THE Millennium Seed Bank (MSB) bills itself as the most botanically diverse place on earth. Banking 10 per cent of the world’s wild plant species by 2010, the MSB currently holds approximately 2.25 billion seeds: 40,000 seed species in 93,000 individual collections coming from 190 countries.

The Wellcome Trust Millennium Building opened in 2000, although collections have been stored at Wakehurst since the 1970s.

Following the Future Trees Trust Annual Supporters Day Seminar, attendees were offered a whistle-stop tour of the facility, ‘from field to bank’. Our tour leaders were Steph Miles, who works with threatened species across the UK on the UK Flora Project, and Ian Willey, who works specifically with UK trees (UK National Tree Seed Project).

Forestry Journal: The Millennium Seed Bank building.The Millennium Seed Bank building.

Our first stop was the dry room where seed collections sent from around the world are held at 15 per cent relative humidity and 15°C. Most seeds will be dry enough to freeze within two to six weeks, although some hard-coated species, such as legume seeds, may need longer. 

Steph said: “The important thing is that, when kept cool and dry, a seed’s metabolic rate is minimised to the point where they are barely ageing.

“When seed collections arrive, they should be accompanied by comprehensive field data and a (pressed) herbarium specimen, taken from the same population, to verify the species we are banking.”

All incoming seed packets and field data are logged and, after drying, the collections are processed. Processing includes cleaning, using a variety of methods, to remove debris and empty and infested seeds. Samples of each collection are then X-rayed to examine seed quality.

Forestry Journal: Four glasshouses set at varying temperatures and humidity contain specimens grown on from seed collections submitted by partners across the globe.Four glasshouses set at varying temperatures and humidity contain specimens grown on from seed collections submitted by partners across the globe.

For species such as Salix (willow), whose seeds age quickly, the processing of catkins collected in the field to the seeds being stored (in the vault or cryo-preservation tanks) must take two weeks or less.

From the dry room, seeds are transferred downstairs to a pre-storage lab where they are kept for a month, again at a constant relative humidity of 15 per cent. This final drying stage is just in case they have picked up moisture while being worked on in the labs during processing.

Following a hygrometer test to check that the seeds are dry enough to freeze, they are banked in the vault at a constant temperature of minus 20 degrees centigrade (-20°C). 

For every 10 per cent decrease in seed equilibrium relative humidity a seed’s lifespan is doubled. For every 5-degree drop in temperature (down to -20°C) a seed’s lifespan is doubled.

Forestry Journal: Leucospermum conocarpodendron subsp. conocarpodendron.Leucospermum conocarpodendron subsp. conocarpodendron.

Just how long Salix will last (be capable of germination) in storage is not yet known. An MSB specialist is working on the germination protocols for all UK tree species to determine optimal requirements for germination, as well as their estimated longevity under standard storage conditions (-20°C). 

If further seed is required for a species or the collection quality starts to decline, rather than grow them on from existing stored material, collectors will – where possible – revisit the original plant populations and re-collect them.

“We would need to grow out a large number of plants to ensure that genetic diversity is included in an ex-situ population. It is usually more effective to re-collect from a wild population,” explained Ian.

The MSB is a working seed bank. In each collection, they aim to conserve 10,000 seeds from one population growing in one location. Where seeds are rare, this is not always achievable.

Steph said: “10,000 seeds gives us two collections, a [permanently stored] base collection and a large and usable active collection. Seeds in the active collection are accessed regularly, for germination tests and by researchers from around the world, as long as users can demonstrate that they are from a bona fide organisation and that their work will benefit research, conservation, restoration (reintroduction) or education.”

Forestry Journal: The germination lab.The germination lab.

Where a base collection contains 259 seeds or less, no seeds will be accessible in an active collection. “This is a continuing conundrum for us as we need seeds for ongoing monitoring of viability in storage. We are actively seeking to get bigger and better collections of those under-represented species where we can.”

For the UK programme, the MSB is working to establish multi-provenance collections, particularly of trees.

“It is not really sufficient to have species from just one location in the UK. We want to make sure we have those fragmented populations represented in the seed bank.” 

In the germination lab, Steph explained that germination testing is the MSB’s way of assessing the viability of a collection and establishing protocols to help with subsequent use of the seeds. The data also helps researchers to gain an understanding of how seeds behave after being placed in cold storage. For germination tests, seeds are placed on a Petri dish containing one per cent agar gel and placed in incubators at optimal temperatures for germination. Dormancy-breaking techniques may be required, such as temperature stratification or physically removing a piece of the seed coat, which is usually required, for example, for germination of legumes.

Forestry Journal: Central lab space around which cold rooms are located. Temperatures are set to -20°C, with the average temperature kept with a slight fluctuation of one degree. Protective clothing must be worn.Central lab space around which cold rooms are located. Temperatures are set to -20°C, with the average temperature kept with a slight fluctuation of one degree. Protective clothing must be worn.

For Salix, optimal storage protocols are still being worked on. A pre-storage test on seeds collected locally by Ian a few weeks ago will be repeated on two batches of seeds, one having been stored at -20°C and the other placed in the MSB’s cryo facilities, to see how both behave over time and which storage conditions they prefer.

The MSB’s cryo room houses three silver chest-height chambers installed five years ago. Running at  -196°C they contain seeds from families such as Orchidaceae (orchids) and Arecaceae (palms) where greater longevity may be achieved in cryo storage.

Steph said: “We will monitor the viability of this method of storage versus conventional (-20°C) over time. It is still early days.

“When seeds have passed germination tests with 85 per cent germination or more, the collection has an [established and] accepted protocol and will be monitored at 10-year intervals. Our oldest collections are 50 years old and we have found that germination viability is generally maintained over time. Very few are showing decreasing viability.”

In the research lab, senior research leader Hugh Pritchard elaborated on the areas being examined by staff in the Department of Comparative Plant and Fungal Biology.

“Our primary interest is looking at the functional traits in seeds, primarily because they enable us to look at germination performance in the natural environment. We have learned that seeds store meteorological signals from the year they developed and were banked and the trait plays out in the physiology of the seed as it performs in the natural environment.

“In the MSB, the environment control of germination is modelled and then validated through seed burial experiments. This enables a link to be made to the potential impacts of climate change (as projected in IPCC modelling scenarios). One example is seeds of the shrubby species Rhamnus persicifolia from the Sardinian mountains. It is possible, with a thermal model, to predict the cold temperature removal of seed dormancy through winter, the timing of the germination event in springtime, and how these events might change with global warming.”

Forestry Journal: Some seeds are contained in fruit heads and it can be too time consuming to separate one from the other. Where they are uncontaminated, they can be X-rayed ‘as is’ and stored as units.Some seeds are contained in fruit heads and it can be too time consuming to separate one from the other. Where they are uncontaminated, they can be X-rayed ‘as is’ and stored as units.

In addition to concerns about thermal thresholds for seed germination under differing climate change scenarios, scientists in comparative seed biology are running hydro-time (drought stress) and (most recently) a halo-time tests (salt stress).

“Thresholds vary with seed lots of species and provide insight to the species’ regeneration niche and competitiveness. How much adaptability there is amongst species is important and we can judge how that will relate to performance downstream.”

Another research focus is desiccation tolerance. Ian explained: “Quercus (oak) seeds are recalcitrant and desiccation sensitive and we cannot use a conventional bank to preserve them. What we are trying to do is not give the seed tissues long enough to die as the water is taken out by reducing tissue volume and increase the speed of drying to a few hours and then plunge the tissue (not the whole seed) into deep cooling. This system is not replicable for every recalcitrant seeded species, so we are investigating the morphological features of embryonic axes that might contribute to success.” 

One way to support the preservation of species with recalcitrance could be to preserve dried pollen from multiple paternal lines.

Hugh added: “It has relevance as the pollen could be used for siring. This has been tried for cryo-preserved pollen of Castanea dentata (American chestnut), a species that suffered from blight in the last century.”

Future research projects include looking at the important role that halophytes play in coastal resilience, for example in South East Asia. Not only do they occupy a particular niche and display traits present in only (about) one per cent of the world’s flowering plants, they grow in an environment increasingly subject to extreme tsunami and hurricane-type weather events. Ian said: “We are saying look at the environments that are most at risk, and the coastal zone is one that needs urgent attention.”

Finally, four glasshouses set at varying temperatures and humidities contain specimens grown on from seed collections submitted by partners across the globe.

“Plants are grown on for a few reasons,” said Steph. “When seed collections are not accompanied by a (pressed) herbarium specimen, we grow a number of plants to create that herbarium specimen for verification. We also work with conservation organisations across the UK – for example with Back from the Brink on their arable plant project – growing out threatened flora as part of this work. We host PhD students working with flora from their particular country who may need to grow out plants for their research.”

Forestry Journal: Cold storage in the vault.Cold storage in the vault.

One plant growing in these greenhouses supports the need to create herbarium specimens for verification and possibly the establishment of the MSB itself. In 1803, Jan Teerlink filled 41 packets with seeds collected from Cape Town and surrounding environs. These packets, kept in a red leather-bound wallet, survived life on a ship and its capture, storage in the Tower of London, a move to Chancery Lane and a further move to the National Archives, where researcher Roelof van Gelder rediscovered them in 2005.

The seeds were brought to the MSB and scientists tried a number of methods (including treating the seeds with liquid smoke in order to replicate Cape Region wildfires) to facilitate germination. Of the three seed species successfully germinated, what was initially thought to be Protea conocarpa grew into the mature Leucospermum conocarpodendron subsp. conocarpodendron we saw at the end our tour, 216 years later.