Dwarfing

Stephen S. Miller

The culture of dwarf tree fruit dates from early times. By definition, a dwarf plant is one that is smaller than normal size at full maturity. A dwarf tree usually has other characteristics in addition to stunted growth or a reduced stature. For example, precocity, canopy architecture, time of flowering, and fruit size may be altered in the dwarf tree, although tree size is certainly the predominate character and the one most often associated with genetically based or culturally induced dwarf trees.

Interest in dwarf trees is based on the many advantages they offer in orchard management and enhanced fruit quality. Due to smaller stature, dwarf trees provide labor savings in pruning, harvesting, and spray application. Light penetration is generally greater in dwarf tree canopies, which improves photosynthesis and fruit quality. Discussions on orchard management of dwarf fruit trees are found in other parts of this book.

From an anatomical standpoint, dwarf trees are not different from standard trees, but horticultural practices can alter the physiology of a tree, resulting in dwarfing. What causes a tree to be dwarfed and how can a fruit tree be manipulated to produce a dwarfed tree? In practice, dwarf trees may result from genetic changes (natural or imposed through select breeding) or through horticultural manipulations (e.g., rootstock, pruning, training, scoring, cropping, deficit irrigation, plant bioregulators, etc.). These methods will be briefly discussed with emphasis on the physiology of the dwarfing process.

68 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT GENETIC DWARFING

Within a large population of trees, a certain number of genetic dwarf variants will occur (Schmidt and Gruppe, 1988; Scorza, 1988), generally from less than 1 percent up to 2 or 3 percent of the total population. The mechanism(s) for dwarfing may be one or more of several inherent structural or physiological characters, such as a spreading growth habit, shortened internodes, a change in hormone levels, a tendency toward basitonic (from the base) growth, or decreased vigor. Despite significant advances in genetic engineering through biotechnology, the genes responsible for dwarfing have not been identified, although recent studies are providing a better understanding of the mechanisms that cause dwarfing. Mutations in nature can also result in dwarf trees. Spur strains of apple are a result of limb or whole-tree mutations. It has been suggested that high light intensity may induce these mutations, but there are no supporting data. Spur growth habit trees are often 25 to 50 percent smaller than standard trees from which they mutate.

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