Optimum conditions for the rapid, efficient, nondestructive determination of rubber producing potential in guayule (Parthenium argentatum) were established. The rubber producing potential may be defined as the ability of the plant material to synthesize rubber from a precursor under specified conditions. To achieve this, stem slices taken from the first 5 centimeters of branches were incubated with [14C]acetate as precursor in 0.1 molar phosphate buffer (pH 6.5) at 26°C for 16 hours in the light. The 14C from labeled acetate and acetyl coenzymeA were efficiently incorporated into rubber whereas the 14C from both mevalonic acid (MVA) and isopentenylpyrophosphate (IPP) were poorly incorporated. Incorporation of 68.6% of the 14C from labeled IPP into the acetone extractable material suggests that most of the IPP was channeled down the lower terpenoid branch of the polyisoprene biosynthetic pathway. The incorporation of 14C from labeled acetate into rubber was most efficient at temperatures between 20 and 25°C. The rubber producing potential was also found to be dependent on light intensity. The roots which represent about one-third of the plant biomass not only had the highest rubber producing potential but also contained the highest amount of rubber (7.6%), indicating that the root system could be a major source of rubber. The mature stem bark also had a high rubber content and rubber producing potential, whereas the young stem had a low rubber content and a lower potential for producing rubber. The leaves showed little potential to incorporate labeled acetate into rubber and no more than 0.5% rubber was found in guayule leaves.
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