Adventitious shoot formation from leaf explants of Rhododendron
A protocol for high frequency adventitious shoot regeneration adventitious shoot regeneration from leaf explants of Rhododendron spp. has been developed. The highest percentage of regeneration and the greatest number of shoots were obtained when leaf explants were cultured on Anderson's medium containing 4.9 μM IBA and 73.8 μM 2iP. Genotypic variation was observed for adventitious shoot regeneration potential among the seven cultivars tested. Regeneration frequencies ranged from 0 to 96%. ‘Lodestar’ had the highest rate of regeneration after 3 months of culture with 96% shoot regeneration and an average of 14 shoots per explant. Regenerated shoots were rooted in soil in about 2 months. This protocol should be useful in applying gene transfer techniques to Rhododendron improvement.
Axillary and Adventitious Shoot Proliferation of Exbury Azaleas in Vitro.1

Shoot tips of Exbury azalea (Rhododendron sp.) were proliferated on Anderson's Rhododendron medium supplemented with 6-benzylamino purine (BA), 6-(= ϒ, = ϒ-dimethylallylamino) purine (2iP), 6-(= ϒ, = ϒ dimethylallylamino) purine riboside (2iP-riboside), 6-furfurylaminopurine (kinetin), or 6-(4-hydroxy-3-methylbut-2-enylamino) purine (zeatin). The greatest number of shoots was produced on medium containing zeatin, a lesser number on 2iP and 2iP-riboside, and the least on BA and kinetin. Shoot proliferation was affected by genotype. Few adventitious shoots formed on detached cultured leaves. Shoot tips rooted when subcultured on medium without cytokinin or when inserted in moist milled sphagnum.
Micropropagation and in vitro plant regeneration of rhododendrons
approved: Dr. Tony H.H. Chen The majority of tissue culture studies on Rhododendron have focused on micropropagation by the induction of axillary shoot growth. This type of shoot multiplication has high genetic stability since somaclonal variation is minimized. There has been no report concerning micropropagation of Vireya rhododendrons. In vitro propagation by adventitious shoot regeneration from somatic tissue offers a more efficient method for massive plant multiplication since a greater number of plants can be produced from a single explant. Furthermore, the ability to regenerate whole plants from somatic tissue is a fundamental prerequisite for applying modern gene transfer techniques to the genetic improvement of Rhododendron. The objectives of this study were: 1) to develop an efficient micropropagation system for the hybrid R^ laetum x aurigeranum; and 2) to explore the feasibility of plant regeneration from somatic tissues of R^. laetum x aurigeranum and commercial cultivars of Rhododendron. To realize objective I, shoot tips taken from greenhouse grown plants were cultured on Anderson's medium supplemented with different combinations of IAA and 2iP concentrations. Optimal axillary shoot proliferation was observed after three months of culture in medium supplemented with 15 mg/1 2iP. After 6 months of culture, brown callus developed at the cut surfaces of the original explant. This callus produced a large number of adventitious shoots. Clumps of adventitious shoots that were divided and subcultured lead to greater shoot proliferation. In this study three types of shoot multiplication were observed in R^ laetum x aurigeranum; 1) multiple shoot formation from shoot tip culture (axillary shoots); 2) adventitious shoot formation from callus produced at the cut surfaces of the primary shoot tip explant; and 3) adventitious shoot regeneration from the base of individual shoots. We estimated a 137, 1600 and 7.8 fold increase in shoot number over six months for type 1, 2 and 3 shoot multiplication types respectively. Of these shoot forms, the second type was the most efficient for micropropagation of Ri. laetum x aurigeranum. In vitro rooting was accomplished by lowering the Anderson's medium to 1/4 ionic strength and by adding IAA (5 mg/1). Plants adapted well to the soil environment and did not show any visible morphological abnormality. Plant regeneration from different somatic tissues of R. laetum x auriaeranum was tested. The highest percent of regeneration was obtained from leaf strips and leaf sections cultured on Anderson's medium supplemented with 2 mg/1 IAA and 2 0 mg/1 2iP. The leaf strip explants required a long intermediary callus phase (6 months) before organogenesis occurred. When whole leaves devoid of petiole end were used, we found that shoot primordia differentiation could be accomplished in 2 months. Leaf strip explants of 10 commercial Rhododendron cultivars were cultured on Anderson's medium supplemented with various combinations of IAA and 2iP concentrations. Genotypic differences were observed in in vitro shoot regeneration. Among the 10 cultivars tested only three ('Ivory Coast', 'Joe Paterno and 'Lodestar') showed reproducible plant regeneration response. Ivory Coast gave the best results at 22.5% regeneration. This study demonstrates that adventitious shoot regeneration from callus induced at wound sites of shoot tip explants is the most efficient plant propagation method for Rj. laetum x auriaeranum. A system was developed for shoot regeneration from leaf explants of R^ laetum x auriaeranum. Plant regeneration was also accomplished from leaf strips of commercial Rhododendron cultivars. Micropropagation and In Vitro Plant Regeneration of Rhododendrons by Giovanni lapichino A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed April 24, 1990 Commencement June 1991