ABSTRAC1Exposure of guayule plants (Parthenium argeatatum Gray) to 6 months of a night temperature of 7C results in a 2-fold stimulation of cis-polyisoprene (rubber) formation over that of control plants exposed to 21 to 24C night temperature. Control and cold-treated plants contained 2.18% and 5.69% rubber, respectively. Exmination 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 x 10'6 to 2.9 x 10-3 square micrometers. Rubber is synthesized in over 2000 species ofplants distributed throughout the plant kingdom. Ficus elastica, the rubber plant, belongs to the family Moraceae; Hevea brasiliensis, the commercial rubber tree of the tropics, belongs to the Euphorbiaceae; and guayule, Parthenium argentatum, a desert shrub of commercial interest as an alternate source of natural rubber, is in the Asteraceae family.The regulation of rubber biosynthesis has been extensively studied in Hevea and guayule (1-6). An outstanding feature of rubber synthesis in guayule is that it is cyclic (4). Rubber does not accumulate during the summer months but synthesis begins in the fall and winter as the plant experiences low night temperatures. Bonner noted (4) that 27C days followed by 7C nights induces rubber accumulation in guayule and suggested (6) that the low night temperature increase the expression of the genes coding for enzymes involved in rubber synthesis. Four-month-old guayule plants were subjected to a cold induction regime of 27 to 32°C days and 7°C nights as described by Bonner (4). Control plants were grown at 27 to 32°C days and 21 to 24°C nights.Rubber Analysis. Guayule plants were defoliated and the stems and pnmary root cut into small pieces and boiled in H20 for 15 min. The sections were dried in an oven for 24 to 72 h at 75 to 80C. Dried sections were ground in a Wiley mill through a 40-mesh screen. The ground powder was thoroughly mixed and 3-to 5-g samples were taken for further analysis. Ground samples were placed in cellulose extraction thimbles and extracted with acetone for 8 to 24 h in a Soxhlet apparatus. Acetone extracts containing resins were discarded. The thimble and powder were dried at 75 to 80°C and extracted with benzene for 24 to 36 h in a Soxhlet apparatus. Benzene extracts containing the dissolved rubber were evaporated to dryness in vacuo. Weight ofthe rubber was determined and the purity of the samples was determined by '3C NMR spectroscopy. Samples were dissolved in CDC13 and placed into 10-mm NMR tubes for analysis. Spectra were recorded on a Fourier transform NMR spectrometer operating at 22.5 Mhz. The peaks were reported in ppm (5) downfield from tetramethylsilane using the central peak of CDC13 (79.96) as an internal standard. The 13C NMR spectrum of authentic cispolyisoprene has five distinct peaks with chem...

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