A Pivotal Hybrid Azalea
An Experiment With Section Tsutsusi And Brachycalyx Azaleas
Satoshi Yamaguchi and Yoshiki Hirata
Kurume Research Station, Kurume, Japan
Reprinted with permission from The Bulletin of the Vegetable and Ornamental Crops Research Station No. 9, December 1986, Kurume, Japan
Japanese azaleas, belonging to the subgenus Tsutsusi, are divided into two major groups, i.e., section Tsutsusi and section Brachycalyx (Philipson, M.N. and W.R. Philipson, 1982). The former is composed of evergreen type species and is greatly diversified with several horticulturally valuable azaleas, Kurume, Satsuki, Hirado and Miyama, however, the latter, composed of deciduous species, is still left horticulturally under utilized. The Brachycalyx species have good traits, such as, early spring flowering, deep purple flower colour, efc., and the Tsutsutsi species also have useful genes for hose-in-hose and double flower, star with white eye flower, variegated flower, and floriferous habit, etc. (Cox, P. 1979). If those excellent characters could be combined in one plant, it would become a good cultivar in the future.
There is a strong incompatibility barrier between both sections (Yamaguchi, S. etal., 1985). To get several pivotal hybrids between the two sections, repeated pollinations were conducted.
Sincere thanks are due to the many trainee students in the station for their earnest and skillful assistance in raising our experimental materials.
II. Materials and Methods
Parent plants were growing in the display garden of the station. Seed parent is R. scabrum cv. 'Tenshou' (= R. pulchrum cv. 'Tenshou'), about 30 years old, belonging to the Hirado azalea group of the section Tsutsusi. Pollen parent is R. tashiroi, also about 30 years old, belonging to the section Brachycalyx.
In the spring, 1976, pollination was carried out three times at two days intervals to each flower, using fresh pollen grains gathered from the just pollen-shed anthers of R. tashiroi.
3. Seed harvest
Ripened capsules were harvested and stored in a desiccator, at room temperature.
4. Seed sowing
In the spring, stored capsules were opened by hand, and seed grains were gathered. They were sown on the seeding bed, composed of a thick layer of clay soil with thin sphagnum layered on it.
Two years after germination, young seedlings were transplanted into polyvinyl pots (° 7.5cm) from the seeding bed. Liquid fertilizer was given to them every month.
III. Results and Discussion
1. Seed set, sowing and germination
Ten flowers of R. scabrum cv. Tenshou' were hand pollinated with pollen grains of R. tashiroi, and four capsules were harvested in autumn. After a half year storage in the desiccator at room temperature, seed grains were sown. One green and several albino seedlings began to grow, however, only one green seedling matured beyond the several leaved stage, at which time the albino seedlings ceased their growth and gradually declined and died.
Table 1. Character comparison in R. scabrum cv. 'Tenshou', R. tashiroi and their hybrid. Plant Flowering day (M/D) Flower diameter (mm) Petal
Hair colour on the ovary R. scabrum
Blotch distribution Floral injury caused by low during temperature winter
pale pink (69-A)
deep red (60-A)
deep red (60-A)
pale yellow (2-D)
light green (136-A)
light green (146-B)
2. Hybrid plant habit (Table 1)
The hybrid plant grew slowly, reaching 40cm high, 30cm wide in 10 years after germination. The period to flower initiation of the hybrid plant was about twice longer than usual azalea seedlings (minimum duration is one to five years).
The hybrid plant is a shrubby type, branching nearly at the base, with evergreen leaves. Shoot cuttings are easy to root under mist equipment.
3. Flowering character (Table 1)
The hybrid plant flowered in spring, 1986. Flowering of the hybrid plant was two days earlier than that of the seed parent and also one week earlier than that of the pollen parent. Flower colour of the hybrid plant is pale pink with deep red blotches on the front petal. Distribution of the blotch of the hybrid plant is similar to that of seed parent and petal colour of the hybrid plant is the same as its pollen parent. In a preliminary analysis of anthocyanins by paper chromatography (BAW, HAcHCI), delphinidin pigment dominates in the seed parent, cyanidin-diglycoside pigment is present in both parents and hybrid plant, and cyanidin-monoglycoside pigment is present in pollen parent and hybrid plant. These results accord well with the previous reports (Kobayashi, Y. 1980). Flower size of the hybrid plant is smaller than the both parents. Petal shape of the hybrid plant is rather narrow and resembles its pollen parent.
| Comparison of flower of seed parent, R. scabrum
'Tenshou' (upper), hybrid (middle) and pollen
parent, R. tashiroi (lower).
Photo by Satoshi Yamaguchi
4. Flower morphology (Table 1)
Petal thickness of the hybrid is intermediate between the thicker seed parent and the thinner pollen parent. Calyx size of the hybrid plant is also intermediate to both parents, however, its shape is round as is its pollen parent.
Peduncle of the hybrid plant is intermediate in length (12mm) between both parents. Pistil length of the hybrid plant is 32mm, same as the pollen parent, but shorter than that of the seed parent (51mm). The number of stamen is 10 in the hybrid plant and both parents.
5. Hair type (Table 1)
Leaf hair of the hybrid plant is scattered on its surface and apt to fall in an early expanding stage. The leaf of pollen parent is glabrous and seed parent's leaf is rather pubescent with pale brown colour.
The colour of stem hair is dark brown in the pollen parent, pale brown in the seed parent and intermediate light brown in the hybrid plant. The colour of ovary hair is white in the seed parent and the hybrid plant, however, light brown in the pollen parent.
6. Leaf morphology (Table 1)
Leaf size of the hybrid plant is similar to its pollen parent and smaller than that of its seed parent. Leaf of the hybrid is 394 Ám in thickness, thicker than that of both parents. Leaf margin of the hybrid is medially waved, similar to its pollen parent. Leaf venation is distinct in the back surface of the hybrid and seed parent, not clear in the pollen parent. Leaf colour is pale green in the hybrid plant and seed parent, but green in the pollen parent. The leaf lamina is revoluted in the hybrid plant and pollen parent.
7. Cold hardiness (Table 1)
Cold hardiness of floral bud in the hybrid plant is tender, similar to that of the seed parent. This might be the expression of the maternal effect in the heredity of cold hardiness in azaleas (Yamaguchi, S., 1983).
Pollen fertility is absolutely 0% in the hybrid plant. Several back crosses with pollen grains of both parents did not produce any seed grains. This hybrid plant is completely sterile on both female and male sides.
Table 2. Comparison of leaf phenological characters of R. scabrum cv. 'Tenshou', R. tashiroi and their hybrid. Plant Life span of leaves (Month) Peak of leaf falling Autumn flashing of terminal shoots Attaching leaves in winter (%) Mean Maxim. (degree of evergreen) R. scabrum
9. Leaf phenology (Table 2)
The mean life span of each leaf is 9.3 month in the hybrid plant, intermediate of seed parent (6.5 month) and pollen parent (16.4 month). Other characteristics concerning leaf phenology are also intermediate between both parents.
IV. General Discussion The Rhododendron species are generally thought to be self-incompatible. However, this mode is different from species to species. As to S.Y.'s personal observation, the mode of self-incompatibility was weak in R. scabrum, R. pulchrum and R. eriocarpum in the sect. Tsutsusi. So the choice of R. scabrum for the seed parent could be appropriate for successful hybridization between sections Tsutsusi and Brachycalyx. This is the second success in the world of inter-section hybridization between sections Tsutsusi and Brachycalyx following G. Ring, 1976 (Kehr, A.E., 1977).
If this hybrid plant could be tetraploidized with colchicine or other means, it would recover fertility and be valuable as the pivotal plant between both sections.
1. Just one hybrid plant between R. scabrum cv. 'Tenshou' (section Tsutsutsi) and R. tashiroi (section Brachycalyx) grew and flowered.
2. It shows an intermediate habit of both parents and is completely sterile.
3. It will act as a pivotal plant to transfer valuable heredital traits between both members.
COX, P. (1979): The larger species of Rhododendron. Batsford, London, pp. 352.
KEHR, A.E. (1977): Azaleodendron breeding. QBARS 31:226-232.
KOBAYASHI, Y. (1980): Studies on the flower colour in Rhododendron II. On the development of kurume azaleas. Bull. Fukuoka Hort. Exp. St. 18: 33-40 (in Japanese with English summary).
PHILIPSON, M.N. and W.R. PHILIPSON (1982): A preliminary synopsis of the genus Rhododendron III. Notes RBC Edinb. 40: 225-227.
YAMACUCHI, S. (1983):Winter defoliation and cold hardiness of Japanese rhododendron species, Jap. Jour. Breed. 33, suppl. 1: 242-243 (in Japanese).
AMACUCHI, S., M. KUNISHICE and T. TAMURA (1985): Interspecific compatibility in Japanese Rhododendrons. Bull. VOCRS, Ser. C (KURUME), No. 8:87-97 (in Japanese with English summary).