Cryopreservation of cassava shoot tips

Currently, ex-situ conservation of cassava germplasm at CIAT is carried out both in the field and in the laboratory as shoot tip cultures. The in vitro conservation constitutes an active collection and clones are sub-cultured and re-cycled every 12-18 months depending on the genotype. The availability of long-term storage of cassava germplasm in reduced space, free of genetic change, and at a low cost can be achieved by cryopreservation. A research project on cassava cryopreservation began at CIAT in 1988; the project comprises three phases: (i) the first phase was carried out in cooperation with IBPGR (1988-90) and resulted in the recovery of plants from frozen shoot tips in liquid nitrogen. With the cv. MCol 22, recovery rates ranged from 20 to 40 percent. However, several cassava cvs. tested only showed a low response or did not respond at all (Ann. Report, CIAT 1991). (ii) the second phase of the research (1990-present), was designed to improve the previous protocol in order to increase the recovery rate of plants, minimizing genotypic differences. Lower temperature and higher illumination of donor cultures increased recovery from liquid nitrogen with consistent rates 50-60 percent. Use of high concentration of sucrose in lieu of sorbitol and DMSO in the pre-culture stage, allowed high rates of plant recovery from frozen shoot tips. Work continues to adjust the cryoprotection, freezing, and post-thawing culture phases. Ultra rapid freezing, i.e. direct immersion of shoot tips into liquid nitrogen, resulted in similar or higher recovery rates than slow freezing. On the other hand, recovery rate of otherwise unresponsive genotypes has significantly increased with the improved technique. This work has paved the way to the development of a long-term, base gene bank of cassava clones using liquid nitrogen. (iii) the third phase of the research will focus on developing further the technique, especially with regard to genotype response and evaluation of genotypic stability; finally critical logistical aspects of cassava cryopreservation will be tackled.