100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 349

scale of the present plant improvement programs, this variability is created faster
than it can be exploited by breeders [ 1 , 5 , 15 ].
Most guayule germplasm today consists of apomictically reproducing triploid
(3 n

72) accessions because they received most of the
attention in previous breeding programs [ 1 , 5 , 15 , 16 ]. When Gore et al. [ 17 ]
evaluated available germplasm from the GRIN system, they found a natural poly-
ploid series ranging from diploid (2 n

¼ 54) and tetraploid (4 n ¼

2 x 36) to pentaploid (2 n 5 x 90), with

4 x being the predominant ploidy. Interestingly, not all plants sampled from an
accession had the same ploidy level (mixed ploidy). This was recently verified by
Coffelt et al. [ 18 ], when they found similar results with ploidy levels varying from
diploid to octoploid in a breeding population derived from diploid crosses and a
seed increase nursery of additional plant introductions. Results from these studies
[ 17 , 18 ] confirm the variability available in guayule for breeding improvement.
Sexually reproducing, largely self-incompatible diploids (2 n

¼ ¼ ¼ ¼

36) have had only

¼

limited use in guayule breeding programs.
At present, the USDA-ARS, National Arid Land Plant Genetic Resources Unit in
Parlier, California, where guayule is curated has 144 P. argentatum accessions and
five interspecific hybrids of different Parthenium species [ 5 ]. Twenty-five of these
accessions have PI numbers, with the remainder carrying western regional num-
bers, but unfortunately as many as 64 accessions may not have viable seed. This is
an important problem that has been recognized by the National Plant Germplasm
System. A collection trip by T.A. Coffelt, M.A. Foster, and D. Stout (sponsored by
the USDA, NPGS, Plant Exchange Office) was made to Texas in 2005 to try to
recollect some of the original collections made in Texas. However, guayule could
not be found at most of the original collection sites. Where guayule plants were
found, little or no seed was present. Other species were found at many of these sites,
especially P. incanum . It is possible that many of the plants at these original sites
were misidentified or that other species have replaced the original guayule
populations. One site was found with viable seed of guayule near Bakersfield,
Texas. Seed has been collected at this site and added to the USDA collection at
Parlier, California. This represents the only wild source of guayule collected since
the early collection trip of Tipton and Gregg in 1982. Seeds were also collected and
added to the collection from old guayule plantings at the Firestone test track facility
near Fort Stockton, Texas. Many of the old USDA lines and materials collected
fromMexico were planted at this site from 1940 to 1990, but the plots have not been
actively maintained for over 20 years and plot identities have been lost. Seed
collected at this site may be the best chance to recover genetic diversity that
otherwise may be lost. Studies to determine the best methods for long-term storage
of guayule seed to maintain viability have been initiated, but recommendations are
not yet available.