APPLE BUD DEVELOPMENT AS INFLUENCED BY ROOTSTOCK

Abstract

Apple flower formation was studied on mature `Starkspur Supreme Delicious' apple trees growing on 5 rootstocks: B.9, M.26 EMLA, M.7 EMLA, P.18, and seedling. In 1991 and 1992 buds were sampled from the previous seasons growth and dissected. In 1991 10-20% of the buds from trees on M.26 EMLA, P. 18 and seedling rootstocks formed flowers whereas flowers were evident in about 50% of the buds from B.9 and M.7 EMLA trees. In 1992 however, over 90% of buds from trees on all rootstocks formed flowers. Despite very different environmental conditions and degree of flower formation, flower differentiation occurred between 80-110 days after full bloom in both years and among all rootstocks. The critical number of nodes present in buds prior to the appearance of flowers was 18-19 and was not influenced by rootstock. Increases in node number over the course of a season was primarily due to increases in the number of bracts present in the bud. In 1991 (a season characterized by high temperatures and low rainfall) buds from trees growing on B.9 and M.7 EMLA rootstocks exhibited a higher final node number and a greater degree of flower initiation than buds from other trees, but no differences were observed during the more favorable conditions in 1992.