Chilling requirement, (the number of hours below 7 °C necessary to break dormancy) has been shown to vary with genotype in blackberry (Rubus subgenus Rubus). Previous work has demonstrated that the chilling requirement of field-grown plants could be accurately determined from stem cuttings of lateral shoots taken at 100-hour intervals of chilling up to 1000 hours, by placing them in a mist chamber maintained at 26 °C with a daylength of 16 hours, and observing budbreak over a period of 5 weeks. This technique has previously demonstrated clear differences in the chilling requirements of thorny and thornless floricane-fruiting cultivars. In the current study, a comparison of floricane-fruiting and primocane-fruiting blackberries using the stem-cutting technique illustrated a lower chilling requirement associated with the primocane-fruiting trait. The use of the stem-cutting technique can be a simple and effective tool for assessing blackberry adaptation to different hardiness zones.
The chilling requirements of the University of Arkansas blackberry cultivars Apache, Ouachita, and Prime-Jim*, and the primocane-fruiting selections APF-25, APF-27, APF-40, APF-42, APF-44, APF-46, APF-52, and APF-53 were investigated using stem cuttings from field-grown plants. A biophenometer was used to measure chilling (hours below 7 °C) in the field and 12-node cuttings of lateral shoots were taken from the cultivars every 100 hours up to 1000 hours below 7 °C. However, only 500 chilling hours had occurred at the time of this writing, and the response of budbreak to higher chilling levels could not be reported. The cuttings were placed in a mistchamber in the greenhouse with a daylength of 16 hours and air temperature of 26–29 °C. Percent budbreak was measured weekly. The cultivar × chilling interaction was significant (P = 0.05). `Apache' and `Ouachita' showed little or no budbreak up to 500 h, indicating a higher chilling requirement. The chilling requirement of Prime-Jim was determined to be between 300 h and 400 h, and that of the APF selections appeared to be between 300 h and 500 h. The chilling requirement of APF-53 could not be determined since budbreak was consistent at all levels of chilling up to 500 h. In general, the primocane-fruiting genotypes appeared to require less chilling than floricane-fruiting `Apache' and `Ouachita', and they would therefore be more suitable for low-chill locations.
Southern highbush blueberry, a hybrid of northern highbush (Vaccinium corymbosum) and southern-adapted Vaccinium species, has the potential to meet the need for an early-ripening blueberry in the southern U.S. southern highbush cultivars can ripen up to one month earlier than the earliest rabbiteye (Vaccinium ashei) cultivars currently grown in the southern U.S. However, chilling requirement and cold-hardiness are cultivar-dependent for southern highbush and cultivar testing has been necessary to determine the cultivars best adapted to specific hardiness zones. In a 4-year study at Hope, Ark. (hardiness zone 7b), several southern highbush cultivars were evaluated for productivity, fruit quality and reliability of cropping. Yields were based on 1089 plants/acre (2690 plants/ha) for southern highbush cultivars and 605 plants/acre (1494 plants/ha) for rabbiteye cultivars. `Ozarkblue' and `Legacy' showed the most adaptability at this location, yielding on average 11,013 lb/acre (12,309 kg·ha-1) and 10,328 lb/acre (11,543 kg·ha-1) respectively, compared to 4882 lb/acre (5456 kg·ha-1) for `Premier' (rabbiteye) over 4 years. `Ozarkblue' and `Legacy' also rated well for plant vigor and fruit quality. We would recommend `Ozarkblue' and `Legacy' for commercial planting in southwest Arkansas and believe these cultivars have production potential for other areas of the southern U.S. that have similar hardiness zones and soil type to southwest Arkansas.