GERMINATION AND CHARACTERIZATION OF GUAYULE (Parthenium argentatum GRAY) SEED

Abstract

Guayule (Parthenium argentatum Gray) is a natural rubber producing plant presently being commercialized for production in arid and semi-arid regions of the world. Inconsistent seed (achenes) quality and the lack of methods to discriminate between good and bad seed lots are significant problems, allowing commercial stands to be established only by transplants, rather than by more cost effective direct-seeding. The objective of this study was to investigate why guayule seed and seed lots vary significantly in quality. The laboratory and greenhouse experiments involved separation of seed based on physical attributes (diameter, color and weight) and the confirmation of internal structures (embryo and endosperm) by x-ray analysis. The effect of different conditioning treatments on seed lots was also tested. Separation of seed by size (diameter) was shown to be less efficient than separation by color. Color was the most effective means of separating heavier and darker (opaque-black and gray color grades) seed, and these seed were confirmed to be filled, containing internal structures by x-ray analysis. Germination percentage and rate were greater for seed with these characteristics than for yellow and bright-black seed, which are lighter and predominately partially filled or unfilled. Conditioning treatments used for guayule seed older than two years had no effect on seed germination and emergence percentage or rate. However, differences in germination and emergence were found among lines, therefore, differences among seed lots appear to be due to variation in the number of filled seed. The quality of seed lots was significantly improved when seed were separated based on color first, then by weight. The separation of good quality seed (filled) from poor quality seed (unfilled) is crucial to lower the costs of stand establishment by direct seeding. Therefore, the quality of seed lots can be improved significantly (higher proportion of filled seed) by separating first by color, then by weight.