Susceptibility of blackberry flowers to freezing temperatures

Authors: Takeda, Fumiomi; Glenn, David

Submitted to: European Journal of Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2016
Publication Date: 4/1/2016
Citation: Takeda, F., Glenn, D.M. 2016. Susceptibility of blackberry flowers to freezing temperatures. European Journal of Horticultural Science. 81(2):115-121.

Interpretive Summary: Spring freeze can be devastating to blackberry growers. In this study, we determined injuries in blackberry buds and flowers after exposure to freezing temperatures in a radiation frost chamber that mimicked overnight radiation frost. An infrared thermal-imaging camera showed that once ice formed in the cane it spread quickly to individual flower buds. Temperature recording equipment connected to miniature sensors measured air temperature surrounding the blackberry plant and inside unopened flower buds and opened flowers. These recordings revealed that inside buds that were still unopened the temperature dropped much slower than in opened flowers. However, unopened flowers froze at slightly higher temperatures than opened flowers. Above 28 F, no injury was visible, but when the internal temperature dropped below 27 to 27.5 F, all buds and flowers were killed. Frosts in which temperatures drop to about 27 F for a short time may not injure buds that are still tight because temperature inside them remain above the lethal range. This study provided information that leads researchers to reconsider the relationship between development stages of flower bud, actual tissue and air temperatures, and susceptibility of flowers to freezing temperatures.

Technical Abstract: Injury of tight buds, open flowers and green fruit often occur in fruit crops during spring frost events. In this study, freezing tolerance of ‘Triple Crown’ blackberry flowers at different reproductive stages of development (tight bud to green drupe) was determined using two methods. One method involved placing whole plants in a radiation frost chamber and taking plants to temperatures ranging from -1.0 to -4.0 degrees C. After freeze treatments, all buds, open flowers, and green fruit were evaluated for damage and the lethal temperature to kill 50 percent of the buds and flowers was determined. In order, from the most sensitive flower part to the least sensitive were the interior ovary (ovule), receptacle, style, stigma, petal, filament, and anther. In the second method, thermocouple wires were inserted into tight bud, open flower, and green fruit and the chamber temperature was slowly lowered to -4.0 degrees C. The thermocouple readings were captured at 5 sec intervals to characterize exotherm. In all runs, tight buds generated an exotherm at -2.5 degrees C and open flowers generated an exotherm at -2.8 degrees C. The tissues comprising the gynoecium (ovary, style, and stigma) were less tolerant to freezing than the androecium and the corolla. The findings suggest that the buds at tight stage are not more resistant to freezing temperatures than those that have already opened. Buds at tight-bud stage are still encased tightly in fused sepals and unfolded petals. These features delay lethal temperatures reaching more freeze-sensitive tissues than in buds that are more developed and their pistils are no longer shielded by the perianth. 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.