Submitted to: HortScience
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/1/2000
Publication Date: 10/31/2001
Citation: Interpretive Summary: Late spring frosts can cause severe injury to many fruit crops, including blackcurrants (Ribes nigrum L.). Specifically, flowering shoots (strigs) and individual flowers are often killed after exposure to a frost. The frost hardiness of different cultivars under field conditions cannot be explained by their ability to tolerate freezing (as measured under labora- tory conditions). Other mechanisms must explain the differential perfor- mance of the cultivars. We used a new method, infrared thermography, to study the freezing response of flowering shoots of blackcurrant. This technique allows one to directly visualize where freezing is initiated in plant tissues and how ice propagates from one plant part to another. Our data indicate that the strigs and individual flowers of blackcurrant have the ability to avoid freezing (supercool)despite the presence of ice in the main shoots. Flowers froze individually over a wide range of temperatures and over a considerable period of time. Therefore, barriers to ice propagation must exist. The frost hardiness of different cultivars observed under field conditions may be due to the type of barrier that exists between the main shoot and the strig. This research demonstrates that supercooling is possible in woody plants, at least in blackcurrant, and could be used as a selection trait in a breeding program whose objective is improved frost hardiness. The research also demonstrates the utility of using infrared thermography to study freezing in plants.
Technical Abstract: Ice formation and movement in stems, leaves, and flowers of blackcurrant (Ribes nigrum L.) were observed by infrared video thermography. Stem sections bearing leaves and racemes were cooled slowly to temperatures between -5 to -8C and allowed to freeze without artificial nucleation. Ice formed in stems first, then moved into leaves and racemes. Patterns of ice movement were complex, and depended upon the temperature of the initia nucleation event. Individual flowers were observed to freeze between -2 to -6C. Survival of flowers after a cooling treatment depended upon whether they froze as well as upon the amount of freezing that occurred in the peduncles to which the flowers were attached. Some flowers survived the initial freezing treatments only to die later because of damage in the peduncles to which they were attached. Movement of ice from stems into peduncles was observed to occur in discrete steps, separated in time and temperature. Several independent freezing events were often observed in a peduncle, rather than a single, continuous freezing event. Pedicels attached to frozen peduncles often remained supercooled for periods ranging from several minutes to over an hr. before freezing. Freezing of individual flowers in a single inflorescence showed no pattern. 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 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 for ice to pass through these barriers is inversely related to the level of supercooling that has occurred prior to freezing.