Threatened by myrtle rust and habitat loss, the swamp maire’s soggy seeds are proving a challenge to preserve, even with the help of liquid nitrogen

When the seeds of swamp maire ripen, the clock starts ticking for Karin van der Walt.

She has six weeks to perform the experiments that could potentially save the species from extinction before the seeds rot, or germinate.

Once widespread throughout swamps in warmer parts of New Zealand, the trees’ numbers have dwindled with the draining of wetlands. The arrival of myrtle rust has added urgency to the swamp maire’s plight. It’s now listed as a threatened species with the status of nationally critical.

Some pockets are classed as living dead – swamp zombies – existing, but in a slow, terminal decline.

Van der Walt is a conservation and science advisor at Otari Native Botanic Garden. For the past three years, the annual ripening of swamp maire seeds has sparked a flurry of activity as she tries to preserve seed as a species back-up plan.

The problem she’s battling is the trees’ seeds aren’t meant to last. 

“They just grow, fall off the tree and they need to germinate. If they can’t germinate they perish.” 

The seeds covered in pulp. Photo: Supplied

They’re termed as recalcitrant seeds by scientists. For a layperson, this translates to soggy, and soggy seeds don’t like to be dried. Soggy seeds also don’t like to be frozen, which is the conventional way seed banks work. Trying to freeze them creates destructive ice crystals.

Cryopreservation is the usual solution to this dilemma. With cryopreservation, seed can be frozen so rapidly ice can’t form. 

However, it’s not as simple as just chucking some seed in a vat of liquid nitrogen and keeping at between -160 C to -196 C. There’s preparation involved and techniques, and temperatures that work for some seed species don’t work for others. That’s why three years on, van der Walt is still working on a solution.

Hurry up, but be methodical

There are several steps in the process of successfully preserving seeds cryogenically. Each step needs to be tested individually to ensure the seed is still viable and then steps need to be tested as a whole.

“It needs to become a normal, vigorous seedling that can grow and become a normal functioning plant, otherwise you have not succeeded in cryopreservation.”

The embryo is removed from the seed, then even with cryopreservation, some moisture is removed, either through rapid drying, or with what van der Walt describes as antifreeze for plants. The embryo can also be snuggled into a physical barrier, a little like a capsule before freezing. Then it’s a matter of freezing them so quickly no ice can form.

“With liquid nitrogen, if you’ve got small enough material we can do it at up to 300 degrees per second. It’s freezing, literally in fractions of seconds.”

Thawing also has to take place at speed to avoid ice crystals, this is done with water at 40C.

Vials of embryos in protective capsules being removed from liquid nitrogen. Photo: Supplied

After three years of a mad rush of methodical experimentation when the seeds are viable, van der Walt has figured some of the steps out correctly.

Embryos have survived removal from seeds, rapid drying of 90 minutes, ‘antifreeze’ and being placed in capsules. Getting them past the freezing stage has been trickier

Embryos with antifreeze didn’t survive after being placed in liquid nitrogen. One year, some embryos in capsules survived freezing, other years they haven’t. It’s something that will be investigated further this year. 

Who is paying to save the zombie trees?

As well as time pressure, there’s money pressure.

Initially cash came from the Ministry for Primary Industry’s response to myrtle rust, which funded 21 projects. Plant and Food Research has helped out with consumables and funding and Wellington City Council has also been supportive of van der Walt’s time spent on the project.

This initial funding has come to an end, meaning the future is uncertain.

“From now on, I will need to look for funding to continue the work.”

While only one tree in Auckland has been reported to have myrtle rust, van der Walt doesn’t think this means the species is mainly escaping infection and efforts can be relaxed. She said she knows of no active surveillance of the scattered populations. Infection for some species can mean rapid death.

“They have species in Australia, big trees, that they are now recording total fatality of that tree within two years.”

She also warns other plant species in New Zealand have soggy seeds, and figuring out the intricacies of preserving each species will take plenty of time, as it has for swamp maire. Building an insurance plan of preserved seeds takes time and resources and very little is being done at present.

“I’m always worried about waiting until a disease strikes, or we see decline or something gets highly threatened before we give it attention to work on.”

She was out surveying trees this week and said the seeds were still too small to collect. In a few weeks she thinks they’ll be ready and the rush will begin. She’s hopeful this season will be the season where a successful method is found.

“We are a long way down the track and I’m feeling really positive for this upcoming season for a significant breakthrough.”

Read more:

Precarious doomsday solution for native plants

Botanic gardens a barometer for loss

Seed banking no lifeline for kauri

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