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Rebecca E. Scoville and Todd P. West

The objective of this study was to investigate the effects of multiple nutrient salt formulations and different plant growth regulator concentrations on initiation and proliferation of axillary shoot culture of tropical hibiscus (Hibiscus rosa-sinensis L.). Combinations of five thidiazuron (TDZ) concentrations (0, 10-6, 10-7, 10-8, or 10-9 M) in conjunction with two 6-benzylaminopurine (BA) concentrations (0, 10-5 M) and two indole-3-butryic acid (IBA) concentrations (0, 10-5 M) were compared to determine which plant growth regulator combination(s) would stimulate the proliferation of the most viable axillary shoots. Also, five nutrient salt formulations (MS, 1/2 MS; Macro MS, WPM, LP, or DKW) ranging from high to low salt formulations were studied to determine a suitable nutrient medium formulation for axillary shoot proliferation. Nodal explants that were 2 cm in length were used to initiate cultures and were maintained on the various medium treatments plus 30 g·L-1 sucrose and 7 g·L-1 agar at a pH of 5.8. Explants were incubated about 30 cm beneath cool-white fluorescent lamps that provide a photon flux of about 40 μM·m-2·s-1 for a 16-hour photoperiod at 25 ± 3 °C. Nodal explants were transferred every 3 weeks for a total culture period of 12 weeks. At each transfer date data were collected on node number, axillary shoot number and length. Initial results indicate that high nutrient salt formulations coupled with low TDZ concentrations performed better at axillary shoot initiation. Poor shoot elongation was observed and further research needs to be performed to address this issue.

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W. David Lane, Basdeo Bhagwat, Susan Wahlgren, and John D. Armstrong

A protocol for micrografting shoot tips harvested from in vitro shoot cultures directly to transplanted rootstock plants in the greenhouse was developed. Shoot tips of the apple (Malus domestica) cultivars Golden Delicious, Granny Smith and Fuji clone, Nagafu12, were harvested, stored in a water bath then prepared for grafting by cutting the stem immediately below the tip into a wedge shape leaving the tip approximately 3 mm (0.12 inch) long. The rootstock cultivar, Malling 9 (M.9) (M. domestica), was prepared by cutting into a young fast growing side branch to expose the cambium, creating a pocket into which the shoot tip was inserted. The cut section of the tip was oriented so as to contact the exposed rootstock cambium and was held in place by wrapping with a strip of pliable plastic film. Two weeks later the wrapping was loosened and the grafted branch cut back. Side branches of the rootstock were not removed until later in order to support rootstock growth. The scion shoots developed into nursery whips suitable for transplanting to a screen house or field after 2 months. The protocol proved to be a simple efficient way to rapidly grow nursery trees from tissue culture clones developed in genetic modification experiments and was used to propagate several hundred plants. Grafting success was often 100% but was reduced if quality of shoot tips was poor due to injury indicated by brown tip color. The protocol eliminates the steps of rooting, acclimatizing and growing shoots into plants to serve as a scion wood source.

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X. Cao and F.A. Hammerschlag

As part of a program to develop transgenic highbush blueberry (Vaccinium corymbosum L.) cultivars, studies were conducted to determine optimum conditions for high efficiency shoot regeneration from leaf explants of shoots propagated in vitro. The effects on shoot organogenesis of age of explant source, length of dark treatment, the addition of either thidiazuron (TDZ) at 1 or 5 μm, or zeatin riboside at 20 μm to the regeneration medium, and a photosynthetic photon flux (PPF) of either 18 ± 5 or 55 ± 5 μmol·m–2·s–1 were investigated. A maximum of 13.0, 13.0, 12.6, and 4.6 shoots regenerating per explant for cultivars Duke, Georgiagem, Sierra, and Jersey, respectively, occurred on regeneration medium with zeatin riboside and under a PPF of 55 ± 5 μmol·m–2·s–1. `Duke' regenerated equally well on medium with either zeatin riboside or 1 μm TDZ, whereas the number of shoots per explant for `Georgiagem' and `Sierra' was significantly higher on zeatin riboside. Regeneration of `Duke', `Jersey', and `Sierra' on zeatin riboside was significantly better under a PPF of 55 ± 5 μmol·m–2·s–1 than under 18 ± 5 μmol·m–2·s–1, but the higher PPF inhibited regeneration of `Duke' on 5 μm TDZ. There were no significant differences in percentage of regeneration or the number of shoots per explant from leaf explants derived from either 1-, 2-, or 3-week-old shoot cultures, or when either 1 week or 2 weeks of darkness preceded light treatments. Chemical names used: 1-phenyl-3-(1,2,3-thiadiazol-5-yl)urea (thidiazuron, TDZ); 9-(-β-ribofuranosyl)-6-(4-hydroxy-3-methyl-but-2-enylamino)purine (zeatin riboside).

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Mark H. Brand and Richard Kiyomoto

Tissue proliferation (TP) of Rhododendron sp. is characterized by basal tumors that often develop into numerous dwarf shoots. Growers need to know if the TP condition will recur in plants grown from normal-appearing cuttings collected from plants with TP tumors. Stem cuttings of seven cultivars were collected from stock plants with TP [TP(+)] and without TP [TP(–)] and rooted. Plants were grown in containers outdoors for 2 years and were then evaluated for tumor formation and other TP-related morphological symptoms. Shoots of TP(+) plants were either similar in length to shoots of TP(–) plants, or were shorter, as was the case for `Boule de Neige', `Catawbiense Album', and `Montego'. Plants grown from TP(+) cuttings of all cultivars had more leaves per growth flush than did plants grown from TP(–) cuttings. `Holden', `Montego', and `Scintillation' TP(+) leaves were narrower than leaves from TP(–) shoots and had greater length: width ratios. Leaves of TP(+) `Montego' and `Scintillation' plants were shorter and smaller than leaves from their TP(–) counterparts. Tumors were not observed on any propagated plants, regardless of the TP status of cutting stock plants. To further test the influence of age and TP status of source plants used for cutting propagation, `Montego' plants were grown from cuttings collected from the following sources: 1) in vitro shoot cultures; 2) 3-year-old plants with TP; 3) 6-year-old plants with TP; and 4) TP(–) plants. Cuttings from TP(+) micropropagated plants less than 3 years old were more likely to develop tumors than were cuttings from older plants. Eighty-three percent of plants from microcuttings and 74% of plants from cuttings of 3-year-old TP(+) plants formed tumors, whereas no plants grown from 6-year-old TP(+) or TP(–) cuttings did so. Large tumors that surrounded half or more of the stem were more likely to develop on plants grown from microcuttings than on plants grown from the next youngest, 3-year-old TP(+), stock plants. Growers must be aware that cuttings from TP(+) plants may produce plants that exhibit morphological and growth abnormalities, possibly even including tumor redevelopment.

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Senay Ozgen, James S. Busse, and Jiwan P. Palta

; Sha et al., 1985 ). This condition is typified by browning and death of the shoot tip, loss of apical dominance, and axillary shoot development in an in vitro shoot culture. Transpiration is limited during in vitro culture by the high humidity that

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Dongliang Qiu, Xiangying Wei, Shufang Fan, Dawei Jian, and Jianjun Chen

of disease-free propagules on a year-round basis ( Chen and Henny, 2008 ; Murashige, 1974 ). Blueberries have been micropropagated via shoot culture ( Fan et al., 2017 ; Frett and Smagula, 1983 ; Litwinczuk, 2013 ; Lyrene, 1980 , 1981 ; Ruzic et

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Ruigang Wu, Yi Wang, Ting Wu, Xuefeng Xu, and Zhenhai Han

materials and treatments. Root, stem, leaf, flower, and fruit tissues were collected from a 5-year-old ‘Golden Delicious’ own-rooted apple tree for tissue-specific expression of MdMYB4 gene under nonstress conditions. In vitro apple shoot cultures of

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Samir C. Debnath

establishment of shoot cultures. Eight-week-old shoot tip cultures of cultivar Fundy developed at the Atlantic Food and Horticulture Research Center, Agriculture and Agri-Food Canada, Kentville, Nova Scotia, Canada, and two wild clones, ‘NB1’ and ‘QB1

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Seong Min Woo and Hazel Y. Wetzstein

with either 100 or 150 μ m IBA for 2, 5, or 8 d and then transferred to the same basal medium in baby food jars. Shoot culture and pulsing was under white fluorescent lights (70 μmol·m −2 ·s −1 ) at 25 °C in a growth room. The percentage of shoots and

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Richard J. Henny, James R. Holm, Jianjun Chen, and Michelle Scheiber

( Tambong et al., 1998 ). Colchicine has been reported to induce tetraploidy in nine Zantedeschia cultivars ( Cohen and Yao, 1996 ). Rapidly multiplying in vitro shoot cultures were exposed to 0.05% (w/v) colchicine on solid Murashige and Skoog (MS) media