DNA analysis will show whether three cultivars of kumara -- to which Maori have claimed the intellectual property rights -- arrived in New Zealand before Europeans, or are derived from varieties brought by early whalers and sealers.
Massey University scientist Andrew Clarke -- who last year made genetic comparisons of modern bottle gourds which produced new evidence of significant Polynesian contact with America -- is now using genetics to trace the family tree of kumara.
Evidence that ancient Polynesians ate kumara, native to South America has helped revive interest recently in whether early Pacific voyagers might have "discovered" or even settled in the Americas.
Mr Clarke has previously said the kumara was probably introduced by Polynesian voyagers who visited the coast of South America about 1000 years ago.
He has used a DNA technique to construct a "family tree" for kumara in Oceania. His research is seeking the patterns of sweet potato dispersal in Pacific: how many cultivars entered the Pacific, and to which islands they were introduced.
His research, under the supervision of Professor David Penny, has already used DNA analysis to show bottle gourds used in the Pacific and America were Asian in origin.
Mr Clarke said the owairaka red kumara -- the most common variety on supermarket shelves -- is definitely related to one introduced by Europeans in the 1860s.
"It's clear that Maori had some kumara varieties before European arrival but it is also clear that American varieties were introduced," he said.
"So we are looking to find exactly when this happened".
He is studying three varieties of kumara -- hutihuti, taputini and rekamaroa -- which are not grown commercially.
At the Waitangi Tribunal, the WAI 262 claim -- initially made 16 years ago by members of six different iwi -- was rooted in Maori claims to cultural and intellectual rights over kumara.
Del Wihongi of Te Rarawa and Saana Murray of Ngati Kuri expressed concern that traditional kumara cultivars were languishing in a "bank" of germplasm in Japan.
The kumara were in an New Zealand collection of hundreds from around the Pacific, with plants transferred to Japan in 1969. A duplicate collection of varieties was held in New Zealand.
In 1988, British celebrity botanist David Bellamy helped stage a trip to Japan to bring back nine cultivars of particular interest to New Zealand.
According to Ms Wihongi the hutihuti, rekamaroa and taputini are the last remaining varieties of the indigenous crop, and it is important Maori are recognised as guardians.
One of the last Crown witnesses in the WAI 262 case, historian Dr Ashley Gould, said it was unlikely that the original kumara variety brought to the country by Maori about 1000 years ago still survived.
He said the popularity of the new higher yielding varieties, introduced probably by American whalers in the early 19th century, apparently replaced the smaller variety of Maori kumara.
Dr Gould said the waina variety introduced in the 1860s -- with distinctive red skin and yellow or cream flesh -- was thought to be the parent of the owairaka red.
"In my view, rekamaroa and, in probability, the ... hutihuti, are not survivals of pre-contact varieties," he told the tribunal.
The modern commercial crop was based on three cultivars: the beauregard, a recent import, the toka toka gold, a 19th-century import, and the owairaka red.
"There is no link between commercial lines and any varieties assumed to have been present in New Zealand pre-contact," Dr Gould said.
The next step in the WAI 262 claim will be closing submissions by both sides -- to be presented between March 19 and 30 -- before the panel led by the tribunal chairman, the chief judge of the Maori Land Court, Joe Williams.
The claim is wide-ranging, covering exclusive and comprehensive rights to indigenous flora and fauna, as well as all Maori cultural knowledge, customs and practices.