Apple Rootstock-Induced Dwarfing is Strongly Influenced by Growing Environment

The primary axis of a compound apple tree is derived from a scion bud that is grafted onto the rootstock. When the grafted bud is vegetative, the primary axis develops directly from the apical meristem. In the case of a mixed floral bud, the apical meristem terminates in an inflorescence and the axis development is continued by outgrowth of a sub-terminal axillary meristem. In plant architecture, these two types of axis development arereferred to as “monopodial” and “sympodial” respectively. In apple, a vegetative bud contains 9-11 preformed nodes whereas a floral bud has only 5-6 preformed vegetative nodes, which may affect the onset and vigour of primary axis development. Anecdotally, it is thought that grafting scion wood comprised mixed floral buds limits young tree growth and therefore final tree size in the first year of growth after propagation. However, no previous studies have determined how grafted bud type may quantitatively modify scion architecture and whole tree dry mass gain.

During the first season of growth after grafting, axillary meristems along the primary axis either grow out to form a syllepticshoot, or develop into either a floral bud or a vegetative bud that remainsdormant until spring of year two. Apple dwarfing rootstocks reduce tree size by altering the development of axillary meristems, which affects the type of growth units that develop during the following season