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.