macrophylla are less cold-hardy than stems and/or that buds dehardened faster than stems, at least under constant warm temperatures. Because opening buds are generally expected to have lost most of their acclimated cold-hardiness ( Kalberer et al., 2006), this supports the suggestion that buds of H. macrophylla started after 5 d of dehardening, when stems were still hardy to –10.9 ☌. macrophylla followed a sigmoid curve with a short lag phase (less than 3 d) followed by a fast dehardening rate. Data demonstrated that dehardening of stems of H. were estimated in response to a simulated warm spell at constant 22/17 ☌ day/night. macrophylla and the considerably more cold-hardy Hydrangea paniculata Sieb. In a previous study ( Pagter et al., 2011), the timing and rate of dehardening of stems of H. macrophylla should ideally deharden slowly or late in response to unseasonable transient increases in temperature. In addition, shifting phenological patterns such as an earlier start to the growing season and earlier flowering ( Fitter and Fitter, 2002 Karlsen et al., 2007), consistent with climate warming, may enhance the risk of frost injuries caused by increasing temperature variation. Although temperate winters are becoming progressively milder, the temperature patterns have become increasingly irregular with an increased frequency of warm spells, during which plants tend to lose cold-hardiness, thereby increasing the risk of subsequent freezing injury ( Gu et al., 2008). The risk of premature dehardening may be increasing as a result of global climate changes. macrophylla, but it is possible that late hardening in fall and/or premature dehardening in spring also limits the successful cultivation and flowering of H. Insufficient midwinter-hardiness may account for some of the frost injuries encountered in H. macrophylla buds have, to our knowledge, never been quantified. macrophyllas ornamental and commercial value, cold-hardiness of H. Despite the key role of flower buds in H. macrophylla cultivars varies between –17 and –24 ☌ ( Adkins et al., 2003 Pagter et al., 2008), but buds seem to be more susceptible to freeze injuries than stems ( Pagter et al., 2008). Previous studies have shown that maximum stem cold-hardiness of different H. Flowering will therefore only occur the next year if terminal and/or lateral flower buds are present and undamaged. macrophylla varieties are formed during the fall and overwinter on dormant stems. The consequences of bud freezing injuries in terms of quality and ornamental value are of horticultural importance. However, even in Denmark, which has a rather mild climate with a daily mean temperature of 0.9 ☌ in December, January, and February (The Danish Meteorological Institute), frost injury or winter kill of buds and current-year shoots is a common problem. macrophylla is native to Japan ( McClintock, 1957) and thrives in maritime regions but grows and flowers in most temperate regions where it is not damaged by cold temperatures. Hydrangea macrophylla is a popular flowering shrub, widely used and commercially important in landscape horticulture ( Adkins et al., 2003). Keywords: freezing tolerance deacclimation reacclimation electrolyte leakage warm spells macrophylla are more sensitive to frost injury than stems and the vulnerability of stems to frost injuries, caused by an unstable temperature regime, changes during the winter season. Results of this study indicate that buds of H. In stems, dehardening proceeded faster in March than in January, and the capacity of the stems to reharden seemed reduced, indicating that both dehardening and rehardening were influenced by the progression of winter. In both stems and buds, the dehardening resistance and the rate of dehardening were influenced by temperature, but buds appeared to be less resistant to dehardening and dehardened faster than stems. macrophylla were slightly less cold-hardy than stems. Dehardened plants were rehardened for up to 12 days in an unheated greenhouse (January) or in the field (March). Plants were acclimated in the field and dehardened in the greenhouse at controlled warm temperatures for various durations. ‘Alma’ during dehardening in response to simulated warm spells and subsequent rehardening in January and early March. This study investigated cold-hardiness of stems and buds of Hydrangea macrophylla ssp. Warm spells may induce premature dehardening, increasing the risk of subsequent freezing injuries. As a result of climate warming, temperate winters are becoming progressively milder, and temperature patterns are becoming increasingly irregular with an increased frequency of warm spells. Hydrangea macrophylla is a popular and commercially important flowering shrub, but frost injury of buds and current-year shoots is a common problem in some of its cultivars.
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