Chilling Requirements to Relieve Bud Dormancy in Black-fruited Aronia Taxonomic Groups Is Related to Ploidy and Geographic Origin

The genus Aronia Medik., also known as chokeberry, is a group of deciduous shrubs in the Rosaceae family, subtribe Malinae. The two commonly accepted black-fruited Aronia species are black chokeberry [Aronia melanocarpa (Michx.) Elliott] and aroniaberry [Aronia mitschurinii (A.K. Skvortsov & Maitul)]. The geographic range of wild A. melanocarpa is the Great Lakes region and the northeastern United States, with a southerly extension into the higher elevations of the Appalachian Mountains. Wild A. melanocarpa found in New England are diploids, whereas plants throughout the rest of the range are tetraploids. A. mitschurinii is a cultivated hybrid between xSorbaronia fallax (C.K.Schneid.) C.K.Schneid. and A. melanocarpa and exists as a tetraploid. There is currently limited diversity of Aronia genotypes in the ornamental and fruit industries, and many of the current cultivars are not adapted to the southern United States and similar environs with limited chilling to break winter dormancy. The goal of this study was to determine 1) the chilling requirements for A. mitschurinii 'Viking' and 2) the range of chilling requirements for wild A. melanocarpa genotypes from different geographic origins. Two experiments were conducted in which plants were subjected to various chilling accumulation treatments and then moved to a greenhouse for observation of budbreak and subsequent growth. Expt. 1 was conducted at the University of Maryland at Wye, MD, and focused solely on the commercial cultivar A. mitschurinii 'Viking'. Outdoor, ambient fall and winter temperatures were used to achieve the chilling treatments. In Expt. 1, we determined the optimal chilling requirements for A. mitschurinii 'Viking' to be greater than 900 h using the single temperature model. Expt. 2 was conducted at the University of Connecticut and focused on wild genotypes, plus A. mitschurinii 'Viking'. A fixed temperature cold room was used to achieve chilling treatments. In Expt. 2, we found A. melanocarpa genotypes from southern regions in the United States required chilling accumulation of 600 h (single temperature model), compared with genotypes from northern regions that required more than 900 h of chilling accumulation. Tetraploid A. melanocarpa required 900 h of chilling to break bud, but diploid A. melanocarpa required 1200 h of chilling to break bud. Expt. 2 confirmed the 900-h chilling requirement for A. mitschurinii 'Viking'. For both experiments, the rate of budbreak and shoot growth was positively correlated with increasing amounts of chilling.