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Raguel Cano—M and Rebecca L. Darnell

The effects of different GA3 concentrations and application times on fruit set, fruit development period, and fruit quality in rabbiteye blueberry (Vaccinium ashei Reade) were studied. Flower clusters were sprayed with 100 or 250 ppm GA3 at 90% full bloom and again 7 days later, or with 125 ppm GA3 at 90% full bloom and again 7, 21, and 42 days later, under greenhouse conditions. Fruit set was monitored every 10 days and fruit weight, fruit development period, soluble solids, and titratable acidity were measured at harvest.

Fruit set in GA3 treatments averaged 69 to 76% compared to an average of 43% for the pollinated control. Weight of GA3 treated berries averaged 1.2, g while that of pollinated berries averaged 2.0 g. However, calculated total yield was greater for the GA3 treatments compared to the pollinated control, averaging 244 and 206 g/treatment, respectively. GA3 increased the fruit development period by 2 to 11 days, depending on the treatment. Soluble solids and titratable acidity were not affected by any treatment.

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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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Donald J. Merhaut and Rebecca L. Darnell

The influence of stage of vegetative flush development on NH4NO3-N uptake and carbon and nitrogen partitioning was evaluated in two-year-old 'Climax' rabbiteye blueberries using dual labeling with 15N and14C. Plants were grown in sand and fertilized with a modified Hoagland's solution. Plants were pruned to induce three stages of vegetative development: flush initiation, mid -flush, and flush maturity.

Total nitrogen uptake did not differ for the different stages of growth. However, N allocation to leaves was greatest at mid-flush, possibly due to higher transpiration rates of developing leaves. Total 14C partitioning to roots was reduced at mid-flush, compared to the other growth stages, reflecting the increased demand for carbon by growing shoots. Although less carbon was allocated to roots at mid-flush,this did not limit N uptake.

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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.

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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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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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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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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.

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Rebecca L. Darnell and Keith B. Birkhold

Rabbiteye blueberry (Vaccinium ashei Reade) cultivars differ in timing of floral and vegetative budbreak and in final fruit size. For example, `Bonita' exhibits concomitant floral and vegetative budbreak and has relatively large fruit size, while floral budbreak precedes vegetative budbreak in `Climax' and fruit size is smaller. Mobilization of carbohydrate before and during fruit development in `Bonita' and `Climax' rabbiteye blueberries was examined to determine if differences in carbohydrate availability between these two cultivars were correlated with differences in fruit size. Root dry mass (DM) of both cultivars decreased from dormancy (31 days before anthesis) through fruit development. Sugar concentrations in roots and stems of both cultivars decreased significantly between dormancy and anthesis, then remained relatively steady throughout fruit development. Starch concentrations in roots and stems of `Bonita' decreased significantly between dormancy and anthesis. The extent of total starch depletion in `Climax' was similar; however, the decrease was more gradual, extending from dormancy to 28 days after anthesis (DAA); at which time, vegetative budbreak in `Climax' occurred. Thus, although total reserve carbohydrate pool sizes were similar between the two cultivars, remobilization patterns were different, resulting in increased starch mobilization in `Bonita' compared to `Climax' in the period leading up to anthesis. Concentration of 14C from reserve carbon sources was similar in flowers of both cultivars at anthesis. These values declined throughout fruit development as a result of dilution of the labeled carbon by unlabeled carbon from current photosynthesis. There was a sharper decline in 14C concentration of `Bonita' fruit compared to `Climax' fruit between anthesis and 51 DAA. This, coupled with differences in timing of vegetative budbreak between the two cultivars, suggests that `Bonita' fruit were accessing current (unlabeled) assimilate earlier (i.e., before 51 DAA) than `Climax' fruit. Smaller fruit size in `Climax' compared to `Bonita' may be a consequence of a decrease in reserve carbohydrate mobilization to `Climax' flower buds before anthesis relative to `Bonita', as well as a delay or reduction in the availability of current carbohydrates to developing `Climax' fruit between anthesis and 51 DAA.

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Horacio E. Alvarado, Rebecca L. Darnell, and Jeffrey G. Williamson

Raspberry root growth during fruiting appears to be a strong sink for assimilates, and may decrease carbon availability for fruits and, consequently, cane yield. Both floricanes and primocanes may contribute to root carbon supply in raspberry during fruiting. To test this, `Tulameen' raspberry canes were grown outdoors in containers filled with perlite and peat (1:1). One-half of the plants were girdled and the rest were nongirdled. Within each girdling treatment, either 0 or 3 primocanes were allowed to grow. Treatments were applied at early bloom (10 May), and 50% fruit harvest occurred the first week in June. Fruit number and yield per plant decreased in girdled plants and plants without primocanes compared with nongirdled plants and plants with primocanes. Individual fruit fresh weight was not affected by treatments, but individual fruit dry weight and the dry weight to fresh weight ratio was higher in girdled plants without primocanes than in the other treatments. Neither girdling nor the presence of primocanes affected dry weight allocation to primocanes or floricanes. Root dry weight was higher in girdled plants with primocanes than in nongirdled plants without primocanes. It appears that primocanes supply carbon to roots during fruiting, and subsequently, roots mobilize carbon to floricanes. Thus, roots appear to serve primarily as a translocation pathway for carbon from primocanes to floricanes. However, when primocane growth is suppressed, root carbon is mobilized to support floricane development. If carbon flow from roots to floricanes is restricted, fruit number and yield is significantly decreased.