Abstract - Paclobutrazol was originally developed as a fungicide (Sugavanam 1983). Later, it was found to have a high level of plant growth regulatory activity (Sugavanam 1983). Specifically, paclobutrazol is a growth retardant. Several of its registered names "Clipper" and "Bonzi" (Keever et al. 1990) reflect this property. Paclobutrazol has been used as a pruning replacement in the culture of fruit trees (Curry and Williams 1983), the production of landscape shrubs (Keever et al. 1990), and the maintenance of electrical transmission and distribution lines (Arron 1985). Paclobutrazol has also been used to control height growth in the culture of poinsettias, chrysanthemums, geraniums, and bedding plants (Keeveret al. 1990). Although new to the forest nursery industry in Canada, a number of researchers have investigated various aspects of the physiology of paclobutrazol in conifer seedlings. The effect of paclobutrazol on drought resistance in Douglas fir, lodgepole pine, white spruce [van den Driessche (1990, 1996)], and jack pine (Marshall et al. 1991) has been reported. The effect of paclobutrazol on shoot and root growth in Douglas fir (Wheeler 1987), loblolly pine (Wheeler 1987, Barnes and Kelly 1992), jack pine, red pine, and eastern larch (Rietveld 1988) has been documented. In Atlantic Canada, where there are few nurseries with black-out curtains, the use of paclobutrazol as a dormancy-induction treatment in black spruce seedling culture is being promoted. Recognizing the conservative nature of nursery culture in the Atlantic region, the approach of this study was to use paclobutrazol application in the context of extended greenhouse culture. The objective of the study reported in this paper was to determine the effect of three application rates of paclobutrazol on seedling morphological attributes at the end of culture and after one growing season under greenhouse conditions.

Materials and Methods

Seedling material

The study was conducted at the Mount Pearl Nursery of the Newfoundland Forest Service in 1994. In an effort to reduce crop variability, seed from a half-sib black spruce family was used. In late June, seed was sown in Multipot 67 containers and germinated under supplemental lighting.
During the establishment and rapid shoot elongation phases, the crop was maintained under supplemental lighting. In late August, seedlings received either (1) extended greenhouse culture (EGC), or (2) extended greenhouse culture supplemented with paclobutrazol application (PCZ).
Under extended greenhouse culture, supplemental lighting ended and growing medium was leached of nutrients. Paclobutrazol was applied using a back-pack sprayer at one of the following three rates: 1.0, 2.5, and 5.0 mg per seedling. These rates were selected from the rates suggested for trial use in the 1993 CONFER Technical Data Sheet (ZENECA Agro, 1993). Paclobutrazol, back-pack sprayer, mixing instructions, and application instructions were provided by ZENECA Agro.
The design structure was a split-plot with four blocks to account for within greenhouse variability. Each block contained 56 Multipot containers. Treatments were randomly assigned to containers in each block. Treatment containers were randomly assigned to various component studies.
Seedlings remained in the greenhouse during September and October. In November, cold-hardiness testing began. Seedlings were moved to an outdoor compound in late November, once cold hardiness had been achieved.
In late April 1995, containers were moved to the Canadian Forest Service Experimental Greenhouses in Mount Pearl. Seedlings were removed from containers and dibbled into pots. Pots were placed in a heated greenhouse and watered on a regular basis until September.

Sampling

Eighty seedlings (randomly sampled from eight containers per treatment) were harvested prior to treatment application in late August 1994 and after cold hardiness had been achieved in late November 1994. Morphological attributes (height, caliper, shoot dry weight, and root dry weight) were determined on seedlings from both sampling dates.
Thirty-two seedlings (randomly sampled from eight containers per treatment, but then culled if plugs fell apart) were grown under greenhouse conditions from late April 1995 to early September 1995. After harvesting, morphological attributes were determined where possible. Leading shoots were missing on one or two seedlings from each of the paclobutrazol treatments.

Presentation of results

Means and standard errors of the variables collected for the EGC treatment and each of the PCZ treatments are presented graphically. When marked differences occurred between the EGC treatment and the PCZ treatments, the mean of the three PCZ treatments was first calculated and then the difference was expressed relative to the mean of the EGC treatment.

Results

Pre-treatment and post-treatment morphological attributes.

Prior to treatment in late August 1994, the seedling crop was uniform. There were only slight differences in height (Figure 1), caliper (Figure 2), shoot dry weight (Figure 3), and root dry weight (Figure 4).
In late November 1994, there were only slight differences in height between ECGtreated seedlings and PCZ-treated seedlings (Figure 5). Caliper of PCZ-treated seedlings was 13% larger than caliper of EGC-treated seedlings (Figure 6); whereas, shoot dry weights of the EGC-treated and PCZ-treated seedlings were similar (Figure 7). Dissections through shoots revealed that PCZ-treated seedlings had more cortical tissue than EGC-treated seedlings and thus accounted for the larger caliper. Root dry weight of the PCZ-treated seedlings was 27% lighter than that of the EGC-treated seedlings (Figure 8).

Morphological attributes after one growing season

In September 1995, after one growing season under greenhouse conditions, all morphological attributes of PCZ-treated seedlings were substantially less than those of EGC-treated seedlings. Height of PCZ-treated seedlings was 36% shorter than that of EGC-treated seedlings (Figure 9). The reduction in height of PCZ-treated seedlings was caused by the reduction in length of the 1995 leader. Length of the leader of PCZ-treated seedlings was 61% shorter than the leader of the EGC-treated seedlings (Figure 10). This reduction in leader length was due to the reduction in number of needles on leaders. Leaders of PCZ-treated seedlings bore 37% fewer needles than leaders of EGC-treated seedlings (Figure 11). The impact of this reduction in photosynthetic tissue was evident in seedling caliper and dry weights. Caliper of PCZ-treated seedlings was 18% lighter than caliper of EGC-treated seedlings (Figure 12). PCZ-treated seedling shoot dry weights and root dry weights were, respectively, 40% and 37% lighter than those of EGC-treated seedlings (Figure 13 and Figure 14, respectively).

Discussion

Relative to the extended greenhouse culture treatment, the paclobutrazol treatments did not control height growth in the year of application. More importantly, however, the paclobutrazol treatments reduced seedling growth in the subsequent year relative to the extended greenhouse culture treatment. Reduction of morphological attributes of conifer seedlings treated with paclobutrazol has been reported by other investigators. Wheeler (1987) reported that paclobutrazol application at germination reduced height, caliper, shoot dry weight, and root dry weight of 22-week-old Douglas-fir and loblolly pine seedlings. Rietveld (1988) found that paclobutrazol applied in August to 1-0, 2-0, and 3-0 jack pine, 2-0 red pine, and 1-0 and 2-0 eastern larch reduced height, caliper, shoot dry weight, and root dry weight the following year. Van den Driessche (1990, 1996) observed that paclobutrazol, applied to 1-0 seedlings prior to planting, reduced height in white spruce, as well as, shoot and root dry weight in white spruce, lodgepole pine, and Douglas-fir. Given the results of this study and those of other studies, the use of paclobutrazol as a dormancy-induction treatment in forest nurseries needs further investigation before it widely replaces currently used and better understood dormancy-induction treatments.

Acknowledgements

I sincerely thank Neil Benson, Manager of the Mount Pearl Tree Seedling Nursery of the Newfoundland Forest Service for providing generous greenhouse space and for culture of the seedling material, Kathy Tosh and Michelle Fullarton of the Tree Improvement Group of the New Brunswick Department of Natural Resources for providing improved seed from a half-sib family, and Greg O'Neill of ZENECA Agro for detailed discussions of formulations of CONFER and application instructions for CONFER, and Sandra Morgan of the Canadian Forest Service for technical assistance. ZENECA Agro generously supplied CONFER and the back-pack sprayer for use in thisexperiment.

Literature Cited

Arron, G.P. 1985. Effect of bunk injection of three growth regulators on sprout growth in silver maple. Journal of Arboriculture 11 (10): 301-306.

Barnes, A.D. and Kelly, W.D. 1992. Effects of a triazole, uniconazol on shoot elongation and root growth in loblolly pine. Canadian Journal of Forest Research 22: 1-4.

Curry, E.A. and Williams, M.W. 1983. Promalin or GA3 increase pedicel and fruit length and leaf size of 'Delicious' apples treated with paclobutrazol. HortScience 18 (2): 214-215.

Keever, G.J., Foster, W.J., and Stephenson, J.C. 1990. Paclobutrazol inhibits growth of woody landscape plants. Journal of Environmental Horticulture 8 (1): 41-47.

Marshall, J.G., Scarratt, J.B., and Dumbroff, E.B. 1991. Induction of drought resistance by abscisic acid and paclobutrazol in jack pine. Tree Physiology 8: 415- 421.

Rietveld, W. 1988. Effect of paclobutrazol on conifer seedling morphology and field performance. Pp. 19-23 In Proceedings, Combined Meeting of the Western Forest Nursery Associations: Western Forest Nursery Council, Forest Nursery Association of British Columbia, and Intermountain Forest Nursery Association, August 1988, Vernon, B.C. Coordinated by Landis, T.D. USDA, Forest Service, Rocky Mountain Forest and Range Experiment Station, General Technical Report No. RM-167.

Sugavanum, B. 1983. Diastereoismers and enantiomers of paclobutrazol: their preparation and biological activity. Pesticide Science 15: 296-302.

van den Driessche, R. 1990. Paclobutrazol and triadimefon effects on conifer seedling growth and water relations. Canadian Journal of Forest Research 20: 722-729.

van den Driessche, R. 1996. Drought resistance and water use efficiency of conifer seedlings treated with paclobutrazol. New Forests 11: 65-83.

Wheeler, N.C. Effect of paclobutrazol on Douglas fir and loblolly pine. Journal of Horticultural Science. 62 (1): 101-106.

ZENECA Agro. 1993. CONFER. Experimental plant growth regulator for conifer seedling conditioning. Technical data sheet. ZENECA Agro, a business of ZENECA Corp., Stoney Creek, Ont., 9 pp. 08/93.

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