Germination characterization of guayule seed by morphology, mass and, X-ray analysis

Guayule is a shrub native to northcentral Mexico and southwestern Texas and is economically important because it produces non-allergenic latex. Seed quality is an important factor in guayule stand establishment and found to vary greatly among seed lots. We hypothesized that this variation was due to the presence or absence of internal structures (e.g., embryo and endosperm) within the seed. The objective of this study was to characterize seed lots as to seed size, color and weight, and then analyze the relationship between X-ray image pattern of each and germination percentage. X-ray was used to discriminate between filled, partially filled, and unfilled seed, and thus, the relationship between internal structures and seed viability. Two seed sizes, 1.80 and 1.57 mm, of breeding line 11591 were separated into four color grades (bright-black, opaque-black, gray and yellow). Seeds were weighed and each category X-rayed. Germination tests were then performed. Results showed that yellow and bright-black seeds tended to be lighter in weight than gray and opaque-black seeds. It was possible to separate color grades by weight. In addition, germination percentage was higher with heavier seed than with lighter seed. After separation by X-ray and germination analysis, results showed that filled seed had a higher germination percentage than partially filled and unfilled seeds. Data from the X-ray image patterns and germination tests strongly suggest that the presence of internal structures plays a major role in germination of guayule seed. We conclude that X-ray could be used in seed lots to improve the quality of guayule seed germination by separating the filled seeds from the unfilled or partially filled seeds.