Lauren Fuge
New research from the Kings Park Seed Bank in Perth has shed light on the surprising life and death of seeds, important for the conservation of species and research into plants which can cope with climate change.
Seed banks around Australia store the seeds of thousands of species, but even when stored in ideal conditions, some species live longer than others – and even seeds within the same species can outlive each other.
“Seeds are living organisms, and they’re like human beings or any other animals – they age and die in storage,” Emma Dalziell, seed biologist at the University of Western Australia. “But we don’t have a really good understanding of how long we can actually store these seeds for.”
Since many of these seeds are vital for saving threatened species, restoring degraded ecosystems or ensuring the future of our agricultural crops, it’s crucial to know how long they can stay alive in a seed bank.
But the answer, as Dalziell and team found, is complex.
Seed bank scene-setting
The Kings Park Seed Bank is part of a network of frozen vaults that are protecting the future of plants on this continent. Called the Australian Seed Bank Partnership, the network stores millions of individual seeds in 14 facilities, from the wild species in places like Alice Springs Desert Park to the crop species in the Australian Grains Genebank in Horsham.
An estimated 10,000–15,000 species of Australia’s native plants are represented in these seed banks – a large proportion of the 24,000 known species in total.
“Seed banks are incredibly useful things because we can store a huge amount of genetic diversity of plant populations in a relatively small amount of space,” says Dalziell.
And as climate change, land clearing, pests and disease increasingly threaten Australia’s species, these collections act like ‘insurance policies’ to prevent extinction, support genetic diversity and help restoration efforts.
Building and maintaining a seed bank is a lot of hands-on work. Scientists collect seeds from the field, dry them, and store them in temperatures of around -18°C, which slows down the seeds’ metabolic processes.
In the wild, Dalziell explains, seeds are exposed to the weather, climate, pests and diseases. “If we get them out of the environment and store them under cool and dry conditions, we increase the life span exponentially,” she says.
But just how long a seed can last in storage is a tricky question. Not all seeds are created equal. Tough species like eucalypts or acacias might live 100s or even 1000s of years in a storage, but the majority of species are expected to live for 10s of years, and some species much less. This means there’s a limited time that scientists can take the seeds out of the bank and use them for research or conservation.
Knowing each seed’s “use-by” date is crucial for prioritising the time and resources of seed bank, so Dalziell and colleagues set out to discover how long different seeds can last in storage.
Their new paper in the Australian Journal of Botany, by CSIRO Publishing, found that seed lifespans can last wildly different times in the same storage conditions – even seeds from the same species.
The science of seed storage
The team tested seeds between 4 and 34 years old from the Kings Park Seed Bank, which holds more than 4000 species native to Western Australia.
Researchers first started collecting and storing seeds at Kings Park in the 1960s – not in a freezer, but in an old shed at the back of the nursery, which could heat up to more than 40°C (104 °F) in summer.
“[The seeds] weren’t dry, they weren’t cool, so they certainly weren’t stored in a way that we would expect to increase their lifespan,” Dalziell says.
The team tested these older seeds along with many more samples from modern collections. Instead of waiting for seeds to age naturally and die, the team put the seeds through “rapid ageing” process by exposing them to hot, wet environmental conditions.
Then they checked whether the seeds were alive or dead. This required a sacrifice, because the process requires cutting open the seeds to inspect the health of the tissues inside.
“We essentially perform what would be the equivalent of a post-mortem examination on humans or on animals on our seeds,” Dalziell says. “We’ll actually dissect them out and have a look at the tissues inside the seeds.”
Of course, the seeds then can’t be returned to the collection.
“It is a destructive process,” Dalziell says. “It’s part of the million-dollar question in seed biology at the moment – can we get [a] non-destructive technique going that will allow us to just take a measurement of those seeds, figure out how many are alive or dead, and then they can go back into the bank?”
But back to the original research question: how long did the rapidly aged seeds live?
When testing the early collections (the ones from the hot shed), the team was surprised to find that although some species had predictably perished, the seeds of other species were doing remarkably well, despite being stored in such punishing conditions.
Many of the other older seeds also had higher survival rates than more recently collected seeds. For example, 80% of the small yellow WA wildflower Myriocephalus gueriniae seeds collected in the 1990s were alive, compared to less than 20% of M. gueriniae seeds collected more recently.
And some collections – like the slender, flowering herb species Hyalosperma cotula, Podotheca angustifolia and Trachymene pilosa – proved hardy regardless of how long they had been stored.
Interestingly, with the exception of a few examples of poor storage, the team found that there wasn’t a clear, consistent link between storage conditions and seed longevity.
So what does affect a seed’s life span?
Lots of factors, according to Dalziell: “It’s even down to the maternal environment of the mother plant – so whether that mother plant was exposed to a particularly contracted drought, or periods of high temperature, could potentially result in her passing down a shorter lifespan to her seeds.”
Seed storage is also impacted by how they are collected in the field. Perhaps researchers took the seeds while they were still too immature – though this is difficult to determine for historically collected seeds, due to lack of records.
Seeding the future
The fundamental takeaway from the new research, Dalziell says, is that “all of our seed collections are potentially valuable, irrespective of how old they were or how well they were packed in the past.
“It’s really important that we have the resources available to be able to continuously test these collections to make sure they continue doing well into the future.”
Understanding the lifespan of different species is a key step to prioritise resources.
The Kings Park Seed Bank currently contains 14,000 individual collections from more than 4000 species, with more species collected and stored every year as plants become more and more threatened.
“But that entire collection is all aging and dying at different rates, and somehow we’re supposed to be able to manage that,” Dalziell says.
“We’ve got to make sure that we’re looking after those seeds that have got short life spans, and making sure that the collection doesn’t go to waste and just die on us without us understanding it.”