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Peter A.W. Swain and Rebecca L. Darnell

Two cultivars of southern highbush blueberry (Vaccinium corymbosum interspecific hybrid) were grown in containers under the traditional deciduous production system, or the dormancy-avoiding evergreen production system. In the dormancy-avoiding system, plants are maintained evergreen and do not enter dormancy in the winter. This alleviates the chilling requirement, thus extending the potential growing area of blueberries into subtropical regions. Plants in the evergreen production system were maintained in active growth through weekly or biweekly N fertilization (≈21–23 g N/ plant per year). Keeping foliage through the year lengthens the duration of the photosynthetic season of the plant and is hypothesized to improve the carbohydrate (CHO) status of the evergreen plants. This, in turn, may decrease source limitations to reproductive development and potentially increase fruit number and/or size. In both cultivars, the evergreen production system advanced the time of anthesis by 3 to 4 weeks compared to the deciduous production system. Plants in the evergreen system initiated 10% to 25% more flower buds than plants in the deciduous system, depending on cultivar. Average leaf area, leaf fresh weight, total above-ground fresh weight, bud density, and cane length were greater in the evergreen plants than deciduous. The evergreen production system increased plant fresh weight and flower bud number compared to the deciduous system, and may ultimately increase yield.

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Rebecca L. Darnell and Gary W. Stutte

Strawberries (Fragaria xananassa Duch. .Osogrande.) were grown hydroponically with three NO3-N concentrations (3.75, 7.5, or 15.0 mM) to determine effects of varying concentration on NO3-N uptake and reduction rates, and to relate these processes to growth and fruit yield. Plants were grown for 32 weeks, and NO3-N uptake and nitrate reductase (NR) activities in roots and shoots were measured during vegetative and reproductive growth. In general, NO3-N uptake rates increased as NO3-N concentration in the hydroponics system increased. Tissue NO3-. concentration also increased as external NO3-N concentration increased, reflecting the differences in uptake rates. There was no effect of external NO3-N concentration on NR activities in leaves or roots during either stage of development. Leaf NR activity averaged ~360 nmol NO2 formed/g fresh weight (FW)/h over both developmental stages, while NR activity in roots was much lower, averaging ~115 nmol NO2 formed/g FW/h. Vegetative organ FW, dry weight (DW), and total fruit yield were unaffected by NO3-N concentration. These data suggest that the inability of strawberry to increase growth and fruit yield in response to increasing NO3-N concentrations is not due to limitations in NO3-N uptake rates, but rather to limitations in NO3 - reduction and/or assimilation in both roots and leaves.

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Donald J. Merhaut and Rebecca L. Darnell

Nitrogen uptake and N and C partitioning were evaluated in `Sharpblue' southern highbush blueberries fertilized with different N forms. Plants were grown in acid-washed silica sand and fertilized with a modified Hoagland's solution supplemented with 5.0 mm N as NH4 + or NO3 -. Nutrient solution pH was adjusted to 3.0 and 6.5 for the NO3 - and NH4 +-treated plants, respectively. After 12 months of growth, plants were dual labeled with 14CO2 and 10% enriched 15N-N as either NaNO3 or (NH4)2SO4 and harvested 12 hours after labeling. Fertilization with NO3 --N increased leaf, stem, and root dry weights compared to NH4 + fertilization. Total 15N uptake did not differ between N fertilization treatments, thus whole plant and root 15N concentrations were greater in NH4 +-fertilized vs. NO3 --fertilized plants. Fertilization with NO3 --N increased C partitioning to new shoots compared to NH4 +-fertilized plants. However, C partitioning to other plant parts was not affected by N form. Although NO3 - uptake in blueberry appears to be restricted relative to NH4 + uptake, this limitation does not inhibit vegetative growth. Additionally, there appears to be adequate available carbohydrate to support concurrent vegetative growth and N assimilation, regardless of N form.

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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.

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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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Rebecca L. Darnell and Steven A. Hiss

Most Vaccinium species have narrow soil adaptation and are limited to soils that have low pH, high available iron (Fe), and nitrogen (N) primarily in the ammonium (NH4+) form. Vaccinium arboreum Marsh. is a wild species that can tolerate a wider range of soil conditions, including higher pH and nitrate (NO3-) as the predominant N form. This wider soil adaptation may be related to the ability of V. arboreum to acquire Fe and NO3- more efficiently than cultivated Vaccinium species, such as V. corymbosum L. interspecific hybrid (southern highbush). Nitrate and Fe uptake, and nitrate reductase (NR) and ferric chelate reductase (FCR) activities were compared in these two species grown hydroponically in either 1.0 or 5.0 mm NO3-. Nitrate uptake rate (on a whole-plant and FW basis) and root NR activity were significantly greater in V. arboreum compared with V. corymbosum. Iron uptake on a FW basis was also greater in V. arboreum, and was correlated with higher root FCR activity than was found in V. corymbosum. Increased Fe and NO3- uptake/assimilation in V. arboreum were reflected in increased organ and whole-plant dry weights compared with V. corymbosum. Vaccinium arboreum appears to be more efficient in acquiring and assimilating NO3- and Fe than is the cultivated species, V. corymbosum. This may partially explain the wider soil adaptation of V. arboreum.

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Keith T. Birkhold and Rebecca L. Darnell

The relative contribution of storage and currently assimilated N to reproductive and vegetative growth of `Bonita' and `Climax' rabbiteye blueberry (Vaccinium ashei Reade) was estimated immediately before and during the fruit development period. Total and storage N decreased in roots and shoots of both cultivars during dormancy and early fruit development. The principle N storage form appeared to be protein, as indicated by a significant decline in total shoot and root protein during this same period. Storage N from roots and shoots in both cultivars was remobilized to flowers and/or fruit and new vegetative growth. At anthesis, 90% of the total N present in reproductive organs was estimated to come from storage N. By fruit maturity, ≈ 50% of the accumulated N was derived from storage pools. Storage N contributed 90% of the total N in developing vegetative growth of `Bonita' at leaf budbreak, which is concomitant with floral budbreak for this cultivar. Developing vegetative growth of `Climax' at leaf budbreak, which occurs ≈ 4 weeks after floral budbreak, derived ≈ 65% of its total N from storage and 35% from currently assimilated N. By fruit maturity, contribution of storage N to new vegetative growth had decreased to ≈ 20% in both cultivars, indicating that currently assimilated N became the principal N supply as vegetative growth became more established. Differences in timing of floral and vegetative budbreak between the two cultivars did not appear to affect allocation of either storage or currently assimilated N to new vegetative or reproductive growth.

Free access

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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Rebecca L. Darnell and Jimmy G. Cheek

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; approximately 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. Although most programs cited financial support as the biggest obstacle to recruitment, there was not a strong correlation between graduate student number/program and stipend amount. However, other funding factors besides stipend amount; such as stipend number, the guarantee of multiple years of support, the funding of tuition waivers, and health insurance costs, likely impact student number. 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 not be directly due to low stipend amounts, but rather 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 towards molecular biology/genetics, leaving the more applied plant science areas particularly vulnerable to low graduate enrollment.

Open access

Rebecca L. Darnell and George C. Martin

Abstract

Single applications of auxins that normally induce parthenocarpic fruit set in June-bearing and everbearing strawberries (Fragaria × ananassa Duch) (e.g., NAA, 2-NOA, and IAA) do not induce set in the day-neutral cultivar Fern. The only auxin found to date that induces set in day-neutrals with a single application is Et-IAA. The translocation and metabolism of [14C]Et-IAA applied to receptacles was followed in order to gain insight into mechanisms involved in set. Ninety to ninety five percent of the recovered 14C activity remained in the receptacles throughout set and initial development (0 to 6 days) after Et-IAA application. Growth of treated receptacles followed the rapid metabolism of [14C]Et-IAA. Twelve hours after application, 16% of the radioactivity remained in the Et-IAA fraction. This level decreased to <5% by 144 hr after application. The majority of the radioactivity (55–60%) was contained in highly polar compounds. Little activity was recovered in the free IAA fraction. Chemical names used: 1-naphthaleneacetic acid (NAA); 2-naphthoxyacetic acid (2-NOA); 1H-indole-3-acetic acid (IAA); IAA ethyl ester (Et-IAA).