Comparison of Formulations of Auxins As Rooting Stimulants for Three Rhododendron Cultivars
John J. McGuire, Professor
W. Johnson, Research Associate
C. Dawson, Laboratory Aide
Department of Plant Sciences, University of Rhode Island
Kingston, Rhode Island
Published as Journal paper no. 2590 of the Rhode Island Exp. Sta.
Abstract
Three commercially important cultivars of Rhododendron were used to test efficacy of auxin formulations. Formulations containing polyethylene glycol or potassium hydroxide solvents were found superior to those containing water or talc. Cultivars responded similarly.
Introduction
Recent pressure by federal authorities to require labeling of indole-3-butyric acid (IBA) formulations could lead to a reluctance by commercial producers of IBA formulations to stay in the market. While other compounds are being tested such as the aryl esters (6) (7), it will be some time before specific treatments are developed for the vast numbers of woody plants being propagated. It would be prudent for all plant propagators to be prepared to formulate their own auxins so they can avoid a problem if their favorite product no longer is available.
It has long been proposed that a combination of IBA and naphthaleneacetic acid (NAA) in one formulation is superior as a rooting stimulant than either one used alone(4) (5). Research at this experiment station has supported that finding and our recommendations have included a mixture of the two at a ratio of two parts IBA to one part NAA. We have used polyethylene glycol (PEG 400, a Union Carbide product) as a solvent although recent research now shows propylene glycol as being an equally good solvent which may be less toxic to plants (1) (2) (3). Both are reputed to be less toxic than alcohol. While the potassium salt (K-salt) of IBA has been available for several years it is now also possible to get NAA in this form, so we can now compare the mixture of IBA/NAA in this water soluble form to the older conventional formulations in solvents. A further complication arose when researchers found that formulations of IBA dissolved in a 1 normal solution of potassium hydroxide in distilled water (1N KOH) appeared to be more effective root promoters than equal concentrations of IBA derived from the K-salt dissolved in water (4). We compared the K-salts of IBA/NAA to similar concentrations dissolved in 1N KOH. These were also compared to the same concentration in polyethylene glycol and to a commercial talc formulation containing 45,000 ppm IBA. All treatments were also compared to solvents containing no auxin.
Procedure
Three rhododendron cultivars were used: 'Boule de Neige', 'Nova Zembla', and 'Scintillation'. All were propagated from the first summer flush of growth as semi hardwood cuttings in outdoor intermittent mist beds under 50% shade. Propagating dates varied with cultivar but all were propagated in either the last week of June or the first week of July.
Twenty-five cuttings of each cultivar were placed in replicates of five cuttings per treatment. They were randomized in the mist bed. Cuttings were selected from a stock block for uniformity of stem diameter. Each was trimmed to a length of 3.5 cm and all but five mature leaves were removed. The remaining leaves were trimmed to half their length. The basal end of the cutting was wounded on two sides. Each wound was 2 cm long and 0.5 cm wide. Aqueous dips were applied for five seconds. Cuttings treated with talc were first dipped in water, then dipped into the talc. Excess talc was tapped off.
Cuttings were placed in a randomized block in a medium of sphagnum peat moss and medium grade vermiculate (1:1, v/v). Medium was 18 cm deep. Mist was applied during the daylight hours for 6 sec. every 6 min. Bottom heat was applied whenever the temperature was below 20 °C. Shade was provided by black saran at 50%.
Aqueous dips of IBA and NAA were formulated from stock solutions prepared by dissolving crystalline IBA and NAA in one of two solvents: 1N KOH or PEG 400. Distilled water was added to bring final concentrations to 10,000 ppm IBA/5,000 ppm NAA in 40% (v/v) 1N KOH and 10,000 ppm IBA/5,000 ppm NAA in 40% (v/v) PEG 400. Stock solutions were appropriately diluted with 40% (v/v) 1N KOH and 40% (v/v) PEG 400 containing no auxin to produce concentrations of 5,000 ppm IBA/2,500 ppm NAA, 3,333 ppm IBA/1,667 ppm NAA and 2,500 ppm IBA/1,250 ppm NNA. A third stock solution of 10,000 ppm K-salt IBA and 5,000 K-salt NAA dissolved in distilled water was also prepared. This formulation was diluted with distilled water to produce concentrations of 3,333 ppm IBA/1,667 ppm NAA, and 2,500 ppm IBA/1,250 ppm NAA.
Cuttings were harvested after 8 weeks and root ball diameters were measured. It is not practical to remove organic media from a rooted rhododendron cutting so cuttings were harvested and were gently tapped to remove excess media. The root ball was then compressed to form a symmetrical ball and measured for average diameter. It should be noted that all records were made after 8 weeks. If cuttings were allowed to remain in the bed for 12-16 weeks, as is the normal commercial procedure, much higher rooting percentages and large root balls would be attained but differences in treatments may be lost. An analysis of variance was performed on the root ball diameters with the means separated by Duncan's multiple-range test.
Results
R. x 'Boule de Neige' (Table 1). The PEG 400 formulation was superior followed by the auxin treatment in KOH. Formulation of K-salts in water were much less effective.
R. x 'Nova Zembla' (Table 2). The trend was the same except that the talc formulations had the highest root ball diameter.
R. x 'Scintillation' (Table 3). A similar trend is evident. The PEG 400 and KOH formulations were superior.
| Table 1: Comparison of IBA/NAA Formulations as Rooting Stimulants for R. x 'Boule de Neige' | |||
| Formulation | Average Root Ball Diameter (cm) | % Rooted | |
| 2,500 ppm IBA, 1,250 ppm NAA in 40% PEG 400 | 5.0 | A | 72 |
| 3,333 ppm IBA, 1,667 ppm NAA in 40% PEG 400 | 4.2 | AB | 72 |
| 5,000 ppm IBA, 2,500 ppm NAA in 40% 1N KOH | 3.2 | BC | 56 |
| 5,000 ppm IBA, 2,500 ppm NAA in 40% PEG 400 | 2.9 | BCD | 56 |
| 3,333 ppm IBA, 1,667 ppm NAA in 40% 1N KOH | 2.7 | CD | 56 |
| 2,500 ppm IBA, 1,250 ppm NAA in 40% 1N KOH | 2.4 | CD | 60 |
| HORMEX 45 (4.5% [45,000 ppm] IBA in talc) | 2.0 | CDE | 64 |
| 3,333 ppm K-salt IBA, 1,667 ppm K-salt NAA | 1.6 | DEF | 32 |
| 2,500 ppm K-salt IBA, 1,250 ppm K-salt NAA | 0.8 | EF | 24 |
| TALC | 0.0 | F | 0 |
| WATER | 0.0 | F | 0 |
| 40% 1N KOH | 0.0 | F | 0 |
| 40% PEG 400 | 0.0 | F | 0 |
| Means within a column followed by the same letter are not significantly different at the 5% level as determined by Duncan's Multiple Range Test | |||
| Table 2: Comparison of IBA/NAA Formulations as Rooting Stimulants for R. x 'Nova Zembla' | |||
| Formulation | Average Root Ball Diameter (cm) | % Rooted | |
| HORMEX 45 (4.5% [45,000 ppm] IBA in talc) | 3.1 | A | 72 |
| 5,000 ppm IBA, 2,500 ppm NAA in 40% PEG 400 | 3.0 | A | 80 |
| 3,333 ppm IBA, 1,667 ppm NAA in 40% PEG 400 | 2.8 | A | 88 |
| 2,500 ppm IBA, 1,250 ppm NAA in 40% PEG 400 | 2.2 | AB | 76 |
| 5,000 ppm IBA, 2,500 ppm NAA in 40% 1N KOH | 2.1 | AB | 52 |
| 3,333 ppm IBA, 1,667 ppm NAA in 40% 1N KOH | 2.0 | AB | 48 |
| 2,500 ppm IBA, 1,250 ppm NAA in 40% 1N KOH | 1.7 | B | 60 |
| 2,500 ppm K-salt IBA, 1,250 ppm K-salt NAA | 0.05 | C | 4 |
| 3,333 ppm K-salt IBA, 1,667 ppm K-salt NAA | 0.05 | C | 4 |
| WATER | 0.03 | C | 4 |
| 40% 1N KOH | 0.0 | D | 0 |
| TALC | 0.0 | D | 0 |
| 40% PEG 400 | 0.0 | D | 0 |
| Means within a column followed by the same letter are not significantly different at the 5% level as determined by Duncan's Multiple Range Test | |||
| TABLE 3: Comparison of IBA/NAA Formulations as Rooting Stimulants for R. x 'Scintillation' | |||
| Formulation | Average Root Ball Diameter (cm) | % Rooted | |
| 3,333 ppm IBA, 1,667 ppm NAA in 40% PEG 400 | 4.2 | A | 84 |
| 2,500 ppm IBA, 1,250 ppm NAA in 40% PEG 400 | 3.9 | A | 84 |
| 5,000 ppm IBA, 2,500 ppm NAA in 40% 1N KOH | 3.9 | A | 56 |
| 5,000 ppm IBA, 2,500 ppm NAA in 40% PEG 400 | 2.2 | B | 68 |
| 3,333 ppm IBA, 1,667 ppm NAA in 40% 1N KOH | 1.8 | BC | 40 |
| HORMEX 45 (4.5% [45,000 ppm] IBA in talc) | 1.7 | BC | 32 |
| 2,500 ppm IBA, 1,250 ppm NAA in 40% 1N KOH | 1.2 | BCD | 40 |
| 2,500 ppm K-salt IBA, 1,250 ppm K-salt NAA | 0.8 | CD | 28 |
| 40% PEG 400 | 0.7 | CD | 24 |
| 3,333 ppm K-salt IBA, 1,667 ppm K-salt NAA | 0.6 | CD | 20 |
| 40% 1N KOH | 0.2 | D | 12 |
| WATER | 0.1 | D | 4 |
| TALC | 0.1 | D | 4 |
| Means within a column followed by the same letter are not significantly different at the 5% level as determined by Duncan's Multiple Range Test | |||
Discussion
It is apparent that solvents of PEG 400 or KOH improved auxin formulations as rooting stimulants and both appear to be safe to rhododendron stems. The talc formulations did work but produced fewer roots. The formulations of K-salts were less effective which is in agreement with Dirr (2) (3). While it is unlikely that commercial growers will resort to KOH formulations, they could use the glycols which are relatively easy to obtain and to use. If the commercial formulations are removed from the marketplace it might be wise for commercial propagators to start making their own and testing them in their own greenhouses. Further work needs to be done to determine optimum levels for K-salt IBA/K-salt NAA in water. It may be necessary to use them at higher concentrations or expose cuttings to soaks longer than 5 seconds.
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