Sitka Spruce and Douglas fir Seedlings in the Nursery and in Cold Storage: Root Growth Potential, Carbohydrate Content, Dormancy, Frost Hardiness and Mitotic Index

Abstract

Seedlings (transplants) of 2+1 Sitka spruce ( Picea sitchensis (Bong.) Carr.) and 1 + 1 Douglas fir ( Pseudotsuga menziesii (Mirb.) Franco) were grown in a nursery at the Bush Estate, Scotland. Batches were lifted and cold stored at 0.5°C in November, December and January. Changes in growth, shoot apical mitotic index, root growth potential (RGP), carbohydrate content, bud dormancy and shoot frost hardiness were monitored throughout the winter by taking samples at intervals from the nursery and from cold storage. Frost hardening occurred during the later stages of bud development (as mitotic indices decreased); autumn hardening was arrested when seedlings were put in cold store, and some dehardening occurred in cold storage, especially in spring. Bud dormancy started , and was greatest, just after bud growth (mitotic activity) virtually ceased; chilling in cold store was almost as effective in releasing dormancy as natural chilling. The concentration of total nonstructural carbohydrates stayed more or less constant at 100–150mg g −1 from September to April in the nursery; in cold storage carbohydrates were depleted at 0.4–0.6mg g −1 d −1 (corresponding to respiration at 0.03–0.05mg CO 2 g −1 h −1 ) until there was only 40–50mg g −1 . Root growth potentials in the nursery increased in December, once the buds ceased growth, became dormant and had received some chilling. Sitka spruce was ‘storable’ in November, before RGPs increased, but they then failed to achieve maximal frost hardiness or ROP. Winter RGPs were high in Sitka spruce and were increased or maintained in cold storage, whereas RGPs were low in Douglas fir and decreased immediately after storage (except when stored in January). By the end of April, the RGP of cold stored Sitka spruce was much higher than that of direct lifted plants. ROP changes in the nursery and in cold storage were not consistently related to changes in seedling carbohydrate contents, shoot frost hardiness or bud dormancy. In practical terms, it was concluded that (1) the optimum date to start lifting bare- rooted conifer transplants in the autumn is when their shoot apical mitotic indices have decreased to near zero, and their RGPs have risen sharply; (2) high RGPs may depend as much on the morphology of the roots (e.g. number of undamaged root apices) as on the physiology of the shoots (e.g. carbohydrate status, dormancy and frost hardiness); and (3) in spring, transplants kept in cold storage since November, December or January are more frost hardy, slightly more dormant, and (in May) have higher RGPs than transplants lifted from the nursery.