid variety and applications of gamma irradiation to the breeding programme could have major economic benefits for the South African Wattle Industry. – [+27 (0)33 386 2314, sascha.pay@icfr.ukzn.ac.za,   http://www.icfr.ukzn.ac.za]
©2017 S&V Publications
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