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Interactive effects of low night temperature (15 degreesC) and growth light intensity on rubber particle proteins and rubber transferase activity were investigated in stem bark tissue of guayule (Parthenium argentatum Gray, var 11591), a desert shrub of commercial interest as an additional source of natural rubber. Three-year-old guayule plants were subjected to 60 cycles of 30/15 degreesC day/night temperature regime with 12h photoperiod in a controlled environment growth chamber. Plants were also treated with 30/20 degreesC and 30/30 degreesC day/night temperatures for 60 cycles under varying day light growth regimes of 450, 900, and 1500 mu Em(-2) s(-1). Rubber particles were extracted and the associated proteins were characterized in stem tissues of guayule. SIDS-PAGE protein profile of the rubber particles showed an abundant rubber particle protein (RPP) with a molecular weight of 50 kDa. This protein accumulated at higher concentrations after 60 cycles of cold night temperature (15 degreesC) treatment. Rubber transferase (RuT) activity was estimated in the rubber particle fractions at an optimum pH of 7.4. Plants exposed to 60 cycles of 30/15 degreesC temperature showed 80% more rubber transferase activity compared to the control plants that were grown in the field (34/28 degreesC). The results indicate that rubber biosynthesis in plants treated with 15 degreesC night temperature for 60 cycles was superior to that of those grown under natural photoperiod. A radiation of 900 mu Em(-2)s(-1) had a positive effect on guayule plants subjected to 30/15 degreesC temperature treatment, showing the highest rubber transferase activity compared to the growth light intensities of 450 mu Em(-2)s(-1) or 1500 mu Em(-2)s(-1) under similar temperature regimes. In conclusion, low temperature modulation of the rubber particle proteins under optimum growth light intensity could enhance the rubber production in guayule plants throughout the year and not just restricted to the winter months.