Recalcitrant seeds become tolerant

31 October 2006

Dutch-sponsored researcher Oblé Neya has developed techniques to extend the lifespan of dried seeds from tropical trees. That is good news for the reforestation of the Sahel countries. According to Neya these seeds are more tolerant to desiccation than was previously thought.

Recalcitrant seeds are seeds that do not or only poorly germinate after they have been dried and/or dry stored. These seeds are widely prevalent in the tropics and are often sensitive to cold. It was assumed that they do not tolerate desiccation well. Oblé Neya, who used modern, physical techniques to test the viability of seeds of four different tree species from his home country of Burkina Faso, discovered that there are various reasons for their controversial germination behaviour.

Village pharmacy

Seeds from the Neem tree (due to its considerable healing power also called the 'village pharmacy') survive drying well but are sensitive to low temperatures if they come into contact with water. The longer they remain dried for, the more sensitive they become to rehydration. Yet according to Neya, the negative effects of rapid water uptake are less if rehydration takes place at a temperature of between 25 and 35 degrees Celsius. Then the seeds can germinate well, as long as no more than 30 percent of their embryonic axes cells have incurred damage during rehydration.

In the case of seed from the Marula tree and the Sabga tree, fruit trees from which a range of useful products is made, the poor germination is due to other causes. For these species dormancy blocks the germination of fresh seeds (Marula) or of both fresh and dried seeds (Sabga). According to Neya that problem can be solved by opening the seed coat prior to germination or by waiting until the germination inhibitors are released upon drying or dry storage. All the seeds investigated were found to tolerate desiccation well and could easily survive dry storage for several years, especially at deep freeze temperatures.

Seed aging

In the temperature range zero to ten degrees Celsius the seeds age very quickly. Neya therefore calls for a detailed investigation into the specific reasons for the loss of the viability of tropical tree seeds at this temperature range, especially as they can tolerate desiccation much better than was previously thought. Drying is the ideal method for prolonging the lifespan of seeds. A supply of viable tree seeds is vitally important for efforts to counteract deforestation in the Sahel and for the preservation of genetic sources.

Neya's research formed part of the programme 'Conservation of tree seeds in tropical arid areas' (the Netherlands and Burkina Faso) of the Foundation for the Advancement of Tropical Research (NWO-WOTRO).

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For further information please contact:

• Oblé Neya (Wageningen University and Research Center, Centre National de semences forestières, Burkina Faso)
• t: +226 50 358013, neyaoble@hotmail.com  
• The doctoral thesis will be defended on 15 November 2006
• Supervisor Prof. L.H.W. van der Plas, associate supervisor Dr F.A. Hoekstra (Wageningen University)