Assessment of phenotypic diversity in the USDA, National Plant Germplasm System (NPGS) guayule germplasm collection

Abstract

Phenotypic characterization on historical breeding lines and wild genetic resources from the 1950′s to 1990′s of the rubber-bearing plant guayule (Parthenium argentatum A. Gray) and hybrids was conducted. Variation among germplasm on seed weight and germination was determined along with leaf traits and trichomes using image analyses. Flow cytometry was performed to estimate diversity in ploidy levels. Tetraploid guayule was the predominant type in the germplasm collection (64%), followed by triploids (34%), pentaploids (1.5%), and hexaploid (1%). No plants with octaploid or higher ploidy level were observed. Guayule 100-seed weight averaged 57.5 mg and was determined to have significant positive correlation to 7 and 14-day seed germination and seed size for this seed lot. Phenotypic similarity among germplasm was determined from quantitative analysis of leaf traits and trichomes. Measurements of other traits on young leaves were highly correlated to those on mature leaves. Most leaf traits within each development stage were found to be positively correlated. Guayule leaves averaged 6.85 cm long and 2.06 cm wide, with 1–6 serrations. Leaf serration height ranged from 2.9 to 6.4 mm. Germplasm lines AZ-2, AZ-3, and N566 had the greatest number of serrations, while R1093 and W6–429 with the least number. Trichome density was consistent between young and mature leaves with average density of 17 trichomes per 0.04 mm2. Accessions R1100, AZ-3, and CAL-4 had the highest trichome density (>21 trichomes per 0.04 mm2). The results from analyzing the relatively high number of plants for seed parameters, leaf traits and ploidy are useful for optimizing germplasm collection and to better aid agronomic management practices and utilization of guayule germplasm for crop improvement activities.