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Tree Physiology 17, 715--721
© 1997 Heron Publishing----Victoria, Canada

Control of longitudinal and cambial growth by gibberellins and
indole-3-acetic acid in current-year shoots of Pinus sylvestris
Q. WANG,1 C. H. A. LITTLE2 and P. C. ODÉN1,3
1
2
3

Department of Silviculture, Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden
Department of Natural Resources Canada, Canadian Forest Service, P.O. Box 4000, Fredericton, New Brunswick E3B 5P7, Canada
Author to whom correspondence should be addressed

Received June 6, 1996

Summary We investigated the involvement of gibberellins
(GAs) and indole-3-acetic acid (IAA) in the control of longitudinal and cambial growth in current-year shoots of Pinus
sylvestris L. Elongating terminal shoots, located at the apex of
previous-year (1-year-old) branches in the uppermost whorl on
the main stem, were variously decapitated (apical 5 to 10 mm

removed), defoliated (all developing needle fascicles removed)
and treated with exogenous GA 4/7 or IAA, or both. Shoot length
and the radial widths of xylem and phloem were measured, and
the concentrations of GA1, GA3, GA 4, GA9 and IAA in the stem
were determined by combined gas chromatography--mass
spectrometry with deuterated GAs and [13C6]-IAA as internal
standards. Decapitation decreased the production of xylem and
phloem and the IAA concentration, but did not alter either
longitudinal growth or the concentrations of GAs. Defoliation
markedly inhibited shoot elongation, as well as cambial
growth, and reduced the concentrations of GA1, GA3, GA 4,
GA9 and IAA. Application of GA 4/7 to defoliated shoots promoted longitudinal growth and phloem production, without
affecting xylem production or IAA concentration. Application
of GA 4/7 and IAA together to decapitated + defoliated shoots
increased shoot elongation, xylem and phloem production and
IAA concentration, whereas applying either substance alone
had a smaller effect or none at all. We conclude that, for
elongating current-year shoots of Pinus sylvestris, (1) both the
shoot apex and the developing needle fascicles are major
sources of the IAA present in the stem, whereas stem GAs

originate primarily in the needle fascicles, (2) GAs and IAA
are required for both shoot elongation and cambial growth, and
(3) GAs act directly in the control of shoot growth, rather than
indirectly through affecting the IAA concentration.
Keywords: decapitation, defoliation, phloem, Scots pine, shoot
elongation, vascular cambium, xylem.

Introduction
Experiments investigating the relationship between endogenous gibberellin (GA) concentrations and shoot growth, and
the effects of applying GA1, GA3, GA 4, GA 4/7 or inhibitors of
GA biosynthesis on shoot growth, indicate that GAs play a

major role in the mechanisms regulating shoot elongation and
the production of xylem and phloem by the vascular cambium
in woody species (Junttila 1991, Little and Pharis 1995, Moritz
1995, Olsen et al. 1995, Wang et al. 1995a, 1995b, 1996).
Because it is widely considered that GA1, and perhaps also
GA3, are the GAs that are active per se in the control of shoot
growth, Pharis et al. (1991) proposed that inherently rapid
growth in woody species, as reflected in height growth, stem

volume and stem dry weight, is causally related to the endogenous concentrations of these GAs. In contrast, Lanner
(1993) argued that shoot growth, particularly the elongation of
stem units in fixed-growth Pinus spp. shoots, is regulated by
the amount of auxin, presumably indole-3-acetic acid (IAA),
exported from developing needle fascicles. Primary evidence
for this view was the finding that the removal of the apical
meristem from elongating Pinus ponderosa Dougl. ex Laws.
shoots did not decrease longitudinal growth, whereas defoliation had a marked inhibitory effect (Lanner and Connor 1988).
However, the relative impacts of decapitation and defoliation
on the concentrations of IAA and GAs in the stem have not
been investigated in elongating woody shoots. Moreover, although it is well established that IAA is important for cambial
growth in shoots of both conifers and woody angiosperms
(Little and Pharis 1995), and for shoot elongation in nonwoody
angiosperms (Yang et al. 1993, McKay et al. 1994), its involvement in the control of longitudinal growth in woody species is
uncertain. Thus, exogenous IAA has been observed both to
promote the elongation of hypocotyl sections excised from
various Pinus spp. (Zakrzewski 1975, Carpita and Tarmann
1982, Terry et al. 1982) and to inhibit the elongation of currentyear Pinus sylvestris L. shoots when applied around the circumference of the subjacent previous-year (1-year-old) shoot
(Sundberg and Little 1990). In addition, both a positive relationship and no relationship have been reported between the
concentration of endogenous IAA and the rate of shoot elongation in woody species (Dunberg 1976, Wood 1983, Sandberg and Ericsson 1987, Buta et al. 1989, Rodríguez et al.

1991, Browning et al. 1992, Yang et al. 1992, Rinne et al. 1993).
To clarify the involvement of GAs and IAA, and to investigate their possible interaction (Wang et al. 1995a, 1995b), in
the regulation of longitudinal and cambial growth in current-

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WANG, LITTLE AND ODÉN

year conifer shoots, we determined the relationships among
shoot elongation, xylem and phloem production, and stem
concentrations of GAs and IAA in elongating Pinus sylvestris
shoots subjected to decapitation and defoliation. The amounts
of GA 4 and GA9, as well as of GA1 and GA3, were measured
because it is known that the GA9 → GA 4 → GA1 pathway is
an important route for GA biosynthesis in conifer shoots
(Moritz et al. 1989, Moritz and Odén 1990, Odén et al. 1995,
Wang et al. 1996). In addition, we investigated the effects on
shoot elongation, cambial growth and stem IAA concentration
of applying GA 4/7 to defoliated shoots, and GA 4/7 and IAA,
alone and together, to decapitated + defoliated shoots.

Materials and methods
Plant material and treatments
Four experiments were performed with Scots pine (Pinus
sylvestris) trees that were about 10 years old and were located
in a natural stand at the Swedish University of Agricultural
Sciences, Umeå, Sweden (63°50′ N, 20°15′ E). Early in the
shoot elongation period, groups of five or six vigorous trees
about 2-m tall and bearing four or five similar-sized previousyear (1-year-old) branches in the uppermost whorl on the main
stem were chosen. One branch per tree in a particular group
was assigned to a specific treatment, described below, such that
the average length of the current-year terminal shoot elongating at the branch apex was the same for each treatment.
In Experiment 1, five trees with four branches in the uppermost whorl were selected, and one elongating current-year
terminal shoot per tree was either (1) left untreated (control),
(2) decapitated (DC), i.e., the apical 5 mm was removed with
a scalpel and 0.8 g lanolin was applied to the cut surface, (3)
defoliated (DF), i.e., all expanding needles (3- to 4-mm long)
were removed and 200 µl of water containing 0.05% Tween 20
was painted over the whole shoot, or (4) decapitated + defoliated (DC + DF). In Experiment 1A, a supplement to Experiment 1, five trees with three branches in the uppermost whorl
were selected, and one elongating current-year terminal shoot
per tree was either (1) left untreated (control), (2) decapitated,

or (3) defoliated, as described in Experiment 1, except that
10 mm rather than 5 mm was removed from the apex of
decapitated shoots. In Experiment 2, six trees with five
branches in the uppermost whorl were selected, and one elongating current-year terminal shoot per tree either was left
untreated (control) or was defoliated (DF) and painted with
200 µl of water containing 0, 0.1, 1 or 10 mg GA 4/7 l −1 and
0.05% Tween 20. In Experiment 3, five trees with five
branches in the uppermost whorl were selected, and one elongating current-year terminal shoot per tree either was left
untreated (control) or was decapitated + defoliated (DC + DF)
and treated apically with 0.8 g lanolin containing 0 or 1 mg
IAA g −1 and painted on all sides with 200 µl of water containing 0 or 1 mg GA 4/7 l −1 and 0.05% Tween 20. All applications
were renewed at weekly intervals. After 3 weeks (Experiments
1, 2 and 3), or after 0, 1, 2 and 3 weeks (Experiment 1A), the
lengths of the current-year terminal shoots were measured, and
the shoots were harvested to measure cambial growth and the

concentrations of IAA (Experiments 1, 1A, 2 and 3) and GA1,
GA3, GA 4, and GA9 (Experiment 1). At the time of harvest, the
basal 2 cm of each shoot was removed for the cambial growth
measurement, and the remaining portion was defoliated (if

needles were present) and saved for hormone extraction. The
sample for hormone measurement was obtained by peeling the
bark, scraping the exposed surface on the xylem side with a
scalpel, and pooling the bark peelings and the scrapings. This
material was frozen in liquid nitrogen and stored at −80 °C
until analyzed.
Measurement of shoot elongation and cambial growth
The length of each current-year terminal shoot was expressed
as a percentage of its initial length (mean of 63, 120, 54 and
74 mm in Experiments 1, 1A, 2 and 3, respectively), to obviate
initial differences in shoot size.
Cambial growth was measured as the radial widths of xylem
and phloem, which were recorded at eight equidistant points
around the circumference of transverse handcut sections obtained 2 cm from the basal end of the current-year terminal
shoot. The sections were stained in a saturated aqueous solution of phloroglucinol in 20% hydrochloric acid and mounted
in glycerol on microscope slides. The xylem measurement
started from the outside of the pith and included only those
xylem cells that reacted with the stain, i.e., in which at least
some lignification had occurred. The phloem measurement
spanned the cambium to the outside of the phloem, with the

phloem accounting for at least 95% of the total width measured.
Measurement of IAA
The concentration of IAA was determined as described by
Sundberg (1990). In brief, after homogenizing the stem sample
in liquid nitrogen, an aliquot (about 0.5 g fresh weight) was
extracted at 4 °C for 2 h in 0.05 M sodium phosphate buffer,
pH 7.0, containing 0.02% diethyldithiocarbamic acid (Sigma,
St. Louis, MO) as antioxidant and 50 ng [13C6]-IAA (Cambridge Isotopes Laboratories, Andover, MA) as internal standard. The extract was purified by neutral and acidic diethyl ether
partitioning, and the acidic ether-soluble part was methylated
and subjected to reverse-phase high-performance liquid chromatography (HPLC). The HPLC mobile phase consisted of
50% methanol in 1% acetic acid and was delivered at a flow
rate of 1 ml min −1 by Waters M 501 pumps and a M 680
gradient controller (Waters Associates AB, Partille, Sweden).
The sample was introduced by a Waters 712 WISP onto a
10 cm × 8 mm i.d. 4-µm Nova-Pak C18 cartridge fitted in a
RCM 8 × 10 module (Waters Associates AB). The IAA-methyl
ester fraction was collected, silylated and injected splitless into
a Hewlett-Packard 5890 gas chromatograph (GC) (HewlettPackard, Palo Alto, CA) fitted with a fused silica glass capillary column (25 m long, 0.25 mm i.d.) with a chemically
bonded 0.25 µm SE-30 stationary phase (Quadrex, New Haven, CT). Helium was used as a carrier gas at a flow rate of
1 ml min −1. The GC was linked to a Hewlett-Packard 5770

mass selective detector equipped with a 9133 data system.
Quantifications were made by gas chromatography-selected

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CONTROL OF SHOOT GROWTH BY GAS AND IAA

ion monitoring-mass spectrometry (GC--SIM--MS). The ratio
of m/z 202:208 was used to calculate the endogenous concentration of IAA by the isotope dilution equation (Cohen et al. 1986).
Measurement of GAs
To measure the concentrations of GA1, GA3, GA 4 and GA9, an
aliquot (about 1 g fresh weight) of the homogenized stem
sample was extracted at 4 °C for 4 h in 80% aqueous methanol
containing 0.02% (w/v) diethyldithiocarbamic acid as antioxidant and 10 ng 17,17[2H2]-GA9, 100 pg 17,17[2H2]-GA 4,
100 pg 17,17[2H2]-GA3 and 100 pg 17,17[2H2]-GA1 as internal
standards. The methanol was filtered, and the tissue residue
was washed with fresh 80% methanol and re-extracted under
the same conditions. After evaporating the methanol from the
combined filtrates at reduced pressure at 35 °C, the aqueous
residue was applied in 0.5 M sodium phosphate buffer, pH 8.0,

to a polyvinylpolypyrrolidone column (20 × 1.0 cm i.d.) and
eluted with 0.1 M sodium phosphate buffer, pH 8.0. The eluate
(0--75 ml) was acidified to pH 2.7 with 6 M HCl and extracted
five times with 75 ml ethyl acetate. The acidic ethyl acetate
extracts were combined, the remaining water was removed by
freezing and filtering, and the organic phase was evaporated to
dryness. The residue was dissolved in 5 ml water, adjusted to
pH 8.0, and applied to a 40 × 20 mm i.d. QAE Sephadex A-25
column (Pharmacia, Uppsala, Sweden). The free GAs were
eluted with 50 ml of 0.2 M formic acid, which was run through
a 100-mg Bond Elute C18 column (Sorbent AB, Västra
Frölunda, Sweden) that had been washed with methanol and
0.2 M formic acid. The column was eluted with 5 ml of
methanol to collect the free GAs, and the methanol was evaporated to dryness. The samples were dissolved in about 100 µl
methanol and subjected to HPLC. The chromatograph consisted of two Waters M 501 pumps connected to the column by
a 7125 injector with a 250-µl loop (Rheodyne, Cotati, CA).
The pumps were controlled by a Waters M 680 gradient controller. The samples were purified by a reversed-phase column
packed with 5 µm Nucleosil C18 (200 × 4.6 mm i.d.) (Skandinaviska Genetec AB, Kungsbacka, Sweden). The mobile phase
consisted of a 60-min linear gradient from water and acetic
acid (99:1%, v/v) to methanol and acetic acid (99:1%, v/v).

The samples were run at a flow rate of 1 ml min −1 and 1-ml
fractions were collected. Fractions known to contain the GAs
of interest were collected, methylated with ethereal diazomethane and trimethylsilylated in 15 µl N-methyl-Ntrimethylsilyltrifluoroacetamide (MSTFA) and 15 µl pyridine
at 70 °C for 30 min. The derivatization mixture was reduced to
dryness and dissolved in about 10 µl n-heptane, and an aliquot
was injected splitless into a Hewlett-Packard 5890 GC fitted
with a fused silica glass capillary column (25 m long, 0.25 mm
i.d.) with a chemically bonded 0.25 µm SE-30 stationary
phase. The injector temperature was 270 °C. The column
temperature was held at 60 °C for 2 min, then increased by
20 °C min −1 to 200 °C, and by 4 °C min −1 to 260 °C. The
column effluent was introduced into the ion source of a JMSSX102 mass spectrometer (JEOL, Tokyo, Japan). The interface and ion source temperatures were 270 °C and 250 °C,
respectively. Ions were generated with 70 eV at an ionization

717

current of 300 µA. Quantifications were made by selected
reaction monitoring (Moritz and Olsen 1995). The reactions
m/z 298 to m/z 270 and m/z 300 to m/z 272 for GA9, m/z 418
to m/z 284 and m/z 420 to m/z 286 for GA 4, m/z 506 to m/z
448 and m/z 508 to m/z 450 for GA1, and m/z 504 to m/z 446
and m/z 506 to m/z 448 for GA3 were recorded. All data were
processed by a JEOL MS--MP7010D data system.
Statistical analysis
Analysis of variance was applied to each data set, and where
appropriate the difference between means was determined by
Duncan’s multiple range test. In the Figures, within each
measured variable, means accompanied by the same letter are
not significantly different at P ≤ 0.05.

Results
In Experiment 1, the control shoots elongated throughout the
experimental period, and by the end of the study they had
increased threefold in length and produced wide bands of
xylem and phloem at their base (Figure 1). Decapitation did

Figure 1. Longitudinal growth and production of xylem and phloem in
current-year terminal shoots that were either untreated (control), decapitated (DC), defoliated (DF) or decapitated + defoliated (DC +
DF). Experiment 1, means ± SE, n = 5.

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WANG, LITTLE AND ODÉN

not affect longitudinal growth, but it decreased production of
xylem and phoem by about one-third. In contrast, defoliation
almost totally arrested shoot elongation, and also reduced
cambial growth. The combined decapitation + defoliation
treatment inhibited longitudinal growth and xylem production
to the same extent as the defoliation treatment, but it reduced
phloem production more than defoliation did alone. Compared
with controls, decapitation reduced the IAA concentration in
the stem without altering the concentrations of GA9, GA 4, GA3
or GA1, whereas the defoliation and decapitation + defoliation
treatments decreased the IAA concentration to an even greater
extent than the decapitation treatment and also lowered the
concentrations of all the GAs (Figure 2). On a percentage
basis, decapitation and defoliation both decreased the IAA
concentration more than the concentration of any GA, and
defoliation reduced the concentration of GA9 more than GA 4,
GA3 or GA1.

Figure 2. Concentrations of IAA, GA9, GA 4, GA1 and GA3 in the stem
of current-year terminal shoots that were either untreated (control),
decapitated (DC), defoliated (DF) or decapitated + defoliated (DC +
DF). Experiment 1, means ± SE, n = 5.

In Experiment 1A, the inhibition of longitudinal growth
induced by defoliation was evident 1 week after the start of
treatment, as were the reductions in cambial growth and stem
IAA concentration caused by decapitation and by defoliation
(Figure 3). In control shoots, the IAA concentration was threeto fourfold higher in the stem than in the developing needle
fascicles. It was also observed that defoliating the upper half
of an elongating current-year shoot decreased longitudinal
growth and the stem IAA concentration more than did defoliating the lower half of an elongating current-year shoot (data
not shown).
In Experiment 2, defoliation decreased shoot elongation,
cambial growth, and the stem IAA concentration, as was also

Figure 3. Longitudinal growth, production of xylem and phloem, and
IAA concentration in the stem (s) or needle fascicles (n) of currentyear terminal shoots that were either untreated (control) ( h, e for
stems and needle fascicles, respectively), decapitated (DC) ( o) or
defoliated (DF) (s). Experiment 1A, means ± SE, n = 5.

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CONTROL OF SHOOT GROWTH BY GAS AND IAA

719

observed in Experiments 1 and 1A, except that the decrease in
xylem production was not statistically significant (Figure 4).
Applying GA 4/7 to defoliated shoots promoted longitudinal
growth, the optimal concentration being 1 mg GA 4/7 l −1. Exogenous GA 4/7 also stimulated phloem production, and the
stimulation increased throughout the entire concentration
range tested. The application of GA 4/7 did not increase xylem
production or IAA concentration.
In Experiment 3, decapitation + defoliation in the absence
of exogenous hormones reduced shoot elongation, xylem production and the IAA concentration as observed in Experiment 1; however, the treatment did not significantly decrease
phloem production (Figure 5). Applying IAA or GA 4/7 alone
to decapitated + defoliated shoots promoted the production of
xylem and phloem, without significantly increasing longitudinal growth or the IAA concentration. In contrast, the application of IAA and GA 4/7 together increased shoot elongation,
cambial growth and the IAA concentration.

Figure 5. Longitudinal growth, production of xylem and phloem, and
stem concentration of IAA in current-year terminal shoots that were
either untreated (control) or decapitated + defoliated (DC + DF) and
treated with 0 or 1 mg IAA g −1 lanolin and 0 or 1 mg GA 4/7 l −1 water
containing 0.05% Tween 20. Experiment 3, means ± SE, n = 5.

Discussion

Figure 4. Longitudinal growth, production of xylem and phloem, and
stem concentration of IAA in current-year terminal shoots that were
either untreated (control) or defoliated and painted with 0, 0.1, 1 or
10 mg GA 4/7 l −1 water containing 0.05% Tween 20. Experiment 2,
means ± SE, n = 6.

The relative importance of the shoot apex and the developing
needle fascicles as sources of the GAs and IAA present in the
stem of elongating current-year Pinus sylvestris shoots can be
deduced from the extent to which decapitation and defoliation
altered the concentrations of GAs and IAA in the stem. Decapitation did not affect the concentrations of GAs, whereas
defoliation markedly reduced the concentrations of all GAs
measured, particularly GA9 (Figure 2). However, decapitation
and defoliation both decreased the IAA concentration, and the
impact of defoliation relative to decapitation decreased with
increasing shoot length at the time of the treatment (compare
Figures 2 and 3, in which the initial shoot length was 63 and
120 mm, respectively). High concentrations of GAs and IAA
have been measured in current-year needles of several conifers, including Pinus sylvestris (Figure 3, Little and Wareing
1981, Savidge and Wareing 1984, Pharis et al. 1992, Wang
et al. 1992). We conclude that the IAA found in elongating

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WANG, LITTLE AND ODÉN

stems originates in both the shoot apex and needle fascicles,
and the contribution by needle fascicles declines as the shoot
ages, whereas needle fascicles are the major source of the
GAs. Based on decapitation and defoliation experiments with
1-year-old shoots of conifers, it was concluded that expanding
current-year shoots were the major source of IAA present in
the 1-year-old stem, with the 1-year-old foliage being a relatively minor source (Little and Wareing 1981, Savidge and
Wareing 1982, Sundberg and Little 1987, 1990). Whether the
shoot apex and needle fascicles actually synthesize GA9 and
IAA, or supply a precursor or conjugate that is metabolized in
stem tissues, has not been determined. Also, it is not known if
the reduction in GA 4, GA3 and GA1 concentrations in the stem
of defoliated shoots (Figure 2) is the result of (1) removal of
the needle fascicles resulting in a decreased import into the
stem of these specific GAs or their precursors (GA9 for GA 4
and GA1 and an unknown presursor for GA3 (Wang et al.
1996)), or (2) an increase in their conjugation or degradation
in the stem.
Our results provide additional evidence that GAs are involved in the control of both longitudinal and cambial growth
in conifers (Little and Pharis 1995, Moritz 1995, Wang et al.
1995a, 1995b). First, comparing the control, decapitated and
defoliated shoots in Experiment 1, stem GA concentrations
were positively related to shoot elongation and, less obviously,
to the production of xylem and phloem (Figures 1 and 2).
Second, applying GA 4/7 alone to shoots deficient in endogenous GAs, i.e., defoliated shoots, stimulated longitudinal
growth and phloem production (Figure 4). The most effective
GA 4/7 concentration was lower for shoot elongation than for
phloem production (1 mg l −1 and 10 mg l −1, respectively),
which we attribute to the 10 mg l −1 concentration resulting in
a supra-optimal amount of GA reaching the shoot apex, but not
the cambial region (Wang et al. 1992, 1996). Third, the application of GA 4/7 alone to shoots deficient in both GAs and IAA,
i.e., decapitated + defoliated shoots, increased the production
of xylem and phloem and also tended to promote shoot elongation (Figure 5). In all cases where the application of GA 4/7
alone stimulated shoot growth, the stem IAA concentration
was not significantly increased (Figures 4 and 5), thus providing additional evidence that GAs control shoot growth directly,
rather than indirectly through altering the IAA concentration
(Wang et al. 1995a, 1995b).
Four observations support the view that IAA plays an important role in the regulation of both shoot elongation and cambial
growth in conifers and, moreover, is required for the action of
GAs in these processes. First, decapitation + defoliation concomitantly decreased xylem and phloem production, longitudinal growth and the stem IAA concentration, whereas
applying IAA apically to decapitated + defoliated shoots increased cambial growth and partially restored shoot elongation
and IAA concentration (Figures 1, 2 and 5). Similarly, it was
observed that exogenous IAA countered the reductions in
tracheid production and cambial region IAA concentration
induced by debudding in previous-year conifer shoots (Little
and Wareing 1981, Sundberg and Little 1987, 1990). Second,
decapitation decreased both cambial growth and the stem IAA

concentration (Figures 1 to 3). However, decapitation did not
inhibit shoot elongation, presumably because the supply of
endogenous IAA from the needle fascicles was sufficient to
maintain normal subapical meristem activity, although it was
limiting for cambial growth. Third, applying IAA together
with GA 4/7 to decapitated + defoliated shoots enhanced the
stimulatory effect of GA 4/7 or IAA alone on shoot growth
(Figure 5). Similarly, exogenous GA3 or GA 4/7 promoted cambial growth in 1-year-old shoots of Pinus sylvestris (Hejnowicz
and Tomaszewski 1969, Wang et al. 1995a, 1995b) and other
woody species (Little and Pharis 1995) only when a source of
endogenous or exogenous IAA was present. Fourth, the
1 mg l −1 GA 4/7 treatment promoted shoot elongation in defoliated shoots (Figure 4) relatively more than in decapitated +
defoliated shoots (Figure 5). We speculate that this difference
in response was caused by a greater supply of endogenous IAA
in defoliated shoots than in decapitated + defoliated shoots,
because decapitation removed only one IAA source whereas
decapitation + defoliation removed two IAA sources. The
marked increase in phloem to xylem ratio in defoliated shoots
treated with 1 or 10 mg l −1 GA 4/7 (Figure 4), and in decapitated
+ defoliated shoots treated with GA 4/7 alone (Figure 5), can be
attributed to the induction of a high GA/low IAA ratio in the
cambial region favoring phloem production (Digby and Wareing 1966).
We conclude that GAs and IAA are involved in the regulation of both longitudinal growth, as manifested in stem unit
elongation, and cambial growth, as reflected in xylem and
phloem production, in the current-year shoots of Pinus
sylvestris. However, because the potential for shoot growth in
Pinus spp. depends more on the number of stem units than on
their length (e.g., Lanner and Connor 1988, Norgren et al.
1996), it remains to be determined if GAs and IAA also play a
role in the mechanism controlling the initiation of stem unit
primordia at the shoot apical meristem (Lanner 1993).

Acknowledgments
We thank Karin Ljung for technical assistance, Thomas Moritz for
helpful advice, and the Swedish Council for Forestry and Agricultural
Research and the Jacob Wallenberg Research Foundation/Lars-Erik
Thunholm Foundation for Promotion of Scientific Research for financial support.

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