Effects of temperature and salinity on germination of non-pelleted and pelleted guayule (Parthenium argentatum A. Gray) seeds

Guayule (Parthenium argentatum A. Gray) is an important domestic source of natural rubber. Commercial field plots are currently established using greenhouse grown seedlings that are hand sown as plugs. However, this practice is expensive and laborious. Direct sowing of guayule seed in the field would reduce time and cost significantly, and yet the effects of seed pelleting, temperature, salinity level, and their interactions on guayule seed germination are not well established. To test germination requirements, non-pelleted (control) and pelleted seeds were planted in solutions having electrical conductivity (EC) of 0, 2, 4, 6, 8, and 10 mS/cm at 10, 20, 30, and 40 °C for 7 days. After 7 days, the non-germinated seeds were transferred to distilled water plates in a 20 °C environment. Seed pelleting, temperature, salinity, and their interactions significantly affected guayule germination. The optimal conditions for seed germination (i.e., highest germination rates) were found to be 20 °C and EC 0-2 mS/cm, regardless of pelleting. Both temperature and salinity delayed germination and decreased viability. Germination was inhibited at both 10 and 40 °C. Salinity effects on seeds decreased as germination temperature became optimal. Lowest germination percentages were observed at EC 6-10 mS/cm and at 30 and 40 °C. Germination percentages increased for treatments after seeds were transferred to optimal conditions. Importantly, pelleted guayule seeds exhibited higher germination than non-pelleted seeds in all treatments. Our results provide important new insights that can help guide the selection of optimal seasonal and soil conditions for field establishment with new direct seeding methods.