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  • Author or Editor: Mark H. Brand x
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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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Studies initiated in Spring 1998 analyzed the influence of division size, shade, and temperature on the growth rate of the ornamental grass Hakonechloa macra `Aureola' in nursery-container production. To determine the optimum division size for production, container-grown stock plants were used to make early spring divisions of four sizes (1-2, 4-6, 8-10, and 12-15 buds). Divisions were established in 325-ml pots for 1 month before being transplanted to 3.7-L nursery containers. Plants were grown outdoors and received topdressed 17-6-10 slow-release fertilizer (containing micronutrients) and drip irrigation from June through September. Average leaf area, shoot number and bud count increased linearly as division size increased, but average height remained the same between each treatment. Plants of all division sizes exhibited healthy growth, with 50% of the plants in the 4-6 buds/division treatment growing to marketable size compared to 45%, 35% and 15% in the 8-10, 12-15, and 1-2 buds/division treatments, respectively. Four shade densities (0%, 30%, 50%, and 70%) were tested to determine which promoted optimum growth. As shading increased, average shoot number per plant decreased, average height and shoot length increased and bud count remained the same. To determine the optimum growing temperature for Hakonechloa, divisions were grown in 325-ml pots under four different day/night temperatures (15/10, 21/16, 27/22, and 33/28 °C) for 12 weeks in growth chambers. Plants were fertigated daily with a 5-25-5 liquid fertilizer. Average bud count, leaf area, plant height, plant width, shoot length, and shoot number increased as temperature increased to 27/22 °C, then decreased significantly beyond this temperature optimum.

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Survey data from 788 single-family residences from New England were analyzed to evaluate purchasing preferences and gardening habits. Particular attention was focused on plant attributes and choices of independent garden centers vs. mass merchandisers. Independent garden centers, magazines, and friends were the most important sources of gardening information, while mass merchandisers were relatively unimportant information sources. While consumers trusted information received at independent garden centers, they did not trust mass merchandiser information as much. The most important product and service attributes of retail establishments were well-maintained plants, informative signage, knowledgeable staff, and a wide selection of plant material. Gardening chemicals and fertilizers were purchased at mass merchandisers due to price. Consumers preferred to purchase high-value, long-lived plants (trees and shrubs) at independent garden centers due to higher plant quality and access to knowledgeable staff. When making plant purchases, plant appearance was the most important consideration regardless of whether the plant was an annual, perennial, or woody plant. The presence of flowers on plants was not ranked as influential in making purchase selections, but evidence of new growth, the presence of dark green foliage, and knowledge of a northern-grown source were important. For trees and shrubs, the significance of a plant guarantee and knowledge of a northern-grown source increased in importance in comparison to annuals and perennials.

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Hakonechloa macra Makino 'Aureola' is an ornamental, shade tolerant landscape grass that grows slowly and commands high prices. Hakonechloa plants grown from four initial division sizes, of 1-2, 4-6, 8-10, or 12-15 tiller buds, were evaluated following a complete growing season (105 days). Based on visual observation, we rated 100% of plants grown from the two larger division sizes to be salable compared with only 30% of those from divisions containing 4-6 growing points, and none from the smallest division size. However, divisions of 1-2 tiller buds produced twice as many new shoots and tiller buds per initial tiller bud as did larger division sizes. To produce salable plants in one growing season, results suggest the use of 8-10 tiller bud divisions, but for propagation and increase of stock material, where it is important to obtain the greatest number of new growing points per initial growing point, use of the smaller division sizes is indicated. Hakonechloa plants were grown under shading densities of 0%, 30%, 50%, or 70% provided by polypropylene shade cloth. Shading increased overall growth and improved the appearance and leaf color of Hakonechloa, but at 70% shade density, plants appeared languid and open. For this reason, 50% shading is recommended for nursery production of Hakonechloa macra 'Aureola'.

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Optimum growing temperatures were determined for Hakonechloa macra Makino 'Aureola' and Chasmanthium latifolium (Michx.) Yates, two shade-tolerant ornamental grasses found naturally in regions differing in temperature conditions. Plants were grown in four growth chambers at average daily temperatures of 13, 19, 25, and 31 °C for 12 weeks. After the treatment period, plants were destructively harvested to quantify growth and shoot tissue concentrations of N, P, K, Ca, and Mg. Optimal growth occurred at an average daily temperature of 25 °C for both grasses, but Hakonechloa was better able to tolerate lower temperatures. Hakonechloa died at 31 °C, while Chasmanthium growth was only slightly reduced at this temperature. Nutrient concentrations in shoot tissue for both species increased with increasing temperatures up to the temperature supporting optimal growth. At 13 and 19 °C, the concentrations of most nutrients were higher for Hakonechloa than for Chasmanthium, possibly reflecting the greater growth (higher nutrient demand) of Hakonechloa at lower temperatures. When compared on a per plant basis at each grasses' optimum temperature for growth, Chasmanthium has a much greater demand for nutrients than Hakonechloa, reflecting the greater growth potential of Chasmanthium.

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Uniform Rhododendron `Roseum Elegans' plants were potted into 3-gal plastic containers and place atop eight identical 1-m3 soil-filled boxes (six per box). The bottomless boxes were recessed into a grassed field and filled with Woodbridge fine sandy loam. Soil samples were taken in 30-cm layers to 90 cm from each box and analyzed for NO3-N. Samples were taken at 14-day intervals from 9 June 1994 to 10 Nov. 1994. All plants received 40 g of Sierrablen 17N–6P–10K 8–9 month controlled-release fertilizer as a topdressing on 13 June 1994. Half of the boxes received a high irrigation rate of 5 gal/min for 4 min and half received a low rate of 5 gal/min for 1 min. Irrigation was provided every other day from above the plant canopy to simulate typical irrigation practices. For the low irrigation treatment, NO3-N levels reached 19.5 mg of NO3-N/kg of soil in the first 28 days of the study, while increases in the deeper layers lagged behind. For the high treatment, NO3-N levels followed a similar pattern, but only reached 8.9 mg of NO3-N/kg of soil in the same time period. These results suggest that the high irrigation volumes cause NO3-N to leach at a faster rate, posing a threat to water resources.

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Rhododendron `Montego' shoot cultures initiated from plants with and without tissue proliferation (TP and NTP) served as explant sources for all studies (Note: in vitro TP shoot cultures produce primarily dwarf shoots, some long shoots, and stem tumors). Calli induced from TP leaves and tumors and NTP leaves were cultured on woody plant (WP) medium containing NAA and 2-iP. During the first 4 weeks of culture, calli from NTP leaves had higher relative growth rates than calli from TP leaves or tumors. However, calli from TP leaves and tumors grew faster than calli from NTP leaves for all subculture periods that followed. Shoot tips (5 mm) were excised from TP dwarf shoots, TP long shoots, and NTP shoots and were cultured on WP medium with or without 15 μM 2-iP. Shoot tips from TP dwarf and long shoots multiplied on medium without 2-iP, averaging 18.4 and 1.7 shoots per shoot tip in 12 weeks, respectively. Shoot tips from NTP shoots only multiplied when maintained on 2-iP-containing medium. When placed on 2-iP-containing medium, both types of TP shoot tips produced clusters of callus-like nodules that gave rise to highly tumorized, short shoots or leafy meristems.

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While Japanese barberry (Berberis thunbergii) is an acknowledged invasive plant, the danger posed by its garden cultivars is unknown. This work analyzed the reproductive potential and seedling traits of wild type Japanese barberry and four important cultivars: `Atropurpurea', `Aurea', `Crimson Pygmy', and `Rose Glow'. The germination capacity of cleaned and stratified seeds was determined for all accessions in a greenhouse and seedling foliage color was noted. A subpopulation of seedlings from each accession was grown further in containers outdoors for a full season to ascertain seedling vigor. The average number of seeds produced per landscape specimen ranged from 75 and 90 for `Aurea' and `Crimson Pygmy' to 2967 for `Atropurpurea', 726 for `Rose Glow', and 1135 for wild type B. thunbergii. The vigor of 1-year seedlings—as measured by dry weight of top growth—for progeny derived from `Aurea' (2.29 g) and `Crimson Pygmy' (2.74 g) was less than `Atropurpurea' (3.45 g), `Rose Glow' (3.88 g) and wild type (3.73 g). Seedlings derived from purple-leaf cultivars displayed variable ratios of green and purple leaf phenotype correlated to the proximity and identity of likely Japanese barberry pollinators. `Rose Glow' specimens located among other purple-leaf B. thunbergii produced up to 90% purple seedlings, while other samples growing in isolation or near green-leaf plants produced less than 10% purple progeny. This suggests that some invasive green-leaf Japanese barberry could be derived from cultivars. The results also show that these cultivars express disparate reproductive potential.

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Experiments were conducted on tissue proliferation (TP) development and in vitro and ex vitro growth of tissues from plants with (TP+) and without TP (TP-). In 1993 the increase in TP in one-, two-, and three-yr-old `Holden' and `Besse Howells' was 3%, 52%. and 32% and 10%, 26% and 21%, respectively. No differential mortality was observed. Shoot tip cultures initated from TP+ and TP- `Montego' showed 10-12 mo were required for miniaturiziation and multiplication in TP- shoot tips and 4 mo in TP+ shoot tips. TP- cultures require 10 uM 2-iP for normal shoot proliferation; whereas TP+ cultures had to be transferred to hormone-free medium after 6 mo to maintain normal shoot morphology. Cutting propagation from TP- and TP+ plants older than 5 yr, showed persistence of morphological aberrations associated with TP+ plants.

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