Genetic variation for bulb size, soluble solids content and pungency in the Spanish sweet onion variety Fuentes de Ebro. Response to selection for low pungency C. M ALLOR 1,4 , M. B ALCELLS 1 , F. M ALLOR 2 and E. S ALES 3 1 Centro de Investigacio´ n y Tecnologı´a Agroalimentaria de Arago´ n, Avda,Montan˜ ana930,50059 Zaragoza, Spain; 2 Universidad Pu´ blica de Navarra, Campus de Arrosadı´a, 31006 Pamplona; 3 Dpt. Agricultura y Economı´a Agraria, Universidad de Zaragoza, Escuela Polite´cnica Superior, Ctra. Cuarte s/n 22071 Huesca, Spain; 4 Corresponding author, E-mail: cmallor@aragon.es With 4 tables Received April 15, 2009/Accepted October 16, 2009 Communicated by M. Havey Abstract The cultivar ÔFuentes de EbroÕ is a long-day onion grown in the northeast of Spain, which is characterized by its succulence and low pungency. However, to match the market demand the size, pungency, and storability need to be improved. We have evaluated these quality-related bulb traits in 15 growerÕs open-pollinated lines of this cultivar. Phenotypic variation observed for bulb weight, size and soluble solids content was significantly affected by location, growing season and line, while pungency depended on line and plot location. We found higher levels of genetic variation for bulb size and pungency than for soluble solids content, and significant phenotypic correlations indicated that milder onion tend to show larger size and lower soluble solids content. After one cycle of selection, we have obtained progeny with significantly lower pungency levels, and therefore, we estimated a realized heritability of 0.67. As conclusion, it is feasible to obtain a sweet cultivar after some selection cycles in the ÔFuentes de EbroÕ onion, although considering indirect consequences in size and soluble solids content. Key words: Allium cepa — breeding — flavour — heritability — long-day onion Onion (Allium cepa L.) is the second most valuable vegetable in the world, following only tomato. It is one of the oldest cultivated vegetables, being primarily consumed because of their unique flavour or for their ability to enhance the flavour of other foods, since onion bulbs do not have a highly significant nutritional value (Randle 1997). Onions are also recognized for their health-giving properties such as providing no digestible dietary fibre from fructans, antiplatelet activity from organosulfur compounds and antioxidant activity from flavonoids like quercetin (Galmarini et al. 2001). Selection by growers and breeders has produced cultivars that vary widely in adaptation, sweetness, storability and processing quality. Bulb composition, namely its content in carbohydrates (glucose, fructose, sucrose and fructans) and sulphur derivatives (1-propenyl, methyl- and propyl-cysteine sulfoxides; Randle and Lancaster 2002), determines its quality for an intended use. Varieties known as Ôstorage onionsÕ have high dry matter content (15–20%) and relatively high amounts of fructans but lower levels of reducing sugars, besides relatively high rates of organosulfur compounds. They have an intense, pungent flavour and desirable characteristics for cooking or for industrial processing. ÔFresh onionsÕ are consumed directly in salads as they do not store well because of its low dry matter content (5–9%), particularly in fructans. They have a mild or sweet flavour, as they contain low levels of organosulfur compounds which are related to pungency. Less pungent onion varieties with higher content of reducing sugars, known as Ôsweet onionsÕ have become preferred for fresh consumption, especially in the USA, Japan and many coun- tries of Europe. The repeating demand of the consumer has generated a large niche market (Pike 1997, Phaff 2007). Breeders involved in the development of sweet onions have first to choose cultivars with low pungency, since pungent flavour can mask a high level of sugars, so that the onion is not perceived as sweet. The genetic basis of pungency has been studied for more than 50 years and heritability estimates of 0.4–0.7, with predominantly additive effects, have been reported for this quantitatively inherited trait (Lin et al. 1995, Simon 1995, Wall et al. 1996, Galmarini et al. 2001). More recently, molecular markers have contributed to find in the large genome of this plant species regions regulating bulb quality traits. Two quantitative trait loci have been found to exert pleiotropic effects on pungency, dry matter and soluble solids content (Galmarini et al. 2001, McCallum et al. 2007). These findings constitute genetic evidence for experimental data, since dry matter content, soluble solids content and pungency show significant positive phenotypic correlations (Galmarini et al. 2001). Development of sweet onions with good storage ability (high dry matter content) is therefore a difficult challenge for breeders. Dry matter content is usually estimated by soluble solids content, a character that shows high heritability rates (0.6–0.8). Quantitative trait loci affecting fructans and reducing sugar contents have also been reported (Havey et al. 2004, McCallum et al. 2006). Although onion flavour has a genetic component, environ- mental factors such as water supply, temperature, and espe- cially nitrogen and sulphur fertility can affect flavour intensity (Randle 1997, Hamilton et al. 1998, McCallum et al. 2005, Yoo et al. 2006). Pungency can be determined indirectly using a colorimetric test for pyruvic acid concentration, since this compound is formed in the reaction that produces the volatile sulphur compounds that are responsible for onion flavour. The National Onion Labs Inc. (NOL) certificates in the USA sweet onions according to its pyruvic acid content: varieties are described as Ôvery mildÕ or Ôextra-sweetÕ when pyruvic acid content is lower than 3.5 lmol/g FW and as ÔmildÕ or ÔsweetÕ when this content is 3.6–5.5 lmol/g FW. Besides, they consider bulbs Ôof moderate pungencyÕ when pyruvic acid content is 5.6–7.5 lmol/g FW and ÔpungentÕ when this content is higher than 7.5 lmol/g FW, respectively. Plant Breeding 130, 55—59 (2011) doi:10.1111/j.1439-0523.2009.01737.x Ó 2010 Blackwell Verlag GmbH wileyonlinelibrary.com