Abstract

Ice formation and movement in stems, leaves and flowers of black currant cultivars Blackdown, Ojebyn, Ben Tirran, Ben Sarek, Ben Alder and Ben Lomond were observed by infrared video thermography. Stem sections bearing leaves and racemes were cooled slowly to as low as -6.4C, and allowed to freeze without artificial nucleation. Ice formed in stems first, then moved from the stems to the leaves and racemes. The patterns of ice movement were complex and depended upon the temperature of the initial nucleation event. Individual flowers froze between -1.6 and -5.5C. The survival of flowers after a cooling treatment depended upon whether they froze and the amount of freezing that occurred in the peduncles to which they were attached. Some flowers survived the initial freezing treatments but later died because of peduncle damage. The movement of ice from stems to peduncles was sometimes observed to occur in discrete steps, separated by time and temperature. Several independent freezing events were often observed in a peduncle, rather than one continuous event. Pedicels attached to frozen peduncles often remained supercooled for several minutes to over an hour before freezing. No consistent pattern was evident during freezing of individual flowers in an inflorescence. The range of temperature over which flowers in a single inflorescence froze was in some instances over 4C. Both mature and immature flowers supercooled. Barriers to the movement of ice appeared to exist at certain anatomical junctions within the plant, notably where the peduncle of an inflorescence attaches to a stem and where a flower pedicel joins a peduncle. The time required for ice to pass through these barriers was inversely related to the degree of supercooling that had occurred prior to freezing.