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Rebecca L. Darnell

Containerized `Climax' and `Beckyblue' rabbiteye blueberry plants (Vaccinium ashei Reade) were exposed to 5 weeks of natural daylengths (i.e. gradually decreasing daylengths from 12 to 11 hr) or shortened daylengths (i.e. gradually decreasing daylengths from 10 to 8 hr) starting October 1. `Beckyblue' initiated twice as many flower buds under short days compared to longer days. The following spring, `Beckyblue' plants exposed to shortened photoperiods the previous fall had a greater percentage of floral budbreak (based on the number of flower buds formed within each treatment) and a shorter, more concentrated bloom period than did plants exposed to longer photoperiods the previous fall. Fresh weight per berry increased following the short fall photoperiod treatment, despite the fact that fruit number was higher. `Climax' did not respond to the photoperiod treatments in any way. Leaf carbon assimilation rates of both cultivars increased under short days, but there was no detectable effect of photoperiod on current carbon partitioning in either cultivar, suggesting that flower bud initiation is not limited by current source leaf assimilate supply under these conditions.

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Rebecca L. Darnell

Containerized `Climax' and `Beckyblue' rabbiteye blueberry plants (Vaccinium ashei Reade) were exposed to 5 weeks of natural daylengths or shortened daylengths starting 30 Sept. `Beckyblue' plants exposed to short daylengths in the fall initiated more flower buds and had a shorter, more concentrated bloom period than did plants exposed to natural fall daylengths. Reproductive development of `Climax' was not influenced by photoperiod treatments. Leaf carbon assimilation of both cultivars increased under short days. Partitioning of translocated 14C-labeled assimilates to stem tissue increased under short photoperiods for `Beckyblue'; however, partitioning patterns in `Climax' were not affected. Increased carbon fixation and increased partitioning of carbon to stem tissue under short days may contribute to the observed effect of short days on enhancing reproductive development in `Beckyblue'.

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Umpika Poonnachit and Rebecca L. Darnell

Vaccinium corymbosum, one of the cultivated blueberry species, is not well-adapted to mineral soils, which are generally marked by high pH, the predominance of NO3-N over NH4-N, and limited iron availability. A wild species, V. arboreum, grows naturally on mineral soils, and thus may be better adapted than V. corymbosum. This adaptation may be related to the ability of V. arboreum to assimilate NO3 and/or iron more efficiently than V. corymbosum. Both species were grown in a hydroponic solution containing 5.0 mM N as (NH4)2SO4 or NaNO3, and buffered to pH 5.5. Nitrate reductase (NR) and iron reductase (FeR) activities were measured. NR activity was higher in V. arboreum compared with V. corymbosum when grown with N03-N, while no difference between species was observed when grown under NH4-N. Activity of FeR was higher in V. arboreum compared with V. corymbosum, and higher under NO3-N compared with NH4-N. After 5 months in hydroponics, Fe was removed from one-half of the solutions. The activity of NRA in both species was higher under Fe-sufficient compared with Fe-limited conditions, but in both cases, activity was higher in V. arboreum compared with V. corymbosum. FeR activity continued to be higher in V. arboreum compared with V. corymbosum, and under NO3 compared with NH4-N. Activity decreased in both species under limited Fe conditions, and there were no interactions between species and Fe. These data indicate that V. arboreum possesses higher NR and FeR activities than V. corymbosum, under both Fe-sufficient and Fe-limited conditions. This may play a role in the better adaptability of V arboreum to mineral soil conditions.

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Keith T. Birkhold and Rebecca L. Darnell

Partitioning of carbon and nitrogen reserves were examined in two cultivars of rabbiteye blueberries (Vaccinium ashei] differing in their timing of vegetative budbreak relative to floral budbreak. Floral budbreak precedes vegetative budbreak in `Climax', while floral and vegetative budbreak occur concomitantly in `Bonita'. Twenty eight containerized plants from each cultivar were dual labeled in the fall with 105 μCi of 14C02 and 0.6 g of nitrogen enriched with 5% 15N. Plants were grown outdoors throughout the winter and the following growing season. At five dates, beginning 27 days prior to full bloom and ending at fruit maturity, plants were harvested into old shoots, roots, fruit, and vegetative growth.

Fall leaf drop accounted for loss of 12% of applied N and 20% of applied 14C. In the first harvest, approximately 73% of the recovered 15N and 50% of the recovered 14C was in the roots for both cultivars. By fruit maturity, approximately 8% of the recovered 15N was in the fruits, 51% in new vegetative growth, and 41% in old shoots and roots. Approximately 1.2% of the recovered 14C was in fruit, 1.5% in vegetative growth, and 97% in old shoots and roots. Data suggest that differences in the timing of vegetative budbreak between these two cultivars do not influence overall partitioning patterns of reserve carbon and nitrogen.

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Rebecca L. Darnell and Frederick S. Davies

Potted `Tifblue', Woodard', and `Climax' rabbiteye blueberry plants (Vaccinium ashei Reade) were exposed to artificial or natural chilling regimes (< 7C) ranging from 100 to 1000 hours during the dormant season to determine the effects on budbreak and fruit set. Insufficient chilling increased the days to 50% vegetative and floral budbreak in all three cultivars. The amount of floral budbreak increased in `Tifblue' and `Woodard', but decreased in `Climax' as chilling increased. Insufficient chilling did not decrease percent fruit set of hand-pollinated flowers in any cultivar, indicating that the fruit-setting potential of these cultivars is unrelated to chilling.

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Donald J. Merhaut and Rebecca L. Darnell

Ammonium and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\) \end{document} uptake and partitioning were monitored in `Sharpblue' southern highbush blueberry plants (Vaccinium corymbosum L. interspecific hybrid) using 10% 15N-enriched N. Shoots and roots were harvested at 0, 6, 12, 24, and 48 hours after labeling. The rate of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\mathrm{-}\mathrm{N}\) \end{document} uptake was higher than that of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\mathrm{-}\mathrm{N}\) \end{document} uptake, averaging 17.1 vs. 8.6 g N/g plant dry weight per hour during the 48-hour period for \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\mathrm{-}\) \end{document} and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\mathrm{-treated}\) \end{document} plants, respectively. At the end of the 48 hours, \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\mathrm{-}\mathrm{N}\) \end{document} accumulation averaged 79 mg N/plant compared to 40 mg accumulated by the \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\mathrm{-}\mathrm{N}\mathrm{-treated}\) \end{document} plants. Similarly, the translocation rate of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\mathrm{-}\mathrm{N}\) \end{document} to shoots was higher than translocation of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\mathrm{-}\mathrm{N}\) \end{document} to shoots (7.7 vs. 1.9 g N/g shoot dry weight per hour, respectively) during the 48 hours. Shoot accumulation of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\mathrm{-}\mathrm{N}\) \end{document} averaged 40 mg N/plant at the end of 48 hours, while accumulation in shoots of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\mathrm{-}\mathrm{N}\mathrm{-treated}\) \end{document} plants averaged 10 mg N/plant. Short-term \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NO}_{3}^{-}\) \end{document} uptake and translocation to shoots appears to be limited relative to \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{NH}_{4}^{+}\) \end{document} uptake and translocation in southern highbush blueberry when plants are previously fertilized with NH4NO3.

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Donald J. Merhaut and Rebecca L. Darnell

Commercial blueberry production is limited primarily to soils where ammonium, rather than nitrate, is the predominant N form. However, Vaccinium arboreum, a species native to northern Florida, often is found growing in soils where nitrate is the major N form. This species may serve as a breeding source or rootstock for commercial blueberries, expanding the potential soil types that may be used for blueberry cultivation. In our study, in vivo nitrate reductase activity (NRA) was measured in roots and leaves of 2-year-old seedlings of V. arboreum and a commercial cultivar, V. corymbosum `Sharpblue'. Plants were grown hydroponically in sand culture and fertilized with a modified Hoagland's solution containing N as either ammonium, ammonium nitrate, or nitrate. Vaccinium arboreum averaged nitrite at 200, 60, and 20 nmol/g fresh weight per h for nitrate, ammonium nitrate, and ammonium fertilized plants, respectively. `Sharpblue' root NRA was significantly lower, averaging nitrite 50, 38, and 8 nmol/g fresh weight per h for nitrate, ammonium nitrate, and ammonium fertilized plants, respectively. NRA was much lower in leaves than roots of V. arboreum, averaging nitrite at ≈15 nmol nmol/g fresh weight per h across N treatments. No NRA was detected in the leaves of `Sharpblue', regardless of N treatment. These data suggest that V. arboreum may be used as a rootstock or breeding source to expand blueberry production into soil types that are higher in nitrate than the soils typically used for blueberry production.

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Raquel Cano-Medrano and Rebecca L. Darnell

To determine if multiple applications of GA3 would increase size of parthenocarpic fruit, and to assess the interaction between GA3 applications and pollination, `Beckyblue' rabbiteye blueberry (Vaccinium ashei Reade) flowers were treated with single or multiple applications of GA3 alone or in combination with full or partial pollination. Single or multiple applications of GA3 resulted in similar or increased fruit set compared with pollination, and increased fruit set compared with no pollination. GA3 applications decreased fruit mass and increased the fruit development period in comparison with pollination alone. Multiple, late applications of GA3 were ineffective in overcoming these effects. Partial (nonsaturating) pollination resulted in an average fruit set of 60%, while set following GA3 treatment in combination with full or partial pollination averaged 85%. Fruit mass was greater in the full pollination ±GA3 treatments than in all other treatments. The number of large seeds and seed mass per fruit were greatest in the full pollination treatment, and were significantly decreased by all treatments in which GA3 and/or partial pollination were used; however, there were no concomitant effects of GA3 in delaying the fruit development period. Our results indicate that under optimal pollination conditions, no detrimental effects of GA3 applications on fruit set, fruit size, or fruit development period in blueberry are to be expected, even though GA3 reduces seed number and seed mass. Furthermore, GA3 applications appear to be beneficial in increasing fruit set under suboptimal pollination conditions, although smaller fruit are to be expected under such conditions. Chemical name used: gibberellic acid (GA3).

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Raouel Cano-Medrano and Rebecca L. Darnell

Exogenous applications of GA, have induced pathenocarpic fruit set in blueberry; however, size of GA,-treated fruit is smaller than pollinated fruit. The small fruit size in GA3-treated fruit may be related to either cell number and/or cell size. Thus, these parameters were examined throughout development in pollinated, non-pollinated and GA3-treated fruits. Fruit growth followed a double sigmoid pattern. During Stage I (0-25 DAA), fruit size in GA,-treated, pollinated, and non-pollinated fruits averaged 0.33, 0.39, and 0.16 g, respectively. There was little change in fruit size in Stage II (25-45 DAA). At ripening, fruit size averaged 1.7 g for GA,-treated and 2.6 g for pollinated fruits. Non-pollinated fruit abscised in Stage II. At anthesis, mesocarp cell number averaged 9910 cells per median cross sectional area and remained constant up to ripening. In Stage I, cell size in G A3-treated and pollinated fruits increased 7X and 9X respectively. Cell size in both fruit types increased 1.5X and 2.8X during Stage II and Stage 111, respectively. Fruit cell number was set at anthesis and differences in fruit size were due to differences in cell ellongation in Stage I.

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Kevin R. Kosola* and Rebecca L. Darnell

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.