How genebanks work
The most valuable food crops--wheat, rice, and maize, which between them provide all of us with half of our calories--produce seeds that are easy to store. So do many other valued crops. For conservation purposes, the seeds are merely cleaned and dried, and placed in a sealed jar or packet. For medium term storage (20 to 30 years), the seeds are maintained at the comparatively modest temperature of 5°C. For long-term storage (up to 100 years) they are kept at -18 to -20°C.

Cool dry seed should remain viable (capable of germination) for many years, and in some cases for decades or even centuries. But viability does decline over time so the seed must be planted and grown out every few years to provide a fresh stock. In addition, stocks of seed have to be replenished to meet demand on them by breeders, farmers and other users of the genebank collection. The plants need to be cultivated carefully and in environments to which they are well adapted so that they remain genetically 'true' to type. In the case of cross-pollinated species, such as maize, the plants
must be isolated so that they do not pick up pollen from any surrounding plants and so become genetically 'polluted'. All this is technically straightforward but of course adds to the expense of conservationWith some plants, however, including some of the world's most important crops, the technical problems are nothing like so easy.

For one thing, many important crops--including potatoes, yams and cassava--are not generally propagated by means of seeds at all. Instead farmers grow them from tubers or rhizomes or other storage organs. Potatoes, yams, and cassava and others do of course reproduce sexually as well, to produce true seed; but when they do, they scramble the genes so that the potatoes or yams that grow from the true seed are not genetically identical with the plants that produced them. The particular qualities of particular selections of potatoes (or yams or whatever) are lost unless the tubers themselves are stored. Nature designed tubers to be storage organs, and again it is not difficult technically to keep them, albeit for the short term; they too merely require to be cool and at appropriate humidity. But tubers at least take up more space than true seed, and must in general be regenerated (replanted) much more frequently. So storage of tubers tends to be more expensive than storage of seed.

Other plants raise even greater difficulties. Many cultivated bananas and plantains produce no seed at all - they are sexually sterile -but they do not produce natural storage organs either. They are propagated by some form of cutting or off-shoot. Traditionally, varieties of bananas and plantains are simply maintained in the field by the farmers themselves who propagate them year on year. They can also be conserved as whole plants in field genebanks. But as with any variety, some more protected ex situ banking is desirable too. Although it is clearly a challenge to keep plants in store that produce neither seed nor natural storage organs, the technical problems have largely been solved. Since the 1960s, techniques of tissue culture have been developed whereby cells can be grown on a gel and, if plied with suitable nutrients and hormones, can give rise to entire plants. In an in vitro genebank, plant tissue is maintained on culture medium in test tubes. In addition, it is becoming more and more possible to store cells for long periods by cryopreservation, a specialized form of freezing in liquid nitrogen at -196° C. But again, as the technicalities increase, so does the cost.

Finally, a wide range of plants and particularly tropical trees produce seeds that are in various ways "recalcitrant". For example, the seeds of some tropical trees germinate while still on the parent: the "seed" that drops to the ground is already a seedling. Recalcitrant seeds cannot be stored simply by drying and cooling. Indeed they regard such treatments as a severe insult and quickly die. They require more sophisticated methods, which to a large extent must be tailored individually to their needs. Again, the special problems associated with conserving these species adds to the expense.