| Cassava, or Manihot esculenta Crantz, is grown in over 90 countries and
provides a livelihood for half a billion people in the developing world. While this hardy
root crop serves as a staple food for many poor farm families, it is also a source of
commercial animal feed, fiber for the paper and textile manufacturers, and starch for the
food and pharmaceutical industries. Global production of cassava is around 152 million
tons per year. Half the 16 million hectares devoted to cassava cultivation is in Africa,
with 30 percent in Asia and 20 percent in Latin America.
The CIAT cassava germplasm collection consists of 6,000 clones: landraces from Latin
America and Asia, elite clones selected by CIAT and the International Institute of
Tropical Agriculture (IITA) in Nigeria,
and several wild Manihot species. These are stored in the form of slow-growth in
vitro plantlets. For each of the accessions, five test-tube plantlets are conserved to
accommodate various purposes. These include long-term conservation, distribution to fill
germplasm orders, security backups, and the provision of duplicates for conservation in
the germplasm’s country of origin. Some cassava germplasm is also conserved as seed.
With the current in vitro method, plantlets have an effective lifespan of only 12 to 14
months (in contrast to the decades-long viability of frozen seeds). After that, tissues
must be removed from the aging plantlets and recultured in fresh growth medium.
CIAT is currently testing two alternative methods for improving cassava germplasm
conservation and prolonging the storage period. The first is to extend the life of in
vitro plantlets through better culturing—for example, by altering the growth medium
used in test tubes. This option has the potential to more or less double the cassava
plantlets’ period of viability.
The second option is to make and freeze "artificial" seeds. Cassava shoot
tips (meristems) are given a protective coating of sodium alginate and then made to
coagulate into tiny beads by immersion in a calcium chloride solution. After having their
water content reduced in a two-step process, the beads are stored in liquid nitrogen. In
this extreme environment of -196 degrees C, all biological activity is effectively halted.
The technique allows the cassava to be conserved for 30 years or more with no maintenance
other than periodic monitoring. Furthermore, it is less labor-intensive than in vitro
culturing, requires less storage space, and allows for easy duplication and transport of
the collection to other sites. The main disadvantage is that generating whole cassava
plants from frozen beads is much more difficult than from test-tube plantlets, and the
success rate is still rather low.
The standard farmer method of propagating cassava is to plant cuttings called stakes.
Although stakes have some practical advantages as a germplasm storage and propagation
medium, they are sources of plant disease and may not be transported across international
borders.
Types of reproductive material: in vitro plantlets; seed; cryopreserved meristems.
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