Determining Redundancy within Six Groups of Current and Collected Short-Day, Onion Accessions

Efficiently maintaining a representative and genetically-diverse collection of germplasm is the mission of the U.S. National Plant Germplasm System. Reducing genetically-duplicate accessions and adding new and different germplasm to the collection maximizes its genetic diversity. This study was conducted to determine redundancies in the current short-day, onion (Allium cepa L.) germplasm collection in the U.S. Some accessions appear to be duplicates as indicated by their similar cultivar names. A second objective of this study was to evaluate recently-collected germplasm to determine if there were duplications among them, as well as, between them and the current accessions in the collection. Twenty different germplasm accessions and collected cultivars, assembled into six different similarity groups, were seeded in October of 2007 and in September of 2008. Plants from each accession were measured for 18 different morphological characters once the plants from each entry neared maturity. These characters measured were bolting percentage, sheath length and diameter, plant and leaf height, leaf width and thickness, and time when 20% of plants in the plot had reached tops down (20% tops down), 50% tops down, and 80% tops down. At the time of 80% tops down, bulbs were harvested and harvest date, pink root disease severity, and bulb number, height and diameter were recorded. Average bulb weight, bulb firmness, and the percentage of bulbs with a single growing point were measured two weeks after harvest. Some groups had a high degree of bolting and seven additional characters were measured from bolting plants of those entries. These characters were pink root disease severity, bulb height and diameter, shape index, scape length and diameter, and umbel diameter. No accessions should be removed from the collection, as none were identified as duplicates of one another. Ten newly-collected lines were deemed to be different from accessions already in the collection and may add some diversity to the short-day onion collection. Of the traits measured, bulb diameter and days to 20% tops down were the most discriminating among entries, while the least discriminating characters were sheath length and uniformity of maturity.