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Storage of seeds at low water content and sub-zero
temperature in genebanks is the most convenient way of conserving the genetic
resources of most spermatophytes. However, the seeds of many, particularly
tropical, species are desiccation sensitive and are not amenable to storage under
these conditions. Not only is long-term conservation of this germplasm
difficult, but the characteristic of desiccation sensitivity also places limits
on short-term storage of the seeds. This paper briefly reviews the physiology
of these ‘non-orthodox' seeds, and considers the progress that has been made in
extending storage lifespan of the seeds, and in the cryopreservation of the
germplasm. Short- to medium-term storage lifespan of hydrated seeds can be
extended by reducing temperatures as low as possible — where chilling
sensitivity permits — although not below zero. Long-term cryopreservation of
whole seeds is precluded because of ice crystal damage to the hydrated tissue,
but a novel approach in dealing with this problem is described. Excised embryonic
axes are dried very rapidly (flash dried) to water contents low enough to
reduce ice crystal formation on freezing, but not so low as to introduce
desiccation damage, and the partially dried axes are frozen very rapidly by
plunging in sub-cooled liquid nitrogen (‘nitrogen slush') to reduce potential
ice crystal damage. Subsequent storage in liquid nitrogen affords the means for
long-term conservation of the zygotic axes (and buds, apical meristems, callus
or somatic embryos) and hence the genetic resources of species producing even
the most recalcitrant of non-orthodox seeds.

South African Journal of Botany
2004, 70(1): 102–108