The genetic diversity of 64 tropical and subtropical onion accessions (Allium cepa L.) belonging to the Embrapa Hortaliças collection was estimated using 23 morphological, agronomic, and biochemical descriptors. Methods of multivariate statistical analysis (Mahalanobis' D² distances, Tocher's cluster analysis and nearest-neighbor method) were employed in order to obtain the average genetic distances among accessions and to perform the clustering. The onion germplasm displayed a relatively large genetic base. Cultivars EX 3000 and Régia were the two most similar accessions. The highest mean genetic distance was observed between cultivars VAL 14 and Beta Cristal. The Tocher's optimization method revealed the formation of 13 groups, whereas the nearest-neighbor method revealed the formation of 12 groups (cutting point of 43% of dissimilarity. The majority of the accessions belonging to Grano and Granex cultivars (agronomically classified as short-day and early-maturing types) clustered together in the same group. The white bulb accessions for processing (Beta Cristal; Dehydrator # 8; Dehydrator # 6; Primero and White Creole), with the exception of Tampico White, displayed a small genetic divergence. The accessions TX 24, Baia Periforme Super Precoce, EX 3001, Excel, Red Creole, H 688, Franciscana IPA 10 and Roxa IPA 3 displayed a large genetic divergence. The descriptors total sugar content, bulb length, number of days from planting to harvesting (i.e. plant cycle), percentage of bulbs with transversal diameter between 70 and 90 mm, and average bulb weight were responsible for 58% of the genetic variability present among accessions. Studies of divergence among accessions are important to choose divergent parentals but complementary genetic materials would allow the development of useful segregating populations with wide genetic basis for tropical onion breeding programs.
Allium cepa L.; genetic distance; tropical onion; sub-tropical onion; grouping analysis; germplasm
