Farmers are researching the best way to grow crops of yacon, an exotic tuber from the Andes that is increasingly being marketed to affluent consumers as a functional food with special health benefits.

The Yacon Growers Group, which includes Gisborne's Sunshine Coast and NZ Biotechnology, has planted trial crops in different soil types and weather conditions around Gisborne, Hawkes Bay and Waikato.

It is using a $120,000 "agribusiness innovation" grant from the Agricultural and Marketing Research Development Trust to fund research costing more than $200,000.

Bruce Clark, of Sunshine Coast, said the yacon growers were testing production systems for the potato-like vegetable to be exported to Japan and Korea.

Yacon is a South American root vegetable with dark brown skin that looks like an elongated potato, and is crunchy like a water chestnut, but refreshingly sweet and juicy.

Left in the sun, its sweetness intensifies, and it can be eaten as a fruit, consumed in drinks, syrups, cakes or pickles or in stir-fries.

The sweetness comes from high levels of fructo-oligosaccharide, a sugar the human body does not metabolise, so it can be eaten by diabetics, and it is also reported to play a role in improving the health of the colon.

The tuber has been the subject of considerable controversy in South America, where it has been cultivated for centuries as a food, because in 1999 the Japanese Government sought from the International Potato Centre in Peru, germplasm from which it could grow new plants.

The Japanese interest stemmed from New Zealand exports of yacon - scientifically known as Smallanthus sonchifolius and also known as Bolivian sunroot - which were reported in scientific papers to have been imported from the Andes, outside Peru. Research work done in Japan on the New Zealand yacon revealed up to 65 per cent of the carbohydrate content was fructo-oligosaccharides, and laboratory work since then has shown that substances in the vegetable are good for the gut, potentially safeguard against cancer, help absorption of calcium and vitamins and can lessen the blood sugar peaks that trouble diabetics.

Its potential status as a "super food" promoted extensive Japanese interest in it, which led to the controversial bid to acquire germplasm from Peru, an effort labelled by some critics as "bio-piracy".

Yacon expert Michael Hermann, at the International Potato Centre, has described it as both a diet food and a diabetic food because the specific fruit sugars in yacon syrup have about half the calories as an equivalent amount of honey.

Yacon is cheap and easy to grow and contains little protein.