Effect of thawing time, cooling rate and boron nutrition on freezing point of the primordial shoot in Norway spruce buds

BACKGROUND: Effects of cooling rates on bud frost hardiness have been studied but there is little information on bud responses to thawing. Since the cell wall pore size has been found to increase with boron (B) deficiency, B deficiency may affect the supercooling ability of buds in winter. METHODS: The effects of duration of thawing time and rate of cooling on bud frost hardiness of Norway spruce (Picea abies) were studied in a B fertilization trial in February 2003 and March 2005. Frost hardiness of apical buds was determined by differential thermal analysis (DTA) and visual scoring of damage. KEY RESULTS: In 2003, the freezing point of primordial shoots of buds (T[subscript f]), i.e. the low-temperature exotherm (LTE), was, on average, -39 °C when buds were thawed for less than 3 h and the T[subscript f] increased to -21 °C after 18 h of thawing. During the first 4 h of thawing, the rate of dehardening was 6 °C h⁻¹. In 2005, buds dehardened linearly from -39 °C to -35 °C at a rate of 0·7 °C h⁻¹. In 2003, different cooling rates of 1-5 °C h⁻¹ had a minor effect on T[subscript f] but in 2005 with slow cooling rates T[subscript f] decreased. In both samplings, at cooling rates of 2 and 1 °C h⁻¹, T[subscript f] was slightly higher in B-fertilized than in non-fertilized trees. By contrast, at very short thawing times in 2003, T[subscript f] was somewhat lower in B-fertilized trees. CONCLUSIONS: There was little evidence of reduced frost hardiness in trees with low B status. This study showed that buds deharden rapidly when exposed to above-freezing temperatures in winter, but if cooled again they reharden more slowly. According to this study, rapid dehardening of buds has to be taken into account in assessments of frost hardiness.