Physiologia Plantarum 134: 473–485. 2008 Copyright ª Physiologia Plantarum 2008, ISSN 0031-9317 Changes in carbohydrates, ABA and bark proteins during seasonal cold acclimation and deacclimation in Hydrangea species differing in cold hardiness Majken Pagter a, *, Christian R. Jensen b , Karen K. Petersen a , Fulai Liu b and Rajeev Arora c a Department of Horticulture, University of Aarhus, Kirstinebjergvej 10, DK-5792 Aarslev, Denmark b Department of Agricultural Sciences, University of Copenhagen, Hoejbakkegaard Alle ´ 9, DK-2630 Taastrup, Denmark c Department of Horticulture, Iowa State University, Ames, IA 50011, USA Correspondence *Corresponding author, e-mail: majken.pagter@agrsci.dk Received 13 March 2008; revised 25 June 2008 doi: 10.1111/j.1399-3054.2008.01154.x Cold injury is frequently seen in the commercially important shrub Hydrangea macrophylla but not in Hydrangea paniculata. Cold acclimation and deac- climation and associated physiological adaptations were investigated from late September 2006 to early May 2007 in stems of field-grown H. macrophylla ssp. macrophylla (Thunb.) Ser. cv. Blaumeise and H. paniculata Sieb. cv. Kyushu. Acclimation and deacclimation appeared approximately synchronized in the two species, but they differed significantly in levels of mid-winter cold hardiness, rates of acclimation and deacclimation and physiological traits conferring tol- erance to freezing conditions. Accumulation patterns of sucrose and raffinose in stems paralleled fluctuations in cold hardiness in both species, but H. macro- phylla additionally accumulated glucose and fructose during winter, indicating species-specific differences in carbohydrate metabolism. Protein profiles differed between H. macrophylla and H. paniculata, but distinct seasonal patterns associated with winter acclimation were observed in both species. In H. paniculata concurrent increases in xylem sap abscisic acid (ABA) con- centrations ([ABA] xylem ) and freezing tolerance suggests an involvement of ABA in cold acclimation. In contrast, ABA from the root system was seemingly not involved in cold acclimation in H. macrophylla, suggesting that species-specific differences in cold hardiness may be related to differences in [ABA] xylem . In both species a significant increase in stem freezing tolerance appeared long after growth ceased, suggesting that cold acclimation is more regulated by temperature than by photoperiod. Introduction Geographical distribution and cultivation of horticultural crops in temperate climates are strongly dependent on their freezing tolerance. Increased freezing tolerance (beyond a ‘constitutive’ level) can be acquired by the process of cold acclimation, which involves physiolog- ical and biochemical changes where plants become increasingly tolerant to subzero temperatures (Guy 2003, Li et al. 2004, Weiser 1970). Cold acclimation is a sea- sonal process, with freezing tolerance increasing during the autumn, reaching its maximum mid-winter and declining in the spring. Susceptibility of plants to frost injury may therefore not only be ascribed to insufficient maximum freezing tolerance, but also to the timing and rates of acclimation and deacclimation (Suojala and Linde ´n 1997). Parallel to cold acclimation temperate Abbreviations – ABA, abscisic acid; DW, dry weight; FW, fresh weight; LT, low temperature; LT 50 , temperature representing 50% REL; REL, relative electrolyte leakage; SD, short photoperiod. Physiol. Plant. 134, 2008 473 Physiologia Plantarum An International Journal for Plant Biology