Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 21, 1997
Publication Date: N/A
Interpretive Summary: Apple trees bear fruit on short lateral branches called spurs. Spur-type trees have more spurs than usual and have the spurs spaced closer together on the branches. These trees are smaller, more compact and fruit earlier and more heavily than standard trees. Spur-type trees grown for commercial apple production originated as chance mutations that were discovered by growers in orchards. When 'Redspur Delicious' apple was propagated in tissue culture (TC), the resulting trees were variable with only 22% retaining the spur-type growth habit; the remaining trees were larger, more vigorous and had varying shapes. Three of the spur-type trees from the first TC propagation were then propagated again by TC. The resulting trees were all spur-type and uniform in size and appearance. These results may be due to the TC procedure, but similar results were not found with any of the other nine spur-type or 25 standard apple varieties propagated in TC. Another possible cause could be the chimeral nature of the original mutation to the spur-type growth habit. A chimera occurs where the mutation occurs in one cell layer that then produces a new bud and shoot. If another bud on the mutated shoot then grows from non-mutated tissue, a reversion to the original type of growth occurs. It is also possible that the TC procedure stimulated such a reversion. This information will be used by scientists studying reversion, and by extension agents and producers working with tissue cultured plants and others likely to show reversion.
Technical Abstract: Micropropagated trees of 'Redspur Delicious' apple (Malus X domestica Borkh.), planted as small actively growing trees in May, 1982, showed a lack of uniformity in tree size, appearance and flowering by the spring of 1986. Only four of the 18 trees had a typical spur-type growth habit; these four trees had significantly more spurs per meter of shoot, flowered sooner, had higher early yields, remained significantly smaller after 13 years in the orchard, but had significantly less cumulative yield than non-spur types. Shoots from the spur-type trees were recultured in 1988 and the resulting trees planted in an orchard in 1990. These latter trees were uniform in appearance and all had typical spur-type growth with about 30% more spurs per meter of shoot growth than the original trees from which they were propagated.