Germination and Growth Inhibitors in Guayule (Parthenium argentatum Gray) Chaff and Their Possible Influence in Seed Dormancy

Guayule (Parthenium argentatum Gray) is a rubber-producing plant native to the semi-arid region of north central Mexico and southwestern Texas. As a member of the family Asteraceae, the seed dispersal unit in guayule is comprised of an achene enclosed by two staminate florets and a bract. The seeds in this condition are dormant and need special treatment in order to germinate. Threshing, prolonged washing, and/or chemical treatments prove effective in improving the germination. Experiments showed that chaff influenced germination and seedling emergence. Bioassays of aqueous extracts of guayule chaff and the seed coat showed a significant inhibition of germination and radicle growth of guayule, lettuce, and tomato seeds. High-performance liquid chromatography enabled the identification of at least seven phenolic acids from guayule chaff extract. These, in order of decreasing concentrations, included p-hydroxybenzoic, protocatechuic, p-coumaric, ferulic, benzoic, vanillic, and cinnamic acids. All of them are well-known germination and growth inhibitors, and seem to play an important role in the dormancy of guayule seed.