Winter-Hardy Tetraploids of R. carolinianum and R. racemosum, and their Tetraploid Hybrids
W. L. Tolstead, Ph.D.
Professor of Biology
Davis and Elkins College, Elkins, West Virginia
Joseph F. Glencoe
Professor of Biology
West Virginia Wesleyan College, Buckhannon, West Virginia
Winter-hardy and drought resistant plants were selected from large populations of colchicine induced polyploids of Rhododendron carolinianum Rhed. These plants were then hybridized with Rhododendron augustinii Hemsley and Rhododendron racemosum Franchet. The resulting hybrids are worthy of description, show some horticultural promise, and provide a genetic base for further breeding experiments. This is the first known report of a tetraploid form of Rhododendron racemosum based on morphological characteristics.
The work reported here took place over the thirty year period from the spring of 1958 through the summer of 1988. The original goal was to develop through natural selection a winter hardy form of Rhododendron carolinianum Rhed., and then either select for tetraploidy or induce tetraploidy into the hardy stock. Colchicine induced tetraploidy in R. carolinianum was accomplished by August E. Kehr (1971), but his plants were not hardy in the climate at the Tolstead nursery. The hybridization projects were started in 1984.
Twelve original bushes of Rhododendron carolinianum Rhed. were purchased from Tingles Nursery of Pittsville, Maryland, in 1958 and planted in the cold greenhouse at the Tolstead nursery near Elkins, West Virginia. The climate in the Elkins area is very erratic, especially in late winter and spring, with warm-up spells in February and March and late frosts in April and May. Minimum winter temperatures as low as -28 degrees Fahrenheit have been recorded at the nursery.
After five years, an excellent seed crop was produced by these original plants and approximately 150,000 seedlings were germinated. Eighteen of these seedlings were selected as possible tetraploids based on the morphological characteristics of deeper green color, thicker stems, and fleshier cotyledons as reported for tetraploid R. carolinianum by Kehr (1971). These eighteen plants and their descendants were selected for hardiness over the period 1963 to 1985. Winter-hardy and drought resistant strains were obtained. These were also considered hypothetical tetraploids based on careful morphological comparisons with known tetraploids (Kehr, 1966, 1971).
Chromosome counts were attempted with minimal success. Microsporocytes, root tips, and terminal shoot tips were pretreated with various concentrations of colchicine (0.01% to 0.1%) for 0.5 to 3 hours, fixed in Carnoy's solution and stained with acetocarmine and iron-propriono-carmine using the squash technique. The chromosomes were very thin and short and difficult to count. However, in microsporocytes of these suspected tetraploids there are most definitely more than n = 13 and certainly fewer than n = 39 chromosomes. This leaves us to speculate that the plants are very likely tetraploid as their general morphology indicates.
An attempt to induce polyploidy into R. carolinianum was made using the following technique. Approximately 100,000 seeds were obtained from a single diploid bush and germinated on felt which was pretreated with 0.02 percent colchicine. Most of the seeds germinated, but only two survived to maturity. These plants showed all the same morphological characteristics of the other suspected tetraploids, having thicker and stouter stems, and thicker, greener leaves which average one centimeter wider and longer than the typical diploid leaves. Flowering in mid May is one to two weeks later than the original diploid plants, and the corollas average 0.5 mm. longer and wider. The flower color is almost white rather than pink as in the original stock.
Using Rhododendron augustinii Hemsely as the pollen parent, hybridization with tetraploid R. carolinianum was accomplished resulting in fertile F1 plants. R. augustinii is reported as a tetraploid by E. K. Janaki Ammal, et. al. (1950).
This hybrid has a delicate, light pink corolla with orange-brown blotches and only a slight trace of the blue or purple of its augustinii parent. The flowers average 0.5 cm. larger than those of R. augustinii and are about the same size as those of R. carolinianum. The stamens protrude farther from the corolla tube than those of either parent. The leaves and stems of R. carolinianum X R. augustinii are about the same size and texture as those of the R. carolinianum parent. Blooming is in mid May and the inflorescences are quite showy. As far as we can tell, the plants are hardy in this area, but recent winters have been rather mild, and perhaps a true test is yet to come.
|R. carolinianum x R. augustinii|
The Rhododendron racemosum Franchet stock is a dwarf form of the species. Several thousand seeds obtained from these plants were germinated on felt which was moistened with 0.02 percent colchicine. Selection for possible polyploids was done in the same manner as for R. carolinianum. The selected plants are definitely genetically distinct from their parents. They are more compact, have stouter and stiffer stems, and lack the distinct glaucous condition of the abaxial leaf surfaces. Flowering is in early May, and the blooms are so prolific that some of the buds must be removed to keep the plants from flowering themselves to death. The corollas are almost pure white with little or no trace of the pink in the unselected stock.
These selected specimens of R. racemosum were used as the egg parent in a cross with tetraploid R. carolinianum. The resulting hybrid has terminal flowers, as in R. carolinianum, which are a soft pink with yellowish to greenish-brown spots. The stigmas are brilliant red. Characteristics of R. carolinianum predominate, but the leaves are intermediate in size between the parents and are somewhat stiffer in texture. Flowering is in mid-May about one week later than R. carolinianum and two weeks later than R. racemosum.
Both of the hybrids may have some horticultural potential, at least as a genetic base for further breeding projects. While they appear to be hardy in this climate, they bloom a little early for here and perhaps would do better in the mid to northern coastal states of both the east and west coasts. They are definitely more showy than the parents from which they are derived.
Duncan, W. H. and R. M. Pullen. 1962. Lepidote rhododendrons of the southeastern United States. Brittonia 14:290-298.
Janaki Ammal, E. K., I. C. Enoch, and Margery Bridgwater. 1950. Chromosome numbers in species of Rhododendron. The Rhododendron Year Book, The Royal Horticultural Society. 5:78-91.
Kehr, A. E. 1966. Breeding for a purpose. Amer. Rhod. Soc. Bull. 20:131-141.
1971. A tetraploid Rhododendron carolinianum. Amer. Rhod. Soc. Bull. 25: 4-7.
*Some taxonomists (Duncan and Pullen, 1962) do not recognize Rhododendron carolinianum as distinct from R. minus. R. carolinianum is used here because it appears to be the familiar name to American growers.