Effects of the cryopreservation procedures on recovered rice cell populations.

Cryogenic storage of plant cells allows the long-term maintenance of valuable genotypes. Cryopreservation of calli and cell suspensions is often performed using cryoprotectants and slow cooling rates. Rice calli (Oryza sativa L.) were cryopreserved by this procedure as well as by direct immersion in liquid nitrogen without cryoprotection. Subsequently, the characteristics of the recovered cells as well as the effects of putative cryoselection were investigated by microscopic observations and flow cytometric analyses. For this purpose, protoplasts were prepared from calli that had been cryopreserved by direct plunging into liquid nitrogen and from their unfrozen controls. Results show that direct immersion in liquid nitrogen of calli pre-treated with abscisic acid is a fast and highly efficient freezing procedure that maintains the main characteristics of the cell populations and appears to increase their metabolic activity