100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 355

new ones as new germplasm becomes available. Now that there are significant
breeding efforts by industry, these trials should again be implemented.
Another important aspect of yield trials is their use in estimating genotype,
environment, and genotype x environment interactions. Coffelt et al. [ 34 ] found
that location, line, and plant age effects were significant and the interactions not
significant for all traits measured in a study at two locations with two plant ages.
Environment accounted for over 50 % of the variability observed in all traits,
followed by plant age (16 %), and line (10 %). These results point to the tremendous
impact that environment has on guayule plant growth, biomass, and latex content.
Coffelt et al. [ 34 ] could not determine from these tests whether temperature, soil
type, moisture, fertility, or a combination of these or other environmental factors
were responsible for this response. Some of the nonsignificant interactions may
have been significant if a larger or wider germplasm base could have been evalu-
ated. Dierig et al. [ 32 ] also observed significant environmental effects even within a
single field. Additional studies are needed to determine the environmental factor
(s) responsible for the large environmental response observed in these studies.
It is important to breeders that the genotype x environment interaction is not
significant since this means selection for superior lines can be done at one location.
The superior lines should be superior at other locations where guayule is grown.
Antidotal evidence supports this conclusion, since the AZ lines [ 22 ] tested by
Coffelt et al. [ 34 ] have been observed to give similar results when evaluated
under diverse environments such as Spain, Australia, South Africa, and China [ 5 ].
Progress in selection for rubber/latex traits has been hampered because of the
difficulty in determining rubber and latex yield in single plants. The analyses for
rubber and latex contents are labor intensive, time consuming, and expensive,
greatly limiting the number of samples that can be processed. The amount of leaves,
the moisture content of the shrub, and deterioration of the latex during processing
all can interfere and must be considered in the analysis of rubber and especially
latex [ 35 ]. In addition, morphological traits have not been identified that consis-
tently correlate with rubber or latex content. Improvements in these areas could
greatly speed the breeding progress.
Research is needed to establish the relationship between latex and rubber
concentrations and yields. If rubber and latex concentrations and/or yields are
closely related, then previous relationships established between rubber concentra-
tion/yield and the various yield components can be expected to be the same as their
relationships with latex concentration/yield. However, if rubber concentration is
not closely related to latex concentration, then studies will need to be conducted to
establish the relationships between latex concentration and traits such as plant
biomass, latex yield, rubber concentration and yield, resin concentration and
yield, plant height and width, etc. Recent studies [ 36 , 37 ] have indicated inconsis-
tent relationships between latex and rubber concentrations. In one study [ 36 ], latex
concentration and yield varied with storage conditions prior to chipping, whereas
rubber concentrations and yield did not. In another study [ 37 ] of the effects of plant
population and planting dates over several harvest dates, rubber and latex concen-
trations were similar. These studies suggest

that more research defining the