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  • Author or Editor: R.L. Darnell x
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Two-year-old, container-grown `Misty' southern highbush blueberry plants were sprayed to drip with two concentrations of hydrogen cyanamide (HCN) (20.4 g·L–1 and 10.2 g·L–1) after exposure to 0, 150, or 300 hr of continuous chilling at 5.6°C. All plants were sprayed immediately after chilling and placed in a greenhouse for several weeks. The plants were moved outdoors during flowering to increase cross-pollination from nearby `Sharpblue' blueberry plants. HCN sprays killed some of the more advanced flower buds on shoot terminals and on small-diameter wood from the previous spring growth flush. Significantly greater flower bud mortality occurred for the 20.4 g·L–1 HCN sprays than for the 10.2 g·L–1 sprays. Flower buds subjected to 0 hr of chilling were more susceptible to spray burn than flower buds receiving 150 or 300 hr of chilling. Very little flower bud death occurred with the 10.2 g·L–1 HCN rate on plants receiving 300 hr of chilling. Vegetative budbreak was advanced for both HCN treatments compared to controls, regardless of chilling treatment. HCN-treated plants were heavily foliated at full bloom, while non-treated plants had very few to no leaves during bloom. HCN may be useful for stimulating vegetative growth in some southern highbush blueberry cultivars that suffer from poor foliation during flowering and fruit set.

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Six-year-old, field-grown `Beckyblue' and `Bonita' rabbiteye blueberries were sprayed to drip with Pro-Gibb (250 ppm GA3, 0.1% surfactant, pH 3.1). Two spray applications were made. The first spray was applied at 80-90% full bloom followed by a second spray 10 days later. Fruit were harvested at five dates, from 21 May until 1 July, 1992. GA3 increased fruit set and doubled total fruit yield for both cultivars compared to the control. Fruit yield was greater for the GA3 treatment than for the control at harvest dates 3 through 5 for 'Beckyblue', and dates 4 and 5 for 'Bonita'. Average berry weight for both cultivars and for both treatments declined as the season progressed. For `Beckyblue', average berry weight did not differ between treatments at most harvest dates. For 'Bonita', average berryweight was less for the GA3 treatment than for the control at harvest dates 3 through 5. GA3 increased yield of rabbiteye blueberry with little detrimental effect on fruit size. However, results from Georgia suggest that greater positive effects on fruit set should be possible.

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Mature `Sharpblue' southern highbush and `Beckyblue' rabbiteye blueberry plants were mechanically pruned at two heights on three dates after fruit harvest during the 1994 growing season. No pruning had occurred for at least 3 years (`Sharpblue') or 5 years (`Beckyblue') before initiating experiment. Pruning heights were 45 and 85 cm and nonpruned for `Sharpblue' and 45 and 95 cm and nonpruned for `Beckyblue'. Pruning dates were 3, 6, and 9 weeks after peak harvest for each cultivar. Regrowth was measured in Mar. 1995 before initiation of spring growth. Pruning `Sharpblue' bushes to 45 cm increased new shoot number and mean and total shoot length but decreased fruit yield compared to the 90-cm pruning treatment. No difference in yield occurred between the 90-cm pruning treatment and the nonpruned control. As time between fruit harvest and pruning increased, new shoot number, mean and total shoot length, plant height, canopy volume, and fruit yield decreased. There was no difference in yield between the earliest pruning treatment and the control. For `Beckyblue', mean and total shoot length of regrowth and flower bud density decreased with increasing time from harvest to pruning. Yield data for `Beckyblue' were not collected in 1995 because of gall midge infestation.

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Cultivated Vaccinium species (e.g. highbush blueberry, Vaccinium corymbosum, or cranberry, V. macrocarpon) commonly require acidic soil (pH 4.5 to 5.5) for optimum growth. Under these conditions, ammonium (NH4 +) is the dominant form of inorganic N. In contrast, V. arboreum, the sparkleberry can tolerate higher-pH mineral soils, where nitrate (NO3 -) is typically the predominant inorganic N form. This tolerance may be related to increased ability to acquire and utilize NO3—N. Measurements of 15NO3 - and 15NH4 + influx kinetics in excised roots of V. arboreum, V. corymbosum, and V. macrocarpon did not support this hypothesis. NO3 - influx kinetics measured from 10 micromolar to 200 micromolar NO3 - were similar among all three species. NO3 - influx was consistently lower than NH4 + influx at all concentrations for all three species.

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`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.

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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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Variability in sucrose levels and metabolism in ripe fruit of several Vaccinium species were examined. The objective was to determine if sufficient variability for fruit sucrose accumulation was present in existing populations to warrant attempts to breed for high-sucrose fruit, which potentially would be less subject to bird predation. Three-fold differences in fruit sucrose concentration were found among species, ranging from 19 to 24 mg·(g fw)-1 in V. stamineum and V. arboreum to about 7 mg·(g fw)-1 in cultivated blueberry (V. ashei and V. corymbosum) and V. darrowi. Soluble acid invertase activity was negatively correlated with fruit sucrose concentration. There was no apparent correlation between fruit sugar concentration and either sucrose phosphate synthase or sucrose synthase activities, both of which were low for all species studied. The degree of variability in fruit sucrose accumulation among Vaccinium species supports the feasibility of developing high sucrose fruit, which would be a potentially valuable addition to current strategies of minimizing crop losses to birds.

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Blueberry plants fertilized at 3-week intervals with nitrogen (N) throughout the year and protected from freezing temperatures avoid dormancy and produce an off-season “winter” crop. Southwestern Florida offers a climate where this production system can be implemented without undue fear of freezes. `Sharpblue', `Gulfcoast', and `Warmabe' southern highbush blueberry cultivars have been planted at high density (10,000 plants/ha) to determine the feasibility of successfully establishing an evergreen production system for blueberry. Three rates of N fertilization (84, 168, and 252 kg·ha–1) and the use of peat or municipal solid waste (MSW) compost as soil amendments are being evaluated in this study. Initial data on plant growth indicate that, during the first 9 months of the planting, 168 kg N/ha will produce plants similar in height, but with significantly less volume, to those receiving 252 kg N/ha. MSW compost appears to be a beneficial soil amendment for blueberry establishment despite an increase in soil pH associated with the compost amendment.

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Vegetative budbreak and subsequent canopy development in some southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrid) cultivars are delayed and/or reduced as flower bud density increases. This delay/reduction in vegetative growth has been correlated with decreased weight and soluble solids of individual fruit. In the present study, the effects of flower bud density (FBD) on vegetative budbreak and canopy development, starch reserves, and whole-canopy net CO2 exchange rate (NCER) were assessed to determine how FBD affected the source supply for fruit development. A range of flower bud densities was established in two cultivars of containerized southern highbush blueberry during dormancy. Vegetative budbreak was delayed and vegetative budbreak, leaf area, and leaf area to fruit ratios decreased as FBD increased. In general, increasing FBD increased the rate of root and cane starch depletion during the first four weeks after bloom. Whole-canopy NCER was similar across the range of FBD during early fruit development, but between four weeks after bloom and fruit ripening, NCER decreased as FBD increased. Although FBD explained only a small proportion of the variability observed in carbohydrate concentration and NCER, the data suggest that both the rapid depletion of starch reserves early in fruit development and the decrease in whole canopy NCER later in fruit development contribute to the detrimental effects of increased FBD on fruit development.

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Water and carbon budgets of individual citrus fruit were determined throughout their growth to quantify the demand for sucrose and water relative to developmental changes. Fruit transpiration, water accumulation, photosynthesis, respiration, and C gain were measured during this period for grapefruit (Citrus paradisii Macf.) and calamondin (Citrus madurensis Lour.). On a whole-fruit basis, estimated rates of grapefruit transpiration and mean daily water inflow decreased after the first third of development, whereas water apparently was lost freely throughout growth of the smaller, thin-peeled calamondins. Estimates of daily fruit C import remained relatively similar during the majority of grapefruit growth, increasing rapidly only as fruit neared maturation. A similar trend was observed in calamondins, although rates were more variable. Overall, estimated mean daily water inflow into “developing grapefruit decreased relative to that of sucrose inflow, resulting in a progressively higher ratio of sucrose transport to net water inflow. Values for these ratios rose from ≈; 10 to >300 g sucrose/liter of water, reaching levels of net water and sngar transfer that could both be accommodated by citrus phloem alone. Any additional entry into grapefruit appears to have been offset by xylem back-flow, because no other net water influx was observed.

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