Over The Horizon

Island Citrus Mysteries

Scientists are helping to unravel the 150-year-old mysteries of Raoul Island, a speck in the South Pacific which is the focus of a major conservation project. The Department of Conservation is trying to establish the historic importance of the island by assessing its archaeology, flora and fauna, and HortResearch is helping with a study of citrus found there.

The island was used by early explorers and whalers, and the first settlers arrived in the 1850s. Harsh conditions, including floods, drought and cyclones meant it was populated only sporadically, and the last inhabitants left in 1914. Since then it has been a weather station.

Budwood cuttings from the island have been propagated by HortResearch's citrus breeding group at Kerikeri Research Centre.

"There are mandarins, oranges, grapefruit and citrons, all very old cultivars, but we don't even know their names. We need to find out more about them so we can decide what to do with them -- manage them in situ or perhaps to cultivate them somewhere else, closer to hand," says DoC's Carol West.

Andrew Harty, of the Plant Breeding Group, is excited about the project, citing the sheer age of the trees and the fact there's no record of where they came from or when.

"They are older than almost all the common cultivars grown today in New Zealand. With the diverse sea traffic that visited the island they may have come from Asia, which would make them of interest to us as Asia is an evolutionary centre of origin of citrus species," he says.

"They could have immediate commercial potential, or be useful in our citrus breeding programme."

The Kerikeri researchers will compare the growth and fruiting habit of the trees with others in their large collection of citrus cultivars. They want to see what time of season the fruit mature, how large and how sweet the fruit is, and for the mandarins, how easily the rind can be peeled.

"We would also like to test the virus status of the trees -- it could be that old' strains of citrus tristeza virus are resident in the trees that could be useful in our mild strain cross-protection programme -- that is, they could protect trees from severe strains of this aphid-spread disease, rather like the way flu inoculations work."

But the island is set to keep its secrets a while longer -- it will take four or five years to grow the trees and evaluate the fruit.