Since September 2012 Anaité López (Instituto Nacional de Bosques, Guatemala), Tim Marks (Millennium Seed Bank) and Wolfgang Stuppy (Millennium Seed Bank, do visit his amazing blog) have been working at the Millennium Seed Bank to develop a long-term storage protocol for the seed of the Maya Nut tree (Brosimum alicastrum).

This is important because Maya Nut is a significant famine food for the rural poor in northern Central America and is being actively used in reforestation projects as it is fast growing and relatively tolerant of disturbance.  At the moment, however, it is not possible to store the seed for more than a couple of weeks making it very difficult to deploy seed as and when it is needed outside of the fruiting season.

Left: Maya Nut (Brosimum alicastrum) fruit - Fruit as it is when still on the tree

Right: Maya Nut (Brosimum alicastrum) seed - Seed once stripped of the fleshy outer layer by bats or birds (left), with both papery coverings removed (centre), with only the outer of the two papery covering removed (right)

The work involved collecting about 5,000 seeds from Maya Nut forests in the Peten of north eastern Guatemala. The seed was then shipped to the Millennium Seed Bank by courier where it was unpacked and studied by Anaité who had flown over from Guatemala for six weeks to undertake the work under the supervision of Tim Marks and Hugh Pritchard, specialists in seed research and storage. This was an opportunity for her to take back the skills and knowledge that she had learnt to Guatemala. The work has been funded by Defra through the Darwin Initiative (project 18-010) and is taking place incollaboration with the Maya Nut Institute.

A batch of Maya Nut seeds being germinated following controlled storage. Image courtesy of Anaité López.

Our plan was to devise a regime of cooling and drying the seed that would enable it to be cryogenically stored (frozen) and so stored for many years. To do this we needed to identify the best rate at which to drop the temperature and humidity whilst keeping the seed viable. This involved storing the seed at different temperatures and humidities and taking a batch out and germinating it every month.

Given that this seed does not survive for long in the wild and that it is protected by two very thin, papery coverings (see above) we expected it to be very prone to dessication and so sensitive to low humidity. In fact we found quite the reverse. Six months later we are still germinating seed that has retained most of its moisture after having being stored in a dessicant for all this time!

In order to understand how this could happen we contacted seed morphologist Wolfgang Stuppy, world expert on seed morphology. He made the following images of sections through the seed which show a thin but dense layer of cells that control waterloss from the seed:

Section through Maya Nut seed showing the dark outer layer that might reduce water loss. Image courtesy of Wolfgang Stuppy.

So we now have a partial explanation as to how the seeds can show such remarkable tolerance to drying out. It does, however beg two questions:

1. Why do seeds not survive under the conditions of the forest floor?
2. Why would a tropical tree that grows in the humid tropics develop such a remarkable resistance to drying out?

To be continued maybe...

I have just spent a week collecting samples of Brosimum alicastrum (Maya Nut) with colleague Tonya Lander in Panama. It was a bit scary at first as we were not sure that we would find it, or recognise it in the field. Although it is relatively common, tropical forests have a lot more different species of tree than you would find at home. It is not uncommon to find over 100 species of tree in a 100 m x 100 m patch of forest. Fortunately the leaves have quite distinctive venation (see below) and we were able to collect samples from most of the places that we went to.

Tonya and I are documenting geographical patterns in the populations of this tree across Central and South America with the aim of making reforestation with this species sustainable. We were especially keen to collect samples from both the Caribbean and Pacific coasts as further north in Mexico there appear to be physical differences in the bark and branching between these coasts. This involved a lot of driving, over 1,500 km and at one site in the Pacific province of Los Santos, avoiding a large number of crocodiles that were basking on the river bank close to which our tree was growing. Although to be fair to them they seem a lot more scared of us than we were of them!