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Lack of consistent flower formation is the underlying cause of biennial bearing. Flower formation in apple (Malus ×domestica Borkh.) has been associated with different factors, such as leaf area, shoot growth, bourse length, crop load, and seed number. However, it is unclear how these different factors interact to promote or inhibit flower formation. The effect of spur defoliation, fruit removal, and their interaction were evaluated on spur flower formation and bourse length in annual-bearing ‘Gala’ and the biennial-bearing ‘Honeycrisp’. Eight different combinations of spur defoliation and fruiting treatments were applied in three consecutive springs, 2013–15. Bourse shoot defoliation and fruiting treatments inhibited spur flower formation in both cultivars, but in different patterns from year to year. In addition, spur leaf defoliation did not affect flower formation in either cultivar. Furthermore, local defoliation and fruiting treatments did not affect bourse length. We propose that bourse leaves play a major role in both producing and transporting flower formation signals, but the effect depends on cultivar.

Irregular flowering and biennial bearing are challenging in many apple (Malus ×domestica) cultivars such as Honeycrisp. Apple flowering is influenced by many factors including crop load, fruit weight, seed number, and bourse shoot length. However, it is unclear how these factors exert their control. We investigated flowering in ‘Honeycrisp’ and whether flower formation is regulated locally within the spur or if it is under the control of the whole tree system. Treatments consisting of 30 to 240 fruit per tree with one or two fruit per spur were applied, and seed number, fruit weight, bourse shoot length, bourse number, and resulting flower formation measured. In 2013, flowering was affected by fruit number per tree, fruit number per spur and their interaction, and with lower total tree crop loads, spurs bearing two fruit had fewer flowers than those with a single fruit. In 2014, few spurs formed flowers regardless of treatment. In 2013, flowering was unaffected by seed number on single-fruited spurs but flowering inhibition was correlated with fruit weight. In spurs bearing two fruit, flowering was inhibited by higher seed numbers but fruit weight per spur had no effect on flowering. Our data suggest that both whole tree and within-spur characteristics contribute to local flower formation. Therefore, ‘Honeycrisp’ spurs can be considered semiautonomous organs because inhibition of flower formation appears to be related to the depletion of resources both locally within the spurs, and systematically within the whole tree. The main factors associated with flower formation were fruit number per tree, fruit number per spur, bourse shoot length, and bourse number per spur. In contrast to previous reports, our data show that seeds do not play a direct role in regulating flower formation.

Adequate flower formation limits dependable apple (Malus ×domestica) production and is a major challenge for apple industries around the world. ‘Honeycrisp’ is a high value apple cultivar, but consistent flowering is difficult to achieve. Apple flower formation is affected by factors including defoliation, girdling, and gibberellin (GA₄₊₇) and 1-naphthaleneacetic acid (NAA) applications. However, the molecular mechanisms that regulate the effects of these factors are not well understood. We studied the effect of local spur defoliation, GA₄₊₇ and NAA applications on ‘Honeycrisp’ flower formation. Furthermore, we investigated the effect of local defoliation and local GA₄₊₇ application on the transcript levels of two major flower formation genes in the meristems of apple spurs. The floral inhibition gene terminal flower1-1 (MdTFL1-1) and floral promoting genes flowering locus T (MdFT1 and MdFT 2) of apple. Local application of GA₄₊₇ and defoliation treatments inhibited flower formation, but NAA applications were without effect. Defoliation treatments were accompanied by a significant reduction in MdFT1, 2 transcript levels compared with controls early in the growing season. Conversely, GA₄₊₇ application was accompanied by a significant increase in MdTFL1-1 transcripts compared with controls throughout the growing season. These results indicate that GA₄₊₇ inhibits flower formation by upregulating the inhibitory MdTFL1-1, and defoliation acts by downregulating transcript levels of MdFT1, 2 early in the growing season. We also provide evidence that defoliated bourse buds may receive flowering promotion signals from other parts of the tree in the absence of their local leaves.

Metamitron is a relatively new postbloom thinning compound for pome fruits that inhibits the photosystem II (PS II) pathway of photosynthesis. Reduced assimilation of carbohydrate by metamitron action may lead to a carbohydrate deficit that promotes fruit abscission. We have evaluated the thinning efficacy of metamitron rate and, to a limited extent, application timing for ‘Bartlett’ pear in five separate trials over 3 years (2015–17) in northern Oregon. Comparisons were made to a nontreated control and, depending on the trial, a commercial standard thinning compound [benzyladenine (6-BA) or abscisic acid (ABA)]. Application timings ranged between 7- and 16-mm fruit diameter depending on the trial. Metamitron markedly inhibited photosynthesis (PN), typically for a duration of 2 to 3 weeks, although longer persistence was observed in two trials. Generally, PN was reduced linearly with increasing metamitron rate, but the effect varied by rate and year and may have been enhanced by high temperatures. Metamitron effectively thinned in four of five trials whereby pear fruit set was negatively and linearly related to rate. Rates of 200 to 300 ppm were efficacious and produced target crop loads. In only one trial, increasing concentrations (600 ppm) led to greater thinning. Metamitron significantly thinned ‘Bartlett’ pears when applied between 10 and 13 mm. In contrast, early application timing (≈7 mm) had little effect on fruit abscission. Fruit size increased with decreasing crop load and thus was significantly improved by metamitron. Interaction between biological and environmental factors likely contributed to year-to-year variability in efficacy.

Uniform annual apple (Malus ×domestica) fruit production is highly dependent on consistent flower formation from year to year, as inconsistent flowering can lead to the biennial bearing observed in some high-value cultivars. The presence of fruit on a spur has been considered the main cause of the expression of biennial bearing and the inhibition of flower initiation, with a number of theories being introduced to explain the phenomenon. In the current experiment, individual spurs of annual bearing cultivars (Gala, Ruby Jon, and Pink Lady) and biennial bearing cultivars (Honeycrisp, Fuji, and Golden Delicious) were thinned to a single fruit or completely defruited at petal fall. Spurs were sampled at the end of the growing season. Effects of fruiting on spur characteristics such as spur and bourse leaf area, stomatal density, leaf gas exchange, and flower formation were determined. Across all cultivars, the presence of fruit on a spur did not affect spur characteristics or flower formation compared with nonfruiting spurs. Similarly, flowering was unaffected by those factors associated with greater spur carbohydrate status, such as bourse leaf area and assimilation rate. Cultivars with greater transpiration and stomatal conductance (g _s) rates had lower rates of flower formation. Future studies should focus on xylem flow and expression of genes regulating flowering and plant growth regulators in annual and biennial bearing cultivars.