Deacclimation kinetics and carbohydrate changes in stem tissues of Hydrangea in response to an experimental warm spell.

Temperate winters are becoming progressively milder due to climate warming, and temperature patterns are becoming increasingly irregular with risk of unseasonable warm spells. Warm spells may cause premature loss of plant cold hardiness and increase the risk of subsequent freezing injury. This study investigated the timing and rate of deacclimation and associated changes in soluble carbohydrates and water status in stems of Hydrangea macrophylla ssp. macrophylla (Thunb.) Ser. 'Alma' and Hydrangea paniculata Sieb. 'Vanille Fraise' in response to a simulated warm spell (22 °C/17 °C day/night). In H. macrophylla, deacclimation kinetics showed a sigmoid course with a short lag-phase followed by a fast deacclimation rate. In H. paniculata, the deacclimation pattern could not be determined precisely, but H. paniculata, the hardier genotype based on mid-winter freezing tolerance, deacclimated to a greater extent than H. macrophylla. These results imply that dehardening resistance is not related to mid-winter hardiness. In both species deacclimation was associated with rehydration and decreasing sugar levels, but species-specific quantitative and qualitative differences in the accumulation patterns of specific sugars were observed. In H. paniculata cold hardiness may be associated with 1-kestose, an oligofructan frequently associated with overwintering in herbaceous plants, but not previously related to freezing tolerance in woody perennials.