Ch. 7— Maintaining Plant Diversity Offsite • 171 



cessful rice variety IR36 developed by the In- 

 ternational Rice Research Institute resulted 

 from the crossbreeding of 13 accessions in their 

 collections of rice from the United States and 

 several Asian countries (79). Wild plant collec- 

 tions help to preserve endangered species, sup- 

 ply materials to restore degraded lands, and 

 provide material for genetic improvement of 

 crops. 



Botanic gardens and arboretums are the pri- 

 mary repositories for w^ild plant species (69, 

 112). Arboretums have been particularly impor- 

 tant for maintaining individual trees and shrubs 

 that may have little commercial significance 

 (69). These facilities may also have commit- 

 ments to public education and display that can 

 result in selecting plants with special or unusual 

 characteristics rather than those representing 

 the genetic diversity within the species. How- 

 ever, many such institutions are now giving 

 greater attention to the potential contributions 

 they can make to maintaining plant diversity 

 (12,34). 



Considerations in Selecting 

 Teciinelogies 



No single technology is appropriate for all 

 the plants stored in offsite collections. Several 



considerations affect the selection of appropri- 

 ate technologies such as biological limitations 

 of the species, reliability of the technology, and 

 cost. 



Biological Limitations 



Seeds are the most commonly and easily 

 stored propagules of plants. When placed in 

 conditions that reduce their moisture content 

 to approximately 5 to 6 percent, seeds of many 

 species will remain viable for years. Lowered 

 temperatures can further extend storage life. 

 Seeds able to withstand reduction in moisture 

 and temperature are called orthodox seeds. 

 Most of the major food crops (e.g., cereals and 

 legumes) have orthodox seeds, and many, when 

 properly dried, withstand cooling to — 196° C, 

 the temperature used in cryopreservation (58, 

 85,89,100,108). 



Seeds that cannot survive a reduction in mois- 

 ture content are called recalcitrant seeds. Recal- 

 citrant seeds are found in many important trop- 

 ical species, a few temperate tree species, and 

 some aquatic plants (8,42,83,100,108,118). 



Reducing the water content of recalcitrant 

 seeds severely shortens their life span. Thus, 

 they cannot be stored like orthodox seeds: cool- 

 ing to subfreezing temperatures would lead to 



Photo credit: International Board for Plant Genetic Resources 



Many temperate and tropical species such as coffee and oil palm have seeds that cannot be stored for long periods; 



for this reason, collection of woody cuttings is preferred. 



