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Mokhles A. Elsysy, Michael V. Mickelbart and Peter M. Hirst

One of the most challenging problems associated with fruit tree production is biennial bearing. Biennial bearing is “the fluctuation in cropping from year-to-year caused by irregular flowering” ( Hirst, 2017 ). Considerable research has been

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Mokhles A. Elsysy and Peter M. Hirst

Consistent fruit production is highly dependent on flowering frequency. Flower formation is related to biennial bearing and continues to be a major challenge for the apple industry and many other fruit tree industries worldwide. Biennial (or

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B.C. Strik, T.R. Roper, C.J. DeMoranville, J.R. Davenport and A.P. Poole

Biennial bearing has long been thought to occur in cranberry (Vaccinium macrocarpon Ait). Researchers have shown that percent return bloom on fruiting uprights can range from 12% to 65% depending on year, bed vigor and cultivar. Resource limitation and/or hormonal factors in a fruiting upright may be related to flower bud initiation and, thus, percent return bloom the following year. This research was undertaken to determine the extent of biennial bearing by cranberry cultivar and growing region. Seven cultivars were studied; three found in all states (MA, NJ, WI, OR), two common to MA and NJ, and two different cultivars in WI and OR representing cultivars commercially grown in these areas. In the fall or winter of 1989/1990 six 2-m transects were randomly selected within a cranberry bed for each cultivar. Along the transect, 60 uprights that fruited in 1989 were tagged. In the summer of 1990, fifty of the uprights will be sampled to determine percent return bloom and percent set.

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B.C. Strik, T.R. Roper, C.J. DeMoranville, J.R. Davenport and A.P. Poole

Biennial bearing has long been thought to occur in cranberry (Vaccinium macrocarpon Ait). Researchers have shown that percent return bloom on fruiting uprights can range from 12% to 65% depending on year, bed vigor and cultivar. Resource limitation and/or hormonal factors in a fruiting upright may be related to flower bud initiation and, thus, percent return bloom the following year. This research was undertaken to determine the extent of biennial bearing by cranberry cultivar and growing region. Seven cultivars were studied; three found in all states (MA, NJ, WI, OR), two common to MA and NJ, and two different cultivars in WI and OR representing cultivars commercially grown in these areas. In the fall or winter of 1989/1990 six 2-m transects were randomly selected within a cranberry bed for each cultivar. Along the transect, 60 uprights that fruited in 1989 were tagged. In the summer of 1990, fifty of the uprights will be sampled to determine percent return bloom and percent set.

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Frank G. Dennis Jr. and John C. Neilsen

The evidence for several hypotheses regarding the mechanism(s) controlling biennial bearing in apple (Malus×domestica Borkh.) are reviewed, citing relevant evidence from work with citrus (Citrus sp.) species and pear (Pyrus communis L.). The view that flowering is inhibited by withdrawal of nutrients, primarily carbohydrates, by apple fruit is questionable, given the effects of seed development in inhibiting flowering in facultatively parthenocarpic (normally seedless) apple cultivars. The hypothesis that seeds inhibit flowering by exporting hormones, chiefly gibberellins (GAs), is an attractive one, given a) the effects of application of GAs in inhibiting flowering and b) the high concentrations of GAs in seeds. However, an alternative hypothesis, namely that seeds compete with apices for hormones that are required for flowering, is equally tenable.

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David C. Ferree

`Jonathan'/M.26 apple (Malus domestics Borkh.) trees were root-pruned annually on two sides, 60 cm from the trunk, to a depth of 40 cm for 6 years while dormant, at bloom, or in mid-June. Root pruning reduced terminal shoot growth by ≈30% in 1985-89 with no influence in 1990. Cumulative yield was reduced by root pruning at bloom (14%) or mid-June (20%), and cumulative yield efficiency [kg·cm-2 trunk cross-sectional area) was reduced by root pruning with no difference among pruning times except in 1 year, where abundant moisture throughout the season appeared to negate the effect. The intensity of biennial bearing was reduced by root pruning with no relationships to time of pruning. Root pruning resulted in a decrease in large fruit and an increase in small fruit in 3 of the 6 years. A covariant analysis with yield showed that root pruning reduced average fruit size. Root-pruned trees produced firmer fruit with an increased soluble solids concentration and had less preharvest drop than nonpruned trees. Under severe drought conditions in 1988, root pruning reduced net photosynthesis and transpiration; supplemental water (57 liters·week-1) increased transpiration and fruit size at harvest.

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Steven McArtney, Duane Greene, Tory Schmidt and Rongcai Yuan

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

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Lisa Wasko DeVetter, Rebecca Harbut and Jed Colquhoun

the next year, whereas vegetative buds lack flower initials and only contain a vegetative meristem. Alternating patterns of mixed bud formation on individual uprights leads to biennial bearing and it has been shown that fruiting uprights are less

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Steven McArtney, Dick Unrath, J.D. Obermiller and Ann Green

Many apple cultivars, including ‘Golden Delicious’, ‘Fuji’, ‘Cameo’, ‘Pacific Rose’, ‘York Imperial’, ‘Braeburn’, and ‘Mutsu’, are prone to develop a biennial bearing habit: a repeating cycle of heavy and light blooming/cropping years. Biennial

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Yicheng Tu, Peter Hirst, Ronald Coolbaugh and Richard Pharis

It is believed that gibberellins (GA) produced in apple seeds act as an inhibitory signal to flower induction in the neighboring buds. The alternate bearing behavior of an apple cultivar is therefore likely to be associated with the activity of endogenous GAs in the seeds of that cultivar. To elucidate the impact of GAs on the flowering of biennial and non-biennial apple cultivars, fruits were sampled from `Fuji' (biennial, on-year) and `Gala' (non-biennial) trees periodically during the early part of the growing season. Seeds were removed from fruits immediately, frozen in liquid Nitrogen and freeze dried. Full scan analysis for GAs using GC-MS identified 16 GAs: GA1, GA3, GA4, GA7, GA20, GA31, GA34, GA35, GA44, GA50, GA54, GA61, GA63, GA68, GA80, and GA88. In addition, we also traced a number of GA-like mass spectra that do not match any published GA mass spectrum reference. The possible structures of these GA-like compounds were also proposed. More types of GAs were found in `Fuji' seeds than in those of `Gala'. This suggests higher GA activity in `Fuji' considering almost all the GAs identified are biologically active. Unlike the results of recent researches on GAs in other apple cultivars, we found that the major GA types in both cultivars are GA80 and GA63, rather than GA4 and GA7. `Fuji' contained significant amounts of GA88, which did not appear in `Gala' samples. Other studies are currently underway to quantify specific GAs from these seeds using deuterated internal standards.