Translocation of Dehardening and Bud-Break Promoters in Climatically 'Split' Douglas-fir

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Abstract
Forked Douglas-fir seedlings were exposed to warm (20 °C) and cold (2 °C) environments simultaneously, by inserting one branch in each environment. All received the same light conditions. Hardiness of foliage after 1–5 months was measured as the freezing temperature causing 50% visible injury in excised needles.The chilling stimulus for breaking rest and inducing hardiness was confined to the chilled branch, but the warm branch apparently transmitted a factor which prevented full hardening in the chilled one. A factor moving in the same direction also promoted flushing in branches chilled only at night from December to June (and receiving greenhouse temperatures and natural photoperiods by day). Factors from the expanding shoot caused loss of short-day-induced hardiness in previous year's foliage and stimulated cambial division. Chilling at night prevented the dehardening but did not prevent cambial activity. The dehardening factor was translocated to an opposite branch whereas movement of cambium stimulator was strictly basipetal.It is suggested that promoter–inhibitor levels controlling dormancy are independently regulated, and that a two-stage dehardening process might protect against premature loss of hardiness in nature. The simultaneous induction of cold acclimation and dehardening in the same plant provides material for comparing properties of hardy and nonhardy tissue.

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