LAUSR

Abstract Guayule (Parthenium argentatum A. Gray) is an alternative source of natural rubber. Direct-seeding is critical to commercialization of guayule and quantifying variability in seed germination response to temperature is necessary. A thermogradient table was used to measure germination rates of the 44 most commonly available Parthenium accessions under 10 temperature gradients. The optimal temperature for seed germination ranged from 17.8–26.9 °C, with an average temperature of 21.8 °C. The sensitivity of seed germination to temperature range varied significantly among accessions. When compared with the highest germination rate at optimal temperature, relative germination rate at 10 °C and 35 °C represented low and high temperature tolerance for seed germination in guayule accessions, respectively. The two accessions with >70 % relative germination rate at 35 °C, R1109 and P. incanum (mariola), had the optimal temperature for germination above 26.6 °C. Six accessions with >70 % relative germination rate at 10 °C, 11591, R1110, PARL 935, CAL-3, 11605, and PARL 920, had a range of 17.8–21.7 °C optimal temperature for germination. This study indicates that guayule and other Parthenium accessions varied significantly in temperature requirements for seed germination. These temperatures can serve as a guide to seasonal timing for direct seeded field planting.