Leaf Bud Propagation: Working Around Dormancy
Dr. Mark C. Konrad
One of the most perplexing aspects of vegetative propagation is dealing with the dormancy factor that occurs in plants. The deciduous azaleas have the most pronounced cycle, and next in line are the elepidote rhododendrons. Rooting the plant is sometimes only half the battle. Getting it to break bud and grow on can be another story.
At this point I would like to mention a few methods that might be helpful in moderating the dormancy period: 1) the application of continuous light, 2) after-chilling following rooting, 3) removal of the apical bud to alter the hormone control, and 4) removal of the apical bud tissue after rooting deciduous azaleas.
With the first point, it is well known that some plants are greatly affected by the length of the photo period, especially true with the deciduous azaleas. Clement Bowers in his book Azaleas and Rhododendrons, Second Edition, 1960, called attention to the work of Dr. Henry T. Skinner while he was director of the U.S. National Arboretum in Washington D.C. Skinner's success was noted with early spring rooting of deciduous azaleas under mist followed by continuous fluorescent lighting. It was recorded as a way to keep the plant in active growth with the assumption that dormancy could be delayed or moderated.
In addition to the above, the quality of light could also be an important factor, especially when indoor lighting is used. The red part of the spectral light band can easily be augmented with incandescent light bulbs.
With the second point, it has become standard practice to use after-chilling following rooting (Hartman, Kester, Davies, Plant Propagation, Sixth Edition, p. 593, 1990). After rooting, the cuttings (elepidotes) should be held at 40°F for about 20 days before forcing on with increased warmth and supplementary light. This again is a way to work through the dormancy cycle. It may, however, not be necessary for all clones.
Weldon Delp of the Great Lakes Chapter also uses after-chilling in his accelerated seedling program. On two separate occasions the seedlings are allowed to go through a dormancy cycle, the first time at the end of the first year and the second time after budding at which time the plants are placed in a cool greenhouse (below 50°F). Normal flowering then occurs the following spring.
With the third point, I would like to describe some of my experimentation, which has produced interesting results with deciduous azaleas. To work around the dormancy issue, I decided to try the following:
After early new growth occurred, the apical bud or terminal part of the shoot was either snipped off with a sharp scissors or cut out with a sharp blade. What followed was stimulation of the dormant axillary or lateral buds into new growth, since there no longer was any inhibition from the naturally occurring auxin in the terminal tissue.
As the upper buds elongated into new growth (best at least 5-10 mm), leaf bud cuttings were made. The use of a surgical scalpel was an ideal tool for the procedure. Small closed containers were used for rooting with equal parts of finely screened perlite and Canadian peat moss. The hormone treatment was one-tenth strength Dip 'N Grow. Additional wounding was done on the under side of the petiole near the leaf bud stem. The containers were left outdoors in a shaded area.
Results: Much to my surprise, rooting took place within six weeks. Would I be ahead of the game since the buds had already broken dormancy? Further to my delight, growth commenced one month after being placed under the indoor fluorescent lighting including incandescent bulbs. Eighteen-hour periods were used. The immediate favorable response was more than 50 percent. Actually, one plant threw up a new shoot from the root mass.
With the fourth point, surprising results were obtained when the terminal bud tissue of newly rooted deciduous azaleas was snipped out with a sharp pointed scissors. Under 18-hour indoor fluorescent and incandescent lighting, the axillary buds started into new growth within one month. Had the control center for dormancy been altered? The immediate favorable response was around 75 percent.
At this point it is interesting to note the earlier work of Weldon Delp in the article "Pruning for Bushy Growth" (J. Amer. Rhod. Soc. 41 :37; 1987). The method described was to remove the tips of newly formed deciduous azalea shoots and then wait five days before taking the cuttings. He felt that better bud break could occur after rooting.
Comment: Until we learn more about plant physiology and all the control factors, we will have to work around the dormancy cycle as best we can; methodology described here might be helpful. Only time will tell whether it can be useful commercially.
Dr. Mark Konrad, a frequent contributor to the Journal, is a member of the Great Lakes Chapter.