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  • Author or Editor: Hazel Wetzstein x
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It has been shown that perennial woody plants exhibit marked seasonal changes in nutrient content, carbon metabolism, and organ development. A knowledge of seasonal nutrient allocation and temporal accumulation patterns can be useful in the development of fertilization regimes that reflect the biology of a tree crop. Maintenance of optimum leaf nutrient status is an important priority in pecan cultural practice. However, a systematic evaluation of nutrient resorption is lacking in pecan. In this work, seasonal changes in nutrients and carbohydrates were evaluated in pecan trees grown under orchard conditions. In addition, resorption efficiencies of eight pecan cultivars were evaluated. Significant levels of resorption were observed in all essential elements, but cultivar differences were not significant. Seasonal patterns of nutrient and carbohydrate content in leaf, stem, and shoot tissue, will be presented as well as a structural evaluation of abscission zone formation.

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Pollination is essential in the production of many agricultural crops. Insufficient pollination can lead to reduced yield and lower harvest quality in many fruit and vegetables. Recent declines in insect pollinators and the use of cultural systems where compatible pollen is limiting have caused pollen-related production problems in many crops. Supplemental mass pollination (SMP) may be beneficial in such cases. However, the high cost of pollen may prohibit its use unless pollen is efficiently and uniformly applied. Our objective was to evaluate the feasibility of using selected dry particulate materials as pollen diluents for SMP. Viability was assessed in apple pollen mixed and held with selected powders (i.e., two formulations of Rilsan® nylon, polyester resin, diatomaceous earth, wheat flour, and CaCO3). Also, an assessment of inhibitory substances was made using in vitro germination tests with extracts obtained from liquid suspensions of the different particulates. Several powders, viz., Rilsan® nylon formulations, polyester resin, and wheat flour were identified as nontoxic to pollen held for 1 h as dry pollen: particle mixtures. Likewise, leachates from these diluents had no significant effect on pollen germination. Diatomaceous earth exhibited slight, but statistically significant, inhibitory effects on germination, while CaCO3 completely inhibited germination. The morphology and size of particulates were evaluated using scanning electron microscopy and will be discussed vis-a-vis pollen dispersion and metering requirements.

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Recently, the release of Hydrangea cultivars with the capacity to produce a second flush of blooms has created a great expectation in the ornamental industry. However, the lack of fundamental information on flower development of big leaf Hydrangea does not allow a descriptive explanation of why re-blooming capacity occurs. The objectives of this study were to characterize the timing and location of flower initiation and development in several H. macrophylla cultivars throughout an annual cycle. Four cultivars were evaluated: 2 exhibiting re-flowering capacity (Penny Mac-PM and Endless Summer-ES) and 2 without (Madame Emile Mouillere-MEM and Nikko Blue-NB). Plants were managed under outdoor nursery conditions and harvested at each of four different time periods. These periods represented key developmental stages: 1) Pre-induction: late summer, after completion of shoot expansion; 2) Post-induction: late fall, following short day and cold temperature exposure; 3) Dormancy: winter, post leaf abscission; and 4) Post-dormancy: early spring, just prior to bud break. At each sampling time, bud location (terminal or lateral) and stem origin (basal, lateral, terminal, or secondary) were established. All buds >;2 mm were dissected under a stereomicroscope and the degree of floral induction was determined. Floral primordial were initiated not only in the terminal buds but also within axillary buds. The degree of induction and development varied according to the stem origin, bud location and cultivar. Cultivars with re-blooming capacity had floral primordial initiated within buds at the first sampling period prior to receiving inductive conditions. This suggests they may have minimal or no photoperiodic/temp requirements for flowering.

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The general doctrine of flowering in Hydrangea is that floral induction occurs during the previous season on last year's growth and usually at the stem's terminal bud. However, Hydrangea cultivars widely differ in their relative abundance and duration of flower production. The objective of this study was to determine how developmental flowering patterns compare among different genotypes. Flowering was characterized in 18 H. macrophylla cultivars by assessing the extent of flower initiation and development in terminal and lateral buds of dormant shoots (i.e., after they have received floral inductive conditions.) Plants were managed under outdoor conditions. Dormant, 1-year-old stems were collected and characterized for caliper and length. All buds >2 mm were dissected and the vegetative or floral bud stage of development was categorized for each bud microscopically. Flower development occurred in 100% of the terminal buds for all the cultivars with the exception of `Ayesha' (33%). In contrast, lateral buds showed a wide variation in flower development. For example: `All Summer Beauty', `David Ramsey', `Kardinal', `Masja', and `Nightingale' showed high levels of floral induction (>92 % of lateral buds induced.) In contrast, `Ayesha', `Blushing Pink', `Freudenstein', and `Nigra' had 10% or fewer lateral buds with floral initials. Thus, the degree of floral induction in lateral buds varied tremendously among different cultivars. In addition, flower initiation and development were not related to the size (length and caliper) of individual buds. Thus, bud size does not appear to be a good indicator of flowering potential.

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Although somatic embryogenesis in vitro has been carried out successfully in a number of plants, a limiting factor in many somatic embryogenic systems is that plantlet regeneration is not obtainable or restricted to low frequencies. We have developed a repetitive, high frequency somatic embryogenic system in pecan (Carya illinoensis) and have identified effective treatments for improved somatic embryo conversion. A 6 to 10 week cold treatment followed by a 5 day desiccation, promoted enhanced root germination and extension, and epicotyl elongation. Light and transmission electron microscopic evaluations of somatic embryo cotyledon development will be presented and related to conversion enhancing treatments and their possible roles in embryo maturation.

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Although somatic embryogenesis in vitro has been carried out successfully in a number of plants, a limiting factor in many somatic embryogenic systems is that plantlet regeneration is not obtainable or restricted to low frequencies. We have developed a repetitive, high frequency somatic embryogenic system in pecan (Carya illinoensis) and have identified effective treatments for improved somatic embryo conversion. A 6 to 10 week cold treatment followed by a 5 day desiccation, promoted enhanced root germination and extension, and epicotyl elongation. Light and transmission electron microscopic evaluations of somatic embryo cotyledon development will be presented and related to conversion enhancing treatments and their possible roles in embryo maturation.

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Georgia plume, Elliottia racemosa Muhlenb. ex. Elliott, is an extremely rare small tree or shrub endemic to Georgia, which is being severely affected by habitat loss and lack of sexual recruitment. In vitro plant regeneration of Georgia plume has not been previously reported and may be a method for the conservation and propagation of this threatened species. Studies evaluated the effects of sterilization methods, explant types, medium composition, and light environment on plant regeneration. An efficient plant regeneration system was developed in which adventitious shoot buds were induced using young, expanding leaf explants placed on an induction medium supplemented with 10 μm thidiazuron and 5 μm indole-3-acetic acid with Gamborg's B5 salts. Shoot elongation was promoted on a medium with 25 μm (2-isopentenyl) adenine incorporated into Woody Plant Medium. In vitro rooting studies evaluated continuous and pulse auxin treatments and ex vitro rooting trials after KIBA (indole-3-butric acid, potassium salt) dips. A 5-day pulse treatment on 100 or 150 μm indole-3-butyric acid produced ≈90% rooting of shoots with greater shoot and root dry weight than other pulse times. High rooting frequencies were obtained under in vitro and ex vitro conditions with over 85% survival of plantlets transferred to greenhouse conditions. The culture protocol was found to be effective with material collected from mature specimens in the wild from divergent populations. Tissue culture appears to be a promising approach for the propagation and conservation of this rare and threatened plant.

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Pomegranate fruit is valued for its juice-containing arils and is consumed and marketed as whole fresh fruit, extracted arils, juice, syrup (grenadine), wine, teas, seed oil, and other products. Recent consumption has rapidly increased attributable in part to reported health benefits that include efficacy against coronary heart disease, atherosclerosis, cancer, hypertension, and infectious diseases. Within commercial orchards, the size of fruits produced can be quite variable even with trees of the same genotype grown under similar cultivation practices. Although pomegranates have been cultivated since antiquity, fruit attributes, particularly those related to size, are poorly defined. In this study, compositional changes in pomegranate fruits of the Wonderful cultivar, including volume and weight, aril weight and number, pericarp weight, seed weight, and juice/pulp content, were evaluated in fruits of variable sizes. Correlations between fruit characteristics were determined, and factor analysis established fruit and aril indices. Results indicated that because fruit volume, fruit weight, and total aril weight are closely correlated, any of these characteristics can be used as an indicator of fruit size. The number of arils per fruit was highly correlated with fruit size with larger fruit containing greater numbers of arils. This is in contrast to individual average aril weight, which showed no significant relationship to fruit size. Crop production strategies aimed at increasing aril numbers may be a means for obtaining larger fruit in pomegranate.

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Somatic embryogenic cultures of pecan (Carya illinoinensis) were induced on medium with either naphthaleneacetic acid (NAA) or 2,4-dichlorophenoxyacetic acid (2,4-D). Percent embryogenesis, embryo development, and subsequent performance were assessed. Cultures induced on medium with NAA were more zygotic-like, with a higher frequency of embryos that had well-defined shoot apices. In contrast, cultures induced with 2,4-D exhibited more extensive callusing and more fused and/or abnormal embryos. Adjustment of the auxin used during induction may be a means of obtaining higher quality embryos, that have higher rates of conversion into plants.

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Pomegranate [Punica granatum (Punicaceae)] is characterized by having two types of flowers on the same tree: hermaphroditic bisexual flowers and functionally male flowers. This condition, defined as functional andromonoecy, can result in decreased yields resulting from the inability of male flowers to set fruit. Morphological and histological analyses of bisexual and male flowers were conducted using light and scanning electron microscopy (SEM) to characterize the different flower types observed in pomegranate plants and to better understand their developmental differences. Bisexual flowers had a discoid stigma covered with copious exudate, elongated stigmatic papillae, a single elongate style, and numerous stamens inserted on the inner wall of the calyx tube. Using fluorescence staining, high numbers of pollen tubes were observed growing through a central stylar canal. Ovules were numerous, elliptical, and anatropous. In contrast, male flowers had reduced female parts and exhibited shortened pistils of variable heights. Stigmatic papillae of male flowers had little exudate yet supported pollen germination. However, pollen tubes were rarely observed in styles. Ovules in male flowers were rudimentary and exhibited various stages of degeneration. Pollen from both types of flowers was of similar size, ≈20 μm, and exhibited similar percent germination using in vitro germination assays. Pollen germination was strongly influenced by temperature. Maximal germination (greater than 74%) was obtained at 25 and 35 °C; pollen germination was significantly lower at 15 °C (58%) and 5 °C (10%).

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