- New research finds that seed banking alone is not sufficient to conserve the world’s threatened plant and tree species.
- According to a paper published in the journal Nature Plants this month, researchers at the UK’s Royal Botanic Gardens, Kew found that 36 percent of critically endangered species produce “recalcitrant seeds,” which means that they cannot tolerate being dried out and thus can’t be frozen at -20°C, the process required for them to be preserved in a seed bank.
- On the other hand, very few wild relatives of crop species and medicinal plants were found to be unsuitable for conventional seed banking.
New research finds that seed banking alone is not sufficient to conserve the world’s threatened plant and tree species.
According to a paper published in the journal Nature Plants this month, researchers at the UK’s Royal Botanic Gardens, Kew (Kew Gardens) found that 36 percent of critically endangered species produce “recalcitrant seeds,” which means that they cannot tolerate being dried out and thus can’t be frozen at -20°C, the process required for them to be preserved in a seed bank.
Target 8 of the Global Strategy for Plant Conservation (GSPC) calls for 75 percent of threatened plant species to be conserved ex situ (outside of natural habitats) by 2020. Because of the focus on seed banking in ex situ conservation, the researchers write in Nature Plants, “The 75% target is unattainable without urgent investment into alternative techniques.”
On the other hand, very few wild relatives of crop species and medicinal plants were found to be unsuitable for conventional seed banking. Target 9 of the GSPC requires that 70 percent of the genetic diversity of crops and their wild relatives, as well as other socioeconomically valuable plant species, be conserved.
“We found that the lists of crop wild relatives and medicinal plants are likely to only have a small number of species that can’t be stored in a conventional seed bank, and therefore that seed banking can make a strong contribution to achieving Target 9 of the GSPC,” lead author Sarah Wyse told Mongabay. Wyse was a scientist at Kew Gardens when the research was done, and is now with the Bio-Protection Research Centre at Lincoln University in New Zealand.
“However, we also found that the threatened species on the IUCN Red List contain a large proportion (36% for the “critically endangered” category) of species that cannot be conserved using traditional seed banking methods,” Wyse added. “This is likely to be because many of these threatened species are trees from tropical moist forests — the group of plants that contain the highest numbers of recalcitrant species. Many of the world’s most threatened species are those that can’t be preserved using conventional seed banks.”
Wyse explained that Kew Gardens has been looking into plant conservation for over 20 years and has compiled a database of information on close to 20,000 tree and plant species. The research team she led used that database, together with information on species’ habitats, plant and seed traits, and other factors, to develop a model for predicting what she called “likely seed storage behavior” — whether or not a species can be stored in a conventional seed bank. The model was published last year in the Annals of Botany.
“For our current research, we combined the existing data with predictions from our model for species that have not yet been tested to estimate the likely proportions of species on various plant lists that can be stored in conventional seed banks,” Wyse said. “These lists were the IUCN Red List, a list of known medicinal plants, crop wild relatives, and also a list of all known tree species.”
Recalcitrant seeds are shed from their mother plant with a high moisture content and already metabolically active, whereas orthodox seeds are dried-out upon reaching maturity and are shed with a low moisture content and in a dormant state, Wyse noted. “Species with recalcitrant seeds are typically main-canopy tree species from tropical moist forests. The theory is that there is an evolutionary advantage for recalcitrant seeds in these conditions — by remaining metabolically active they are able to germinate and the seedlings establish very quickly, giving them an advantage over seeds that need to ‘wake up’ before getting going.”
In the highly competitive conditions of a tropical forest’s understory, early establishment can make a big difference in a young seedling’s chances of survival, allowing it to hold its own in newly opened light gaps, for instance. “In tropical forests, where the environmental conditions are pretty stable throughout the year, the seeds don’t need to have a dormant phase to survive over winter, unlike in temperate regions,” Wyse said. Oaks are a temperate species that produces recalcitrant seeds, but the theory for why they do so is similar to the theory for tropical tree species: “They germinate and put down a tap root very quickly, then the developing seedling enters a dormant phase over winter. Come spring the root is already down and it can quickly put up its leaves and get going — again giving it the advantage of time over its competitors when it comes to monopolising light gaps.”
Wyse and her co-authors suggest cryopreservation — which involves removing the embryo from a seed and then using liquid nitrogen to freeze it at -196°C — as an alternative to seed banking. One of the reasons seed banking is considered such a good option for ex situ conservation is that it allows for the preservation of large numbers of individuals, and therefore genetic diversity, in a relatively small space. “Cryopreservation has this same benefit,” Wyse said. “However, it is much more research- and labour-intensive than conventional seed banking, and more costly. If it is a technique that is to become a major player in ex situ conservation we need to increase research efforts and investment.”
The researchers also emphasize the value of in situ conservation, in which species are preserved in their natural habitats. “Ex situ conservation like seed banks are just a final back stop, or for use in propagating species for restoration projects to get them back into the wild,” according to Wyse. “For tropical moist forests, for example, we suggest that it may well be naïve and dangerous to assume that ex situ conservation is a valid means of safeguarding these species from extinction — in situ conservation may be the only feasible tool and we really need to increase conservation efforts in these areas before it’s too late.”
Wyse also notes that, even if an estimated 36 percent of critically endangered species have recalcitrant, non-bankable seeds, the remaining 64 percent are likely to have orthodox, bankable seeds. “It’s a matter of knowing how individual species’ seeds will behave before we plan to collect and conserve them; and that’s what the predictive model we used here was designed to do.”
• Wyse, S. V., & Dickie, J. B. (2017). Taxonomic affinity, habitat and seed mass strongly predict seed desiccation response: a boosted regression trees analysis based on 17 539 species. Annals of botany, 121(1), 71-83. doi:10.1093/aob/mcx128
• Wyse, S. V., Dickie, J. B., & Willis, K. J. (2018). Seed banking not an option for many threatened plants. Nature Plants 4. doi:10.1038/s41477-018-0298-3
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