A method is described for cryopreservation of cell suspension lines of rice (Oryza sativa L.) for use in protoplast research and as a way of retaining desirable characteristics of cell lines. The procedure involves pre-culture with mannitol, addition of a cryoprotectant solution of sucrose, dimethyl sulfoxide, glycerol and L-proline, two step freezing and storage in liquid nitrogen. Cells have been preserved for up to 14 months (the longest period tried in these experiments). Cryopreserved cells proliferated after plating on solid medium and new cell suspensions could be initiated within 15 days. Viable protoplasts, capable of divisions and callus formation, could be obtained 15-21 days after thawing. Variation between cell lines in terms of recovery rate after cryopreservation occurred. Differences between cell lines in plating efficiencies on solidified medium, however, contributed to this variation. Protoplasts from cryopreserved regenerable cell lines gave rise to embryogenic callus from which plants could be regenerated. These plants developed to maturity. A transformed cell line was also cryopreserved and it had retained the hygromycin resistance and regenerative capacity of the original cell line.

Vitrification and two-step freezing were comparatively tested for cryopreservation of tobacco cell suspensions. The optimal growth phase of the culture and the optimal length of the protecting preculture period were determined. With both methods, late-exponential cells showed higher survival rates compared to early exponential and growthlimited cells. Under optimal conditions vitrification yielded higher survival rates than two-step freezing (55% and 36%, respectively). Using two-step freezing a preculture period of 72 h in medium supplemented with 0.3 M mannitol was necessary to obtain maximal survival, whereas for vitrification 24 h of preculture sufficed. Heat shock treatment prior to the cryopreservation procedure could improve survival when mannitol precultured cells in a non-optimal growth phase were used. Heat-shocked cells, which were not precultured with mannitol, did not survive vitrification. Vitrification is the method recommended for cryopreservation of tobacco cell suspensions, in view of the shorter preculture period and higher survival rates resulting in quicker regrowth.

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