Hydrangea macrophylla is a popular flowering shrub, widely used and commercially important in landscape horticulture (Adkins et al., 2003). H. 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. However, even in Denmark, which has a rather mild climate with a daily mean temperature of 0.9 °C in December, January, and February (The Danish Meteorological Institute), frost injury or winter kill of buds and current-year shoots is a common problem. The consequences of bud freezing injuries in terms of quality and ornamental value are of horticultural importance. Flower buds of most H. macrophylla varieties are formed during the fall and overwinter on dormant stems. Flowering will therefore only occur the next year if terminal and/or lateral flower buds are present and undamaged. Previous studies have shown that maximum stem cold-hardiness of different H. macrophylla cultivars varies between –17 and –24 °C (Adkins et al., 2003; Pagter et al., 2008), but buds seem to be more susceptible to freeze injuries than stems (Pagter et al., 2008). Despite the key role of flower buds in H. macrophyllas ornamental and commercial value, cold-hardiness of H. macrophylla buds have, to our knowledge, never been quantified.
Insufficient midwinter-hardiness may account for some of the frost injuries encountered in H. 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. macrophylla (Adkins et al., 2003). The risk of premature dehardening may be increasing as a result of global climate changes. 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). 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. To reduce the risk of frost injuries, H. macrophylla should ideally deharden slowly or late in response to unseasonable transient increases in temperature. In a previous study (Pagter et al., 2011), the timing and rate of dehardening of stems of H. macrophylla and the considerably more cold-hardy Hydrangea paniculata Sieb. were estimated in response to a simulated warm spell at constant 22/17 °C day/night. Data demonstrated that dehardening of stems of H. macrophylla followed a sigmoid curve with a short lag phase (less than 3 d) followed by a fast dehardening rate. It was also observed that budbreak in H. macrophylla started after 5 d of dehardening, when stems were still hardy to –10.9 °C. 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 are less cold-hardy than stems and/or that buds dehardened faster than stems, at least under constant warm temperatures.
In addition to the stability of the hardiness and the rate of dehardening, the effect of an unstable temperature regime on the frequency and severity of frost injuries also depends on the ability of the plants to reharden in response to low temperatures after a period of dehardening (Kalberer et al., 2006). Rehardening may be an important winter survival strategy in plants that deharden quickly on exposure to increased temperatures. Seasonal fluctuations in cold-hardiness (decreases followed by increases and vice versa), which indicate some capacity to reharden, have been documented in many plants (Cox and Stushnoff, 2001; Neuner et al., 1999; Sauter et al., 1996), and rehardening in controlled conditions after considerable losses of cold-hardiness has also been documented in some species (Eagles and Williams, 1992; Kalberer et al., 2007a, 2007b; Suojala and Lindén, 1997). However, in H. macrophylla, the potential rehardening capacity is largely unknown.
This study was conducted to 1) determine the hardiness of buds of H. macrophylla and to determine if buds are less cold-hardy than stems; 2) investigate dehardening resistance and loss of cold-hardiness of stems and buds of non-dormant H. macrophylla under different temperature conditions; and 3) determine if stems and buds of non-dormant H. macrophylla have an ability to reharden after a period of dehardening.
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