Nuts ain't nuts: fine-tuning the macadamia industry

In 1881 an American, William Purvis, pocketed a handful of macadamia nuts from a tree in the Gold Coast hinterland and spirited them away to Hawaii.

It's now folklore that Australia gave Hawaii the kernel of a lucrative new industry, and ended up with the shell.

It still rankles that Australia's peerless native nut is still widely known as the 'Hawaiian nut', but Australia recently overtook Hawaii as the world's largest producer, and Australian interests now own many of Hawaii's farms.

The Australian industry was largely founded on high-yielding Hawaiian cultivars. Purvis' precious pocketful appears to have been a local form of one of the two edible species, Macadamia integrifolia; most Hawaiian cultivars are of M. integrifolia ancestry.

In contrast, many Australian cultivars are of mixed ancestry, being derived from hybrids between M. integrifolia and M. tetraphylla.

Today more than 2 million trees, representing some 700 named cultivars and 300 different genotypes, are grown in orchards along the east coast between Gladstone and Lismore, according to Dr Cameron McConchie, of CSIRO Tropical Agriculture in Brisbane. Twenty selections account for the bulk of production. McConchie says some of the original Hawaiian cultivars grow rapidly to reach 10-15 metres at maturity, making them difficult to spray and harvest. Others are susceptible to disease or insect pests, or have other undesirable production traits.

He says such variety is remarkable for an industry still in its infancy, but the macadamia remains relatively untamed by comparison with other nut species like the pistachio, cashew almond and walnut.

He says there is still enormous potential within the gene pool of cultivated and wild macadamias to improve and diversify the crop, to cater for specialised markets or differing consumer preferences.

How much room? Using the latest DNA-based fingerprinting techniques, CSIRO and University of Queensland researchers sampled and genotyped all the cultivars currently grown in Australia and overseas, and all the remaining wild populations of the four Macadamia species that grow in between Maryborough, in south-east Queensland, and Ballina, on the NSW north coast.

McConchie says the data indicate that cultivated macadamias represent only 60 per cent of the wild genetic diversity of the genus.

That 60 per cent still offers enormous potential for hybridists in major producer countries like Australia, Hawaii, California, and South Africa to develop new cultivars.

But the remaining 40 per cent is available only to local breeders, and, cleverly exploited, should ensure Australia will remain the industry leader in the 21st century, McConchie believes.

The CSIRO-UQ study used the polymerase chain reaction to genotype DNA fragments from the macadamias.

McConchie says the study has yielded "massive amounts of information" that have clarified a turbid gene pool.

'We've been able to work out where each species grows, where natural hybrid populations occur, which trees are probably escapees from gardens or farms, and where the trees now in cultivation originally came from.

Just one of the 700 "name" varieties, a cultivar designated 791, contains genes from a third species -- M. ternifolia. It contributed about 40 per cent of the genes in 791.

M. ternifolia is a smaller tree, with smaller nuts, made toxic by cyanogenic compounds. Nuts from 791 are smaller than normal, but non-toxic, auguring well for a proposal to breed a peanut-sized macadamia as a snack food.

A fourth species, M. jansenii, unknown until a small population was discovered in a valley north of Bundaberg, is not represented in any cultivar.

McConchie says all four of the southern species can hybridise, but only three are represented in natural hybrid populations in the wild -- the remnant M. jansenii population is isolated from the other species.

McConchie's group performed some experimental crosses between 12 selected breeding lines in 1993, including a successful cross between M. jansenii and M. integrifolia cross.

Specialist plant breeder Dr Craig Hardner was appointed in 1997 to develop a broader systematic hybridisation program involving 40 parent lines -- all of them commercial cultivars.

In parallel with the breeding program, McConchie's team has established a comprehensive germplasm collection from the remaining wild populations of the four species and their hybrids.

He says the Australian Macadamia Society, which is helping to fund the group's research, is interested in developing smaller, high-yielding trees with a range of nut sizes -- small, medium and large -- for different markets.

Results of a parallel research project into nut quality, and the chemical changes that occur during roasting, will also be incorporated into the breeding project.

"With the macadamia market becoming more sophisticated, the industry wants to differentiate its product range," McConchie says. "The Japanese, for example, prefer a lightly roasted nut, while the Germans prefer a darker roast. The roast influences the flavour."

McConchie says that in the past, most Australian macadamias were exported raw. When the industry began roasting nuts for export a few years ago, it encountered a problem with nuts darkening and developing off-flavours.

Researchers at the Bread Research Institute in Sydney traced the problem to a reaction between sugars and amino acids in nut proteins that occurred during roasting. Research into how seasonal conditions and cultivation techniques and growers' methods interact with genetic factors will help breeders minimise the problem in new cultivars.

Hardner says the breeding program is emphasising traits like high yield and high kernel-to-shell ratio, resistance to pests and diseases and improving kernel quality.

Some of the new hybrids from the breeding program will be planted out next year; they should begin bearing in three to five years, and the first new cultivars should become available to the industry from around 2010 onwards.

Before any wild parents can be introduced to the breeding program, Hardner says it's necessary to determine how they perform under orchard conditions in their own right.

"We have taken the material we have collected from the wild and propagated plants from it to do some assessments that will indicate their potential as a source of new genes," he said.

"Macadamias grow very differently in the wild to the way they grow in plantations when water and nutrients are freely available, and there's little competition. For example, macadamias flower very little in the wild, yet can flower prolifically in cultivation."