Content uploaded by Shuju Bai
Author content
All content in this area was uploaded by Shuju Bai on Apr 28, 2014
Content may be subject to copyright.
137Journal of Arboriculture 30(3): May 2004
RESPONSE OF CAMBIAL AND SHOOT GROWTH IN
TREES TREATED WITH PACLOBUTRAZOL
by Shuju Bai1, William Chaney2, and Yadong Qi3
Paclobutrazol (PBZ), which inhibits gibberellin synthesis and
consequently cell elongation (Kimball 1990; Grossmann
1992; Rademacher 2000), is known to reduce the growth of
woody plants and has been used successfully for this purpose
by horticulturists and utility arborists (Keever et al. 1990;
Werblow 1998; Fletcher et al. 2000). The use of PBZ by
utility arborists lengthens the time between trimming cycles,
reduces the amount of time at the job site, and lowers the
amount of biomass removed during trimming for many tree
species (Redding et al. 1994; Burch et al. 1996). Shoot
growth reduction along with other benefits to tree health
reported in recent years has led to the expansion of PBZ use
solely by utility foresters to its use by commercial arborists
for management of other trees in the urban landscape
(Chaney 2003).
Although PBZ is known to suppress shoot growth of
most tree species (Davis and Curry 1991) and to increase
tolerance to drought conditions and resistance to fungal
organisms (Chaney et al. 1996; Fletcher et al. 2000), the
response of cambial growth in PBZ-treated trees has not been
clearly elucidated. Because the principal focus of research
with growth retardants such as PBZ has been on growth in
height, only a few observations have been recorded concern-
ing growth in diameter. Previous research, although limited
in scope, has generally indicated a suppression of cambial
growth (Gilliam et al. 1988; Estabrooks 1993; Schnurr et al.
1996). Growth in diameter of trees is a major cause of
damage to hardscape in urban areas, accounting for a
significant portion of annual tree program expenditures by
municipalities (McPherson and Peper 1995). Hence, the
main objective of this study was to determine the effect of the
tree growth regulator PBZ on cambial growth of several tree
species growing in the different environmental conditions of
Indiana and Louisiana, U.S. Growth in length of terminal and
lateral shoots also was measured to relate the acknowledged
reduction of primary growth to any effect on secondary or
cambial growth.
MATERIALS AND METHODS
Trees used in this study were in two locations; one at the
Horticulture Farm at Southern University and A&M
College in Baton Rouge, Louisiana, U.S., and the other at
Martell Experimental Forest near Purdue University in West
Lafayette, Indiana, U.S. . Three experiments were designed,
two at the Indiana and one at the Louisiana site, using a
combination of nine tree species. For Experiment I con-
ducted in Indiana, two species, red oak (Quercus rubra L.)
and white oak (Q. alba L. [10 cm (4 in.) average caliper]
were treated by the soil drench method in April 1995.
Experiment II, also in Indiana, involved eight species
ranging from 4 to 8 cm [1.6 to 3.2 in.] caliper treated in
April 1996 using the soil injection method: black walnut
(Juglans nigra L.), European black alder (Alnus glutinosa L.),
red oak, sweetgum (Liquidambar styraciflua L.), white ash
(Fraxinus americana L.), white oak, white pine (Pinus strobus
L.), and yellow poplar (Liriodendron tulipifera L.). Experi-
ment III, conducted in Louisiana, involved two species
treated in March 1997 using the soil drench method:
cherrybark oak (Q. falcata var. pagodaefolia Elliott) and
sweetgum [4 cm (1.6 in.) average caliper].
Abstract. Three experiments were conducted to investigate the
effect of paclobutrazol on shoot and cambial growth of nine tree
species located in Indiana and Louisiana, U.S. In Experiment I,
white oak (Quercus alba L.) and red oak (Q. rubra L.) in Indiana
were treated in April 1995 with paclobutrazol using the soil
drench method. In Experiment II, paclobutrazol was applied in
April 1996, using the soil injection method, to eight species
growing in Indiana: white oak, red oak, sweetgum (Liquidambar
styraciflua L.), black walnut (Juglans nigra L.), European black
alder (Alnus glutinosa L.), yellow poplar (Liriodendron tulipifera L.),
white ash (Fraxinus americana L.), and white pine (Pinus strobus
L.). In Experiment III, two species, sweetgum and cherrybark oak
(Q. falcata var. pagodaefolia L.) in Louisiana, were treated with
paclobutrazol by the soil drench method in March 1997. At the
end of the 1998 growing season, annual shoot growth and annual
xylem ring increment were measured for each of the two to four
growing seasons after treatment in the three experiments. The
effects of paclobutrazol on cambial growth and annual shoot
growth at various positions along the trunk and in the canopy
varied with species and treatment duration. Paclobutrazol
reduced cambial growth in white oak, red oak, cherrybark oak,
sweetgum, European black alder, and white pine with the amount
varying among species, vertical location in the tree, and year after
treatment.
Key Words. Paclobutrazol (PBZ); growth retardant; cambial
growth; annual shoot growth; xylem ring increment.
138 Bai et al.: Cambial and Shoot Growth in Trees Treated with Paclobutrazol
Twenty (Experiment I), 12 (Experiment II), and 14
(Experiment III) plantation trees of each species growing at
2 × 2 m [6.6 × 6.6 ft] spacing were selected based on their
size and condition in 1995, 1996, and 1997, respectively.
Adjacent trees were not selected for the studies to avoid
root interaction and the unintended exposure to PBZ
treatment. Half of the trees for each species used in the
three experiments was randomly assigned to each of two
different treatment groups. The treatments were basal
drench or soil injection with PBZ formulated as Profile 2SC
or water as a control in complete random experimental
designs. The dose of PBZ applied was 9.6 g active ingredi-
ent (g a.i.) (0.34 oz) per tree in all the experiments except
the sweetgum trees in Louisiana, which were treated with
4.8 g a.i. (0.17 oz.) per tree.
For Experiment I, the annual shoot growth for the 1995,
1996, 1997, and 1998 growing seasons was measured on
the main stem, on four lateral shoots in the upper crown,
and on four lateral shoots in the lower crown of each tree.
For Experiment II, annual shoot growth for the 1996, 1997,
and 1998 growing seasons was measured on the main stem
and four lateral shoots of each tree. For Experiment III, the
annual shoot growth for the 1997 and 1998 growing
seasons was measured as in Experiment II. The amount of
annual growth was apparent by locating the
terminal bud scale scars on branches. All of
the species used normally produce only one
flush of growth each growing season. Shoot
growth for European black alder is not
reported because it was impossible to locate
with certainty the bud scale scars on this
species.
Cross-sections of the trunk were removed
approximately 50 cm (20 in.) from the base of
each tree for observations of annual xylem
ring width in Experiment I. In Experiment II,
three cross-sections were removed from each
selected tree. One was approximately 10 cm
(4 in.) from the base; the second was either 2
or 4 m (6.6 or 13.2 ft) from the base (deter-
mined by the height of the tree), and the third
was from wherever the cross-section would
include the annual rings of xylem for the
1995 through 1998 growing seasons. In
Experiment III, an increment borer was used
to obtain a wood sample for determining the
ring width at a height of 10 cm (4 in.) from
the base of each tree. An increment core also
was taken from the highest point in the trees
that would include the annual rings of xylem
for the 1996 through 1998 growing seasons.
Increment core samples were taken from only
two heights in the trees in Louisiana because
the trees were not tall enough to allow for
additional sampling.
Each cross-section was sanded smooth with a handheld
orbital sander, and each increment core was cut with a sharp
blade to make a flat surface to facilitate microscopic viewing
of the annual rings of xylem. Xylem ring widths for the year
before PBZ treatment through the 1998 growing season were
measured along four radii at approximately right angles for
each cross-section using an Acu-rite III (Jamestown, NY)
digital measuring system. Wood sections were placed on the
table of the manually mobilized scale assembly and viewed
through a lens with 10× magnification. The console was set
for incremental measurement mode, and it gave a digital
readout of the distance across each xylem ring with an
accuracy of 0.001 mm (0.00004 in.).
Data were analyzed using analysis of variance and
differences between means were determined using Tukey’s
Studentized Range (HSD) test, P ≤ 0.05.
RESULTS
Annual Shoot Growth
Annual growth was not affected the first growing season after
PBZ treatment in either red or white oak grown in Indiana
(Experiment I), but it was markedly reduced in the central
leader and in shoots of the upper and lower crown of white
oak for the next three growing seasons (Table 1). Significant
Table 1. Annual and total shoot growth (cm) for 4 years and total
reduction expressed as a percentage of untreated controls for white
and red oak in Indiana untreated and treated in April 1995 with
paclobutrazol using the soil drench method.
Year 4-year Percentage
Treatment 1995 1996 1997 1998 total reduction
White oak
Central leader
Control 42.9 a 46.6 a 41.1 a 40.9 a 170.5 a
Treated 39.0 a 20.0 b 6.6 b 10.5 b 76.1 b 55
Shoots in upper crown
Control 38.5 a 36.9 a 39.6 a 31.3 a 146.3 a
Treated 31.2 a 14.6 b 7.0 b 8.1 b 60.9 b 58
Shoots in lower crown
Control 28.2 a 34.1 a 32.8 a 26.6 a 121.7 a
Treated 20.2 a 9.3 b 3.4 b 5.8 b 38.7 b 68
Red oak
Central leader
Control 91.0 a 51.3 a 62.4 a 45.4 a 250.1 a
Treated 76.9 a 26.3 b 12.5 b 14.9 b 130.6 b 48
Shoots in upper crown
Control 72.5 a 42.3 a 40.6 a 37.4 a 192.8 a
Treated 67.4 a 18.6 b 8.9 b 12.3 b 107.2 b 44
Shoots in lower crown
Control 44.0 a 35.1 a 31.6 a 20.1 a 130.8 a
Treated 51.5 a 20.0 a 5.2 b 7.2 b 83.9 b 36
Pairs of values followed by the same lowercase letter are not significantly different at the
P ≤ 0.05 level.
139Journal of Arboriculture 30(3): May 2004
reduction in annual shoot growth also was observed in the
central leader and the upper crown of red oak for the second
and the next two growing seasons. However, in the lower
crown of red oak, significant reduction in annual shoot
growth occurred only in the third and fourth years after PBZ
treatment. Nevertheless, the total growth in shoot length was
reduced throughout the crown in both species by as much as
36% to 68% compared to the untreated controls (Table 1).
Annual shoot growth of the central leader of the seven
species investigated in Experiment II in Indiana showed large
variations among species and years in response to soil
injection of PBZ (Table 2). Annual growth of the central
leader of sweetgum and white pine was reduced for the three
growing seasons after PBZ treatment. Reduction of annual
growth of the central leader of red oak, white ash, and white
oak occurred in the first and second growing seasons after
treatment, but in the third season, growth suppression
disappeared (Table 2). In yellow poplar, inhibition occurred
only in the first season after treatment. No inhibition of shoot
growth was observed through the three growing seasons in
black walnut. Total growth of the central leader over the 3-
year study period was significantly suppressed in only
sweetgum (99%), white oak (46%), and white pine (30%)
(Table 2).
For growth in length of the lateral shoots in the seven
species in Experiment II in Indiana, only PBZ-treated
sweetgum and white oak trees were suppressed for the three
growing seasons after treatment, resulting in a total reduction
in three growing seasons of 99% and 66%, respectively.
Shoot growth of lateral branches of white pine was inhibited
only in the second and third growing seasons, but this
growth suppression was adequate to result in 19% less
growth of PBZ-treated trees during the 3-year study period
(Table 2). Growth reduction in red oak and yellow poplar
occurred only the first growing season after treatment. No
growth suppression was found in black walnut (Table 2).
Table 2. Annual and total shoot growth (cm) for 3 years and total reduction expressed as a percentage of un-
treated controls of seven species of trees in Indiana untreated and treated in April 1996 with paclobutrazol using
the soil injection method.
Year 3-year Percentage Year 3-year Percentage
Treatment 1996 1997 1998 total reduction Treatment 1996 1997 1998 total reduction
Black walnut White oak
Central leader Central leader
Control 27.5 a 13.5 a 7.5 a 48.5 a Control 37.3 a 32.0 a 28.7 a 98.0 a
Treated 12.5 a 8.9 a 6.8 a 28.2 a 42 Treated 18.2 b 13.7 b 20.7 a 52.6 b 46
Lateral shoots Lateral shoots
Control 10.0 a 10.5 a 8.2 a 28.7 a Control 41.7 a 28.7 a 32.8 a 103.2 a
Treated 7.0 a 5.6 a 4.8 a 17.4 a 39 Treated 14.7 b 10.0 b 10.2 b 34.9 b 66
Red oak White pine
Central leader Central leader
Control 71.7 a 45.7 a 31.2 a 148.6 a Control 95.3 a 82.3 a 89.8 a 267.4 a
Treated 64.3 b 31.3 b 28.1 a 123.7 a 17 Treated 84.0 b 48.0 b 55.7 b 187.7 b 30
Lateral shoots Lateral shoots
Control 55.4 a 23.4 a 21.5 a 100.3 a Control 55.4 a 46.5 a 37.2 a 139.1 a
Treated 40.1 b 17.5 a 16.2 a 73.8 a 26 Treated 50.0 a 34.7 b 28.6 b 113.3 b 19
Sweetgum Yellow poplar
Central leader Central leader
Control 51.5 a 54.7 a 67.0 a 173.2 a Control 53.7 a 43.0 a 63.3 a 160.0 a
Treated 0.8 b 0.3 b 0.3 b 1.4 b 99 Treated 37.0 b 44.1 a 59.5 a 140.6 a 12
Lateral shoots Lateral shoots
Control 34.0 a 27.7 a 21.7 a 83.4 a Control 32.3 a 28.3 a 24.7 a 85.3 a
Treated 0.7 b 0.3 b 0.2 b 1.2 b 99 Treated 21.8 b 22.4 a 18.6 a 62.8 a 26
White ash
Central leader
Control 44.8 a 37.5 a 37.5 a 119.8 a
Treated 31.3 b 26.3 b 35.0 a 92.6 a 23
Lateral shoots
Control 31.0 a 20.5 a 15.2 a 66.7 a
Treated 24.0 a 17.5 a 13.9 a 55.4 a 17
Pairs of values followed by the same lowercase letter are not significantly different at the P ≤ 0.05 level.
140 Bai et al.: Cambial and Shoot Growth in Trees Treated with Paclobutrazol
In Louisiana, annual shoot growth of sweetgum was
markedly inhibited in both the central leader and the lateral
shoots for the two growing seasons after PBZ treatment,
resulting in total growth suppression of 95% and 96%,
respectively (Table 3). For cherrybark oak, inhibition
occurred only in the second growing season after treatment,
and the total reduction in growth in 2 years was significant,
but less than in sweetgum.
Annual Xylem Ring Increment
In Experiment I, the width of the annual ring of xylem
produced 50 cm (20 in.) up from the base of trees was
significantly reduced in both white and red oak during the
first growing season after PBZ treatment by the soil drench
method. This suppression of cambial activity in treated
trees persisted for the four growing seasons of the study,
resulting in a total reduction of cambial growth in white
and red oak of 80% and 60%, respectively (Table 4).
Xylem accumulation at the base of the trunk of trees
treated in spring 1996 using the soil injection method
(Experiment II) showed that PBZ suppressed annual ring
increment in sweetgum for the first and second growing
seasons after treatment, and in white oak for the second
and third seasons (Table 5). Reduction of xylem accumula-
tion occurred in white pine in only the first growing season
after treatment and in European black alder in the third
growing season after treatment. No growth suppression
near the base of the trunk was observed in the other species
(Table 5).
A similar pattern of xylem growth reduction in
sweetgum and white oak also occurred in the cross-sections
2 or 4 m (6.6 or 13.2 ft) above the ground line (Table 5).
Treated European black alder had significantly reduced
annual ring growth for the three growing seasons after PBZ
treatment. In addition, PBZ suppressed
annual ring increment of white pine in the
second growing season. No growth reduction
2 or 4 m high was found in the other species
investigated (Table 5).
Annual ring increment in the upper
crown in the cross sections of the trunk that
included four years of xylem growth of the
eight species treated in 1996 using the soil
injection method (Experiment II) also are
shown in Table 5. PBZ-treated sweetgum
trees had less xylem increment than control
trees for the three growing seasons after
treatment. Growth suppression also occurred
in white oak and white pine during the
second and the third growing seasons.
Annual ring increment in the upper crown in
the other species was not affected by PBZ
(Table 5).
Total cambial growth for the three years following PBZ
treatment was significantly reduced (P ≤ 0.05) only for
sweetgum and white oak. There also was a 3-year reduction
in the mid-trunk section of European black alder and in the
cross-section within the crown of white pine (Table 5).
In Louisiana (Experiment III), the annual ring increment
of cherrybark oak was reduced by PBZ at both the base and
within the crown for the two growing seasons after PBZ
Year
Prior After treatment Total Percentage
Treatment 1994 1995 1996 1997 1998 1995–98 reduction
White oak
Control 4.95 a 4.43 a 5.06 a 4.50 a 6.11 a 20.10 a
Treated 5.30 a 1.60 b 0.99 b 0.70 b 0.71 b 4.00 b 80
Red oak
Control 6.19 a 4.65 a 4.65 a 4.03 a 5.57 a 18.90 a
Treated 6.24 a 2.68 b 1.78 b 1.37 b 1.81 b 7.64 b 60
Pairs of values followed by the same lowercase letter are not significantly different at the
P ≤ 0.05 level.
Table 4. Annual xylem ring increment (mm) 50 cm from the ground
line for white and red oak for the year prior to treatment and for 4
years after treatment with paclobutrazol in April 1995 using the soil
drench method, with total reduction expressed as a percentage of
untreated controls.
Table 3. Annual and total shoot growth (cm) for 2
years and total reduction expressed as a percentage
of untreated controls of sweetgum and cherrybark
oak in Louisiana untreated and treated in March
1997 with paclobutrazol using the soil drench
method.
Year 2-year Percentage
Treatment 1997 1998 total reduction
Sweetgum
Central leader
Control 83.5 a 89.3 a 172.8 a
Treated 6.5 b 1.4 b 7.9 b 95
Lateral shoots
Control 33.3 a 28.0 a 61.3 a
Treated 1.4 b 1.1 b 2.5 b 96
Cherrybark oak
Central leader
Control 36.1 a 30.0 a 66.1 a
Treated 31.2 a 4.2 b 35.4 b 46
Lateral shoots
Control 24.8 a 25.0 a 49.8 a
Treated 23.9 a 4.8 b 28.7 b 42
Pairs of values followed by the same lowercase letter are not significantly
different at the P ≤ 0.05 level.
141Journal of Arboriculture 30(3): May 2004
treatment, resulting in a total reduction compared to
untreated controls of 56% and 53%, respectively. However,
reduction of cambial growth in sweetgum occurred only in
the first growing season after PBZ treatment, but this was
sufficient to cause a 29% reduction at the base of trees
during the two year period of the study (Table 6).
DISCUSSION
The principal focus of research with tree growth retardants
has been on shoot length and growth in height. Reductions
in shoot growth reported range from a low of 20% to a high
of 90% among a broad spectrum of species including
hardwoods, conifers, shrubs, and palms (Wheeler 1987;
Year Year
Prior After treatment Total Percentage Prior After treatment Total Percentage
Treatment 1995 1996 1997 1998 1996–98 reduction Treatment 1995 1996 1997 1998 1996–98 reduction
Black walnut White ash
Within crown Within crown
Control 1.36 a 1.38 a 1.06 a 1.08 a 3.52 a Control 1.55 a 1.63 a 1.95 a 2.07 a 5.65 a
Treated 0.82 a 1.28 a 0.99 a 0.94 a 3.21 a 9 Treated 1.09 a 1.27 a 1.32 a 1.48 a 4.07 a 28
Mid-trunk (2–4 m high) Mid-trunk (2–4 m high)
Control 2.05 a 2.44 a 1.46 a 1.67 a 5.57 a Control 2.21 a 2.11 a 2.21 a 2.15 a 6.47 a
Treated 1.74 a 1.54 a 1.14 a 1.40 a 4.08 a 27 Treated 1.39 a 1.62 a 1.55 a 1.66 a 4.83 a 25
Base of trunk (10 cm above ground line) Base of trunk (10 cm above ground line)
Control 1.86 a 1.42 a 1.25 a 1.66 a 4.33 a Control 2.03 a 2.25 a 1.92 a 2.25 a 6.42 a
Treated 2.06 a 1.61 a 1.05 a 1.04 a 3.70 a 15 Treated 1.67 a 1.57 a 1.30 a 1.71 a 4.58 a 29
European black alder White oak
Within crown Within crown
Control 2.06 a 1.84 a 1.46 a 1.50 a 4.80 a Control 2.42 a 2.17 a 1.84 a 2.68 a 6.69 a
Treated 2.59 a 1.20 a 1.11 a 1.29 a 3.60 a 25 Treated 2.46 a 1.08 a 0.80 b 0.97 b 2.85 b 57
Mid-trunk (2–4 m high) Mid-trunk (2–4 m high)
Control 5.80 a 5.00 a 3.90 a 6.27 a 15.17 a Control 3.02 a 2.98 a 2.69 a 3.26 a 8.93 a
Treated 3.15 a 2.16 b 1.27 b 2.40 b 5.83 b 62 Treated 2.30 a 1.04 a 0.90 b 1.11 b 3.05 b 66
Base of trunk (10 cm above ground line) Base of trunk (10 cm above ground line)
Control 5.19 a 3.85 a 3.30 a 4.45 a 11.60 a Control 4.11 a 2.94 a 2.91 a 4.36 a 10.21 a
Treated 5.00 a 3.25 a 2.73 a 2.25 b 8.23 a 29 Treated 3.63 a 1.40 a 0.95 b 1.16 b 3.51 b 66
Red oak White pine
Within crown Within crown
Control 3.14 a 2.16 a 2.07 a 3.36 a 7.59 a Control 4.71 a 4.95 a 8.67 a 10.50 a 24.12 a
Treated 4.54 a 2.38 a 1.80 a 2.85 a 7.03 a 7 Treated 5.35 a 5.67 a 4.42 b 5.29 b 15.38 b 36
Mid-trunk (2–4 m high) Mid-trunk (2–4 m high)
Control 4.32 a 2.82 a 2.37 a 3.38 a 8.57 a Control 9.77 a 9.05 a 10.39 a 10.77 a 30.21 a
Treated 4.17 a 2.23 a 1.79 a 2.60 a 6.62 a 23 Treated 8.48 a 8.79 a 8.42 b 10.17 a 27.38 a 9
Base of trunk (10 cm above ground line) Base of trunk (10 cm above ground line)
Control 5.87 a 4.39 a 3.39 a 4.91 a 12.69 a Control 9.17 a 11.32 a 9.84 a 12.08 a 33.24 a
Treated 5.97 a 3.19 b 2.04 a 3.57 a 8.80 a 31 Treated 8.95 a 9.33 b 8.22 a 11.46 a 29.01 a 13
Sweetgum Yellow poplar
Within crown Within crown
Control 4.68 a 4.23 a 3.78 a 4.26 a 12.27 a Control 2.83 a 2.64 a 2.98 a 3.65 a 9.27 a
Treated 4.18 a 1.99 b 1.66 b 1.54 b 5.19 b 58 Treated 2.74 a 1.92 a 2.57 a 3.83 a 8.32 a 10
Mid-trunk (2–4 m high) Mid-trunk (2–4 m high)
Control 4.50 a 4.42 a 3.51 a 3.58 a 11.51 a Control 3.28 a 2.55 a 2.71 a 3.99 a 9.25 a
Treated 3.54 a 2.46 b 2.17 b 2.07 a 6.70 b 42 Treated 2.68 a 1.88 a 2.33 a 3.62 a 7.83 a 15
Base of trunk (10 cm above ground line) Base of trunk (10 cm above ground line)
Control 4.10 a 4.44 a 4.97 a 4.93 a 14.34 a Control 2.47 a 2.25 a 2.20 a 3.90 a 8.35 a
Treated 3.78 a 3.37 b 3.45 b 4.13 a 10.95 b 24 Treated 2.25 a 2.16 a 2.63 a 4.16 a 8.95 a (–7)
Pairs of values for each set of control and treated trees followed by the same lowercase letter are not significantly different at the P ≤ 0.05 level.
Table 5. Annual xylem ring increment (mm) for the year prior to treatment and for 3 years after treatment with
paclobutrazol in April 1996 using the soil injection method on eight tree species growing in Indiana, with total
reduction expressed as a percentage of untreated controls.
142 Bai et al.: Cambial and Shoot Growth in Trees Treated with Paclobutrazol
Ruter 1994; Burch et al. 1996; Hensley and Yogi 1996;
Arron et al. 1997).
Of the nine species investigated in this study, only
European black alder and cherrybark oak have not previ-
ously been reported to show reduction in shoot growth
when treated with a gibberellin synthesis inhibitor like PBZ.
Although shoot growth of European black alder was
reduced by PBZ treatment, we could not quantify the
amount because the terminal bud scale scars, which
indicate the beginning and ending point of annual shoot
growth, were not visible.
Our data demonstrate the wide variability among
species in sensitivity to PBZ mentioned above. With the
exception of sweetgum in Louisiana, only one dose rate was
used for all nine species investigated. For example, shoot
growth of black walnut was not affected by PBZ either in
the main stem or in the lateral branches at the 9.6 g a.i.
(0.34 oz.) dose rate. However, shoot growth in sweetgum at
this dose rate was markedly reduced during the three
growing seasons after treatment, whereas other species
showed reductions in shoot growth in only some growing
seasons (Tables 1, 2, and 3).
The differences in response among species found in this
study are consistent with the dose rate charts provided by
distributors of commercial formulations of PBZ that
recommend six rates categories, A through F [1.25 to 4 g
a.i. (0.04 to 0.14 oz) per inch dbh] reflecting different
sensitivities among species to PBZ. Black walnut, white ash,
white pine, and yellow poplar are recognized as the least
sensitive and are in category F for the highest dose rate.
Sweetgum is quite sensitive to paclobutrazol and is in
category B. Both red and white oak are intermediate and are
in category D. Cherrybark oak, although not yet assigned to
a category, probably should be in D based on the response
found in this study. The majority of oaks are in category E,
with a few in F. The dose rate applied in this study was that
for category D or E depending on the diameter of the tree
treated. Because of the small caliper [4 cm (0.16 in.)] of the
sweetgum trees used in Louisiana, the 4.8 g a.i. (0.17 oz)
dose rate also was that for category E.
Although the preponderance of research has focused on
shoot growth, a few observations have been recorded
concerning growth in diameter. Schnurr et al. (1996) found
that both PBZ and flurprimidol (another growth retardant
with the same mode of action as PBZ) reduced height and
stem caliper of Jack pine (Pinus banksiana) seedlings. Reduc-
tion in trunk diameter also was observed in apple (Malus
spp.) (Estabrooks 1993), peach (Prunus spp.) (Liyembani
and Taylor 1989), Colt cherry (Prunus avium ´ P.
pseudocerasus) (Asamoah and Atkinson 1985), and red maple
(Acer rubrum) (Gilliam et al. 1988) with the suppression
continuing for 2 to 3 years after treatment. The different
responses in cambial growth we found among the species to
the same dose of PBZ is consistent with the varied effects
expected from a single dose rate on shoot growth.
Cambial activity at different vertical positions in a tree
may respond to PBZ differently. For example, in white pine,
reduction in xylem ring width occurred in the cross-section
10 cm (4 in.) above the ground line in the year of treat-
ment. In the second year after treatment, reduction shifted
to the cross-sections 2 m (6.6 ft) above the ground line, as
well as within the canopy, and in the third year after
treatment, xylem accumulation only in the uppermost
cross-section was reduced.
This raises the question concerning the mode of action
of PBZ on cambial growth. It was suggested by Elfving
(1984) that the reduction in trunk growth in apple (Malus
spp.) is due either to reduced shoot growth and associated
leaf area or to direct effect of PBZ on cambial growth itself.
Because gibberellins are not known to be produced in the
cambium, but rather in leaf primordia and root tips, any
effect on cell growth in the cambial region would rely on a
disruption of the quantity of gibberellins translocated via
the phloem or xylem (Leopold and Kriedemann 1975). The
varied length of the translocation conduit between the
gibberellin source and the cambial location in the trunk or
tree crown, as well as the relative importance of leaf
primordia or root tip produced gibberellins, could account
for the differences reported.
Table 6. Annual xylem ring increment (mm) for the
year prior to treatment and for 2 years after treatment
with paclobutrazol in March 1997 using the soil
drench method for cherrybark oak and sweetgum
growing in Louisiana, with total reduction expressed
as a percentage of untreated controls.
Year
Prior After treatment Total Percentage
Treatment 1996 1997 1998 1997–98 reduction
Cherrybark oak
Within crown (2–4 m high)
Control 2.00 a 2.22 a 1.82 a 4.04 a
Treated 1.60 a 1.06 b 0.82 b 1.88 b 53
At base of tree (10 cm above ground line)
Control 4.63 a 5.86 a 5.74 a 11.60 a
Treated 4.21 a 3.19 b 1.89 b 5.08 b 56
Sweetgum
Within crown (2–4 m high)
Control 1.75 a 1.85 a 0.99 a 2.84 a
Treated 1.29 a 1.52 b 0.88 a 2.40 a 15
At base of tree (10 cm above ground line)
Control 2.91 a 1.83 a 1.76 a 3.59 a
Treated 2.01 a 1.21 b 1.34 a 2.55 b 29
Pairs of values followed by the same lowercase letter are not significantly
different at the P ≤ 0.05 level.
143Journal of Arboriculture 30(3): May 2004
The climatic differences between Indiana and Louisiana
seemed to have little influence on the efficacy of PBZ with
respect to shoot growth (Tables 1, 2, and 3). However, there
was a marked difference between the two experimental sites
in cambial growth of sweetgum. At both locations, cambial
growth was reduced in the year of treatment with PBZ, but
in Louisiana the effect did not continue into the second
growing season and the percent reduction was much less,
both in the lower trunk and within the canopy in particular
(Tables 5 and 6). This difference may be due to the applica-
tion method, which was soil injection in Indiana and soil
drench in Louisiana, rather than to climatic differences.
To provide a perspective of the potential impact of the
reduction in shoot and cambial growth reported here on
trees in the landscape, the total green aboveground weight
of an untreated and PBZ treated white oak is estimated. The
estimate is based on the average height [750 cm (25 ft)] and
trunk diameter [14 cm (5.6 in.)] of the control white oaks
at the termination of this study. These average measure-
ments were reduced by the 4-year reductions in height and
diameter determined for white oak growing in Indiana
(Tables 1 and 4) . A nonlinear regression equation specific
to white oak was used (Total green weight = 1.3426 ×
D2.2409 × H0.4275 where D is tree diameter and H is height)
(Myers and Polak 1976). Using this equation, the total
weight of an average 16-year-old untreated white oak used
in this study was found to be 109.3 kg (240 lb), whereas
the total weight of a white oak treated with PBZ 4 years
earlier was calculated to be 78.6 kg (173 lb), a 30.7 kg
(67.6 lb) reduction in total weight.
Whether the reduction in cambial growth by PBZ was a
function of a direct effect on meristematic activity and cell
development through gibberellin synthesis inhibition or an
indirect effect caused by reduced shoot growth with
consequent reduced photosynthesis and carbohydrate
partitioning was not determined and needs to be investi-
gated. Additional studies also are needed to determine if
there are any effects on cell structure, cell composition, or
other micro aspects of xylem anatomy that could affect
wood strength and susceptibility of trees to failure under
wind or ice loading.
LITERATURE CITED
Arron, G.P., S. de Becker, H.A. Stubbs, and E.W. Szeto. 1997.
An evaluation of the efficacy of tree growth regulators
paclobutrazol, flurprimidol, dikegulac, and uniconazole
for utility line clearance. J. Arboric. 23:8–16.
Asamoah, T.E.O., and D. Atkinson. 1985. The effects of
(2RS, 3RS)-1-(4-chlorophenyl)-4, 4-dimethyl-2-(1H, 2,
4 triazole-1-yl) pentan-3-ol (Paclobutrazol: PP333) and
rootpruning on the growth, eater use and response to
drought of Colt Cherry rootstocks. Plant Growth Regul.
3:37–45.
Burch, P.L., R.H. Wells, and W.N. Kline III. 1996. Red
maple and silver maple evaluated 10 years after
application of paclobutrazol tree growth regulator. J.
Arboric. 22:61–66.
Chaney, W.R. 2003. Tree growth retardants: Arborists
discovering new uses for an old tool. Tree Care Ind.
14(3):54–59.
Chaney, W.R., G.S. Premachandra, and H.A. Holt. 1996.
Physiological basis for benefits of tree growth regulators,
pp 8–18. In Proceedings, 8th Annual Conference of the
Western Plant Growth Regulator Society, Sacramento,
California, January 24–25, 1996.
Davis, T.D., and E.A. Curry. 1991. Chemical regulation of
vegetative growth. Critical Rev. Plant Sci. 10:151–188.
Elfving, D.C. 1984. Control of apple tree growth and vigour
with growth regulators. Compact Fruit Tree 17:142–
146.
Estabrooks, E.N. 1993. Paclobutrazol sprays reduce
vegetative growth and increase fruit production in
young McIntosh apple trees. Can. J. Plant Sci. 73:1127–
1135.
Fletcher, R.A., A. Gilley, N. Sankhla, and T.D. Davis. 2000.
Triazoles as plant growth regulators and stress
protectants. Hortic. Rev. 24:55–138.
Gilliam, C.H., D.C. Fare, and J.T. Eason. 1988. Control of
Acer rubrum growth with flurprimidol. J. Arboric.
14:99–101.
Grossmann, K. 1992. Plant growth retardants: Their mode of
action and benefit for physiological research, pp 788–797
In Karssen, C.M., L.C. van Loon, and D. Vreugdenhil
(Eds.). Progress in Plant Growth Regulation. Kluwer
Academic Publishers, The Netherlands. 963 pp.
Hensley, D., and J. Yogi. 1996. Growth regulation of some
tropical species. J. Arboric. 22:244–247.
Keever, G.J., W.J. Foster, and J.C. Stephenson. 1990.
Paclobutrazol inhibits growth of woody landscape
plants. J. Environ. Hortic. 8:41–47.
Kimball, S.L. 1990. The physiology of tree growth
regulators. J. Arboric. 16:39–41.
Leopold, C.A., and P.E. Kriedemann. 1975. Plant Growth
and Development (2nd ed.). McGraw-Hill, New York,
NY. 545 pp.
Liyembani, S., And B.H. Taylor. 1989. Growth and
development of young peach trees as influenced by
foliar sprays of paclobutrazol or XE-1019. HortScience
24:65–68.
McPherson, E.G., and P.J. Peper. 1995. Infrastructure repair
costs associated with street trees in 15 cities, pp 49–63.
In Watson, G.W., and D. Neely (Eds.). Trees and
Building Sites. International Society of Arboriculture,
Champaign, IL.
Myers, C., and D. Polak. 1976. Full tree weight equations
and tables for selected central hardwoods, pp 401–407.
144 Bai et al.: Cambial and Shoot Growth in Trees Treated with Paclobutrazol
In Fralish, J.S., G.T. Weaver, and R.C. Schlesinger
(Eds.). Proceedings of the Central Hardwood Forest
Conference, Carbondale, IL, October 17–19, 1976.
Rademacher, W. 2000. Growth retardants: Effects on
gibberellin biosynthesis and other metabolic pathways.
Ann. Rev. Plant Physiol. Plant Mol. Biol. 51:501–531.
Redding, K.D., P.L. Burch, and K.C. Miller. 1994. Growth,
biomass, and trim/chip time reduction following
application of flurprimidol tree growth regulator. J.
Arboric. 20:38–45.
Ruter, J.M. 1994. Growth and landscape establishment of
Pyracantha and Juniperus after application of
paclobutrazol. HortScience 29:1318–1320.
Schnurr, J.P., Z.-M. Cheng, and A.A. Boe. 1996. Effects of
plant growth regulators on sturdiness of Jack pine
seedlings. J. Environ. Hortic. 14:228–230.
Werblow, S. 1998. TGRs slow cycle buster trees. Vegetation
Manage. J. 1(10):4–6.
Wheeler, N.C. 1987. Effect of paclobutrazol on Douglas fir
and loblolly pine. J. Hortic. Sci. 62:101–106.
1Department of Computer Science
Southern University and A&M College
P.O. Box 9221
Baton Rouge, LA 70813, U.S.
2*Department of Forestry and Natural Resources
715 West State Street
Purdue University
West Lafayette, IN 47907, U.S.
3Urban Forestry Program
Southern University and A&M College
P.O. Box 11288
Baton Rouge, LA 70813, U.S.
*Corresponding author.
Résumé. Trois expériences ont été menées afin de
vérifier l’effet du paclobutrazol sur la croissance des pousses
et du cambium de neuf espèces d’arbres localisées en
Indiana et en Louisiane. Dans l’Expérience I, le chêne blanc
(Quercus alba L.) et le chêne rouge (Quercus rubra L.) en
Indiana ont été traités en avril 1995 avec du paclobutrazol
au moyen de la méthode par trempage du sol. Dans
l’Expérience II, le paclobutrazol a été appliqué en avril
1996 au moyen de la méthode par injection dans le sol sur
huit espèces poussant en Indiana, soient le chêne blanc, le
chêne rouge, le liquidambar styracifère (Liquidambar
styraciflua L.), le noyer noir (Juglans nigra L.), l’aulne
européen (Alnus glutinosa L.), le tulipier de Virginie
(Liriodendront tulipifera L.), le frêne blanc (Fraxinus
americana L.) et le pin blanc (Pinus strobus L.). Dans
l’Expérience III, deux espèces en Louisiane—liquidambar et
chêne rouge du Sud (Quercus falcata var. pagodaefolia L.)—
ont été traités par la méthode de trempage du sol en mars
1997. À la fin de la saison de croissance 1998, le taux de
croissance de la pousse et l’épaisseur des anneaux de
xylème ont été mesurés pour chacune des deux à quatre
années de croissance qui se sont écoulées depuis le
traitement avec les trois expériences. Les effets du
paclobutrazol sur la croissance du cambium et des pousses
annuelles, et ce à différentes positions sur le tronc et dans la
cime, ont varié selon les espèces et la durée du traitement.
Le paclobutrazol a diminué la croissance cambiale chez le
chêne blanc, le chêne rouge, le chêne rouge du Sud, le
liquidambar, l’aulne européen et le pin blanc de manière
variable selon l’espèce, l’élévation dans l’arbre et le nombre
d’Années après le traitement.
Zusammenfassung. Es wurden drei Experimente
durchgeführt, um die Wirkung von Paclobutrazol auf das
Trieb- und Kambiumwachstum von neun Baumarten in
Indiana und Louisiana zu erforschen. In Experiment I
wurden Quercus alba und Q. rubra in Indiana im April 1995
mit Paclobutrazol über den Boden durch Tränken
behandelt. Im Experiment II wurde Paclobutrazol im April
145Journal of Arboriculture 30(3): May 2004
1996 durch Bodeninjektion an 8 Arten (Q. alba, Q. rubra,
Liquidamber styraciflua, Juglans nigra, Alnus glutinosa,
Liriodendron tulipifera, Fraxinus americana und Pinus strobus)
in Indiana behandelt. Im Experiment III wurden im März
1997 in Louisiana 2 Baumarten (Liquidamber styraciflua und
Q. falcata) durch eine Tränkung des Bodens behandelt. Am
Ende der Wachstumsperiode 1998 wurden das jährliche
Triebwachstum und der Jahresringzuwachs für jede der 2
bis 4 Wachstumsperioden nach der Behandlung gemessen.
Die Auswirkungen von Paclobutrazol auf das sekundäre
Dickenwachstum und die jährliche Trieblänge an
verschiedenen Positionen entlang des Stammes und in der
Krone variierte mit der Baumart und der Dauer der
Behandlung. Paclobutrazol reduzierte das Dickenwachstum
bei Q. alba, Q. rubra, Q. falcata, Liquidamber styraciflua,
Alnus glutinosa und Pinus strobus in unterschiedlichem
Ausmaß je nach Art, vertikaler Lage im Baum und dem
Zeitraum nach der Behandlung.
Resumen. Se condujeron tres experimentos para
investigar el efecto del paclobutrazol en el crecimiento
cambial y de brotes de nueve especies de árboles
localizados en Indiana y Louisiana. En el experimento I, el
encino blanco (Quercus alba L.) y encino rojo (Q. rubra L.),
en Indiana, fueron tratados en Abril de 1995 con
paclobutrazol usando el método de zanjas en el suelo. En el
experimento II, se aplicó paclobutrazol en Abril de 1996,
usando el método de inyección al suelo, a ocho especies en
Indiana: encino blanco, encino rojo, liquidámbar (Liq-
uidambar styraciflua L.), nogal negro (Juglans nigra L), aile
negro europeo (Alnus glutinosa L.), chopo amarillo
(Liriodendron tulipifera L.), fresno blanco (Fraxinus
americana L.) y pino blanco (Pinus strobus L.). En el
experimento III, dos especies de liquidámbar y encino (Q.
falcata var. pagodaefolia L.), en Louisiana, fueron tratadas
con paclobutrazol por el método de zanjas en el suelo en
Marzo de 1997. A finales de la estación de crecimiento de
1998 se midió el crecimiento de los brotes y el incremento
de los anillos anuales del xilema para las cuatro estaciones
de crecimiento después de los tratamientos en los tres
experimentos. Los efectos del paclobutrazol en el
crecimiento cambial y en los brotes, en diferentes
localizaciones a lo largo del tronco y en la copa, variaron
con las especies y la duración de los tratamientos.
Paclobutrazol redujo el crecimiento cambial en encino
blanco, encino rojo, liquidámbar, aile y pino blanco, en una
cantidad variable de acuerdo a las especies, la localización
vertical en el árbol y el año después del tratamiento.