Effect of light, gibberellic acid and abscisic acid on germination of guayule (Parthenium argentatum Gray) seed

Guayule (Parthenium argentatum Gray) produces high quality, low-allergenic rubber which has commercial potential. The species has small seeds with a high level of dormancy which was investigated in a series of germination experiments. Many researchers have investigated the effect of gibberellic acid and light on guayule seed dormancy but there have been no reports on light quality. In this study, increased germination was found for yellow (82.0%) and red (65.3%) light compared to green (55.3%) and blue (25.3%) light. Effect of light quality indicates evidence of phytochrome-mediated germination and dormancy in guayule. A higher ratio of red to far-red radiation in yellow and red light activates phytochrome that stimulates production of endogenous gibberellins to promote germination. The highest level of far-red radiation found in blue light had similar inhibitory effects as complete darkness (21.3%). An intermediate level of red to far-red radiation for green light produced intermediate germination. Seed coat and light also affected germination of freshly harvested guayule seed. Embryos responded to light compared with darkness (68.0% vs. 34.7%) albeit to a much lower level than intact seed (57.3% vs. 8.6%) indicating the possible presence of inhibitors in the seed coat. Light and GA strongly interact to overcome dormancy in guayule and appear to act on the same pathway. The optimum concentration of GA for seed used in this experiment ranged from 250 to 500ppm