Seed banking no lifeline for kauri

New research has found traditional seed banking methods where seeds are dried and then frozen at -20 C is unsuitable for 21 percent of New Zealand’s native trees.

This includes seeds of iconic native species such as kauri which only last for a few years in traditional cold storage and some “recalcitrant” seeds which can’t be frozen at all including tōtara, rimu, tawa and swamp maire.

These seeds fail to survive the drying process due to their high-moisture content. If seeds can’t be dried, they can’t be traditionally frozen because ice crystals will destroy the cells.

“It’s not straight forward, it’s not just a matter of freezing the seeds and chucking them in a freezer.”

The alternative, cryopreservation, is time-consuming and expensive, requiring liquid nitrogen at -160 C to -196 C to freeze the seed material quickly so ice is unable to form.

Bio-Protection Research Centre’s Sarah Wyse has recently returned to New Zealand after working at Kew’s Royal Botanic Gardens. She is the lead author on the paper and warns cryopreservation comes with complications.

“It’s not straight forward, it’s not just a matter of freezing the seeds and chucking them in a freezer.”

Cryopreservation sometimes requires removing the embryo from the seed before this is placed in the liquid nitrogen. The embryo needs to be revived in a laboratory in culture before it can be planted.

Kauri seeds can be stored in a traditional seed bank, but they don’t last for very long. One estimate puts their shelf-life at around nine years.

“The lifespan of the seed in a seed bank will be shorter than the time for a seedling to become cone-bearing,” said Wyse.

Wyse said she’s not an expert on cryopreservation but thinks there is a chance cryopreservation could be used to extend the shelf-life of kauri seed.

“If the work was put into it, I don’t see why not.”

In New Zealand there is little work being put into cryopreservation.

Massey University leads the New Zealand Indigenous Flora Seed Bank project. While the bank relies on traditional storage, one student project is looking at the best way swamp maire, which is susceptible to myrtle rust, can be cryopreserved. Director of the seed bank project Craig McGill said it would be fair to say Massey is "developing" cryopreserving capability. 

He's not aware of any cryopreserving work under way to extend the shelf-life of kauri seeds. 

Different species have different sensitivity to cryopreservation. What works for one may not necessarily give the same result for another. Exact procedures to ensure the best success need to be established by species and genotypes.

McGill believes cryopreserving seed that can be dried, such as kauri, would be less complicated than seeds which can't be. 

"We're fairly certain kauri seed can be dried down. We would need to immerse them in cryo and take them out and make sure they do survive that immersion and reheating to room temperature."

AgResearch senior scientist and director of the Margot Forde Germplasm Centre, Doctor Kioumars Ghamkhar said the research paper highlighted the limitations of traditional seed banks for moist seeds.

"There are more than 1200 seed banks around the world, and for many of these the economic cost of cryopreservation may be too high. Therefore more frequent regeneration of the stored seed supply – by growing to a plant to create more seed – becomes important."

He said Crown research institutes are aware of the issue.

"This has been an ongoing discussion for some years, and there is no question that a combination of traditional seed banking and use of cryopreservation techniques are needed to ensure the preservation of a broad range of species into the future."

Even when seeds are preserved through seed banking or cryopreservation there are still potential problems. The small amount of material saved means a limited gene pool exists. Genetic diversity is an important factor in successfully dealing with disease, or climate changes. This may mean an entire population with a low natural resistance to a certain pathogen could be wiped out.

The research draws the conclusion, assuming seed banking is a conservation insurance policy, “may even be somewhat naïve and dangerous”.

“In-situ conservation may be the only feasible tool in the conservation tool-box for many such plants.”