Profitable apple production requires regular yields of marketable fruits. The challenge is that many commercially important cultivars are biennial, producing excessive numbers of flower clusters and fruits in one year, which suppress flower initiation for the next year. This leads to oscillation in crops between many small, poor-quality fruits in one year and a few large fruits or no fruits at all in alternate years (Stover et al., 2001). Rootstock and environmental factors such as drought, spring frost, diseases, light interception, and canopy architecture manipulation can also impact the bearing pattern of apple trees (Davis, 2002; Fulford, 1965; Willaume et al., 2004).
High crop yields inhibit flower initiation. Chan and Cain (1967) showed the effects of seeded and seedless fruit on return bloom and concluded that hormones exported from the seeds inhibit floral initiation. They showed that only the pollinated side of the parthenocarpic apple cultivar Spencer Seedless had no return bloom, demonstrating floral inhibition to be a localized effect. Subsequent work has tried to identify the hormones involved.
The distance between flower clusters in one year has long been known to influence the development of floral buds for the next year (Fulford, 1966). ‘Wealthy’, a strongly biennial cultivar, requires a minimum distance of 15 to 25 cm between individual flower clusters, approximately equivalent to 40 flower clusters/m3 of canopy volume, to maintain regular yields (Bobb and Blake, 1938). More recent work on ‘Honeycrisp’, a strongly biennial cultivar, demonstrated that pruning short (1–10 cm) branches known as spurs to reduce flower cluster number to between 40 and 60/m3 of canopy volume is necessary to control biennial bearing (Nichols et al., 2011). These studies and others suggest that biennial bearing may be a result of carbon limitation and/or hormonal inhibition of floral initiation by nearby seeds as reported by Dennis (2000) and Tromp (2000). However, none of these studies conclusively settled the question.
The balance of hormone concentrations has been hypothesized to control floral initiation (Callejas and Bangerth, 1997; Hoad, 1984; Luckwill, 1970). Before 2000, there had been many conflicting reports on the effects of applying exogenous hormones on concentrations of endogenous hormones such as auxins, gibberellins, cytokinins, and abscisic acid (Grochowska, 1968; Ramírez and Hoad, 1981). These are well reviewed by Dennis and Neilsen (1999).
In more recent years, endogenous gibberellins (possibly GA1, GA4, and iso-GA7) produced by seeds have been shown to inhibit floral initiation in ‘Golden Delicious’ (Ramírez et al., 2004a, 2004b). This was confirmed with heavily cropping ‘Fuji’ trees having higher concentrations of endogenous GA1 and GA4 in apical buds at the time of floral initiation than was found on blossom-thinned trees (Kittikorn et al., 2010). Other endogenous hormones [9,10-ketol-octadecadienoic acid (KODA) and jasmonic acid (JA)] have been found at high concentrations in apple trees when GAs are low (blossom-thinned trees) and at low concentrations when GAs are high (heavily bearing trees). KODA and JA are associated with flower induction in other plants and may promote floral initiation in apple (Kittikorn et al., 2010, 2011; Kong et al., 2005; Suzuki et al., 2003; Yokoyama et al., 2000, 2005).
Thus, although more than 70 years have passed since Bobb and Blake identified the effect of a heavy crop on return bloom, the underlying mechanisms remain unclear and hypotheses remain unconfirmed. Mathematical modeling may help researchers better understand the phenomenon and clarify the mechanisms involved. This article models the hypothesis originally developed by Hoad (1984) that a balance of hormones controls flower initiation in apple trees and predicts the effect of flower thinning on percent bloom in the next year.
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Essential R code to create the flowering branch model.