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  • Author or Editor: Rebecca Darnell x
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Most Vaccinium species, including V. corymbosum, have strict soil requirements for optimal growth, requiring low pH, high iron, and nitrogen, primarily in the ammonium form. V. arboreum is a wild species adapted to high pH, low iron, nitrate-containing soils. This broader soil adaptation in V. arboreum may be related to increased efficiency of iron or nitrate uptake/assimilation compared with cultivated Vaccinium species. To test this, nitrate and iron uptake, and nitrate reductase (NR) and ferric chelate reductase (FCR) activities were compared in two Vaccinium species, V. arboreum and the cultivated V. corymbosum. Plants were grown hydroponically for 15 weeks in either 1.0 or 5.0 mm NO3 with 0.09 mm Fe. Root FCR activity was greater in V. arboreum compared with V. corymbosum, especially at the lower external nitrate concentration. However, this was not reflected in differences in iron uptake. Nitrate uptake and root NR activity were greater in V. arboreum compared with V. corymbosum. The lower nitrate uptake and assimilation in V. corymbosum was reflected in decreased plant dry weight compared with V. arboreum. V. arboreum appears to be more efficient in acquiring nitrate compared with V. corymbosum, possibly due to increased NR activity, and this may partially explain the wider soil adaptation of V. arboreum.

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Graduate student enrollment in the plant sciences has decreased over the past several years, and there is increasing interest in recruitment/retention strategies. Before successful strategies can be implemented, however, the status of current plant science graduate programs needs to be determined. Survey data on graduate student demographics, research area, support levels, current recruitment strategies, and career opportunities were collected from 23 plant science graduate programs. Overall, 55% of graduate students in plant sciences were male and 45% were female; about 60% were domestic and 40% were international. Cellular/molecular biology and breeding/genetics were the two disciplines that had the greatest number of graduate students and the greatest number of job opportunities. Most programs cited financial support as the biggest obstacle to recruitment. However, stipend number, the guarantee of multiple years of support, the funding of tuition waivers, and health insurance costs also impact student numbers. As more of these costs are shifted to faculty, there appears to be an increasing inability and/or reluctance to invest grant funds (which support 60% of the plant science graduate students) in graduate student education. These data suggest that the decline in plant science graduate student enrollment may be due to shifting of more of the total cost of graduate training to faculty, who may be unable/unwilling to bear the cost. There is also a clear shift in the research focus of plant science graduate students, as postdoctoral and career opportunities are weighted toward molecular biology/genetics, leaving the more applied plant science areas particularly vulnerable to low graduate enrollment.

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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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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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Dormant two-year old `Bonita' rabbiteye blueberries were subjected to the following treatments: 1) no defoliation, 2) one-half of the canes on a plant defoliated, and 3) complete defoliation. After 300 h at 7C, plants were brought into the greenhouse and vegetative budbreak was assessed. Completely defoliated plants exhibited rapid, extensive budbreak, averaging 70 budbreaks/plant. Plants or individual canes that retained their leaves during chilling averaged 25 budbreaks/plant. Budbreak on defoliated canes of partially defoliated plants (treatment 2) was drastically reduced compared to whole plant defoliation, averaging 30 budbreaks/plant. In a second study, plants were partially defoliated, as in treatment 2 above. The foliated canes remaining on each plant were then either girdled or non-girdled. After 300 h chilling, vegetative budbreak was assessed. Budbreak on defoliated canes of plants that had non-girdled foliated canes was low, averaging 15 budbreaks/plant. Budbreak on defoliated canes of plants that had girdled foliated canes was significantly higher, averaging 35 budbreaks/plant. These results suggest that chilling is inhibited by leaves, and this “inhibition” is phloem translocatable from leaves to buds on other canes, thus inhibiting budbreak on those canes.

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Abstract

Two rabbiteye blueberry (Vaccinium ashei Reade) cultivars Beckyblue and its open-pollinated progeny Aliceblue, both of which are known to be highly male- and female-fertile, were self-pollinated, reciprocally cross-pollinated, and pollinated with pollen from a third cultivar, Climax, which is not closely related to the other two. Fruit set percentage on ‘Aliceblue’ and ‘Beckyblue’ averaged 65% when ‘Climax’ was the pollen parent and < 1% when the cultivars were self-pollinated. Reciprocal crosses of ‘Beckyblue’ and ‘Aliceblue’ averaged 24% set, indicating a high degree of cross-incompatibility. There was no effect of pollen parent on berry fresh weight, although seed weight and seed number were reduced by self-pollination or pollination by the related cultivar.

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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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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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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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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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