Effective crop load management of apple is critical to the viability of commercial orchard operations. Growers routinely use a variety of strategies to produce consistent annual yields of high-quality fruit. Standard techniques such as strategic pruning, nitrogen management, and chemical thinning help maintain appropriate balance between vegetative and reproductive growth, yet many well-managed apple trees still fall into biennial bearing cycles.
The financial and horticultural costs of alternate bearing have been decried for decades in commercial apple industries around the world, including those of New York (Beach, 1903), the Pacific Northwest (Palmer and Fischer, 1937), and Australia (Bowman, 1932; Carne, 1931). Historically, research on biennial bearing focused on effective thinning techniques, but more recently, synthetic bioregulators have proven effective at either promoting or inhibiting floral initiation in apple. By applying a flowering promoter such as naphthaleneacetic acid (NAA) (Harley et al., 1958), naphthylacetamide (Harley and Regeimbal, 1959), or ethephon (Byers and Carbaugh, 1991) in the “on” year and/or a flowering inhibitor such as gibberellic acid (GA) in the “off” year (Marino and Greene, 1981), the peaks and valleys of a biennial cycle can theoretically be diminished to achieve annual yields with less variability.
Chan and Cain (1967) demonstrated the importance of fruit seeds with respect to flowering in a classic experiment on the parthenocarpic apple cultivars Ohio 3 and Spencer Seedless. Manual pollination of blossoms induced a marked reduction in flowering the next season, suggesting a direct correlation between seed development and floral initiation. Careful analyses of apple seed extracts have subsequently identified a variety of endogenous gibberellins (Ramirez, 1995) and exogenous GAs have been shown to reduce flowering in apple the season after application (Bertelsen and Tustin, 2002; McArtney, 1994; Meador and Taylor, 1987; Tromp, 1982).
The presence or absence of gibberellins clearly influences flowering in apple, but the underlying mechanisms for that relationship are not well understood. Low return bloom in poorly thinned apple trees is often blamed on floral inhibition from endogenous gibberellins synthesized in the endosperm of developing seeds (Elfving, 1996), but the systematic model of flowering in apple may involve multiple metabolic mechanisms.
Pointing to the lack of direct proof of GA export from apple seeds, Dennis and Neilsen (1999) argued for consideration of an alternative model for floral initiation in which seeds have high demand for the presumed flowering promoter, florigen; if differentiating buds are poorer sinks for florigen than surrounding fruitlets/seeds, they would be less likely to become reproductive. Using radiolabeled GA and mass spectrometry, Stephan et al. (1999) did later offer evidence of GA export from apple fruits and transport of radiolabeled exogenous GAs to surrounding pedicels and bourses (Stephan et al., 2001). They further found that ‘Spencer Seedless’ exhibited quantities of GA similar to those of seeded cultivars Golden Delicious and Jonica and suggested that GA may be produced in the pericarp of ‘Spencer Seedless’. Despite the findings of Stephan's group, the hypothesis posited by Dennis and Neilsen may still be valid; plant hormones other than gibberellins could logically play a role in floral initiation.
Conducting a similar study to Chan and Cain's on the facultatively parthenocarpic pear (Pyrus communis L.) cultivar Williams Bon Chretien (syn. ‘Bartlett’), Weinbaum et al. (2001) found that return bloom was inhibited on spurs bearing fruit, whether or not those pears contained seeds. Although this phenomenon is common in California, where the study was conducted, in other geographic regions, seeded ‘Bartlett’ pears are believed to be more inhibitory to floral initiation than unseeded fruit. Because of this inconsistency among genotypes in different locations, the authors suggest that “seed-derived hormonal inhibitors may not function consistently as the primary determinant of floral initiation” and that “broad extrapolation of Chan and Cain's results to other apple cultivars and other species may be inappropriate” (Weinbaum et al., 2001).
Callejas and Bangerth (1997) found increased levels of diffusible indoleacetic acid, an auxin, in fruits and shoot tips of ‘Elstar’ and ‘Golden Delicious’ during the period of floral initiation, especially after application of exogenous GA. The study did not report treatment effects on return bloom, but the impact of GA on transport of auxin during bud differentiation suggests a complex model of flowering involving multiple hormonal factors.
Ethylene is known to thin fruit and promote reproductive growth in many tree fruit species (Greene, 1996), but it is unclear whether increased floral initiation is directly controlled by the gas or associated with increased plant resources resulting from reduced vegetative growth (Walsh and Kender, 1982). Regardless of specific mechanisms, ethylene-inducing bioregulators such as ethephon and NAA are widely used by commercial orchardists to promote return bloom.
Ethephon can also be a useful tool when increased florigenesis is desired without increased thinning of the current season's crop; Ferree and Schmid (2000) demonstrated these effects in ‘Fuji’ with weekly applications at 200 mg·L−1 starting at 10-mm fruitlet size for durations of 4 or 6 weeks. Byers (1993) found that 1200 mg·L−1 of ethephon reduced current-season fruit size and trunk cross-sectional area but increased return bloom of ‘Starkrimson Delicious’ when applied at 26, 61, and 103 d after full bloom. Return bloom has also been improved by applications of ethephon around June drop (35 to 50 d after full bloom) on ‘Nured Delicious’ (Byers and Carbaugh, 1991), ‘McIntosh’, and ‘Melba’ (Karaszewska et al., 1986) and ‘Golden Delicious’ and ‘Cox's Orange Pippin’ (Luckwill and Child, 1978). The Crop Protection Guide for Tree Fruits in Washington (Washington State University, 2009) recommends application of 300 mg·L−1 ethephon after June drop has begun to promote flowering in bearing apple trees without excessive thinning of the current crop.
Considerable work has been published regarding effective isomers, rates, and timings for using GA and ethephon to manage bloom in apple, but little has been reported regarding the influence of initial crop on the efficacy of these bioregulators. Greene (1989) sprayed GA in split applications on ‘Empire’ trees of varying crop load. He found that GA4 + 7 decreased return bloom on decropped trees, increased return bloom on fully cropped trees, and had little effect on trees with moderate crop loads, results that do little to clarify the role of gibberellins in floral initiation in apple. If one presumes that a tree with a light crop has a different hormone profile than one with a heavy crop, then it is reasonable to expect that the response to exogenous bioregulators might be different between those trees. This article reports on a series of experiments that further consider the influence of initial crop load on the efficacy of GA and ethephon applications in apple cultivars prone to biennial bearing.
Bertelsen, M.G. & Tustin, D.S. 2002 Suppression of flower bud formation in light cropping trees of ‘Pacific Rose’ apple using gibberellin sprays J. Hort. Sci. Biotechnol. 77 753 757
Bowman, F.T. 1932 Alternate cropping of apples: Its effect upon the industry in New South Wales Agr. Gaz. New South Wales October 777 781
Callejas, R. & Bangerth, F. 1997 Is auxin transport of apple fruit an alternative signal for inhibition of flower bud induction? Acta Hort. 463 271 277
Carne, W.M. 1931 Heavy and light cropping in alternate years: A serious defect of the Australian apple industry J. Council Scientific Ind. Res. 4 65 77
Dennis F.G. Jr & Neilsen, J.C. 1999 Physiological factors affecting biennial bearing in tree fruit: The role of seeds in apple HortTechnology 9 317 322
Elfving, D.C. 1996 Physiological processes and the coordination of vegetative and reproductive plant growth and development 3 10 Maib K. , Andrews P. , Lang G. & Mullinix K. Tree fruit physiology: Growth and development Good Fruit Grower Yakima, WA
Greene, D.W. 1989 Gibberellins A4+7 influence fruit set, fruit quality, and return bloom of apples J. Amer. Soc. Hort. Sci. 114 619 625
Greene, D.W. 1996 Ethylene-based preharvest growth regulators 149 159 Maib K. , Andrews P. , Lang G. & Mullinix K. Tree fruit physiology: Growth and development Good Fruit Grower Yakima, WA
Harley, C.P. , Moon, H.H. & Regeimbal, L.O. 1958 Evidence that post-bloom apple-thinning sprays of naphthaleneacetic acid increase blossom-bud formation Proc. Amer. Soc. Hort. Sci. 72 52 56
Harley, C.P. & Regeimbal, L.O. 1959 Comparative effectiveness of naphthaleneacetic acid and naphthylacetamide sprays for fruit thinning York Imperial apples and initiating blossom buds on Delicious apple trees Proc. Amer. Soc. Hort. Sci. 74 64 66
Karaszewska, A. , Jankowska, B. , Mika, M. & Grochowska, M.J. 1986 Effects of growth regulator treatments on the hormone pattern in the trunk and the collar tissue of apple trees Acta Hort. 179 185 194
Marino, F. & Greene, D.W. 1981 Involvement of gibberellins in the biennial bearing of ‘Early McIntosh’ apples J. Amer. Soc. Hort. Sci. 106 593 596
McArtney, S.J. 1994 Exogenous gibberellin affects biennial bearing and the fruit shape of ‘Braeburn’ apple N. Z. J. Crop Hort. Sci. 22 343 346
Meador, D.B. & Taylor, B.H. 1987 Effect of early season foliar sprays of GA4+7 on russeting and return bloom of ‘Golden Delicious’ apple HortScience 22 412 415
Palmer, R.C. & Fischer, D.V. 1937 Apple thinning investigations: Summerland experimental station 1920–1935 Tech. Bul. Can. Dept. Agr. 10 5 27
Ramirez, H. 1995 Estimation and identification of apple seed gibberellins in the early stages of fruit development Acta Hort. 394 101 103
Schmidt, T. 2006 Manipulation of crop load with bioregulators to mitigate biennial bearing in apple Master's thesis, Wash. State Univ Pullman, WA
Stephan, M. , Bangerth, F. & Schneider, G. 1999 Quantification of endogenous gibberellins in exudates from fruits of Malus domestica Plant Growth Regulat. 22 55 58
Stephan, M. , Bangerth, F. & Schneider, G. 2001 Transport and metabolism of exogenously applied gibberellins to Malus domestica Borkh. cv. Jonagold Plant Growth Regulat. 33 77 85
Walsh, C.S. & Kender, W.J. 1982 Effect of cultivar, strain, and growth regulator treatments on shoot development and ethylene evolution in apple trees J. Amer. Soc. Hort. Sci. 107 198 202
Weinbaum, S.A. , DeJong, T.M. & Maki, J. 2001 Reassessment of seed influence on return bloom and fruit growth in ‘Bartlett’ pear HortScience 36 295 297