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Jane E. Knapp and Mark H. Brand

Horticultural improvements in Rhododendron require long periods of time to produce flowering plants by traditional breeding methods. In addition, new trait development by conventional genetics is limited to existing germplasm. Genetic engineering approaches to horticultural improvement offer the possibility for introduction of new traits using foreign DNA from any source. To this end, we have developed a system for the genetic transformation of Rhododendron based on microprojectile bombardment. Leaves from in vitro-grown plantlets of R. `Catawbiense Album' L. were bombarded with the marker genes uidA (GUS) in combination with nptII or hph. Two days post-bombardment, explants were transferred to shoot iniation medium containing either 50 mg/L kanamycin or 2.5 mg/L hygromycin. After 4 weeks, proliferating tissues were transferred to media containing increased levels of selective agent (100 mg/L kanamycin or 5 mg/L hygromycin, respectively). Shoots that regenerated were then excised from necrotic tissues and transferred to shoot proliferation medium containing the high level of selective agent. PCR analysis of putative transformants revealed the presence of the transgenes. Southern blot hybridization confirmed stable transgene integration. Histochemical GUS assays of transformed tissues indicated uniform expression throughout the transgenic plant. With the development of an efficient transformation system, the introduction of genes to confer useful horticultural traits becomes feasible.

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Mary W. George and Robert R. Tripepi

Plant Preservative Mixture™ (PPM), a relatively new, broad-spectrum preservative and biocide for use in plant tissue culture, was evaluated as an alternative to the use of conventional antibiotics and fungicides in plant tissue culture. Concentrations of 0.5 to 4.0 mL·L-1 were tested with leaf explants of chrysanthemum (Dendranthem×grandiflora Kitam), European birch (Betula pendula Roth), and rhododendron (Rhododendron catawbiense Michx.). PPM had little effect on the percentage of explants forming shoots and the number of shoots formed per explant in birch and rhododendron, but dramatically reduced both responses in chrysanthemum. Therefore, the effects of PPM must be evaluated for each species of interest prior to use.

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David J. Colangelo and Mark H. Brand

Plastic 208-L industrial barrels (14 total) were modified for use as soil-filled lysimeters to study the nitrogen dynamics of a typical container crop production system. The top of each barrel was removed and the bottom was fitted with a drain hole and filter fabric. The drain was then connected via tubing to a 2-L leachate collection vessel made from a length of 15.24-cm-diameter PVC pipe that had been capped on one end. All barrels and connected collection vessels were recessed into a grassed slope. Barrels were filled with homogeneous B and C horizon soil to simulate soil conditions of a typical container nursery. Uniform Rhododendron `Catawbiense Album' plants in 4.5-L containers were arranged atop the barrellysimeters at four plants per barrel. Irrigation/fertilizer treatments included fertilized pulse trickle irrigation (four replications), fertilized overhead irrigation (four replications), and unfertilized controls corresponding to each irrigation treatment (three replications each). All fertilized plants received 10 g of 17N–6P–10K 8- to 9-month controlled-release fertilizer at the beginning of the crop cycle. Leachate from the barrel-lysimeters was collected weekly and total volume, total Kjeldahl N, nitrate-N, and ammonium-N were determined. Peak nitrate-N levels were well above the current drinking water standard for both irrigation treatments at certain times during the year. Cumulative nitrate-N mass output was similar for both irrigation treatments. A nitrogen balance for the complete production system including fertilizer and irrigation water input, plant material, potting media, soil in the lysimeter barrels and leachate output from the barrels has also been determined.

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P. Chris Wilson and Joseph P. Albano

( Rhododendron catawbienseAlbum’) crops of 7.2 mg·L −1 (1.6 mg·L −1 N) for overhead irrigation and 12.7 mg·L −1 (2.9 mg·L −1 N) for trickle irrigation. Those concentrations fell within the 75th and 90th percentiles for the CRF portion of this study