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Volume 38, Number 2
Spring 1984

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Effect of Terminal Bud Removal and Exogenous Indole Butyric Acid Treatments
on the Rooting of Vegetative Rhododendron Cuttings

B. C. Strik1 and C. J. French
Agriculture Canada
Saanichton Research & Plant Quarantine Station
Sidney, British Columbia

        Many factors influence the rooting of rhododendron stem cuttings, including photoperiod (3), the application of exogenous growth substances (4) and water stress (5). Removal of the terminal bud during preparation of the cuttings was reported to improve rooting in Rhododendron 'English Roseum' (6) and has been suggested as of general importance in the propagation of rhododendron hybrids (Wells, unpublished observations). In contrast, no effect of terminal bud removal on the rooting of vegetative stem cuttings was found in R. 'Pink Pearl' (1).
        In view of the uncertainty regarding this technique, the present study was conducted to investigate the effects of terminal bud removal combined with two standard exogenous treatments with indole butyric acid (IBA) on the rooting of four rhododendron cultivars ('Unique', 'Elizabeth', 'Anna Rose Whitney' and 'Vulcan').

Materials and Methods:
Vegetative stem cuttings were taken from the current year's growth on field-grown, 15-year-old stock plants. Terminal cuttings were used for 'Elizabeth', 'Anna Rose Whitney' and 'Unique'; lateral cuttings were used for 'Vulcan' because few terminal cuttings could be taken. Lower leaves and lateral buds were removed. Four apical leaves and their associated axillary buds were retained, leaves being trimmed by approximately one-third to give equal leaf areas. The stem was shortened to 7 cm and wounded by removing a strip of bark from the basal 2.5 cm of one side of the stem. Two standard exogenous growth substance treatments were employed; 0.8% IBA in talc and 0.4% IBA in 95% ethanol. The control cuttings received no exogenous IBA. In each of these treatments, the terminal bud and underlying meristem were excised from one-half of the cuttings using a narrow-bladed scalpel. Care was taken not to damage the lateral buds associated with the apical rosette of leaves.
        Powdered hormone was applied by dipping the cutting to the depth of the wound and shaking gently to remove the excess. IBA in ethanol was applied by dipping the stem base to a depth of 1 cm for 5 sec, followed by air drying before insertion into the rooting medium.
        'Unique' cuttings were taken June 29, 1982 and propagated under mist in a glass greenhouse, shaded to reduce natural light intensity to 50%. The propagating medium used was sphagnum peat : perlite (1:8 v/v) maintained at 24 ± 0.9°C by thermostatically controlled electric warming cables. Misting was from 0400 to 2000 hrs, 5 sec/5 min. Air temperature ranged from a mean minimum of 18 ± 2°C to a maximum of 27 ± 3°C.
        'Vulcan', 'Anna Rose Whitney' and 'Elizabeth' cuttings were taken September 11-13, 1982 and propagated in a wood-framed shade house with polyethylene-lined, wire-mesh walls and a fiberglass roof allowing 20% light transmission. The propagating medium used was peat : perlite (1:1 v/v) maintained at 20 ± 1.0°C. Misting was from sunrise to sunset, 5 sec/15 min. Minimum and maximum air temperatures were, respectively, 6 ± 3°C and 16 ± 6°C.
        Five cuttings were used for each of the six treatments and each treatment was replicated 8 times. A complete randomized block design was utilized.
        Rooting was assessed after three months, except for 'Anna Rose Whitney' (four months). Percent rooted and mean root ball diameter (RBD) were recorded as well as a rooting score as follows:
0  dead
1  no roots and no callus formation
2  no roots but with callus formation
3  1-4 single roots
4  mean RBD 1.5 - 3.0 cm
5  mean RBD 3.0 - 5.0 cm
6  mean RBD 5.0 cm
        The data was analyzed by a 2 x 2 factorial analysis of variance (terminal bud removal and exogenous IBA as main effects), followed by single degree of freedom comparisons, where appropriate (7).

Results:
Exogenous IBA, supplied in powdered form, promoted rooting in all cultivars studied (Table 1). This is in agreement with previous reports (4). The 'quick-dip' method of IBA application also stimulated rooting to a comparable extent in 'Anna Rose Whitney', 'Elizabeth' and 'Unique', but had no effect on 'Vulcan' (Table 2).
        The effects of terminal bud removal were variable, depending on the cultivar and IBA treatment.

IBA in Talc (Table 1):
In 'Unique', terminal bud excision had no effect on rooting, either with or without application of IBA. In contrast, disbudding reduced root development in 'Vulcan' as judged by both root score and mean root ball diameter. Terminal bud excision also slightly inhibited rooting in 'Elizabeth'. Percent rooted was increased by bud excision in 'Anna Rose Whitney'. Root growth in this cultivar was unaffected.

Table 1: The effect of terminal bud removal in combination with exogenous IBA (0.8% in talc) on the rooting performance of rhododendron cuttings.
  With IBA Without IBA Standard
error of means
Level of Significance
Cultivar: TBz TB TB TB   TB IBA &
  Parameter intact removed intact removed IBA removal TB removal
Anna Rose Whitney:                
  Percent Rooted 75 85 23 48 ±7.8 * * N.S.
  Rooting Score 4.3 4.7 2.2 2.9 ±0.3 * N.S.x N.S.
  RBD (cm) 5.8 5.4 0.9 1.9 ±0.5 * N.S. N.S.
Vulcan:                
  Percent Rooted 93 83 58 53 ±9.1 * N.S. N.S.
  Rooting Score 5.1 3.8 2.8 2.5 ±0.3 * * N.S.
  RBD (cm) 4.8 2.8 1.2 0.7 ±0.5 * * N.S.
Elizabeth:                
  Percent Rooted 95 98 98 98 ±2.7 N.S. N.S. N.S.
  Rooting Score 5.5 5.6 5.1 4.8 ±0.2 * N.S. N.S.
  RBD (cm) 7.0 6.5 5.7 4.0 ±0.5 * * N.S.
Unique:                
  Percent Rooted 98 98 65 60 ±7.0 * N.S. N.S.
  Rooting Score 4.3 4.2 2.9 3.0 ±0.2 * N.S. N.S.
  RBD (cm) 2.7 2.6 0.8 0.9 ±0.3 * N.S. N.S.
TBz - Terminal Bud
N.S.x - non-significant at 5% level.
* - significant at 5% level

Liquid IBA (Table 2):
No effect of terminal bud removal on rooting was observed in 'Unique' or 'Vulcan'. In 'Elizabeth', inhibition was noted together with an interaction between hormone treatment and disbudding. Terminal bud removal was inhibiting only in the absence of exogenous IBA. This indicates that the exogenous growth substance reversed the inhibition caused by disbudding. Percent rooted, root ball diameter and rooting score were improved by terminal bud removal in 'Anna Rose Whitney'.

Table 2: The effect of terminal bud removal in combination with exogenous IBA (0.4% in 95% Ethanol) on the rooting performance of rhododendron cuttings.
  With IBA Without IBA Standard
error of means
Level of Significance
Cultivar: TBz TB TB TB   TB IBA &
   Parameter intact removed intact removed IBA removal TB removal
Anna Rose Whitney:                
   Percent Rooted 78 88 23 48 ±7.1 * * N.S.
   Rooting Score 4.0 4.6 2.2 2.9 ±0.3 * * N.S.
   RBD (cm) 3.9 5.2 0.9 1.9 ±0.5 * * N.S.
Vulcan:                
   Percent Rooted 60 43 58 53 ±9.5 N.S.x N.S. N.S.
   Rooting Score 3.0 2.5 2.8 2.5 ±0.2 N.S. N.S. N.S.
   RBD (cm) 1.2 0.7 1.2 0.7 ±0.3 N.S. N.S. N.S.
Elizabeth:                
   Percent Rooted 95 95 98 98 ±3.4 N.S. N.S. N.S.
   Rooting Score 5.2 5.3 5.1 4.8 ±0.2 N.S. N.S. N.S.
   RBD (cm) 5.7 6.3 5.7 4.0 ±0.4 * N.S. *
Unique:                
   Percent Rooted 88 93 65 60 ±7.9 * N.S. N.S.
   Rooting Score 4.1 4.0 2.9 3.0 ±0.2 * N.S. N.S.
   RBD (cm) 2.8 2.3 0.8 0.9 ±0.3 * N.S. N.S.
TBz - Terminal Bud
N.S.x - non-significant at 5% level.
* - significant at 5% level

Discussion:
The results indicate that the effects of terminal bud removal on the rooting of rhododendron cuttings depend on (a)the cultivar, and (b)the application of exogenous hormone. Terminal bud removal stimulated rooting in 'Anna Rose Whitney'. This suggests that the dormant vegetative bud in this cultivar produced a rooting inhibitor. Excision of the bud thus removes this inhibition. Such inhibitors have been implicated in rooting studies with other species; for example, Fadl and Hartmann's (2) work with pear hardwood cuttings. In 'Elizabeth', rooting was reduced by terminal bud removal and the effect was reversed by the application of exogenous IBA. This result may be interpreted as follows; the resting bud produces significant amounts of endogenous rooting hormone, removal thus inhibits rooting; normal rooting may be restored by applying exogenous hormone to the cutting base. In 'Vulcan', disbudding caused a slight inhibition of rooting was observed when the hormone was supplied as 0.8% IBA in talc. Apparently this inhibition was unrelated to hormone levels, since the application of exogenous IBA failed to reverse the inhibition in those cuttings with the terminal bud removed. The liquid hormone treatment failed to stimulate rooting in this cultivar; possibly more concentrated IBA may be required. The application of hormone as either liquid or powder stimulated rooting in 'Unique', in which no effect of disbudding was observed. The results serve to emphasize the diversity of rhododendron cultivars regarding their response to various factors affecting rooting. No general benefits of terminal bud removal were apparent although the slowest rooting cultivar, 'Anna Rose Whitney', showed a positive response. Possibly in the more difficult-to-root rhododendrons there is a rooting inhibitor present. Thus in these cultivars, rooting would be promoted by terminal bud excision.
        At this time, it is doubtful that removal of the vegetative terminal bud is of general benefit in rooting stem cuttings of Rhododendron.

Summary
The effect of terminal bud removal on the rooting of four cultivars of Rhododendron ('Unique', 'Vulcan', 'Elizabeth' and 'Anna Rose Whitney') was investigated. No effect of disbudding was found in 'Unique'. In 'Vulcan' and 'Elizabeth' terminal bud removal reduced root growth. In 'Elizabeth' this inhibition could be reversed by the exogenous application of IBA. Root development and percent rooted was improved by terminal bud excision in 'Anna Rose Whitney'. The effects of terminal bud removal were found to depend on the cultivar and the exogenous application of hormone.

Acknowledgements:
The authors thank Dr. D.J. Ballantyne for helpful discussions. This work was supported by a grant from the American Rhododendron Society to B.C. Strik and was submitted as part of an Honors thesis to the University of Victoria, 1983.

Literature Cited:
1. Adams, D.G. and Roberts, A.N. 1965. Effect of flower buds on rooting response tested. Oregon Ornamental and Nursery Digest 9:1-6.
2. Fadl, M.S. and Hartmann, H.T. 1967. Relationship between seasonal changes in endogenous promoters and inhibitors in pear buds and cutting bases and the rooting of pear hardwood cuttings. Proc. Amer. Soc. Hort. Sci. 37:96-112.
3. French, C.J. 1983. Stimulation of rooting in rhododendrons by increasing natural daylength with low intensity lighting. HortScience 18:88-89.
4. Leach, D.G. 1961. Rhododendrons of the World C. Scribner's Son's, New York. 544 pp.
5. Loach, K. 1977. Leaf water potential and the rooting of cuttings under mist and polyethylene. Physiol. Plant. 40:191-197.
6. Saunders, T., Legg, L. and Coartney, J. 1981. Comparison of various treatments in rooting of Rhododendron 'English Roseum' cuttings, proc. Int. Plant Prop. Soc. 31:681-684.
7. Steel, R.G.D. and Torrie, J.M. 1980. Principles and Procedures of Statistics. 2nd Edition. McGraw-Hill, New York.

1Present address: Department of Horticultural Science, University of Guelph, Guelph, Ontario


Volume 38, Number 2
Spring 1984

DLA Ejournal Home | JARS Home | Table of Contents for this issue | Search JARS and other ejournals