100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 359

their approach to breeding where so much is unknown. The breeder must be
innovative and able to change approaches and methodology rapidly to meet the
opportunities and constraints as they are encountered. Breeding guayule is difficult
because of several factors such as its perennial growth, need to overwinter to initiate
rubber biosynthesis, facultative apomictic reproduction system, and necessity for
evaluating multiple harvests. In spite of these difficulties, there have been successes
through guayule plant breeding resulting in significant increases in yield per area.
Rubber and resin yields have been increased by 300 % in some lines.
These increases have been accomplished mainly through selection of high-
yielding individual plants but also through mass selection. Other breeding methods
such as pedigreed natural selection, interspecific and intraspecific hybridization,
and family selection can also be used in breeding guayule. Utilizing these methods
requires a long-term commitment for the program to be successful. Genetically
modifying guayule by transgenesis is another tool that might be used for improving
guayule; however, initial experiments using this method have not proved successful
as of yet.
Guayule still contains many wild characteristics such as indeterminate
flowering, seed shattering, seed dormancy, and both sexual and asexual reproduc-
tion occurring in the same plant, which will be desirable to change to facilitate
commercialization. To overcome these undesirable characteristics, long-term well-
supported breeding efforts will be needed. Hopefully, the renewed interest in
guayule as a commercial crop will lead to these efforts. The increased interest in
commercialization of guayule should also help breeders as industry identifies those
morphological and other traits needed to produce the ideal guayule plant for needed
for commercialization.
For future progress in guayule breeding to be made, much work remains to be
done. The relationship between solid rubber and latex rubber and the factors
affecting this relationship need to be identified and understood. The inheritance
of important traits needs to be determined, the genes involved identified, and their
location mapped to specific chromosomes. The large environmental effects on resin
and rubber also need to be determined.
Guayule research and development priorities during the past 100 years have
appropriately focused on variety development, agronomic studies, and latex extrac-
tion processes. Rubber remains the primary driver in development. Current com-
mercialization efforts are a culmination of the results obtained from previous
breeding efforts as well as agronomic studies. The next priority for complete
commercialization of guayule is coproduct development for the bagasse and resin.
References
1. Thompson AE, Ray DT. Breeding guayule. Plant Breed Rev. 1988;6:93-165.
2. Siler DJ, Cornish K. Hypoallergenicity of guayule rubber particle proteins compared to Hevea
latex proteins. Ind Crops Prod. 1994;2:307-13.