Fig. 4. Post-thaw viability of R. sylvatica organ samples incubated in PBS (no urea) or PBS containing 80 mmol l^-1 urea and frozen at -4°C. Control samples were incubated in PBS but not frozen/thawed. Metabolic index is the percent reduction of indicator dye per milligram dry tissue per hour at 15°C. Mean values (± S.E.M., N=6-12 replicates per group) identified by different letters differed significantly (ANOVA/Bonferroni; P<0.05).