Wetenschap - 4 april 2011

Knowing your onions: some thrive on less phosphate

Onions need large amounts of phosphate to grow well, but Wageningen researchers have found an onion that thrives on very little phosphate. Once this ingredient of artificial fertilizer becomes scarce, this will be a very interesting find for plant breeders.


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The onion has a shallow root system, which makes it difficult for it to extract sufficient nutrients from the soil. It can manage water-soluble phosphate but not phosphate that is bound to soil particles. There is still plenty of phosphate in Dutch soils but its availability will decrease over the coming decades.
At Wageningen UR Plant Breeding, Olga Scholten and a team of colleagues have been researching onion varieties: the variety mainly eaten here (Allium cepa), a leek-like onion from Asia (Allium fistulosum), a third type of onion, and hybrids of these varieties. She started by planting the onions in sterile soil in pots. Half of the plants were given a teaspoonful of mycorrhizae fungi, and the other half were not. These fungi extract phosphate from the soil for the onion in exchange for sugars. The plants reacted to the fungi in different ways, and those differences are genetically determined, claims Scholten in the March edition of Theoretical and Applied Genetics.
Scholten  then placed genetically identical varieties of onion in beds with either large or small amounts of phosphate. She identified three types of plant: those which grow better with higher phosphate applications, those which do not respond to extra phosphate and - surprise! - plants like Allium fistulosum which thrive on less phosphate. 'This last group seems to deal with phosphate in the soils very efficiently', says Scholten. 'And that is a very interesting characteristic.'
This year Scholten is going to repeat her field trials. If the results match, she aims to try to find out what mechanism underlies good phosphate uptake. Then plant breeders will be able to breed robust onions for consumption which also thrive on low phosphate applications.
Dutch vegetable breeders are interested in this research because they export large quantities of onion seeds. After the tomato, the onion is the biggest vegetable crop in the world.