Many important apple (Malus ×domestica Borkh.) varieties develop a biennial bearing habit, in which there is a repeating cycle of a heavy crop 1 year followed by a light crop or no crop in the next year. Regulating the flower bud formation process to restore a more equal balance between vegetative and reproductive spurs is one strategy for restoring trees from a biennial bearing habit to consistent cropping. Early removal of fruit from apple spurs can promote return bloom (Aldrich, 1932; Aldrich and Fletcher, 1932; Harley and Masure, 1937; Harley et al., 1934). An effective chemical fruit thinning program reduces the number of fruit per tree or per spur, thereby increasing the probability that the terminal axillary meristem on a flowering or fruiting spur will develop reproductive structures. However, cultivars with a strong natural tendency for biennial bearing remain predisposed to an alternating habit, even after a successful chemical thinning program has reduced the fruit number to a commercially acceptable level (personal observation by authors). In such situations, additional strategies are needed to restore consistent cropping.
Application of growth regulators GA3, GA4+7, or GA7 to apple trees in the non-fruiting year of the biennial bearing cycle can inhibit flower bud formation (Fulford, 1973; Marino and Greene, 1981; McArtney, 1994; McArtney and Li, 1998; Meador and Taylor, 1987; Schmidt et al., 2010), whereas application of ethephon (Williams, 1972) or NAA (Harley et al., 1958; McArtney et al., 2007) in the heavy cropping year of a biennial bearing cycle can stimulate flower bud formation. Williams (1972) reported that postharvest application of ethephon to young vigorous ‘Wellspur Delicious’ apple trees stimulated flower bud formation. Thus, the transition from vegetative to floral development may be triggered by a chemical stimuli relatively late in the season.
‘York Imperial’ is an important processing apple in Virginia and Pennsylvania that can develop a biennial bearing habit. Hand-thinning ‘York Imperial’ trees 8 d after bloom resulted in abundant fruit bud formation, whereas thinning later than 25 d after bloom did not effectively increase return bloom on trees with a heavy fruit set (Aldrich and Fletcher, 1932), suggesting that a signal inhibiting flower bud formation emanated from the fruit of this cultivar very early in the growing season. The combination of limb-ringing and fruit removal to establish 100 leaves per fruit stimulated fruit bud formation on ‘York Imperial’ apple trees when it was imposed before 60 d after full bloom but was less effective after this time (Magness et al., 1934). It was proposed that buds on high-vigor trees remain in a meristematic state later into the season compared with buds on low-vigor trees and presumably therefore respond to florigenic stimuli later than buds on less vigorous trees (Magness et al., 1934).
NAA directly stimulates flower development in apple (Harley et al., 1958). Applications of NAA in the heavy cropping year of a biennial bearing cycle increased the proportion of flowering spurs in the next year (McArtney et al., 2007). Applications of NAA to ‘Golden Delicious’ at weekly intervals during the month leading up to harvest (120 to 150 d after bloom) increased return bloom just as effectively as four biweekly summer applications during the period 60 to 100 d after bloom (McArtney et al., 2007). The florigenic activity of foliar NAA sprays applied during the month before harvest (McArtney et al., 2007) and of a postharvest ethephon application (Williams, 1972) is unexpected given these growth regulators were applied after the period when apple buds are believed to transition from vegetative to reproductive development.
Broadening of the meristem apex within the buds developing on 1-year-old wood of apple occurs during the period 21 to 42 d after bloom (Buban and Faust, 1982; Foster et al., 2003; Pratt et al., 1959) and this change provides the earliest physical indication of the transition to floral development. Doming of the apex follows soon after broadening, providing the first easily observable indication of the floral transition. Doming is typically observed 70 to 100 d after full bloom (Foster et al., 2003; Hirst and Ferree, 1995; Hoover et al., 2004; McArtney et al., 2001) and is relatively synchronized within buds sampled from wood of the same age (Foster et al., 2003; Hoover et al., 2004; McArtney et al., 2001). Doming of the meristem apex has not been observed later than ≈110 d after bloom, i.e., well before harvest of most apple cultivars. Thus, the timing of summer NAA sprays for return bloom coincides with the period during which buds are normally doming. In contrast, the application of NAA sprays during the month before harvest occurs well after buds have normally made this transition. The florigenic activity of preharvest NAA sprays for return bloom provides evidence to suggest that NAA can trigger floral development within vegetative buds relatively late in the season. Because ethephon sprays during the postharvest period are also florigenic (Williams, 1972), it may be possible that the positive effect of NAA on return bloom is mediated by an auxin-induced ethylene response (Curry, 1991).
The objectives of the present studies were to 1) evaluate the efficacy of summer NAA sprays for promotion of return bloom of ‘York Imperial’ and ‘Golden Delicious’ apple; 2) verify that weekly NAA applications during the month leading up to harvest are florigenic; 3) establish if ethylene is involved in the florigenic activity of NAA sprays; and 4) determine the time of floral transition in different growing environments and confirm that preharvest NAA sprays are applied after the developing buds have normally made the transition from vegetative to floral development.
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