With respect to seed preservation possibilities, plant species have been divided into 2 categories:
Orthodox seeds, which can withstand dehydration to 5% or less (dry weight basis) without damage. When dry, the viability of these seeds can be prolonged by keeping them at the lowest temperature and moisture possible.
Recalcitrant seeds, which are high in moisture and are unable to withstand much desiccation. They are predominantly seeds from tropical or subtropical species. They can be stored only in wet medium in order to avoid dehydration injury and in relatively warm conditions because chilling injury is very common among these species. They remain viable only for a short time (weeks or months), even if kept in the required moisture conditions (e. g., oil palm, coconut, cacao coffee.
- Moreover, there are practical problems in applying long-term seed storage to most long-live forest trees, including gymnosperms and angiosperms, since their juvenile period is very long and they do not produce seeds for several years. The conservation of plants which are vegetatively propagated, such as cassava, potato and yams also possess considerable problems.
- In situ conservation has been made almost impossible due to the disappearance of large wild areas. Conservation ex situ is very difficult to carry out due to the following problems: an adequate sample has to be taken for the conservation of genetic diversity.
- During the last years, in vitro culture techniques have been extensively developed and applied to more than 1,000 species, including many tropical species. The use of in vitro tissue culture techniques can be of great interest for germplasm collection, storage and multiplication of recalcitrant and vegetatively propagated species. Plant species that are in danger of being extinct can be conserved using tissue culture techniques. Tissue culture systems present several advantages including:
- very high multiplication rates
- aseptic system:
- -free from fungi, bacteria, viruses (after thermotherapy and indexation) and insect pests
- -production of pathogen-free stocks
- reduction of space requirements ,
- genetic erosion reduced to zero under optimal and
- reduction of the expenses in labour costs
- Further, tissue culture systems greatly facilitate the international exchange of germplasm. Indeed, the size of the samples is drastically reduced and they can be shipped in sterile conditions. However, the in vitro storage of large quantities of material induces various problems: laboratory management of plant material which needs to be regularly subcultured, risks of genetic variation, which increase with in, vitro storage duration, and can lead to the loss of trueness to type.
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