186 • Technologies To Maintain Biological Diversity 



fa vitro Cwltwre 



In vitro cultures of plants have been advo- 

 cated for a variety of species, especially those 

 that are clonally propagated (20,53,56). Al- 

 though this technology can be adapted to many 

 species (16,33,93), it is generally unnecessary 

 for those that have orthodox seeds. However, 

 the methods may be necessary if there is a need 

 to maintain specific genetic types, if seed 

 progeny are highly variable, if plants have long 

 juvenile stages (e.g., many tree species), or if 

 seeds are lacking (e.g., clonal crops such as 

 banana, taro, and sugarcane) (20). 



In vitro maintenance is defined as the grow- 

 ing of cells, tissues, organs, or plantlets in glass 

 or plastic vessels under sterile conditions (108). 

 When plants originate from intact isolated 

 meristems, the cultures may be free of patho- 

 gens (108). The media for growing in vitro cul- 

 tures may vary among species and among in- 

 dividuals within a species. By altering the 

 balance of nutrients and growth regulators in 

 the media, in vitro cultures can be made to de- 

 velop unorganized growth (termed callus), pro- 

 duce multiple shoots or plantlets, form struc- 

 tures similar to the embryo in a seed, or develop 

 roots to enable transfer to field conditions. Not 

 all plants, however, are amenable to growth or 

 manipulation by in vitro culture (108). 



One aspect of in vitro technology of particu- 

 lar concern for plant germplasm conservation 

 is the occurrence of genetic modification (so- 

 maclonal variation) in plants derived from 

 callus cultures (67,90). Such variation is con- 

 sidered useful in the development of new varie- 

 tal characteristics but is unacceptable when 

 preservation of specific genotypes is the objec- 

 tive. Although it is known that certain types 

 of cultures and conditions, such as callus cul- 

 tures, can produce higher frequencies of soma- 

 clonal variation, the cellular processes that pro- 

 duce them are not well understood (90,120). 

 Furthermore, growing cultured plants to matu- 

 rity remains the only satisfactory way to exam- 

 ine the consequences of such changes. Conse- 

 quently, each method of culture must be carefully 

 evaluated before it is applied. For germplasm 

 preservation, in vitro plants directly derived 



Photo credit: Inlernational Board for Plant Genetic Resources 



In vitro culture could become an important method of 



long-term maintenance for plants with seeds that cannot 



be stored under dry, cold conditions and for plants that 



can only be maintained in field collections. 



from buds or shoot-tips are considered most 

 suitable. 



No in vitro long-term base collections of agri- 

 cultural crops exist at present, although some 

 active collections are being developed: potato 

 at CIP, cassava at the International Center for 

 Tropical Agriculture (CIAT) in Colombia, and 

 yam and sweet potato at the International In- 

 stitute for Tropical Agriculture in Nigeria (122, 

 123). The NPGS Clonal Repositories are inves- 

 tigating in vitro cultures as backup to field col- 

 lections of some crops (108). 



In vitro cultures can be stored under normal 

 growth conditions, in reduced temperatures or 



