Starting Azaleas From Leaf-bud Cuttings
Maria Toivio and Cecil Stushnoff
Department of Horticultural Science and Landscape Architecture
University of Minnesota, St. Paul, MN
Deciduous azaleas have not been widely used in Minnesota because of insufficient winter hardiness. As a result of several years' breeding at the University of Minnesota, cold-tolerant (-37°C) hybrid plants are now available from reciprocal crosses of Rhododendron x kosterianum Schneid. x R. roseum Rehd. However, difficulty in asexual propagation has delayed rapid multiplication of desired plants. According to Henny and Read (2) softwood cuttings of the hybrids, taken at the proper time, treated with rooting hormone and placed under mist, root satisfactorily within a reasonable time. The strong inhibition of auxiliary bud growth in the azalea shoot was overcome by pinching and/or continuous illumination of the rooted cutting.
It was thought that leaf-bud propagation, similar to that used successfully with blueberry, another ericaceous plant, might provide a useful method for rapid multiplication of the new cultivars. The leaf-bud method allows the use of individual buds instead of stem sections, thus, increase of propagules can be 3-6 times as rapid. In a preliminary study in 1974, azalea leaf-bud cuttings made no new growth after abundant callus formation. Thus, there was a need to explore modifications which might establish this new propagation technique.
Several experiments were carried out during the summer of 1975 to improve the rooting and shoot formation ability of the azalea leaf-bud cuttings. The cuttings were prepared by making a straight cut at 0.5 cm above the bud, and another slanting cut 1.5 cm below the bud. The first experiment was carried out with 4 replications, each replication consisting of 10 cuttings. The cuttings were inserted in the medium so that the bud and the small portion of the stem attached to it were just below the surface. The rooting medium was a peat-vermiculite mixture (1:1, v/v), pH 4.2. The cuttings were planted in flats, covered with a plastic sheet and grown in the same kind of conditions that were optimal for blueberry leaf-bud cuttings (6): 2 weeks in 55°F, after which they were moved to a greenhouse (70-75°F) and grown under solid white cloth shade (light intensity under shade 100-150 ft. candles). The cuttings were misted by hand 2 to 3 times daily.
Several growth regulator treatments were applied to increase the ability of the axillary buds to break. These treatments hopefully would improve the rootability of the cuttings as well. In previous studies we found that the optimal rooting ability of cuttings usually occurs simultaneously with bud break (6). Application of 6-benzylaminopurine (BA, 0.02%) and GA4+7 directly to the buds in lanolin mixture, or 3 repeated spray applications of BA (500 ppm) to the foliage either at the stock plant or cutting stage, turned out to be unsuccessful. These treatments have successfully increased the number of primary branches of apple trees (4,9); however, all the azalea leaf-bud cuttings turned brown slowly, starting from the stem end. This might have been caused by the 2 week cold treatment used in the experiment. In 1975 Kirkham and Emino (5) reported that deciduous azalea cuttings should not be put in cold storage prior to rooting. They concluded that cold storage with auxin treatment appears to promote rooting in evergreen species and inhibits it in deciduous species. Experiments with phenolic cofactors were carried out outdoors in modified Johnston propagation frame boxes (6). Light intensity reaching the cuttings varied from 200-1000 ft; candles (2 to 10% of bright sunlight). The propagation medium consisted of a 1: 1 mixture of peat and vermiculite. The cuttings were misted by hand 3-4 times daily.
|Table 1. The rooting ability of azalea (5749A) leaf-bud cuttings after treatment with several different kinds of phenols, either alone or combined with IAA or IBA (5000 ppm).|
Percent of rooted cuttings
|Control dipped in 50% EtOh||-||-||0||0||5.0|
|Control, no dip||-||-||0||-||-|
|(10)||1802||0||0||20.0 z**, y**|
z- rooting significantly better than in no EtOh dipped control (**LSD0.01= 8.02%)
y- rooting significantly better than in EtOh dipped control
The role of a number of phenolic compounds as synergists of auxin in root formation was investigated. Azalea leaf-bud cuttings were treated with phenolic compounds and auxin dips in a 50% ethyl alcohol (EtOH) mixture of the particular chemicals for 5 seconds. The phenolic compounds employed were chlorogenic acid, caffeic acid, sinapic acid, p-coumaric acid, ferulic acid, p-hydroxybenzoic acid, catechin and salicylic acid. Three concentrations of each chemical were employed (see Table 1). The auxins used were IAA (indoleacetic acid) and I BA (indolebutyric acid) at 5000 ppm concentration. Necessary controls for auxins and phenolic compounds were provided. Twenty cuttings were used per treatment. Cuttings were planted June 25, 1975, and results in terms of the percentage of the rooted cuttings were retarded October 18, 1975.
None of the nontreated cuttings rooted (Table 1). The IBA treatment had a slightly favorable effect, and IBA combined with caffeic acid (10 mM) gave the best response of the cofactor treatments, wherein 20% of the cuttings rooted. Sinapic acid (20 mM) - IBA combination also had a significant, positive influence on the rooting process. The auxiliary bud did not break in any of the azalea leaf-bud cuttings. The use of wounding has been extensively recommended in rooting of azalea cuttings (1,3). On the other hand, wounding is known to cause an increase in the production of phenolic acids (7). Perhaps lignification of the ageing stem tissues may divert available phenolics from root primordium initiation and development. The results of our experiments suggest that some phenolic compounds may prove useful to the plant propagator in the rooting of the hard-to-root azalea hybrids. However, the overall rooting ability of the azalea leaf-bud cuttings was very poor and further research is needed before this technique could be useful.
The authors wish to thank the American Rhododendron Society for the financial support received for this work.
1) Minnesota Agricultural Experiment Station Miscellaneous Journal Series Paper No. 1662.
1. van Elk, B. C. M., 1973, Recent developments in the propagation of Rhododendrons at Boskoop, Comb. Proc. Int. Pl. Prop. Soc., 23:154-161.
2. Henny, R. and Read, P.E., 1971, Propagating the new University of Minnesota hardy deciduous azaleas, Comb. Proc. Int. Pl. Prop. Soc., 21:331-339.
3. Kalkenstron, K. and Dirr, M.A., 1976, Factors affecting the rooting of Rhododendron 'P.J.M.' cuttings, Plant Propagator, 22(1):6-7.
4. Kender, W. J. and Carpenter, S., 1972, Stimulation of lateral bud growth of apple trees by 6-benzylamino purine, J. Amer. Soc. Hort. Sci., 97(3):377-380.
5. Kirkham, M. B. and Emino, E. R., 1975, Effect of cold storage on the rooting of deciduous azalea cuttings, Plant Propagator, 21(4):4-5.
6. Parliman, B., Toivio, M. and Stushnoff, C., 1974, Propagation procedures for new half-high blueberry hybrid, Proc. 3rd N. Amen. Blueberry Res. Workers Conf., Mich. Agric. Exp. Sta. Rep., 17 p.
7. Uritani, I., Asahi, T., Minamikawa, T., Hyodo, H., Oshima, K. and Kojima, M., 1967, The relation of metabolic changes in infected plants to changes in enzymatic activity, pp. 342-356. In C. J. Mirocha and I. Uritani (eds.). The dynamic role of molecular constituents in plant-parasite interactions, American Phytopathological Society, St. Paul, MN.
8. Weiser, C.J. and Blaney, L. T., 1963, Rooting and night-lighting trials with azaleas and dwarf rhododendrons, Amer. Hort. Mag., 42:95-100.
9. Williams, M. W. and Billingsley, H. D.,1970, Increasing the number and crotch angles of primary branches of apple trees with cytokinins and gibberellic acid, J. Amer. Hort. Sci., 95(5):649-651.