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Mark S. Strefeler, Neil O. Anderson, and Peter D. Ascher

Our objective was to determine whether repeated applications of 2-chloroethylphosphonic acid (ethephon) + gibberellic acid (GA3) to stock chrysanthemum plants that are day-neutral for flower bud initiation would increase the number of quality cuttings. Across five cultivars, there were no significant differences between controls and plants receiving 250 ppm ethephon in the total number of cuttings per plant. The percentage of cuttings with crown buds was greater for cuttings from controls than for ethephon-treated plants. Applying 500 ppm ethephon significantly reduced the number of cuttings. We conclude that chrysanthemum clones day-neutral for flower bud initiation and development with low long-day leaf number could be selected to form a 4 to 5 week production group.

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Jose Lopez-Medina, James N. Moore, Kyung S. Kim, and John R. Clark

Floral initiation (FI) was studied both in greenhouse- and field-grown plants of primocane-fruiting (PF) blackberries recently developed by the Univ. of Arkansas. Root cuttings of A-1836 and APF-13 were dug from the field and planted in a greenhouse on 1 Mar. 1997. NC 194 was included only in the field study. Terminal apices were sampled weekly starting at 0 (just before emergence) nodes of growth on 21 Mar. Floral primordia were first seen at five and six nodes of growth in greenhouse-grown A-1836 and APF-13, respectively, 35-42 days after root cuttings were planted (DAP). Under field conditions, the same event was not observed until 21 May when A-1836 and APF-13 reached at least 20 nodes; NC 194 did not show evidence of floral parts until 10 July. Once FI occurred, floral differentiation proceeded uninterrupted until completion. Blooming occurred 32-35 and 40-45 days after FI in APF-13 and A-1836, respectively; NC 194 bloomed in late August. The first fruits of APF-13 were harvested 120 DAP. These findings demonstrate that PF blackberries form flower buds soon after a short period of vegetative growth. This information should be useful for implementing horticultural practices, such as programming of the harvest date.

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Jose Lopez-Medina, James N. Moore, and Kyung-S. Kim

Scanning electron microscopy (SEM) and light microscopy (LM) were used to study the transition of meristems from vegetative to floral phase in erect primocane-fruiting (PF) blackberries [Rubus (Tourn.) L. subgenus Rubus] developed at the Univ. of Arkansas. Dormant root cuttings of A-1836 and APF-13 blackberries were dug from the field and planted on 28 Dec. 1996 and 1 Mar. 1997 to produce plants for use in a greenhouse study. In a field study, terminal buds of field-grown A-1836, APF-13, NC194, and summer-fruiting `Arapaho' were sampled on 21 Mar 1997 (before shoot emergence from soil), and then weekly from 14 to 28 May 1997. Flower bud primordia were first observed at five and six nodes of growth in greenhouse-grown A-1836 and APF-13 plants, respectively, 35 to 42 days after root cuttings were planted (DAP). Under field conditions, floral primordia were not observed until 21 May when A-1836 and APF-13 had at least 20 nodes of growth; NC194 did not differentiate floral structures until 10 July. The developmental patterns of the vegetative apical meristem in the PF selections, both field- and greenhouse-grown plants, were similar to those of `Arapaho'. Opening of the terminal flower of the inflorescence occurred 32 to 35 days after floral initiation in APF-13, and 8 to 10 days later on A-1836. Field-grown NC194 bloomed in late August. The first fruits of greenhouse-grown APF-13 were harvested 120 DAP. These findings demonstrate that PF blackberries form flower buds after a short period of vegetative growth.

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Jeffrey G. Williamson and E.P. Miller

In 1998, representative canes of mature, field-grown, `Misty' and `Sharpblue' southern highbush blueberry were hand-defoliated on 4 Sept., 2 Oct., 6 Nov., 7 Dec., or not defoliated. The experiment was repeated in 1999. Randomized complete-block designs with 11 (1998) or 10 (1999) replications were used. The early defoliation treatments (4 Sept. and 2 Oct.) resulted in reduced flower bud number per unit length of cane for `Misty', but not for `Sharpblue', when compared with later defoliation treatments or controls. A similar response to early defoliation was found both years for both cultivars. The later defoliation treatments (6 Nov. and 7 Dec.) had no significant effect on flower bud number compared to controls. Early defoliation had a negative effect on flower bud development for both cultivars. Flower buds that developed on canes defoliated on 4 Sept. or on 2 Oct. had smaller diameters than flower buds on canes defoliated on 6 Nov., 7 Dec., or on non-defoliated canes. Fruit fresh weight per unit cane length was less for the September and October defoliation treatments than for the December defoliation treatment or controls. These results support the need for summer pruning and a effective summer spray program to control leaf spot diseases that often result in early fall defoliation of southern highbush blueberries grown in the southeastern United States.

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Timothy M. Spann, Jeffrey G. Williamson, and Rebecca L. Darnell

Experiments were conducted with V. darrowi and two cultivars of southern highbush blueberry, `Sharpblue' and `Misty,' to test whether V. darrowi and cultivars derived from it are photoperiodic with respect to flower bud initiation. Plants of each cultivar were grown under three different photoperiod treatments [long days (LD) = 16-hour photoperiod; short days (SD) = 8-hour photoperiod; and short days + night interrupt (SD-NI) = 8-hour photoperiod with 1-hour night interrupt] at constant 21 °C for 8 weeks. Vegetative growth was greatest in the LD plants of both cultivars. Flower bud initiation occurred only in the SD treatments, and the lack of flower bud initiation in the SD-NI treatment indicates that flower bud initiation is a phytochrome mediated response in Vaccinium. Previously initiated flower buds on the V. darrowi plants developed and bloomed during the LD treatment, but bloom did not occur in the SD and SD-NI treatment plants until after those plants were moved to LD. These data indicate that flower bud initiation in both V. darrowi and southern highbush blueberry is photoperiodically sensitive, and is promoted by short days, while flower bud development is enhanced under long days.

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Timothy M. Spann, Jeffrey G. Williamson, and Rebecca L. Darnell

Experiments were conducted with `Misty' southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrid) to test the effects of high temperature on flower bud initiation and carbohydrate accumulation and partitioning. Plants were grown under inductive short days (SDs = 8 hour photoperiod) or noninductive SDs with night interrupt (SD-NI = 8 hour photoperiod + 1 hour night interrupt), at either 21 or 28 °C for either 4 or 8 weeks. Flower bud initiation occurred only in the inductive SD treatments and was significantly reduced at 28 °C compared with 21 °C. The number of flower buds initiated was not significantly different between 4- and 8-week durations within the inductive SD, 21 °C treatment. However, floral differentiation appeared to be incomplete in the 4-week duration buds and bloom was delayed and reduced. Although plant carbohydrate status was not associated with differences in flower bud initiation between SD and SD-NI treatments, within SD plants, decreased flower bud initiation at high temperature was correlated with decreased whole-plant carbohydrate concentration. These data indicate that flower bud initiation in southern highbush blueberry is a SD/long night phytochrome-mediated response, and plant carbohydrate status plays little, if any, role in regulating initiation under these experimental conditions.

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J.G. Williamson and E.P. Miller

Three experiments were conducted in north-central Florida to determine the effects of fall defoliation on flower bud initiation and yield of southern highbush (SHB) blueberry (Vaccinium corymbosum hybrid). In 1998, randomly selected upright shoots of mature, field-grown `Misty' and `Sharpblue' plants were hand-defoliated at monthly intervals beginning 4 Sept. and ending 7 Dec. In 1999, a similar study was conducted using different plants of the same cultivars. Representative shoots were defoliated at monthly intervals beginning 14 Sept. and ending 15 Dec. Additional shoots were also partially defoliated by removing the distal two-thirds of each leaf at monthly intervals from 15 Oct. through 15 Dec. In a third experiment, 2-year-old container-grown `Star' SHB plants were completely defoliated at monthly intervals beginning 13 Sept. and ending 15 Dec. In each experiment, control shoots, or plants ('Star'), were not defoliated. Although there were differences among cultivars and years, all cultivars tested demonstrated negative effects on reproductive growth and development from September and October defoliations. For `Sharpblue', reduced fruit yield from early fall defoliation appeared to be due to fewer fruit set per flower bud. However, for `Misty', reduced fruit yield from early fall defoliation was the result of large reductions in flower bud numbers as well as fewer fruit set per flower bud. September and October defoliations of `Star' reduced yields or delayed fruit ripening. Collectively, these experiments demonstrate the importance of maintaining healthy foliage through October in the lower southeastern United States for adequate flower bud initiation and high yields of SHB blueberry the following spring.

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P.A.W. Swain and R.L. Darnell

Two cultivars of southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrid), `Sharpblue' and `Wannabe', were container-grown outside in either a dormant or nondormant production system to determine how the two production systems affected carbohydrate (CH2O) status, growth, and development. Plants were maintained in the nondormant condition by continuous N fertilization throughout winter (average maximum/minimum temperatures of 17/5 °C). Plants in the nondormant system retained their foliage longer into the winter compared with plants in the dormant system. Flower bud number, density, fruit number, and total fruit fresh weight (FW) per plant were greater in the nondormant compared with the dormant system plants for both cultivars. Mean fruit FW was greater in dormant compared with nondormant `Wannabe' plants, while in `Sharpblue', mean fruit FW was similar in both systems. Cane and root CH2O concentrations in nondormant system plants were generally similar to or lower than those measured in dormant system plants. Assuming that longer leaf retention in nondormant system plants increased CH2O synthesis compared with dormant system plants, the patterns of reproductive/vegetative development and root/shoot CH2O concentrations indicate that the increased CH2O in nondormant system plants was allocated to increased reproductive growth in lieu of CH2O reserve accumulation. It is probable that this increased CH2O availability, combined with longer perception of short days due to longer leaf retention, were major factors in increasing flower bud initiation and yield in the nondormant compared with the dormant system plants.

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Neil O. Anderson

The increasing number of crops being grown for the floriculture market has frustrated educators faced with limited classroom and laboratory time. Time constraints necessitate selection of crops to serve as examples of floral induction treatment(s) and provide an accurate scope of production requirements for all cultivated species. Since flowers are the primary reason for purchasing most floricultural products—with the notable exception of cut and potted foliage—the various treatments required for flower bud initiation and development were used to categorize potted plants. New and old crops (>70 species) are categorized for flower bud initiation and development requirements, including photoperiod (short, long day, day neutral; facultative/obligate responses), vernalization, temperature, autonomous, rest period, and dormancy. Crop-specific temperature, irradiance, and photoperiod interactions are noted, as well as temperature × photoperiod interactions. A course syllabus can be modified to ensure that at least one crop from each category is presented to serve as a model. It is recommended that the class focuses on example crop(s) from each floral induction category and then reviews other crops within each category for differences or similarities. This method allows coverage of floral induction categories without leaving information gaps in the students' understanding. This method was used with students in the Fall 1999, floriculture production class (Hort 4051) at the University of Minnesota, St. Paul.

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P.A.W. Swain and R.L. Darnell

`Sharpblue' southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrid) was grown in either a dormant or nondormant production system to determine the effect of production system on source limitations to fruit and vegetative growth. Source limited stages were evaluated in the two production systems by reducing reproductive sink load during either the fruit cell division or fruit cell enlargement stage. Source limitation during cell division was evaluated by removing 80% of the flower buds in late fall, since the majority of cell division in blueberry ovaries occurs before bloom. Source limitation during cell enlargement was evaluated by removing 80% of the fruit after fruit set the following spring. In the dormant production (DP) system, mean fruit dry weight (DW) was greatest in the flower bud removal treatment and least in the control (nonthinned) treatment, suggesting that cell number, rather than size, is more important in determining blueberry fruit weight in the DP system. Fruit in the dormant flower bud removal treatment may have approached maximum cell number and therefore fruit size; this was supported by the observation that significant depletion of root carbohydrate concentration did not occur in this treatment, as it did in the control treatment. Mean fruit DW in the nondormant production (NDP) system was greatest in the fruit removal treatment compared with the other two treatments, suggesting that cell enlargement played a larger role in determining fruit size in this production system. However, the effect of the flower bud removal treatment (and therefore the effect of cell division) on fruit DW in the NDP system was apparently masked by continued flower bud initiation in this system after flower bud removal in late fall. Continued floral initiation was apparently an alternative sink to increasing cell division in previously formed flower buds. In both systems, fruit removal increased vegetative growth compared with the control and flower bud removal treatments. Thus, both systems exhibited source limitations to fruit and vegetative growth, although the timing and extent of the limitation to fruit growth differed between the production systems.