Ch. 7— Maintaining Plant Diversity Offsite • 183 



varieties. Other factors such as control of pol- 

 hnation can also be important for regenerat- 

 ing certain crops (95,108). Wind-pollinated ac- 

 cessions can readily cross with others, and thus, 

 individual accessions must be grown in widely 

 separated fields to ensure that they are not ge- 

 netically mixed. 



Genetic loss by natural selection during grow- 

 out maybe undetected in regenerated seed. At 

 NSSL, the designated grower is responsible for 

 ensuring that the sample returned is from plants 

 grown under conditions that would minimize 

 genetic loss. No testing beyond visual exami- 

 nation and a viability test of the returned sam- 

 ple is conducted. 



Grow-outs are expensive in terms of facilities 

 and personnel, and they subject stored materi- 

 als to damage from pests, pathogens, and en- 

 vironmental conditions, which may reduce 

 genetic diversity in an accession. But the most 

 effective and least expensive way to maintain 

 diversity is to reduce the frequency of regener- 

 ation through technologies that extend storage. 



Cryogenic Storage off Seeds 



A critical factor in cryogenic storage is the 

 amount of water in the tissue to be frozen. Most 

 orthodox seeds can be easily stored at cryogenic 

 temperatures because their water content is low 

 enough to avoid damage associated with freez- 

 ing (99,100,102,108). 



Cryogenic technologies may be able to extend 

 the storage life of orthodox seeds to more than 

 a century, which would greatly reduce the need 

 for viability testing and regeneration (100,102, 

 121,122). 



However, limitations on cryogenic storage 

 exist for some species depending on a plant's 

 seed coat, oil or moisture content, and seed size 

 (108). Some plants, such as plums and coffee, 

 have orthodox seeds that tolerate low moisture 

 levels but are sensitive to cooling below —40° 

 C (100). If cooled or warmed incorrectly, many 

 seeds can crack (100). Seeds as big as cotton 

 seeds (about eight seeds per gram) are appro- 

 priate for cryogenic storage (99,100,102). Larger 

 seeds, such as beans, can also be frozen, but in- 



Pholo credit: OTA staff 



Cryogenic storage, applicable to seeds of some plant 



species, can reduce dependency on mechanical 



systems and greatly increase storage times. 



creased costs, due in part to the greater amount 

 of space required, may reduce or eliminate po- 

 tential cost-savings over mechanical refriger- 

 ation (102). 



This method could hold considerable cost ad- 

 vantages with regard to operating a seed bank 

 and regenerating seed (table 7-4) (102). Cryo- 

 genic storage facilities will cost about the same 

 to establish, but operation over time would be 

 cheaper, in part due to reduced need for via- 

 bility testing and grow-out. Investment in a 

 facility to produce liquid nitrogen might be nec- 

 essary in some areas, but the operational sav- 

 ings in the seed bank could allow recovery of 

 costs in 6 to 14 years (99). 



Major obstacles to this technology are the lack 

 of appropriate facilities and scientific exper- 



