Storability and root freezing tolerance of Norway spruce (Picea abies) seedlings

The objective of the study was to evaluate if root freezing tolerance and p ost-storage survival of Norway spruce (Picea abies (L.) Karst.) seedlings c an be improved by cool storage prior to frozen storage. Four-year-old bare- root seedlings were lifted and put into cool or frozen storage at 2-week in tervals from mid-September to mid-December. Length of cool (5.5 degrees C) storage prior to frozen (-4.6 degrees C) storage was 0, 2, 4, 6, 8, 10, or 12 weeks. Root freezing tolerance, determined directly by freezing-induced electrolyte leakage and indirectly by shoot and root dry matter content, we re assessed at lifting and biweekly during cool storage. At these occasions , seedlings were also transferred from cool storage to frozen storage. Surv ival of the seedlings was assessed in a cultivation test in April. Root fre ezing tolerance increased for seedlings put into cool storage before Novemb er, whereas seedlings put into cool storage from November onwards did not i ncrease in root freezing tolerance. Electrolyte leakage after root freezing to -5 degrees C at the time of transfer to frozen storage correlated to po st-storage survival of seedlings pretreated up to 6 weeks. Lifting for succ essful overwinter storage could be done up to 1 month earlier if seedlings were pretreated in cool storage 6 weeks before transfer to frozen storage.