Search Results

You are looking at 21 - 30 of 49 items for

  • Author or Editor: Rebecca L. Darnell x
  • Refine by Access: All x
Clear All Modify Search
Open access

Rebecca L. Darnell and George C. Martin

Abstract

The development of phloem sieve tube files in receptacles of pollinated, nonpollinated, and auxin-treated strawberry flowers (Fragaria × ananassa Duch. ‘Fern’) was examined from 0 to 120 hr after emasculation and treatment and correlated to fruit set and growth. The receptacle dry weight of nonpollinated flowers increased very little during the first 120 hr after emasculation. Conversely, receptacle dry weights of the pollinated and auxin-treated flowers began to increase within 24 hr after emasculation and continued to increase throughout the rest of the experimental period. No differences were found in the appearance of the phloem and xylem cells produced in the receptacles among the three treatments. Although the number of sieve tube files in all treatments increased with time, there was no consistent difference in the number of files between growing and nongrowing receptacles. These results indicate that there is no correlation between the degree of phloem sieve tube development and fruit set/initial growth of strawberry receptacles.

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).

Open access

Rebecca L. Darnell and George C. Martin

Abstract

Soluble sugar and starch levels and translocation of 14C-Iabeled photoassimilates in receptacles of pollinated, nonpollinated, and auxin-treated strawberry flowers (Fragaria × ananassa Duch. ‘Fern’) were examined from 0 to 144 hr after anthesis and correlated with fruit set and initial growth. Receptacle dry weight of nonpollinated flowers increased very little during the experimental period, whereas receptacles of pollinated and auxin-treated flowers showed a gain in dry weight within 24 hr, which continued throughout the experimental period. However, 14C accumulation in flower receptacles did not reflect the initial differences in growth, indicating that recently fixed 14C-labeled photosynthate was not the source for the observed dry weight increases. Analysis of carbohydrate pool sizes in treated receptacles indicated no consistent correlation between pool sizes and fruit set and initial growth. Thus, fruit set and initial growth in strawberry is not limited by the capacity of receptacles either to mobilize current source leaf assimilates or accumulate carbohydrates.

Full access

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, L. Carl Greve, and George C. Martin

Abstract

A simple procedure for synthesizing and purifying the [14C]ethyl ester of IAA (Et-IAA) is described. This auxin has been found to stimulate parthenocarpic fruit set in day-neutral strawberries (Fragaria × ananassa Duch. ‘Fern’), which are non-responsive to various other auxins. Et-IAA may prove useful in eliciting physiological responses in systems shown previously to be auxin-nonresponsive. Chemical name used: 1H-indole-3-acetic acid (IAA).

Free access

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.

Free access

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

Low yields have been observed in annual production systems in raspberry (Rubus idaeus L.) compared with annual yields in perennial systems. This yield reduction could be related to a depletion of root carbohydrates and its consequent detrimental effect on fruit number or size. Additionally, primocanes could play an important role in the carbohydrate dynamics in an annual system and may also affect yield. Two experiments were carried out in 2003 and 2004 to determine the importance of root carbohydrates and primocanes in fruiting and yield components of ‘Tulameen’ red raspberry in an annual production system. In the 2003 experiment, girdled floricanes were compared with nongirdled controls. Girdling before bloom decreased fruits per cane and consequently yield per cane compared with controls, whereas girdling at the end of bloom had no effect. Root dry weight accumulation at the end of the fruiting season was significantly less in both early and late girdled compared with nongirdled plants. In 2004, floricanes were completely girdled or nongirdled at midflowering. Additionally, three primocanes were permitted to grow in one-half of the nongirdled and girdled plants, whereas primocanes were completely removed from the other half. Girdling and removal of all primocanes resulted in 100% plant mortality. In the presence of primocanes, floricane girdling had no effect on yield compared with the nongirdled treatments. There was a reduction in root dry weight in the nongirdled + primocane removal treatment compared with the treatments in which three primocanes were present. In the presence of primocanes, however, root dry weights were similar in both girdled and nongirdled plants. These results suggest that root carbohydrates are important in determining fruit number and yield in the annual system, and reductions in root carbohydrate during early flowering results in decreased yield. However, roots appear to quickly convert from source to sink status, and as the season progresses, both floricanes and primocanes act as sources to replenish root carbohydrate reserves. Carbohydrate dynamics appear to be similar between the annual production system and the traditional perennial system; however, because the annual system begins with limited carbohydrate reserves resulting from root pruning that occurs during removal from the nursery, yields are lower than those seen in perennial systems.

Free access

Timothy M. Spann, Jeffrey G. Williamson, and Rebecca L. Darnell

Experiments were conducted with V. darrowi and two cultivars of southern highbush blueberry, `Sharpblue' and `Misty,' to test whether V. darrowi and cultivars derived from it are photoperiodic with respect to flower bud initiation. Plants of each cultivar were grown under three different photoperiod treatments [long days (LD) = 16-hour photoperiod; short days (SD) = 8-hour photoperiod; and short days + night interrupt (SD-NI) = 8-hour photoperiod with 1-hour night interrupt] at constant 21 °C for 8 weeks. Vegetative growth was greatest in the LD plants of both cultivars. Flower bud initiation occurred only in the SD treatments, and the lack of flower bud initiation in the SD-NI treatment indicates that flower bud initiation is a phytochrome mediated response in Vaccinium. Previously initiated flower buds on the V. darrowi plants developed and bloomed during the LD treatment, but bloom did not occur in the SD and SD-NI treatment plants until after those plants were moved to LD. These data indicate that flower bud initiation in both V. darrowi and southern highbush blueberry is photoperiodically sensitive, and is promoted by short days, while flower bud development is enhanced under long days.

Free access

Rebecca L. Darnell, Bruno Casamali, and Jeffrey G. Williamson

Successful blueberry (Vaccinium sp.) cultivation typically requires soils with low pH, high organic matter, readily available iron, and nitrogen (N) in the ammonium form. Growth of blueberry on typical mineral soils (higher pH, low organic matter) is reduced. Although soil pH effects on nutrient availability and uptake are known, it is unclear if the requirement for low soil pH in blueberry production is due to effects on nutrient availability/uptake or is a more direct effect of rhizosphere pH on root function. In addition, it is unclear if the requirement for high organic matter (soil amendments) is related directly to nutrient availability/uptake. Several studies have examined the use of rootstocks to increase soil adaptation of blueberry and some of these rootstocks have been found to increase plant vigor and yield. In particular, we have investigated whether sparkleberry (Vaccinium arboreum)—a wild blueberry species that is adapted to high pH and low organic matter soils—could be used as a rootstock for commercial production of blueberry on mineral soils. Our work indicates that both nitrate (NO3 ) and iron (Fe) uptake and assimilation are greater in sparkleberry compared with southern highbush blueberry [SHB (Vaccinium corymbosum interspecific hybrid)]. This is correlated with increased activity of nitrate reductase (NR) and iron chelate reductase, the rate limiting enzymes for NO3 and Fe acquisition, respectively. Field studies comparing growth and yield of own-rooted vs. grafted ‘Meadowlark’ and ‘Farthing’ SHB in amended vs. nonamended soils are ongoing. In general, own-rooted plants on amended soils exhibit greater growth than own-rooted on nonamended soils, while grafted plants in either soil system exhibit intermediate growth. Yields generally followed this pattern. Our preliminary results suggest that tolerance of SHB to mineral soils is greater when plants are grafted onto sparkleberry than when grown on their own roots. However, growth and yield of grafted plants grown under mineral soil conditions may not equal or exceed that of own-rooted plants grown under optimum soil conditions, at least in the first years after field planting. Longer term studies are necessary to fully evaluate the potential of using sparkleberry and other blueberry species as rootstocks for SHB and northern highbush blueberry (V. corymbosum).

Free access

Timothy M. Spann, Jeffrey G. Williamson, and Rebecca L. Darnell

Experiments were conducted with `Misty' southern highbush blueberry (Vaccinium corymbosum L. interspecific hybrid) to test the effects of high temperature on flower bud initiation and carbohydrate accumulation and partitioning. Plants were grown under inductive short days (SDs = 8 hour photoperiod) or noninductive SDs with night interrupt (SD-NI = 8 hour photoperiod + 1 hour night interrupt), at either 21 or 28 °C for either 4 or 8 weeks. Flower bud initiation occurred only in the inductive SD treatments and was significantly reduced at 28 °C compared with 21 °C. The number of flower buds initiated was not significantly different between 4- and 8-week durations within the inductive SD, 21 °C treatment. However, floral differentiation appeared to be incomplete in the 4-week duration buds and bloom was delayed and reduced. Although plant carbohydrate status was not associated with differences in flower bud initiation between SD and SD-NI treatments, within SD plants, decreased flower bud initiation at high temperature was correlated with decreased whole-plant carbohydrate concentration. These data indicate that flower bud initiation in southern highbush blueberry is a SD/long night phytochrome-mediated response, and plant carbohydrate status plays little, if any, role in regulating initiation under these experimental conditions.