Winter Root Temperature Effects on Vegetative Budbreak, Spur Development, and Flowering of ‘Delicious’ Apple

Abstract

Effects of continuous root temperatures > 15°C on vegetative budbreak, shoot and spur development, and flowering were studied during the first 3 years of growth in ‘Delicious’ apple trees (Malus domestica Borkh.) on MM.106, M.7a, and M.9 rootstocks. Sixteen trees of each rootstock-scion combination were potted and placed on an outside container area with overhead irrigation. Root-zone temperature was maintained at > 15° throughout the winter in half the trees, while the other half were at ambient temperature. Vegetative budbreak and spur development were reduced significantly and flowering was increased during the first 2 years by warm winter root temperatures. Flowering the third year was increased on MM.106 trees due to enhanced spur development the first 2 years. Flower bud to vegetative bud ratio was reduced on ambient-root trees in the second and third year and root : shoot ratio was increased, indicating that root chilling during dormancy favored vegetative growth, particularly root growth.