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Differential thermal analyses (DTA) and freeze viability tests were conducted to investigate the biophysics of freezing in floral buds of ‘Danka’ black (Ribes nigrutn L.) and ‘Red Lake’ red currants [Ribe.s sativum (Rchb.) Syrne] sampled from Nov. 1989 through Mar. 1990. Scanning electron microscopy was also used to determine the relationship between floral morphology and the freezing characteristics of the buds. Floral buds had multiple abrupt low-temperature exotherms (LTEs) and one or two broad LTEs in DTA tests. Abrupt LTEs from DTA were associated with apparent injury to the inflorescence in viability tests. The number of LTEs did not correspond to the number of racemes or flowers per bud, indicating that several flowers froze simultaneously. DTA experiments conducted in Dec. 1990 revealed that the broad exotherm detected between – 14 and – 20C in ‘Danka’ samples resulted from freezing of supercooled water in the outer nonliving region of the periderm of cane tissue attached to the bud. DTA has been used to study freezing in floral buds of several fruit genera. In DTA experiments, floral buds exhibit a nonlethal high-temperature exotherm (HTE), representing extracellular ice formation (Burke et al., 1976; Quamme, 1974). The HTE is followed by one or more LTEs associated with intracellular freezing of supercooled water in the floral prlmordia. In buds containing a racemose in florescente, such as Rubus, LTEs correspond to freezing injury of individual flowers (Warmund et al., 198$) or to the entire floral region (Kraut et al., 1986; Warmund et al., 1990). Previous research on Ribes spp. has focused on fall acclimation (Lobanov and Shcherbinin, 1990) or spring frost tolerance of R. nigrum after budbreak (Dale, 1981; Dale and Heiberg, 1984; Keep et al., 1983; Mather et al., 1980). The purpose of our study was to investigate supercooling in floral buds of ‘Danka’ black and ‘Red Lake’ red currants and to ascertain the relationship between floral morphology and freezing characteristics. Materials and Methods ‘Danka’ black and ‘Red Lake’ red currant samples were collected from 21and 6-year-old plants, respectively, growing at the Horticultural Research Station at Excelsior, Minn. One-yearold wood was obtained at 0.5 to 1 m above the soil surface on 13 Nov. 1989, 17 Jan. 1990, and 19 Mar. 1990 for DTA, viability tests, and scanning electron microscopy (SEM). On 10 Dec. 1990, ‘Danka’ samples were also obtained for DTA. All plant material was placed in sealed plastic bags containing moist paper towels, packed on ice, and shipped by overnight mail to the Univ. of Missouri where the freezing tests were conducted. Samples were stored at 2C for 24 h before being tested. DTA. The DTA system used in this study has three sample chambers and a reference chamber, each containing a Yellow Received for publication 11 Mar. 1991. Contribution from the Missouri Agricultural Experiment Station, Journal Series no. 11323. We thank D. Bedford and J. Luby for providing plant material. Technical assistance by Glen Davis is acknowledged. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. lAssociate Professor, Dept. of Horticulture. ‘Associate Professor, School of Natural Resources. 3Research Horticulturist. 1030 Springs Instrument (Yellow Springs Instrument Co., Yellow Springs, Ohio) Series 400 thermistor probe connected to a signal conditioning network (George, 1982). The analog outputs from the signal conditioning network were converted to digital format and stored on floppy disks using an Analog Device AD363 data acquisition system (Analog Devices, Norwood, Mass. ) interfaced with a Southwest Technical Products (Southwest Technical Products Corp., San Antonio, Texas) 6809 microcomputer. Data were plotted on a Houston Instruments (Houston Instruments, Austin, Texas) 2000 X-Y plotter. The DTA system can resolve freezing events that produce differential temperatures of ≈ 0.001C. Intact buds were removed with a small section of adjacent stem tissue from the middle portion of l-year-old canes for all dates, except ‘Red Lake’ buds tested on 19 Nov. On this date, bud scales and leaves surrounding the inflorescence were removed before being frozen. Each sample was placed in an aluminum foil cup and attached to a thermistor. An empty aluminum foil cup was attached to the reference sensor to balance the heat capacity of the reference and sample chambers. The temperature was lowered rapidly to 0C and cooling was initiated at 3C/h when the bud temperature was within 2C of the reference temperature. Data were collected at 40-sec intervals until the sample temperature reached – 40C. After the DTA test was completed, samples were removed from the chamber and placed in vials containing formalin–acetic acid–alcohol (FAA) fixative. Samples were mailed to the Appalachian Fruit Research Station, Kearneysville, W.Va., where they were dehydrated in a graded ethanol series, dissected, and examined under a stereoscope. Additional nonfrozen control samples were also fixed in FAA and prepared for SEM in Kearneysville. Dehydrated buds were critical-point-dried with CO2, sputter-coated with gold palladium alloy, and examined on a Cambridge StereoScan 120 electron microscope (Cambridge Instruments, Cambridge, England) operated at 8 kV. In Dec. 1990, additional DTA experiments were conducted on the following tissues: 1) intact sections of canes in which the floral bud had been removed; 2) cane samples of pith and Abbreviations: DTA, differential thermal analysis; HTE, high-temperature exotherm; LTE, low-temperature exotherm; SEM, scanning electron microscopy. J. Amer. Soc. Hort. Sci. 116(6):1030-1034. 1991.

Supercooling in Floral Buds of `Danka' Black and `Red Lake' Red Currants