Breeding guayule for commercial production
Introduction
Guayule (Parthenium argentatum Gray) is a potential economic and renewable source of rubber/latex. This paper updates two previous reviews (Thompson and Ray, 1988, Estiali and Ray, 1991) on the genetics, germplasm development, and breeding of this new crop. The primary objective of the guayule breeding programs has been to facilitate successful commercialization by developing higher yielding cultivars. However, other approaches and information are necessary for full commercialization, and much of this research has been led by Dr. Francis Nakayama, who we honor with this symposium. Dr. Nakayama has contributed significantly to guayule's development, especially in the areas of agronomy (planting, irrigation and harvesting), development of chemical procedures to extract rubber/latex (for both laboratory analyses and for commercial production), and most recently in his research on the pesticidal properties of the bagasse and byproducts left after latex extraction. For plant breeding programs, Dr. Nakayama's main contribution has been the development of chemical testing procedures to evaluate new lines.
Guayule is one of over 2000 rubber producing species, however only two, Hevea brasiliensis (A. Juss.) Muell.-Arg. and guayule have been exploited as commercial sources of natural rubber. Today, Hevea is an established and greatly improved crop, acclimated to growth in areas outside of its natural habitat, and essentially the sole source of natural rubber for industry. In contrast, work is still underway to completely domesticate and commercialize guayule as a new or alternative crop for arid and semiarid areas of the southwestern United States, north central Mexico, and regions with similar climates throughout the world (Thompson and Ray, 1988).
Although Hevea is the dominant rubber crop today, Hevea and guayule had parallel early histories of development. In both, commercialization began by harvesting from wild stands, before the establishment of plantations and the initiation of cultural studies. Variability within wild stands lowered yields in both species, and this problem continued through the early attempts at cultivation because populations were established with open-pollinated seeds collected from plants that were very heterogeneous genetically. The differences in commercial development between the two crops can be associated with the initiation of the Rubber Research Institute of Malaya in 1925, which has been responsible for over 75 years of continuous increases in Hevea yields and the production of a uniform and reliable industrial product (Bonner, 1991). Guayule, on the other hand, has suffered from intermittent research efforts, which have been undermined by periods of neglect. Guayule researchers have found themselves more than once in the position of “reinventing the wheel,” whereas Hevea has had a continuous and well supported research effort (Ray, 1993).
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History of guayule domestication and commercialization
Guayule has been known as a source of rubber since pre-Colombian times when native populations in Mexico used it to form balls for their games. In the early 1900s, guayule was considered as an alternative source of natural rubber in the United States because of the high price of Hevea rubber from the Amazon region (Bonner, 1991). Since then, there have been four major efforts to domesticate and commercialize guayule. The first effort to commercialize guayule began at the turn of the 20th
Breeding
Guayule and Hevea have similar early breeding histories. Yields were first increased by planting larger areas and improving cultivation techniques rather than through breeding. Both are difficult species to work with from the breeder's point of view because they are perennials, and thus, need a considerable amount of land for breeding programs. Also, both species are physiologically immature for 3–7 years before the first harvest, and in both reproduction is essentially asexual (clones in Hevea
Reproductive biology
Guayule is one of the dominant perennial xerophytic shrubs found on the limestone hillsides of the Chihuahuan Desert of northcentral Mexico and the Big Bend region of Texas (West et al., 1991). Wild stands contain a natural polyploid series of diploids (2n = 36), triploids (3n = 54) and tetraploids (4n = 72); and under cultivation individual plants have been identified with chromosome numbers up to octaploid (8n = 144) (Thompson and Ray, 1988). Diploids reproduce predominately sexually, and polyploids
Germplasm
The domestication and development of guayule as a crop was initiated in 1910 by W.B. McCallum, who was then employed by the Intercontinental Rubber Company. A breeding and selection program was started in 1916 at Continental, Arizona, and transferred to Salinas, California in 1925. A selection made by McCallum in Salinas, ‘593’, was the principal cultivar utilized in production in the 1920s, 1930s, and the Emergency Rubber Project during World War II (Thompson and Ray, 1988). During the
Genetic diversity in guayule germplasm
Plant materials derived from the preceding germplasm collections have exhibited extreme variability both within and between lines for rubber quality and quantity, dry weight, resin content, latex content, and yield (Naqvi, 1985, Thompson and Ray, 1988, Thompson et al., 1988, Dierig et al., 1989a, Dierig et al., 1989b, Coffelt et al., 2004), chromosome number (Powers, 1945, Bergner, 1946, Thompson and Ray, 1988, Cho, 1993) and isozymes (Estilai et al., 1990, Diallo, 1993, Ray et al., 1993).
Components of yield
Selection in guayule has been significantly aided by the description of the components of yield and their relationships to rubber production (Thompson et al., 1988, Dierig et al., 1989b). In general, rubber content (%) was not found to be highly correlated with rubber yield, and in fact was often negatively correlated. Fresh and dry weights, as well as other characters related to biomass production, were highly and consistently correlated to rubber yield (Thompson et al., 1988, Dierig et al.,
Potential breeding approaches
In many instances, the breeding of new and conventional crops is the same. The major differences are that in new crops: (1) the plant breeder starts with a different, and frequently unique and exotic germplasm base from which to develop a crop; (2) the breeder is often totally unfamiliar with the species, the germplasm, and potential end products; (3) the traits to be improved frequently have not been identified by researchers, industry, or growers; and (4) there is often a paucity of previous
Conclusion
Full commercial production and utilization of guayule will be greatly facilitated by the development of higher-yielding cultivars that fit well in today's mechanized agriculture. In spite of limited personnel and funding resources, yields have been improved significantly. There is abundant genetic variability from which further progress may be made. However, as guayule becomes a commercial crop, the demands upon the plant breeders will be greater. Generally, the more successful a crop, the
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