Do Chemical Growth Regulators Stimulate New Shoot Growth And Improve Overwintering of Deciduous Azalea Cuttings?
Thomas J. Banko1
Hampton Roads Agricultural Research & Extension Center
Virginia Polytechnic Institute & State University
Virginia Beach, Virginia
To improve propagation success of deciduous azaleas authors Banko and Stefani conducted experiments using growth regulators to promote an early flush of growth on cuttings. Although some improvement of bud break and shoot growth was produced, overwinter plant survival was not increased. The limited success, however, indicates possible improvements in survival with treatment modifications.
One problem in propagating deciduous azaleas by stem cuttings is that after the cuttings root they often fail to develop a new flush of shoots before overwintering. Apparently the lack of new foliage to aid in photosynthesis results in depletion of carbohydrate reserves and failure to survive the following winter (Smalley & Dirr, 1986). Researchers have documented this phenomenon in other trees and shrubs, including maples, flowering dogwood, viburnum and stewartia (Goodman & Stimart, 1987; Smalley & Dirr, 1986; Smalley & Lindstrom, 1991).
One technique to promote new shoot growth before overwintering is to bring containerized stock plants into a greenhouse after undergoing a cold period (to break dormancy). Forced shoots are then harvested for cuttings in early spring. Cuttings obtained and rooted by this method are more likely to put on a flush of new growth before the next winter, increasing their survival (Grampp, 1976). This technique, however, is not practical for field-grown stock plants.
Certain chemical growth regulators, such as gibberellins and cytokinins, have been effective in substituting for chilling requirements, breaking bud dormancy of some woody plants (Smalley & Lindstrom, 1991; Maynard et al., 1990; MacCarthaigh, 1983). Because of the success others experienced with promoting shoot growth in woody plants, we wanted to develop similar techniques for deciduous azaleas. If the application of a growth regulator could promote an early flush of growth on rooted azalea cuttings, the overall propagation success of these plants might be improved.
Materials and Methods
On May 25, 1981, cuttings of the deciduous azaleas 'Kilauea' and 'Sahara' were collected from large, field-grown plants at Le-Mac Nurseries, Hampton, Va. The basal ends were treated with a 5-second dip in a 4000 ppm IBA (indole-3-butyric acid) solution in 50% ethanol, then inserted in flats containing a peat/perlite (1:1 by volume) rooting medium. Cutting flats were placed in a high humidity rooting greenhouse, with humidity controlled by a 'Humidifan' unit (Jaybird Mfg., Inc., Centre Hall, Pa.). After rooting was initiated, flats were drenched with a solution containing Peter's fertilizer (20-20-20), 1 teaspoon/gallon, and benomyl fungicide, 2 teaspoons/gallon.
On Aug. 4, rooted cuttings were potted in 5 x 6" containers in a medium of pine bark, peat, and sand (3:1:1 by volume). At this time, there were 283 rooted cuttings of 'Kilauea' and 210 rooted cuttings of 'Sahara'. Each plant received 6 grams of Osmocote slow-release fertilizer (18-6-12) which was followed on Aug. 23 with Peter's fertilizer diluted to 200 ppm nitrogen.
Growth regulator treatments were applied with a hand mister on Aug. 20, 1981. These treatments consisted of 250, 500, or 1000 ppm gibberellic acid (GA3) - a gibberellin; 10 ppm benzyladenine (BA) - a cytokinin ('Sahara' did not receive this treatment); or 250 and 500 ppm GA3 plus 10 ppm BA - gibberellin plus cytokinin (Tables 1 and 2). All treatment solutions also contained 0.1% Tween 20 as a wetting agent. Control plants were untreated. For each treatment, there were four replications, with eight or nine plants per replication per treatment. A randomized complete block experimental design was used.
On Oct. 15, 1981, plants were evaluated by counting the number of new shoots and new leaves that had developed after treatment (see Tables 1 & 2). Then plants were placed in a polyethylene-covered Quonset-type house for overwintering. The temperature was maintained just above freezing with a small, natural gas-fueled heater. The plants were removed from the overwintering house in early spring, and were evaluated again for number of main shoots, overall plant height, and percent survival on June 22, 1981.
Table 1. Growth regulator treatments on rooted cuttings and their effects on shoot numbers, leaf numbers (rating), plant height, and survival. Deciduous azalea, Rhododendron 'Sahara' 1981 1981 Treatment Shoot Number (Mean) Leaf Rating1 Shoot Number (Mean) Plant Height (cm) % Survival Control 0.5 1.7 1.6 37.8 41 250 ppm GA3 1.1* 2.3 1.8 36.1 34 500 ppm GA3 1.1* 2.9* 1.5 43.7 56 1000 ppm GA3 1.1* 2.6* 1.7 36.0 56 250 ppm GA3 + 10 ppm BA l.2* 2.8* 1.7 37.4 66 500 ppm GA3 + 10 ppm BA 1.3* 2.7* 2.0 40.6 50 NS NS NS * Indicates means within a column are significantly different from the untreated control plants using LSD comparison at the 5% level of significance
NS= no significant difference within a column
1 Leaf rating: 1 = no new leaves, 2 = 1-2 fully expanded leaves, 3 = 3-5 expanded leaves, 4 = more than 5 expanded leaves per cutting
Table 2. Growth regulator treatments on rooted cuttings and their effects on shoot numbers, leaf numbers (rating), plant height, and survival. Deciduous azalea, Rhododendron 'Kilauea' 1981 1981 Treatment Shoot Number (Mean) Leaf Rating1 Shoot Number (Mean) Plant Height
% Survival Control 0.5 1.9 2.5 40.5 5 250 ppm GA3 0.5 1.7 3.0 43.0 8 500 ppm GA3 0.7 2.0 3.5 24.5 5 1000 ppm GA3 0.6 2.1 2.0 41.3 19 250 ppm GA3 + 10 ppm BA 0.7 1.7 1.8 37.7 25 500 ppm GA3 + 10 ppm BA 0.5 1.8 2.5 31.8 11 10 ppm BA 0.7 2.2 06 44.7 8 NS* NS * NS = no significant difference within a column. Statistical analysis was not performed on 'Kilauea' treatments in 1994 because of the very low % survival of this variety
1 Leaf rating: 1 = no new leaves. 2 = 1-2 fully expanded leaves, 3 = 3-5 expanded leaves, 4 = more than 5 expanded leaves per cutting
Results and Discussion
Our results are summarized in Tables 1 and 2. The two azalea cultivars responded somewhat differently to the growth regulator treatments. For Rhododendron 'Sahara', the gibberellin and BA treatments significantly improved the initial shoot number and leaf ratings (Table 1). The combination GA3 + BA treatments promoted slightly more new shoots than the GA3 alone. However, the following season, the numbers of new main shoots and plant growth were no longer different, and any improvement in plant survival was not enough to be statistically significant.
For 'Kilauea' (Table 2), no improvement in numbers of new shoots or leaves was seen initially from any treatments, and percent survival overwinter was very poor for all treatments. There appeared to be better survival with the 1000 ppm GA3 or the combination 250 ppm GA3 + 10 ppm BA, but this may be due to chance, and unrelated to the treatments, because of the lack of an initial treatment response.
The results obtained in terms of improved overwinter survival do not currently justify the treatments as applied in this experiment. However, the limited but significant increase in bud break and new shoot development obtained with Rhododendron 'Sahara' suggests some improvement may be possible with treatment modifications. Perhaps increased growth regulator application rates, different combination treatments, different timing, or increased frequency of treatment applications could improve bud break and shoot growth to the extent that overwinter survival could be improved.
This research was supported in part by a grant from the American Rhododendron Society Research Foundation. We also wish to thank Dr. Sandra McDonald of Le-Mac Nurseries in Hampton, Va., for her assistance with this project and for supplying the deciduous azalea cuttings.
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1Associate Professor and Research Specialist respectively. H.R.A.R.E.C. Research Report number 81-5.