Botanists find a cool way to preserve an endangered plant's seeds

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Juglans cinerea is the butternut or white walnut. You’d typically find it in the eastern United States, but it’s getting harder to find. That’s thanks to Ophiognomonia clavigignenti-juglandacearum, a fungus that causes butternut canker, a fatal disease for the tree. Now butternut is endangered. Botanists are gathering seeds to help preserve the plant, but this hasn’t been a huge success because the seeds are difficult to conserve with the usual methods. Now Martin Williams and colleagues have published a new method with a 76% survival rate after seven years. The seeds are cryopreserved.

Cryopreservation means storing the seeds in liquid nitrogen, which boils at -196 ℃. The team used seeds from trees in an ex-situ conservation project in New Brunswick, Canada. The team pulled seeds that had been stored for zero, two and seven years to see how well they germinated.

Top left, a pair of test tubes with axes in them, then a pair of test tubes with germinated seedlings showing a strong stem and radicle. Top-right. a black tray of seedlings. Bottom-left, a tray of considerably bushier seedlings and finally Bottom-right, the seedlings sit in individual pots. — Germination and plantlet growth development of *Juglans cinerea* embryogenic axes after 7 years of cryopreservation. (A) – embryogenic axis after inoculation in WPM medium, (B) – embryogenic axis with visible root pushing off seed coat after 5 days post inoculation, (C) – embryogenic axes showing different levels of root and shoot elongation after 4 weeks post inoculation, (D) – embryogenic axes transfer from culture tube to soil, (E) – plantlet growth after 8 weeks post inoculation, (F) – final transfer to single pots showing fully acclimatized plants. Source: Williams *et al*. 2023.

They found a remarkable number of the seeds germinated, around three-quarters. Also, they discovered there was no obvious decline in the seeds after storage in liquid nitrogen, but the authors point out there are limits to this study. They write: “[W]e did not find any significant viability loss due to cryostorage after 7 years, but we do acknowledge that this is only based on one seed lot. However, the high viability of all the seed lots after 7 years post-LN is encouraging and points to minimal loss due to cryostorage conditions.”

“Although seven years is not considered very long when compared to orthodox seed sources that can be kept for 20+ years, it is better than the current storage duration for the species and validates the efforts done in safeguarding butternut through cryopreservation.”

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Williams, M., Brophy, M. and van der Meer, B.M. (2023) “Cryogenic storage increases the longevity of butternut (Juglans cinerea, L.) seed embryogenic axes,” Cryobiology. Available at: https://doi.org/10.1016/j.cryobiol.2023.01.002.