An Initial Estimation of Chill Requirement in the Dormancy Release of the Deciduous Specialty Fruit Crop Aronia

An initial estimation of chill requirements for the cultivated variety of the black chokeberry Aronia mitchurinii have been developed using three classic temperature‐based models. Aronia, as it is known in the commercial trade, is a shrub in the Family Rosaceae and in the apple subtribe of Pyrinae and is known for the high antioxidant content in the fruit. It is presently gaining interest as a specialty fruit crop in the United States. The purpose of this study was to estimate the minimal chill requirements for dormancy release of A. mitchurinii through the use of three chill models and develop recommendations for the southern extent of orchards in the U.S. Three temperature models were used to calculate chilling hours or chilling units though measured ambient air temperatures during two dormant seasons. Chilling hours were determined by calculating the number of hours below 7° Celsius (H7) and by the number of hours between 7° and 0° Celsius (HB). Chilling units (CU) were calculated through the Utah model which assigns a weighted effectiveness within a range of temperatures. Several times during both seasons, a randomly selected group of dormant plants being held outside were brought into a greenhouse environment being maintained at 23° Celsius with 12 hours of light (both daylight and supplemental). At that point, the accumulated chill hours/units were calculated for that group using the three models. Bud swelling and dormancy break was observed in these plant groups. It was noted, especially in the second year study, that plant groups receiving less than 800 H7 did not exhibit full terminal bud dormancy release. With all groups receiving more than 900 H7, dormancy release occurred, but axillary buds broke dormancy before terminal buds in at least half the plants. Averaging results from the two years, full dormancy release occurred only after 1100 H7, 828 HB and 626 CU. Greater chilling hour or unit accumulation increased vigor and growth rate. This information can set initial guidelines for orchard locations within the U.S. With a commercial preference for organically grown fruit, many orchards would not consider growth regulators to break dormancy, limiting the plant's southern range. Further studies in successive years are needed to more accurately determine chill requirement for Aronia dormancy release, along with examining the best model for that determination.