Characterization of genetic resources of onion (Allium cepa L.) from the Spanish secondary centre of diversity

Onions are the second most-valuable vegetable in the world. Despite its global culinary and economic significance, the knowledge of genetic diversity and resources is limited. The aim of this study was to morphologically and physico-chemically characterize eighty-six onion landraces from Spain, part of the secondary Mediterranean Centre of diversity. The evaluated traits in the bulb included: weight, shape, firmness, soluble solids content (SSC), pungency and sugars content (glucose, fructose and sucrose). The results evidenced a great variability in all the evaluated traits. Correlations were significant between pungency and SSC (r = 0.34), firmness (r = 0.32), and sucrose content (r = 0.34); between fructose and glucose contents (r = 0.79); between sucrose content and SSC (r = 0.57); between SSC and weight (r = -0.35); between fructose and sucrose contents (r = -0.22); and between weight and sucrose content (r = -0.43). The combination of cluster and discriminant analyses resulted in a classification into four clusters (95.3% fit). Cluster 1 is represented by firm, pungent and large-sized bulbs; cluster 2 consists of large-sized, mild and sweet onions; cluster 3 is constituted by pungent, high-SSC and small-sized bulbs and cluster 4 is made up by elongated bulbs. The variability in important agronomical traits found point out that these onion accessions could be candidates for future breeding programs. In addition, the clustering could make the initial plant material selection easier and the correlations among the evaluated traits found could help to establish adequate selection strategies.