Crop Type: Garden Crop
Breeding system: Predominantly allogamous
Allium cepa L. is a biennial but often cultivated as annual species, belonging to Alliaceae (Liliaceae) family. It is the second most important horticultural crop worldwide, after tomatoes, with annual world production in 2017 around 97 million tonnes (FAOSTAT, 2019) and it is produced over an area of about 5.2 million hectares, mainly in Asia (66% of the world production with China and India the largest onion-producing countries in the world). Onion is a vegetable that has been widely used, for more than 4000 years, as food and for medical purposes. The maximum diversity of Allium species is found in a belt from the Mediterranean basin to Iran and Afghanistan, indicating the primary centre of origin. The earliest record comes from Egypt where onions appear as carvings on pyramid walls and in tombs from the third and fourth dynasties (2700 BC). It is thought that the Romans took the onion north of the Alps. The onion was among the first cultivated plants taken to the Americas from Europe. Europeans took the species to East Asia during the last century (Lawande 2012). The distinctive flavour and pungency of onions, which are due to various sulphur compounds, has established the plants as an essential part of the cuisine of the world. It is used as immature or mature bulbs, as vegetable and spice. Onion cultivation focuses predominantly on the production of bulbs and secondly on the production and consumption of green plants (fresh onions), pickles (small size bulbs) and powder or onion flakes (dehydrated bulbs) (Bletsos 2012). It is consumed uncooked in sandwiches and salads, but is also often cooked before eating (Benítez et al. 2011; Lawande 2012).
Very wide variation in shape, size and colour is observed in this species. Intensive selection during domestication and natural hybridization might have created variability (Lawande 2012). Also onion’s chemical composition is variable and depends on cultivar, season of production, stage of maturation, environment, agronomic conditions, storage time, and bulb section (Abayomi and Terry 2009; Rodriguez et al. 2009; Downes et al. 2010). Onions are rich in several groups of plant compounds, such as dietary fiber (DF), fructo-oligosaccharides (FOS), flavonoids, and alk(en)yl cystein sulfoxides (ACSOs) that have perceived benefits to human health (Mogren et al. 2007; Kosseva 2013). Water makes up the majority (80–95%) of the fresh weight of onion. Up to 65% or more of the dry weight may be in the form of non-structural carbohydrates (NSC), which include glucose, fructose, sucrose, and FOS (Davis et al. 2007). Moreover, brown skin showed high concentrations of quercetin aglycone and calcium, and top/bottom showed high concentrations of magnesium (Mg), iron (Fe), zinc (Zn), and manganese (Mn). Moreover, onion is one of the major sources of dietary flavonoids in many countries (Ly et al, 2005).
Onion is a predominantly allogamous species. Plants are propagated by seeds or vegetatively by sets (small bulbs) or bulbs.
References- Abayomi LA, Terry LA (2009) Implications of spatial and temporal changes in concentration of pyruvate and glucose in onion (Allium cepa L.) bulbs during controlled atmosphere storage. J Sci Food Agric 89: 683-687.
- Benítez V, Mollá E, Martín-Cabrejas MA, López-Andréu JF, Downes K, Terry LK, Esteban RM (2011) Study of bioactive compound content in different onion sections. Plant Foods for Human Nutrition 66 (1): 48-57.
- Bletsos F (2012) Onion. In: Practical vegetable production and local varieties. Zitis pubs, Thessaloniki, Greece, pp. 407 (in greek).
- Davis F, Terry LA, Chope GA, Faul CFJ (2007) Effect of Extraction Procedure on Measured Sugar Concentrations in Onion (Allium cepa L.) Bulbs J Agric Food Chem 55: 4299−4306 https://pubs.acs.org/doi/pdf/10.1021/jf063170p
- Downes K, Chope GA, Terry LA (2010) Postharvest application of ethylene and 1-methylcyclopropene either before or after curing affects onion (Allium cepa L.) bulb quality during long term cold storage. Postharvest Biol Technol 55: 36-44.
- FAOSTAT (2019) Food and Agriculture Organization of the United Nations, FAOSTAT database.
- Kosseva MR (2013) In: Food Industry Wastes Assessment and Recuperation of Commodities, Chapter 3-Sources, Characterization, and Composition of Food Industry Wastes. Academic Press, pp: 37-60. Doi: 10.1016/B978-0-12-391921-2.00003-2
- Lawande KE. In Handbook of Herbs and Spices (Second Edition), Volume 1, 2012, Peter KV eds, pp: 417-429. Doi:10.1533/9780857095671.417
- Ly TN, Hazama C, Shimoyamada M, Ando H, Kato K, Yamauchi R (2005) Antioxidative compounds from the Outer Scales of Onion. J Agric Food Chem 53: 8183-8189.
- Mogren LM, Olsson ME, Gertsson UE (2007) Effects of cultivar, lifting time and nitrogen fertiliser level on quercetin content in onion (Allium cepa L.) at lifting. J Sci Food Agric 87: 470-476.
- Rodríguez Galdón C, Tascón Rodríguez C, Rodríguez Rodríguez EM, Díaz Romero C (2009) Fructans and major compounds in onion cultivars (Allium cepa). J Food Comp Anal 22: 25–32.