Ch. 7— Maintaining Plant Diversity Offsite • 187 



in a medium that inhibits growth (53,56,119, 

 120,122). Cultures can thus be maintained for 

 weeks to months without subculture (i.e., trans- 

 fer to fresh medium). However, all treatments 

 that retard growth put additional stress on the 

 culture, which may increase the potential for 

 somaclonal variants. 



In vitro culture techniques could be impor- 

 tant for the long-term maintenance of plants 

 with recalcitrant seeds. One recent proposal 

 has been to excise the embryo from the seed 

 and store it under cryogenic conditions. The 

 embryo could be thawed and then grown and 

 multiplied in vitro (40). Research in coopera- 

 tion with the Royal Botanic Gardens at Kew, 

 England, has demonstrated the feasibility of this 

 procedure for two tree species (Araucaria 

 husteinii and Quercus robur). Further research 

 is needed to apply it to other plants with recal- 

 citrant seeds (40). 



Cryogenic storage may help avoid the stresses 

 of continuous in v'iYro culture (49,62,90,121,122, 

 123). Considerably greater attention would be 

 needed in preparation, freezing, storing, thaw- 

 ing, and subsequent culturing than is the case 

 for orthodox seeds. Although some generali- 

 zations can be made, methods acceptable to one 

 species or variety may not be satisfactory for 

 others. However, research has demonstrated 

 that in vi^ro-cultured shoot-tips from some her- 

 baceous plants (e.g., potato, carnation, and cas- 

 sava); berries (e.g., strawberry, raspberry, and 

 blueberry); and buds of some woody species 

 (e.g., apple) can survive cryogenic storage (108). 



Many questions remain before cryogenic 

 storage of in vitro cultures is widely applied. 

 Among these is whether a single procedure can 

 be developed that works well for an array of 

 plants. Further, it is not yet understood how 

 the process of freezing and thawing affects 

 regeneration of cultures or their genetic con- 

 stitution (108,121). Additional investigation for 

 individual crops is needed, and current tech- 

 nologies have not yet been adapted for handling 

 the large numbers of specimens that might be 

 expected in an offsite facility. 



DNA Storage 



Future storage technologies may include, as 

 a supplemental strategy, the preservation of the 

 isolated genetic information (DNA and RNA) 

 of plants. Existing technologies can locate, ex- 

 cise, and reinsert genes. In some cases, these 

 genes retain nearly normal function (75,108). 

 A much better understanding of gene structure, 

 function, and regulation is needed, however, 

 before isolated DNA can be used for germplasm 

 storage (76,108). 



Management of Stored Materials 



Offsite collections of plants must be well man- 

 aged to guard against loss of materials and to 

 use financial and technological resources most 

 efficiently. Some duplication between collec- 

 tions can prevent catastrophic loss, but exces- 

 sive redundancy can waste resources. Disease 

 organisms that might be brought into a collec- 

 tion by new accessions need to be managed. 

 Finally, information on the accessions must be 

 easily available both to managers and users. 



Duplication of Collections 



Duplication of collections provides the best 

 insurance against natural catastrophes, pests, 

 diseases, mechanical failures, or abandonment 

 (81,108). CIP protects its collection of landrace 

 potatoes with field plantings at other locations, 

 with seed storage, and with in vitro culture (50). 

 At the NPGS Clonal Repositories (see ch. 9), 

 greenhouse collections back-up field-maintained 

 collections of fruit and nut species. Duplica- 

 tion is equally critical for seed banks, where 

 malfunctioning of a mechanical compressor 

 can result in loss of cooling. 



Plants in an offsite collection also can be lost 

 if institutional priorities change, or if the per- 

 son responsible for the species or collection 

 leaves (108). The situation is particularly criti- 

 cal for older varieties of fruits, berries, and 

 vegetables that may be held only by private in- 

 dividuals or groups (112). For wild species, too, 

 a large collection is frequently the result of the 



