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Additional index words. Rubus spp., cold hardiness, differential thermal analysis, flowering Abstract. 'Hull Thornless' and 'Black Satin' blackberry (Rubus spp.) canes were collected from Sept. 1989 through Mar. 1990 to determine the hardiness and supercooling characteristics of buds at various stages of development. Anatomical studies were also conducted to examine the location of ice voids in buds frozen to -5 or -30C. Differ- entiation of the terminal flower occurred in 'Black Satin' buds by 6 Nov., whereas 'Hull Thornless' buds remained vegetative until early spring. As many as nine floral primordia were observed in both cultivars by 12 Mar. The hardiness of the two cultivars was similar until February. Thereafter, 'Black Satin' buds were more susceptible to cold injury than those of 'Hull Thornless'. Flora1 and undifferentiated buds of both cultivars exhibited one to four low temperature exotherms (LTEs) from 9 Oct. to 12 Mar. in differential thermal analysis (DTA) experiments. The stage of flora1 development did not influence the bud's capacity to supercool. The number of LTEs was not related to the stage of floral development or to the number of floral primordia. Extracellular voids resulting from ice formation in the bud axis and scales were observed in samples subjected to -5 or -30C. Low temperature injury is a major factor limiting commercial production of eastern thornless blackberry. Midwinter temper- atures below -20C frequently injure floral buds and reduce yield. Kraut et al. (1986) reported that 'Smoothstem' was the most susceptible to cold injury, 'Hull Thornless' was interme- diate, and 'Dirksen' was the least susceptible when the relative bud hardiness of three thornless blackberries was evaluated. In another study (Warmund and George, 1990), 'Chester' and 'Hull Thornless' buds had similar T50 (temperature at which 50% of the buds are dead) values (-15C) in November, while those of 'Dirksen' were one degree hardier. In blackberry, the time of floral bud differentiation and bloom are variable (Moore and Caldwell, 1985; Robertson, 1957; Waldo, 1933). Takeda and Wisniewski (1989) reported that 'Black Satin' and 'Hull Thornless' blackberry differentiate floral buds in Oc- tober and late March, respectively. In both cultivars, anthesis of the terminal flower of the A 1 axis preceded that of the ter- minal flower on each of the A2 axes. Anthesis then progressed acropetally from the basal floral primordia of the A 1 axis. Most thorny, erect-type blackberries develop floral buds in early fall (Warmund and George, 1990). In 'Darrow' black- berry, the supercooling characteristics of the buds changed as floral differentiation progressed (Warmund et al., 1988). A sin- gle LTE was associated with injury to the entire floral region in September, when buds were at an early stage of floral de- velopment. As many as ten LTEs were observed at later stages of floral differentiation, indicating that individual primordia froze and were injured independently. In contrast, differentiating flo- ral primordia of other blackberry cultivars, such as 'Shawnee' and 'Dirksen', did not freeze independently in midwinter (War- mund and George, 1989, 1990). More recent studies indicated

Supercooling and Extracellular Ice Formation in Differentiating -Buds of Eastern Thornless Blackberry