JARS v43n3 - Factors Affecting Rooting Of Rhododendron 'Britannia' Stem Cuttings

Factors Affecting Rooting Of Rhododendron 'Britannia' Stem Cuttings
C.J. French and J. Alsbury
Saanichton Research and Plant Quarantine Station
Sidney, British Columbia, Canada

Abstract
Acetone, used as a 16 hr overnight treatment in the dark at 4.5 µmolml -1 prior to treatment with 0.8% indole butyric acid (IBA) in talc, stimulated rooting of difficult-to-root stem cuttings of Rhododendron 'Britannia' ('Queen Wilhemina' x 'Stanley Davies') during spring propagation but not in fall. Acetone, used as a 15 sec "quick-dip" prior to IBA treatment, did not promote rooting. Application of IBA in talc at 0.8%, 1.6% and 3.0% stimulated rooting in fall. CO 2 mist inhibited rooting. Treatment with exogenous abscisic acid (ABA) at 0.038 µmolml -1 , night breaks from low irradiance incandescent lamps at 4.6 µmolsec -1 m -2 (2000 to 0400 hr) and removal of the vegetative terminal bud had no effect on rooting.

Rhododendrons are normally propagated commercially by stem cuttings. However, different clones vary greatly in ease of rooting (4). Rhododendron 'Britannia' ('Queen Wilhemina' x 'Stanley Davies') is an example of a difficult-to-root cultivar that generally gives less than 20% rooting under standard propagation conditions (9). Application of exogenous indole butyric acid (IBA) has been reported to stimulate rooting (5) but quantitative details were lacking. Analysis of various rooting hormones in 'Britannia' compared to the easy-to-root R. ponticum indicated little difference in indole acetic acid (IAA), gibberellins or cytokinins (9). However, there were large differences in endogenous ABA leading to the suggestion that this hormone could be a rooting co-factor in Rhododendron (9). Other work has suggested that ABA is inhibitory to rooting of Rhododendron (6).

Acetone treatment has been used as a quick-dip to aid rooting of English holly (Ilex aquifolium ) cultivars that can be difficult to propagate from stem cuttings (2, 5). Removal of vegetative terminal buds stimulates rooting in some Rhododendron cultivars (8) and CO 2 enrichment during rooting can also be promotive (French, unpublished data). Night break treatments (2000 to 0400 hr) from incandescent lamps at 4.6 µmolsec -1 m -2 are known to stimulate rooting in a range of Rhododendron cultivars (1). The current study was conducted to investigate the influence of several of these factors on the rooting of 'Britannia', with the aim of increasing rooting.

Containerized stock plants were maintained in an unheated shade-house (50% light transmission). Containers (6 liter capacity) were filled with milled softwood bark: coarse peat: sand: sawdust (3:1:1:1 by volume) with the following amendments (per m 3 ); dolomite lime, 2.1 kg, 20% superphosphate, 1.8 kg and fritted trace elements, 0.15 kg. A mixture of two types of Nutricote slow release fertilizer (Type 180: Type 40, 3:1 by weight) was incorporated at a rate of 4.7 kg.m -3 . Containers were top-dressed annually in spring at 20 g/container. Lateral cuttings were taken from the stock plants and lower leaves and buds were stripped. Four apical leaves and lateral buds were retained and leaves trimmed by up to one half to give about equal leaf areas. The stems were shortened to 6 cm and given a single wound treatment by removing a 1 mm strip of bark from the basal 2.5 cm. Exogenous hormone was applied as indole butyric acid (IBA) in talc. Rooting medium was 1 peat:1 perlite (by volume), maintained at 21° + 1°C with heating cables.

Propagation was conducted in a glass greenhouse (light transmission 40%). CO 2 mist system was as previously described (2, 3), giving an air enrichment to 1100 µl·liter -1 . Cuttings were misted from 0400 to 2000 hr for 10 sec every 10 min. Water pressure, approximately 2.8 kg·cm -2 (40 psi), was adjusted to maintain equal water supply to cuttings with and without CO 2 mist. Experiments were conducted in spring (Mar-June) or fall (Oct-Jan). Greenhouse temperatures during propagation were 20 to 30°C (day) / 10 to 20°C (night) in spring and 15 to 20°C (day) / 10 to 12°C (night) in fall.

In an initial experiment, cuttings were pre-incubated in the dark at 20°C for 16 hr from 1630 to 0830 hr in either 4.5 µmolml -1 acetone or in 4.5 µmolml -1 acetone containing 0.038 µmolml -1 ABA. This concentration of ABA approximates the endogenous level found in the easy-to-root R. ponticum (9). All cuttings were treated with 0.8% IBA in talc immediately preceding sticking. Cuttings pre-treated with acetone alone were assigned at random to two identical greenhouse compartments with either CO 2 mist or tap water mist. Cuttings pre-treated with ABA were assigned to the tap-water mist compartment. There were four replications of five cuttings per treatment. Percentage data were assessed for homogeneity of variance by Bartlett's test. The result indicated that the variances were not normally distributed and arcsin transformation of the data was required before analysis of variance (7).

Fourteen weeks after sticking the cuttings were assessed by measuring rooting percentage, root ball diameter and depth (Table 1). CO 2 mist reduced rooting percentage compared to the tap water control. ABA had no significant effect on rooting. There was no significant effect of either CO 2 or ABA on root ball dimensions.

Table 1. Effect of ABA and CO2 mist on rooting of R. 'Britannia'.
Variable CO 2 z ABA y Control x
Rooting percentage w 55a v 65ab 90b
Root ball diam. (cm) 1.7a 2.2a 2.3a
Root ball depth (cm) 1.3a 1.6a 1.4a
z Cuttings pre-incubated 16 h in dark at 20° C in 4.5 µmolm -1 acetone prior to IBA treatment; CO 2 mist was supplied from 0400 to 2000 hr, providing air enrichment to 1100 ul liter -1 .
y Cuttings were pre-incubated 16 hr in dark at 20°C with 0.038 µmolml -1 ABA in 4.5 µmolml -1 acetone prior to IBA treatment.
x Cuttings pre-incubated 16 hr in dark at 20°C in 4.5 µmolm -1 acetone prior to IBA treatment.
w Data was subjected to arcsin transformation prior to analysis.
v Mean separations in rows by Duncan's multiple range test, 5% level.

Rooting percentages in the initial experiment were very high considering the reported difficulty in rooting R. 'Britannia' from stem cuttings (9). Since a pre-treatment with acetone had been used in all treatments, it was thought that this might be a stimulatory factor in rooting. Therefore, the effect of an acetone pre-treatment and duration of application (16 hr vs. quick-dip) on rooting was investigated. Sixteen hr treatment was as described in experiment 1; quick-dip was a 15 sec treatment. A distilled water dip served as a control. A 2 x 2 factorial design was used with acetone (+, -) and duration of treatment (16 hr vs. quick-dip) as factors. Following pre-treatment with either distilled water or acetone, all cuttings were treated with IBA in talc (0.8%) immediately prior to sticking.

Acetone, when used overnight as a pre-treatment, was effective in promoting rooting (Table 2). When used as a quick-dip, acetone was ineffective in stimulating rooting and gave similar results to pretreatment with distilled water.

Table 2. Effect of acetone and pre-treatment duration on rooting of R. 'Britannia
Variable Acetone z
16 hr y 		Acetone
Quick-dip x 		Water
16 hr 		Water
Quick-dip
Percentage rooted w 55 25 30 30
Root ball diam. (cm) 3.1 0.5 1.2 0.9
Root ball depth (cm) 2.2 0.2 0.8 0.4
Source of Variation v Percentage
rooted 		Rootball diam.(cm) Rootball depth
(cm)
Acetone NS NS NS
Treatment Duration NS * *
Acetone x Duration NS * *
z Acetone concentration 4.5 µmolml -1
y Iincubation at 20°C in dark from 1630 to 0830 hr.
x Treatment for 15 sec.
w Percentage data were subjected to arcsin transformation prior to analysis.
v NS, *, Non-significant or significant at 5% level.

Earlier reports suggested that R. 'Britannia' could be rooted by increasing the concentration of IBA to 2.0% although quantitative data were not provided (5). Also, McDonald (5) suggested that the effect of acetone on holly cuttings might be to increase the solubility of IBA in powder form, thus increasing its effectiveness. In order to investigate these possibilities, a range of four IBA treatments from 0 to 3.0% was used with and without the addition of a 16 hr pre-treatment with acetone. A 2 x 4 factorial design was used with acetone (+, -) and IBA (four concentration levels) as factors. Treatments contained five replications of five cuttings per treatment. Propagation was conducted from Oct 8 to Jan 26.

The results indicated (Table 3) that acetone had no effect on rooting and that IBA was stimulatory. There was no statistically significant difference in the effectiveness of IBA over the concentration range 0.8 to 3.0% and no interaction between acetone and IBA.

Table 3. Effect of acetone and a range of IBA concentrations on rooting of R. 'Britannia'.
Variable 0% IBA z 0.8% IBA 1.6% IBA 3.0% IBA
+AC y -AC x +AC -AC +AC -AC +AC -AC
Percentage rooted w 4 0 24 56 32 52 48 48
Root ball diam.(cm) 0.2 0 1.3 2.9 1.0 1.7 2.1 1.9
Root ball depth (cm) 0.2 0 0.9 2.1 0.7 1.3 1.4 1.3
Source of Variation v Percentage rooted Root ball diam. Root ball depth
IBA * * *
Acetone NS NS NS
IBA x Acetone NS NS NS
z IBA was applied in talc.
y 16 hr incubation in dark with 4.5 µmolml -1 acetone at 20°C.
x 16 hr incubation with distilled water as above.
w Percentage data were subjected to arcsin transformation prior to analysis
v NS, *, Non-significant or significant at 5% level.

Rooting of R. 'Britannia' in spring was not affected by either a night break treatment (2000 to 0400 hr) from incandescent lamps at 4.6 µmolsec -1 m -2 or by removing the vegetative terminal bud prior to propagation.

The effectiveness of acetone as a root-promoting agent was dependent on season of application. This may be related to seasonal variations in many other factors that can influence rooting (4). The results do not suggest that acetone promotes uptake of IBA. Rooting was stimulated by IBA but no improvement was noted in increasing concentrations above 0.8%, at least in fall propagation. ABA did not stimulate rooting, in agreement with previous results (6). The findings do not support the suggestion that ABA is a rooting co-factor (9), although it is possible that positive effects might be found at other concentrations of ABA.

Rooting of R. 'Britannia' from stem cuttings at commercially acceptable levels can be achieved by using a 16 hr acetone pre-treatment during spring propagation.

Literature Cited
1. French, C.J. 1983. Stimulation of rooting in rhododendrons by increasing day length with low intensity lighting. HortScience 18: 88-89.
2. French, C.J. and W.C. Lin. 1984. Seasonal variations in the effects of CO 2 mist and supplementary lighting from high pressure sodium lamps on rooting of English holly cutting. HortScience 19: 519-521.
3. Lin, W.C. and J.M. Molnar. 1981. Effects of CO 2 mist and high intensity supplementary lighting on propagation of selected woody ornamentals. Can. J. Plant Sci. 61: 965-969.
4. Leach, D.C. 1960. Rhododendrons of the World . Charles Scribner's Sons, New York.
5. MacDonald, B. 1986. Practical woody plant propagation for nursery growers . Timber Press, Portland, Oregon.
6. Pierik, R.L.M. and H.H.M. Steegmans. 1975. Analysis of adventitious root formation in isolated stem explants of Rhododendron . Scientia Hort. 3:1-20.
7 Steel, R.C.D. and J.M. Torrie. 1960. Principles and Procedures of Statistics . McGraw-Hill, New York.
8. Strik, B.C. and C.J. French. 1983. Effect of terminal bud removal and exogenous indole butyric acid treatments on rooting of vegetative rhododendron cuttings. J. Amer. Rhododendron Soc. 38:63-65.
9. Wu, F.T. and M.F. Barnes. 1981. The hormone levels in stem cuttings of difficult-to-root and easy-to-root rhododendrons. Biochem. Physiol. Pflanzen. 176:13-22.

C.J. French is currently at Agriculture Canada, Research Station, Vancouver, British Columbia, Canada.