cis-Polyisoprene Synthesis in Guayule Plants (Parthenium argentatum Gray) Exposed to Low, Nonfreezing Temperatures

Exposure of guayule plants (Parthenium argentatum Gray) to 6 months of a night temperature of 7°C results in a 2-fold stimulation of cis-polyisoprene (rubber) formation over that of control plants exposed to 21 to 24°C night temperature. Control and cold-treated plants contained 2.18% and 5.69% rubber, respectively. Examination of the stem apices by transmission electron microscopy showed extensive formation of rubber particles in the cold-treated plants compared to the control plants. The rubber particles in guayule are formed in the cytoplasm and fuse to form large globular deposits. The surface area of the rubber particles and globules range from 4 × 10⁻⁶ to 2.9 × 10⁻³ square micrometers. The deposition of rubber in the cytoplasm of the cortical parenchyma cells differs from rubber deposition in the vacuoles of laticifers of Asclepias syriaca. Electron micrographs of stem cortical parenchyma in control plants show mature cells with large central vacuoles, thin layers of parietal cytoplasm, and smaller numbers of rubber particles. Radioactive acetate and mevalonate are incorporated into rubber at a faster rate in stem slices from cold-treated plants compared to slices from control plants. A faster rate of these reactions may account for the increase in rubber synthesis in the cold-treated plants.