Most people prefer slow freezing.

I suggest to initiate with gradual freezing experiments and then, if you determine that in one of your plant cell suspension lines a strong tolerance to freezing, move into a rapid freezing experiment.

Regards

Gradual freezing may be helpful to achieve the goal.

If your aim is to evaluate impact of freezing stress on suspension cultures, then you should expose cultures directly to freezing stress. You may let us know regarding the type of suspension culture you are maintaining or generating for getting a better answer. In case if you have callus or non-uniform suspension cultures, you may find somaclonal variation. Under such circumstances, gradual freezing for longer duration might make few cells/units to adapt to freezing and hence few of them may tolerate freezing stress better than rest.

You should be very clear with your objective (i.e. are you interested in evaluating impact of freezing stress on your suspension cultures or you wish to isolate cells/units that have superior potential to withstand freezing stress).