Pushing Rhododendron: Triacontanol Revisited
Donald W. Paden, Urbana, Illinois
In a climate where extremes of temperature range from -25° to 105°F., there is reason to stimulate the growth of rhododendron seedlings and cuttings before exposing them to the realities of a relatively hostile environment. This note reports on two attempts to maximize the growth of seedlings and cuttings in central Illinois, first by keeping the plants indoors under lights for two years rather than one, and second by using 1-triacontanol as a growth stimulant during the second year.
Rooted cuttings of 'Sefton' and 'Nova Zembla' propagated in vitro and seedlings from a cross of (R. fortunei x R. yakushimanum) x R. williamsianum were kept indoors under lights during their first winter. At the beginning of the summer, 40 of the 'Nova Zembla' and 10 of the 'Sefton' were planted in a bed outdoors where they remained throughout the following winter (Year 2). The remaining plants (in pots and flats) which also spent the summer outdoors were returned indoors to a 16-hour day in the fall. All of the indoor plants were treated during Year 2 with liquid fertilizer at least once and subjected to foliar feeding on a fairly regular basis.
The first comparison involves the 'Nova Zembla' and 'Sefton' outside with the plants kept indoors during Year 2. The height of the plants and side branches were measured and the average growth per plant was calculated. The overall averages for the two-year period, shown on the third line of Table 1, are 41 and 13 inches versus 15 and 8 inches, respectively. Unfortunately, a record of the height of the outdoor plants at the end of Year 1 was not recorded, so that some small part of the difference credited to the second winter indoors may possibly have been due to selecting the more vigorous plants to bring indoors. The growth of the indoor plants, however, some with flower buds in less than two years, is encouraging.
Table 1: Average Growth of Untreated Bench-Grown and Untreated Field-Grown Plants Growth at Nova Zembla Sefton End of Inside Outside1 Inside Outside1 Year 2 N = 5 N = 40 N = 20 N = 10 Main stem 25.2" 11.2" 10.1" 6.5" Branches 16.0" 4.0" 3.5" 1.6" Total 41.2" 15.2" 13.6" 1Outside during year 2 only.
Table 2: Average Overall and Net Growth of Plants Treated with Triacontanol and Control Nova Zembla1 Sefton1 Seedlings Growth Untreated Treated Untreated Treated Untreated Treated N = 5 N = 5 N = 20 N = 20 N = 12 N = 14 Year 1 plus 2 41.2" 39.8" 13.5" 13.6" 4.3" 4.6" Less Year 1 23.2" 21.4" 4.8" 5.1" - - Net Growth (Year 2) 18.0" 18.4" 8.7" 8.5" - - 1Plants from tissue culture
A second comparison involving all three of the varieties listed above was prompted by an article which appeared in the Summer 1982 issue of this Journal written by D.L. Hinerman and S.L. Kunkel, "Triacontanol as a Growth Stimulant: A Report of Two Experiments." One of these experiments involved lepidote rhododendron. Various measurements based on three of these small-leaf plants indicated a difference between the untreated and treated plants and favored the latter: Weight, less soil (275%), diameter of trunk (100%), number of branches (150%), number of leaves (269%) and number of flower buds (100%).
The elepidote plants selected for use in the present study were initially fairly evenly matched as between the two treatments, although the differences within each treatment were perhaps larger than one would have expected of individual plants which had been cloned. Table 2 is self-explanatory. The procedure described in the Hinerman and Kunkel article with respect to dates of application was followed with all reasonable care. The five treated 'Nova Zembla' were painted with the 1-triancontanol solution to insure thorough coverage. The 20 'Sefton' and the seedlings were sprayed. The solution used (to be commented upon later) was prepared by a professional chemist who is a member of ARS.
Growth per plant from test tube to final measurement for the two-year period for the three varieties is shown in the first line of Table 2. The results for these large-leaf varieties were mixed, with 'Nova Zembla' and the seedlings favoring the experimental treatment. The dwarfing influence of R. williamsianum is apparent from an examination of the meager growth of the seedlings over the two-year period.
In order to compensate for differences in the growth of the plants at the time the triacontanol was first applied, the average growth (main stem plus branches) at the beginning of the second year was subtracted from the overall growth recorded in the first line of Table 2. The difference, or the net growth in the second year, is shown in the bottom line of Table 2. Only the 'Nova Zembla' seemed to benefit from the treatment and this by a small amount. For purposes of comparison, an estimate of the growth of the main stem and branches shown in a photograph of a treated and an untreated plant in the Hinerman and Kunkel article shows roughly a 70 percent advantage for their treated plant, as compared with the mixed results of the present study.
The evidence presented here does not contradict the findings reported by Hinerman and Kunkel, although the results are not very supportive. One wonders whether the combined effects of the 16-hour day, fertilization, and foliar feeding simply overwhelmed any possible influence of the 1-triacontanol. In interpreting the results one should keep in mind other differences between the two studies. Different types of rhododendron were used (lepidote versus elepidote). Different measurements (already noted) were taken. There was a difference in the number of plants observed. In this experiment, isopropyl alcohol was used rather than chloroform as the solvent for the 1-triacontanol, although the amount of 1-triacontanol per liter of solution was the same.1 Finally, there was perhaps a six-months difference in the age of the plants at the beginning of the critical part of the experiments. In retrospect, it would have been reassuring had the present study included a group of lepidotes (optimally, the Delp hybrids used in the Hinerrran and Kunkel study). The writer admits to being misled by the very noticeable breaking of dormancy in response to triacontanol which was used on a group of seedlings of R. yakushimanum.2 As a result, there seemed every reason to anticipate no trouble in duplicating the spectacular results reported by Hinerman and Kunkel. Herein lies a lesson for future investigators, or at least for amateurs like myself.
1 Dr. Tatum noted, "There is some evidence that higher alcohols, C12, C15, and higher, interfere with the action of 1-triacontanol, but no such evidence for lower alcohols [used here]." Isopropyl alcohol may be somewhat more available than chloroform for amateur researchers.
2 Dropped from the study because of accidental damage to some plants, although at the end there was no noticeable difference between the treated and untreated plants which survived.