100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 350

Related Species
Sixteen species, in addition to guayule, have been identified in the genus
Parthenium , but none of these species produce appreciable amounts of rubber.
Ploidy level in these species is also variable as seen in guayule. However, they have
been identified as potential sources of other useful traits such as disease resistance,
increased biomass, cold tolerance, regrowth potential, and drought tolerance
[ 19 ]. Limited seed of some of the related species is available from the USDA-
ARS, National Arid Land Plant Genetic Resources Unit in Parlier, California,
through the GRIN system ( http://www.ars.grin.gov/npgs/ ). Reviews of the mor-
phology, anatomy, ultrastructure, and relationships among the species are available
[ 20 , 21 ].
Seed collection of these related species should be a high priority for the near
future as native stands of these species are threatened. Seed of three species,
P. incanum, P. hysterophorus , and P. confertum , were collected by Coffelt, Foster,
and Stout during the collection trip in Texas in 2005. Except for P. incanum , the
stands were very small and seed very limited of these species. Large stands of
P. incanum were found at several sites. Ploidy analyses indicated that both triploid
and tetraploid plants were present at these sites [ 18 ].
Breeding Methods
In many instances, the breeding of new and conventional crops is essentially the
same [ 5 ]. 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 research, including the appropriate technology for evaluating,
selecting, and breeding for the commercial production of the products and coprod-
ucts sought. New crop breeders must be flexible in 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.
Guayule yields were first increased by planting larger areas and improving
cultivation techniques rather than through breeding. This is standard in many
new/minor crops because plant breeding programs take time to initiate and start
achieving their desired goals. In addition, many new/minor crops do not have a
large industry capable of supporting a plant breeding program. This was true in
guayule, as described above, but it also is a more difficult species to work with for
some biological reasons: it is a perennial; it is physiologically immature for 1 year
(rubber does not produce any appreciable amounts until it has gone through one