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Kew's Millennium Seed Bank partnership discovers successful storage of recalcitrant sycamore seeds

Kew's Millennium Seed Bank partnership field research across Europe has found that sycamore seeds thrive in long and warm summers are more robust to cope with the freeze drying storage process.

Branch and leaves of the sycamore tree (Acer pseudoplatanus) (Image: Andrew McRobb, RBG Kew)

We’re hoping it may prove possible to keep sycamore seeds deep-frozen and yet viable for years at a stretch, using seeds collected from within the species’ native range.

Hugh Pritchard, Kew's MSBP

Sycamore seeds freeze dried

Great oaks from little acorns grow – as long as you keep those acorns fresh and moist. Once they’re dry, they’re finished. Their cell membranes rupture, toxins build up and not even the smallest oak will grow.

The seeds of thousands of other trees behave in a similar way. And if you can’t dry a seed, then you can’t freeze it and extend its storage life in a seed bank in the usual way.

Not surprisingly, finding ways of storing such ‘recalcitrant’ tree seeds has become a priority for researchers at Kew’s Millennium Seed Bank (MSBP). But there is some good news. In at least one apparently recalcitrant species, the sycamore (Acer pseudoplatanus), the seeds turn out to be far less vulnerable to desiccation when the tree grows in its native European haunts.

Although widespread in Britain today, sycamores were probably introduced in the 16th century and didn’t become naturalised until the 1700s.

Long warm summers

Working with colleagues in Scotland, Norway, Greece, Poland, Italy and France, Hugh Pritchard and Matthew Daws of the MSBP’s Seed Conservation Department have discovered that when summers are long and warm, sycamore seeds are not recalcitrant. Instead, the seeds become more physiologically robust and able to cope with greater degrees of drying, making them candidates for storage under conventional seed bank conditions.


The breakthrough came from field research in half a dozen locations across Europe, where individual sycamores were kitted out with data monitors. As the seeds matured, sensors measured environmental conditions, recording air temperature every 30 minutes.

The quality of the seeds depended on the total amount of heat the seeds experienced during their development: the more heat, the better the seeds. In southern Europe, where sycamores are natives, this ‘heat sum’ was about 50 per cent greater than in northern Europe (CryoLetters, vol 29, p189).

Differing seed development

This extra warmth made all the difference, says Pritchard. The researchers suspect that seeds produced by trees growing in their warmer native range in southern Europe were fully mature by the time they are shed, whereas those developing in northern climes fell from the trees before they were fully ripe.

This crucial difference in maturity may also help to explain why the seeds from crops such as tea, coffee and neem – all grown outside their native range – have sometimes been classified as recalcitrant, and sometimes not.

‘And the predictability of our results means that we can begin to predict the potential impact of future climate changes on seed development,’ says Pritchard.

Author: Gail Vines, Kew magazine

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