Breeding Superior Onions

Onions are the third largest vegetable crop in the world and have been grown and bred for thousands of years. Around 90 million tons of onions are produced globally each year, but genetic studies of onions have been limited. Research is now allowing industry to tap into more diverse genetic resources and breed products adapted to different and changing environments. Onions are biennial plants. When it matures, it stores food reserves in the bulb. The leaves die back and it’s at this point that the onion is harvested. If left in the soil, the middle point of the bulb begins to develop new leaves and the plant will flower.

In New Zealand onions grow in Franklin/Waikato, the Hawke’s Bay, Canterbury and Manawatu/ Horowhenua with around 4,500 hectares in commercial production.  They are our second largest vegetable crop, with around 600,000 tons produced each year, generating in excess of $60 million in exports.

Onions form a bulb in response to lengthening days, but until recently, the mechanisms controlling this response weren’t known. Research undertaken by Plant & Food Research and the University of Otago has identified the gene that controls bulb development, the first step in discovering genetic markers that can be used as tools to screen conventional breeding programmes for new onion varieties with the right genetic profile.

Plant & Food scientist John McCallum says the research is about what makes an onion an onion and what makes it useful for people. If it produces a seed and not a bulb, it’s useless as a consumer product. The aim is also to help breeders produce bulbs in different growing environments. John says some of the unknowns until recently were how they sense winter and “know” when it’s time to make a bulb. What has been discovered is that both temperature and day length play a big part in bulb production.

John’s research is an excellent example of how new genome technologies can enable major discoveries that, in the past, have been difficult. By understanding how onions control development of the bulb, researchers can support the breeding of new cultivars that have the right genetic profile to respond to specific growing conditions, ensuring each plant produces a bulb for sale on the market. For example a single gene has been identified as critical to both the sweetness and storability of onions.

In research undertaken in 2006, John identified the gene responsible for sugar content in onions. He says there is a relationship between sweetness and dry matter level, which is important for onion storability. This research was one of the first major practical applications of the onion gene map. John says the research is applicable in areas other than onions.

Grower Peter Ryan has been tied up with breeding his whole lifetime. He’s now 74 and claims onion breeding is his passion. He says what John has achieved with his recent research is shortening the development of new cultivars by years.