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  Eur.J.Hortic.Sci. 81 (2) 115-121 | DOI: 10.17660/eJHS.2016/81.2.5
ISSN 1611-4426 print and 1611-4434 online | © ISHS 2016 | European Journal of Horticultural Science | Original article

Susceptibility of blackberry flowers to freezing temperatures

F. Takeda and D.M. Glenn
United States Department of Agriculture, Agricultural Research Service, Appalachian Fruit Research Station, Kearneysville, USA

In this study, freezing tolerance of Triple Crown blackberry flowers at different stages of reproduc-tive development (tight bud to green drupe) was determined. One method involved placing whole plants in a radiation frost chamber and taking plants to -1.0C, -2.0C, -3.0C, and -4.0C for 1 h (total of 8 plants for each selected temperature). After freeze treatments, all buds, open flowers, and green fruit were evaluated for damage. Exposure to -1.0C or -2C temperatures for 1 h did not injure the corolla, gynoecium or androecium. However, damage was observed in gynoecium as indicated by tissue discoloration in the ovaries, styles and stigmas when blackberry were exposed to ≤3.0C for 1 h. An exposure to -4.0C for 1 h resulted in the injury of all floral parts. In the second method, thermocouple wires were inserted into tight buds, open flowers, and green fruit of potted blackberry plants and the chamber temperature was slowly lowered to -4.0C on 8 separate runs. The thermocouple readings were captured at 5 s intervals to characterize the exotherms in tight buds and open flowers. In all runs, tight buds and open flowers generated exotherms between -2.5C and -2.8C. These findings suggested that flowers at tight-bud stage and flowers already at anthesis froze at similar temperatures. In another study, an infrared (IR) thermal imaging camera was used to determine ice formation in blackberry plants by capturing thermographic images of plant surfaces at 1 ms intervals. IR thermography revealed that ice nucleation was initiated in the cane and ice propagated into flower shoots and then quickly into several flowers and flower buds. The results of these studies suggested that the buds at tight stage have similar susceptibility to freezing temperatures to those that have already opened. The results of this study suggest that we need to reconsider the relationship between developmental stages of flower bud, actual tissue and ambient temperatures, and susceptibility to spring frost damage in blackberries.

Keywords exotherm, freezing chamber, ice barrier, infrared thermography, radiation frost, Rubus, thermocouple

Significance of this study

What is already known on this subject?

  • In spring blackberry buds, flowers, and young fruit are damaged by sub-freezing temperatures. Previously, convection chambers have been used to study frost resistance, however, radiation freeze chambers provide more realistic simulation of plant response to frost conditions.
What are the new findings?
  • Exothermic events were recorded in tight buds at -2.5C and in open flowers at -2.8C. Infrared video ther-mography revealed that ice formation was initiated in the cane and spread rapidly to the flower shoot and then to individual flower buds. Both flower buds and open flowers were killed when exposed from -2.5C to -3C. Determining the susceptibility of blackberry buds and flowers to freezing temperatures could be im-proved with these procedures.
What is the expected impact on horticulture?
  • Thermocouple and IR thermography can be used to capture freezing events and ice nucleation and propaga-tion. These technologies and radiation frost chambers will provide a precise visualization of ice nucleation and propagation in blackberry plants and simulation of natural frost events.

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Received: 22 September 2015 | Revised: 10 November 2015 | Accepted: 26 January 2016 | Published: 25 April 2016 | Available online: 25 April 2016

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