Effect of Cultivar, Strain, and Growth Regulator Treatments on Shoot Development and Ethylene Evolution in Apple Trees1

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Abstract ‘Miller Sturdeespur Delicious’ apple (Malus domestica Borkh.) trees made less extension growth than ‘Imperial Red Delicious’ (non-spur) under controlled conditions. No differences in extension growth were measured between ‘MacSpur’ and ‘Imperial McIntosh’ (non-spur) trees. Development of the spur-type morphology was determined by the number of lateral buds which grew into spurs during the second growing season. Application of (2-chloroethyl)phosphonic acid (ethephon) alone, or in combination with butanedioic acid mono-(2, 2-dimethylhydrazide) (daminozide) decreased extension growth and increased shoot ethylene evolution in growth chamber studies, and promoted flowering in the field. Application of daminozide plus ethephon was more effective than ethephon alone. Daminozide alone did not affect shoot ethylene. No change in ethylene evolution or shoot development was detected following applications of gibberellin 4/7 (GA) or 2, 3, 5-triiodo benzoic acid (TIBA) to spur or non-spur type trees. Bark scoring had no effect on shoot ethylene. However, this treatment also promoted flowering of young trees in the field. It is suggested that while some manipulations can increase shoot ethylene evolution and also promote flowering, the ethylene evolved following treatment operates indirectly by retarding growth, rather than by directly stimulating flower bud initiation.