Variation in the Molecular Weight Distribution of Rubber from Cultivated Guayule

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Abstract The molecular weight (MW) and the molecular weight distribution (MWD) of natural rubber (NR) are directly related to rubber quality. Although over 2000 species of plants are capable of producing rubber, only about a dozen are recognized as producing sufficient quantities of high quality, high MW rubber necessary for commercial use. Guayule, Pathenium argentatum Gray, can synthesize high MW rubber similar to Hevea and is actively being developed as a new source of NR for the arid regions of the world especially in the United States. Rubber of MW 105 or less is considered unsuitable for much commercial use. Unfortunately, most of the 2000 rubber synthesizing species are believed to produce rubber with a MW of approximately 5×104 and a unimodal MWD4. The MWD of Hevea, however, has a bimodal distribution exhibiting two MW peaks. The major peak exists at a MW of about 7×105 and extends to 106 and beyond. A minor peak or flattened shoulder is also frequently present which corresponds to a MW of 5×104. This bimodal MWD nearly always exists for Hevea, but the amplitude of the two peaks may vary due to variety, age, and tapping frequency. Such variation in the MWD can lead to differences in the average MW. In rubber obtained from native stands of guayule, reports indicate the existence of only a unimodal MWD. Little is known about the environmental or genetic factors influencing rubber MW in stands of cultivated guayule. Since general husbandry and genetic variation seem to affect Hevea rubber, it is not unreasonable to suggest that differences may also appear in guayule, as other work implied.