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

Abstract

The objective of the study was to evaluate if root freezing tolerance and post-storage survival of Norway spruce (Picea abies (L.) Karst.) seedlings can 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 intervals from mid-September to mid-December. Length of cool (5.5°C) storage prior to frozen (-4.6°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, were assessed at lifting and biweekly during cool storage. At these occasions, seedlings were also transferred from cool storage to frozen storage. Survival of the seedlings was assessed in a cultivation test in April. Root freezing tolerance increased for seedlings put into cool storage before November, whereas seedlings put into cool storage from November onwards did not increase in root freezing tolerance. Electrolyte leakage after root freezing to -5°C at the time of transfer to frozen storage correlated to post-storage survival of seedlings pretreated up to 6 weeks. Lifting for successful overwinter storage could be done up to 1 month earlier if seedlings were pretreated in cool storage 6 weeks before transfer to frozen storage.