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  • Author or Editor: Majken Pagter x
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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. 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. Warm spells may induce premature dehardening, increasing the risk of subsequent freezing injuries. This study investigated cold-hardiness of stems and buds of Hydrangea macrophylla ssp. macrophylla (Thunb.) Ser. ‘Alma’ during dehardening in response to simulated warm spells and subsequent rehardening in January and early March. Plants were acclimated in the field and dehardened in the greenhouse at controlled warm temperatures for various durations. Dehardened plants were rehardened for up to 12 days in an unheated greenhouse (January) or in the field (March). Buds of H. macrophylla were slightly less cold-hardy than stems. 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. 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. Results of this study indicate that buds of H. 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.

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Fuchsia (Fuchsia L.) is a popular woody ornamental, but it is very susceptible to frost injury during winter. As drought stress may be used to enhance freezing tolerance in woody plants, the effects of different types of water deficit on growth, selected physiological traits, and freezing tolerance were examined in Fuchsia magellanica Lam. ‘Riccartonii’. Drought responses were investigated after 6 weeks of pretreatment, where individual plants grown in a greenhouse under conditions of unrestricted water supply were compared with plants subjected to cyclic or continuous water deficit. After an additional 4 weeks of treatment at short day (10 h) and low temperature (8 °C day/4 °C night), freezing tolerance was examined. Both continuous and cyclic water deficit plants reduced water loss by reducing aboveground biomass and by efficient stomatal regulation. Continuous water deficit plants tended to adjust osmotically, while cyclic water deficit induced significantly higher xylem sap abscisic acid [(ABA)xylem] and leaf proline concentrations and a lower leaf water potential (ψl) than continuous water deficit, indicating that F. magellanica responds differently to continuous water deficit and to fast drying associated with stress phases of cyclic water deficit. The root water potential (ψr) and (ABA)xylem were negatively linearly correlated, implying that increasing water deficiency stimulated formation of ABA in the roots. An inverse, curvilinear relation between (ABA)xylem and stomatal conductance (g s) indicated that root-originated ABA might control g s during mild water deficits. Neither cold-acclimating conditions alone nor combined with water deficit increased stem freezing tolerance, indicating that F. magellanica lacks cold-acclimation ability under the inductive conditions used in this study.

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