Plant Article

182

American Journal of Botany 87(2): 182–190. 2000.

EXECUTION OF THE AUXIN REPLACEMENT APICAL

DOMINANCE EXPERIMENT IN TEMPERATE

WOODY SPECIES1

MORRIS G. CLINE

The Ohio State University, Department of Plant Biology, Columbus, Ohio 43210 USA

The classic Thimann-Skoog or auxin replacement apical dominance test of exogenous auxin repression of lateral budoutgrowth was successfully executed in both seedlings and older trees of white ash, green ash, and red oak under thefollowing conditions: (1) decapitation of a twig apex and auxin replacement were carried out during spring flush, (2) thedecapitation was in the previous season’s overwintered wood, and (3) the point of decapitation was below the upper largeirrepressible lateral buds but above the lower repressible lateral buds. Although it has been suggested that neither auxin, theterminal bud, nor apical dominance have control over the outgrowth of the irrepressible buds during spring flush, there isevidence in the present study that indicates that such control over the repressible buds exists. In seedlings, second-orderbranching, which resulted from decapitation of elongating current shoots, was also inhibited by exogenous auxin in thethree species. Hence, the auxin replacement experiments did work on year-old proleptic buds (of branches of older trees)that would have entered the bud bank and also on current buds of seedlings. Cytokinin treatments were ineffectual inpromoting bud growth.

Key words: apical dominance; auxin; decapitation; Fraxinus americana; Fraxinus pennsylvanica; lateral bud growth;Quercus rubra.

Apical dominance is the control exerted by the shootapex over the outgrowth of the lateral buds. In 1933 Thi-mann and Skoog removed the shoot apex of the herba-ceous species Vicia faba and repressed subsequent lateralbud outgrowth by application of auxin to the top of thedecapitated stem. This classical auxin replacement ex-periment, which works in many herbaceous plants (Ta-mas, 1995; Cline, 1996), is commonly cited as evidencefor a repressive role of apically produced auxin in thecontrol of lateral bud growth in apical dominance. Sub-sequent studies also have suggested an interaction of cy-tokinin with auxin in bud growth (Sachs and Thimann,1967).

Because of substantial evidence (e.g., forking in shootsfollowing injury to terminal buds) for apical dominancein woody plants, models of apical dominance employedfor herbaceous plants are generally used to explain apicaldominance in these plants as well. This extrapolation isdone in spite of the recognition of the significantly in-creased morphological and physiological complexity ofwoody species with respect to such factors as perennialgrowth habits, the predominance of woody vascular tis-sue, and endodormancy.

The way that these herbaceous models may be appliedto trees has been considered by a number of authors.Brown, Alpine, and Kormanik (1967) have pointed outthat the term ‘‘apical dominance’’ as used for herbaceousplants by Thimann and Skoog should not be employedfor a whole tree but only for the current year’s growth.Similarly, Wareing (1970) has explained that it would be

1 Manuscript received 16 March 1999; revision accepted 11 June1999.

The author thanks Drs. D. Struve, M. Larson, B. Wilson, and R.Harmer for their helpful input as well as to Ms. Cathy Maupin, Super-intendent of Building and Grounds at the Ohio State University for theuse of selected trees.

difficult to envision auxin moving down a considerabledistance from the main shoot apex and then acropetallyfar out to the apices of branches to control lateral budgrowth. Sundberg and Uggla (1998) have also pointedout that in tall trees with the slow rate of polar auxintransport (e.g., 1 cm/h), it takes months for an auxin mol-ecule to move down the trunk from the tip. This com-plicates the explanation of apical control by auxin. Otherworkers (Romberger, 1963; Tomlinson and Gill, 1973;Timell, 1986; Wilson, 1990; Bollmark et al., 1995) havealso recognized the difficulty of explaining the mecha-nism of auxin control of apical dominance at the whole-tree level.

To what degree can we expect the auxin replacementexperiment, which works so well with most herbaceousplants, to be applicable to trees in the field? Although itmay be presumed that the response in the current year’sgrowth of woody plants is similar to that in herbaceousplants, such studies are lacking on intact branches oftrees, particularly of trees beyond the seedling stage.Many of the reports involve in vitro systems, stem cut-tings, and indirect approaches.

Gunckel, Thimann, and Wetmore (1949) demonstratedthat 1% NAA (naphthaleneacetic acid) applied to the cutsurface of decapitated 3-yr-old Ginkgo seedlings inhib-ited short lateral shoots from growing into long lateralshoots. However, since the short laterals have alreadygrown out to some extent it could be argued that this isnot a clear case of repression of apical dominance releasebut rather one of subsequent apical control (Cline, 1997).Similarly, after removing all the laterals except the lon-gest in the whorl of white pine saplings, Little (1969)treated the decapitated terminal with auxin (5–20 mg/g)and observed the prevention of a small amount of com-pensatory growth in the remaining lateral, which was in-terpreted in terms of auxin-directed nutrient diversion.

February 2000] 183CLINE—APICAL DOMINANCE EXPERIMENT IN WOODY SPECIES

Wang, Faust, and Line (1994) found that IAA (indoleac-tic acid, 10–100 nmol) injected into the decapitated tipsof 25-cm-long excised apple shoots collected in Augustand placed in glass jars containing distilled water re-pressed the growth of the most distal lateral buds. Yangand Read (1991) in their in vitro culture experimentshave found a reduction in the percentage of privet budbreak and shoot elongation when IAA or NAA was add-ed.

Wignall and Browning (1988) have reported NAA (20mg/dm) inhibition and BA (benzyladenine, 20 mg/dm)reversal of this inhibition of epicormic bud developmentin stem explants of Quercus robur over a 6-wk period.Leaky and Longman (1986) found a 85–35% reductionin actively growing buds in rooting stem cuttings of Tri-plochiton scleroxylon with 250-mg NAA injections; 1 mgBA had no effect. Wilson (1979) reported that IBA (in-dolebutyric acid 1024 M) totally inhibited epicormic budgrowth in decapitated stem segments of Acer pennsyl-vanicum L., whereas GA (gibberellic acid) and BA hadno effect on initial bud release. Bowersox and Ward(1968) found NAA, IBA, and IAA to equally inhibit ep-icormic branching in white oak stem segments. Recently,House et al. (1998) reported that auxin (NAA, IBA, IAA,and 2,4-dichlorophenoxyacetic acid, 10–0.01%) applica-tions in lanolin to exposed cut surfaces of 3-yr-old hooppine (Araucaria cunninghamii) stocks could temporarilyrepress the outgrowth of orthotropic replacement shoots.

The objective of the present study was to determinewhether the auxin replacement experiment would workon seedlings and on intact branches of older trees (greenash, white ash, and red oak) and to elucidate the role ofauxin in apical dominance in these woody species. Thesethree species are ring-porous with a fixed growth periodin mid-spring and were selected because of their vigorousshoot growth. Cytokinin (BA) effects were also analyzed.

MATERIALS AND METHODS

For the greenhouse seedling studies, 100 seedlings (beginning theirsecond year) of white ash (Fraxinus americana var. americana L.),green ash [Fraxinus pennsylvanica var. subintegerrima (Vahl.) Fern],and red oak (Quercus rubra L.) were obtained (bare root) from theoutside plantings of the Ohio Division of Forestry nursery at Zanesvillein late March 1996 and were planted in (3.8 L, one gallon) pots con-taining Pro-mix, a general-purpose peat-vermiculite growing mediumon 2 April and placed in a greenhouse (168–328 C) with supplementaryGeneral Electric 400-W mercury vapor lamps (total irradiance up to3300 mmol·m22 s21).

The first of three white ash seedling experiments involving decapi-tation, 1% IAA (;60 mmol/L) or NAA (;45 mmol/L) in lanolin and100–200 mg/L (444–889 mmol/L) BA spray treatments (5–10 plantseach) was begun 9 April as the buds were beginning to grow out. TheIAA, NAA, and BA were obtained from Sigma Chemical Co., (St.Louis, Missouri, USA). IAA was dissolved in warm alcohol before mix-ing with hot lanolin. The BA in powder form was dissolved in waterwith glacial acetic acid. BA in solution (1 mg/mL) obtained from Sigmawas also used. The stems were decapitated ;0.5 cm above the fourthnode down from the terminal bud, and auxin was immediately appliedto the top of the stump of the decapitated stem. The BA spray wasapplied to the shoot at least once a week for 2–3 wk. The concentrationof the BA was increased from 444 to 889 mmol/L (pH 7.2) for the lasttwo treatments to increase the likelihood of a a response. The seedlingsfor the second-order branching experiment were taken from the decap-

itated controls of these three white ash experiments. The two growinghighest lateral shoots (resulting from previous decapitation) were them-selves decapitated 7 May below the terminal bud (see Fig. 2). One wastreated with lanolin and the other was treated with auxin (1% IAA orNAA) in lanolin. Determinations were then made of second order out-growth of the next two lower lateral buds on each of the two shoots.The first of two green ash experiments were begun on 18 April andcarried out as with those of white ash.

There was one major experiment carried out for red oak (26 April–8 May) similar to those of the ashes. The oak seedlings were, withrespect to overall morphology, a more heterogenous group than theashes. There were 20 plants employed in three experiments involvingsecond-order growth, which began 14 May and involved seven decap-itated plants. There was a single sprouting lateral shoot on each plant,which in turn was decapitated, and subsequent second-order outgrowthof lower lateral buds was analyzed. In the other two studies of sevenand six plants each, there were two sprouting lateral shoots/plant, onetreated with lanolin and the other with auxin (1% IAA or NAA).

For the 1996 tree field studies, the following selections were made:nine white ash (;15-yr-old), four green ash and four red oak trees, both5- to 6-yr-old. The selections were made before bud break had occurred.As it turned out, all the white ash were vigorous healthy trees. However,a few of the green ash and red oak trees had some dead and unhealthybranches. Data from these latter branches were included in the averageddata.

The white ash decapitation and auxin treatments were begun on 20April, coinciding with the beginning of spring flush and floral bud open-ing, which preceded that of the vegetative buds. For these field studiesthe more persistent auxin, NAA, was used instead of IAA. Decapitationwas carried out above the second or third node below the terminal budon a twig of at least three or four nodes, thus leaving buds located onthe lower nodes for possible sprouting (see Fig. 4). One percent NAAin lanolin was immediately applied to the stump of specific decapitatedtwigs. Final measurements were made on 19 August. No BA treatmentswere given.

The green ash NAA field treatments were begun on 24 April in asimilar fashion to those of white ash. The decapitation was done abovethe third or fourth node down from the terminal bud on a twig contain-ing three or four nodes thus ensuring that buds remained at the lowestnode for subsequent outgrowth by decapitation. Measurements weremade on 16 May and 22 August. Four BA (2.2 mol/L; pH 3.3–3.5)spray treatments were given over a 2-wk period beginning 18 May.

The red oak decapitation and NAA treatments in the field were begunon 29 April as buds were beginning to sprout with a variable decapi-tation point partway down the 1995 growth wood just above medium-to small-size lateral buds. Irrepressible large lateral buds were manuallyremoved. On the following day, 30 April, more twigs were decapitated,this time in the pre-1995 wood, i.e., the 1994 or even 1993 wood justabove any persistent medium- to small-sized lateral buds. Measurementswere made on 16 May and 22 August. BA spray treatments were givento select intact shoots as described for green ash.

RESULTS

Greenhouse studies of seedlings—White ash—Over-wintered seedlings with a mean height of 39.2 6 6.9 cm(N 5 8) were brought into the greenhouse on 2 April.Lateral buds began to grow within a week, at which timetreatments were applied. In a majority of the intact seed-lings lateral bud growth occurred at the upper nodes. De-capitation at any location on the stem released lateralbuds at the node below the point of decapitation (Table1). Stems were decapitated in early and mid-April (as theterminal buds were beginning to grow out) above thefourth node down from the terminal bud (Fig. 1). Therelatively few large lateral buds that were present on the

184 [Vol. 87AMERICAN JOURNAL OF BOTANY


Fig. 2. A test for decapitation release and auxin effects on out-growth of second-order (current) lateral buds in white and green ashseedlings.

←

Fig. 1. One-year-old seedlings. (A) White ash; (B) green ash; (C) red oak. Left, intact; middle, decapitated; right, decapitated 11% NAA inlanolin added to top of stem stump 2 wk after decapitation.

upper portion of intact stems generally grew out withoutdecapitation. The application of IAA in lanolin on thestem following decapitation resulted in the strong inhi-bition of growth of the lower and smaller lateral buds.Periodic spray treatments with cytokinin (BA) to the in-tact shoots had no promotive effect on the lateral buds,but a moderately toxic wrinkling effect was observed onthe leaves. The main stem elongated 28.3 6 6.4 cm (N5 8) during the 1996 growing season.

When elongating primary lateral shoots resulting fromdecapitation of the main stem were themselves decapi-tated partway down (Fig. 2), the formation of second-order branches from outgrown lower lateral buds, whichwere repressible by auxin, was commonly observed (Ta-ble 2). NAA was more effective than IAA in such re-pression.

Green ash—The results with green ash seedlings weresimilar in many respects to those obtained with white ash.However, in the intact green ash seedlings with an initialmean height of 38.1 6 4.7 cm (N 5 8), there was ab-solutely no outgrowth in any of the buds of the controlseedlings observed except for that of the terminal bud.Stems were decapitated above the first node below theterminal bud. Decapitation released lateral shoot growthat several top nodes within 6 d, which was repressible byboth 1% IAA or NAA in lanolin, particularly the latter(Fig. 1, Table 1). The BA spray had no effect other thanto cause wrinkling of the leaves. The main shoot elon-gated 19.1 6 7.0 cm (N 5 8) during the growing season.Green ash exhibited second-order branching (Fig. 2, Ta-ble 2) following decapitation to a greater extent than didwhite ash.

Red oak—The main stems of the red oak seedlingswere leaning and not as straight and upright as those ofthe ashes. The mean initial height was 40.7 6 7.6 cm (N5 8). Distribution of the lateral buds, heterogenous insize, on the stem is alternate and is not uniform as com-pared to that of the ashes. Near the stem apex, there wasoften an unsymmetrical cluster of buds. Although therewas more sprouting on the upper portion of the shootthan lower down, it was not unusual to observe a largegrowing lateral bud lower down. These large buds weremanually removed since they would have grown out any-way, having little or no sensitivity to exogenous auxintreatment at this late stage of development. Decapitationreleased lateral buds high on the stem and close to thepoint of decapitation. The inhibitory effect of the NAAtreatment on lateral bud growth was marginal (Fig. 1,Table 1). The main stems elongated 9.8 1 4.2 cm (N 58) during the growing season. Second-order outgrowth,repressible by auxin (Table 2), was also observed in redoak. No BA effects were seen.

Field studies with trees—White ash—There weresome significant differences in the responses between thestems of the greenhouse seedlings and the twigs on the

branches of the 15-yr-old outdoor trees during 1996. Thetwigs of the latter were oriented in a more or less hori-zontal position, whereas the stems of the greenhouseseedlings were upright. Stems of the greenhouse seed-lings averaged 40 cm with ten nodes, whereas the 1995wood of the older tree branches averaged 16 cm withthree or four nodes in the terminal shoots and two orthree nodes in the side shoots. Another obvious differ-ence was the widespread presence of floral buds in theolder trees.

The terminal bud was always vegetative and alwaysgrew out if present (Fig. 3). Laterally adjacent to the ter-minal bud are two axillary buds, which often do not growout (Gill, 1971). Nearly half of the axillary buds at thefirst node below the terminal bud were vegetative and didgrow out (data not shown). Many of the buds at the sec-ond and third nodes down were floral. Within a fewweeks after sprouting, the floral shoots on all of the ninetrees except one had abscised. Likewise, all external la-tent buds eventually appeared to have abscised so that atthe end of the 1996 growing season, the only visible ap-pendages left on the 1996 wood were vegetative shootsat the terminal position and at some of the upper nodes.

Stems were decapitated at budbreak in April above thesecond or third node below the terminal bud, thus elim-inating the irrepressible buds (Fig. 4, Table 3). Since thetotal number of nodes below the terminal bud usuallyranged from two to four, the point of decapitation wasabove the first or second node from the base of the 1995twig. The breaking of the lower smaller buds, whichwould have remained dormant without decapitation (i.e.,repressible), indicated that apical dominance had been


TABLE 1. Greenhouse studies on effects of decapitation, IAA, NAA,and BA on stems in 1-yr-old seedlings. The white ash main stemwas decapitated just above the fourth node down from the apex.The green ash and red oak stems were decapitated just above thefirst node down from the shoot apex. BA concentration was in-creased from 444 mol/L to 889 mol/L on day 12 and day 2 withwhite ash and green ash, respectively.

Numberof seed-

lings

Numberof seed-

lingswithout-

growinglateralbuds

Mean numberof outgrowinglateral buds/

seedling(6 1 SD)

Mean length(cm) of

outgrowinglateral buds(6 1 SD)

White ash (18 d after decapitation)IntactDecapitatedDecapitated 1 1%

IAAIntact 1 BA

(889 mol/L)

710

9

7

110

1

4

0.3 6 0.84.0 6 1.3

0.2 6 0.7

1.9 6 2.0

4.8 6 5.34.8 6 4.0

1.3 6 0.4

2.0 6 2.8

Green ash (15 d after decapitation)IntactDecapitatedDecapitated 1 1%

IAAIntact 1 BA

(889 mol/L)

810

9

8

010

1

1

02.8 6 0.9

0.2 6 0.7

0.3 6 0.7

03.1 6 2.6

0.5 6 0

0.5 6 0

Red oak (12 d after decapitation)Intact ShootsDecapitatedDecapitated 1 1%

IAAIntact 1 BA

(889 mol/L)

89

9

8

29

7

0

0.4 6 0.72.6 6 1.0

1.4 6 1.2

0

——

—

—

Fig. 3. Twig cuttings of branches of outside trees during spring flushin early May with lower branches removed. Left, red oak; middle, whiteash; right, green ash.significantly released. Applications of 1% NAA to the

decapitated stump counteracted the release of apical dom-inance (Table 3). Decapitation also resulted in stimulatinggrowth of 1995 internal bud-scale buds (Fig. 5, Table 3).This latter response, which was completely repressible byNAA, only occurred in those branches where externalvegetative lateral buds did not grow out (usually becausethey were floral or had abscised).

Green ash—The morphological differences betweenthe twigs of the greenhouse seedlings and those of theoutside trees were similar to those with white ash withrespect to shoot orientation, height, and node number(data not shown). However, the average 1995 elongationin the white ash twig was 15.9 cm, whereas that for greenash was only 2.8 cm. This low green ash value may havebeen due in part to some seasonal metabolic fluctuationwithin the tissue as well as to poor health on the part ofa few of the green ash branches. On the intact stems,only the terminal buds sprouted. Lateral buds did notgrow out except when the terminal bud was damaged.Decapitation of the stem just above the basal node usu-ally triggered the release of both basal lateral buds (Table4). NAA, when applied to decapitated stems, was com-pletely repressive to lateral bud growth. No BA effectswere observed.

Red oak—At the beginning of the growing season,there was an average of 5.2 lateral buds on the 1995

wood of intact branches on red oak trees per twig (Fig.3). About one-fourth of these sprouted. Most of the re-maining buds had abscised. However, it was noticed thatthere were occasional latent buds present on the earlieryear’s wood. Decapitations were sometimes carried outin the 1994 wood because no visible latent buds remainedbelow the sprouting lateral buds on the 1995 wood. It ispossible that some of these 1994 wood buds were inter-nal, i.e., buds remaining from prior years that developedinto epicormic shoots.

There was an average of 1.8 lateral buds left on thetwig after decapitation and of these 1.5 sprouted (Table4). This 83% outgrowth following decapitation was cer-tainly greater than the 23% sprouting that normally oc-curred in the intact stems and strongly suggests that api-cal dominance was released by decapitation. Decapita-tion-induced release of apical dominance was stronglyinhibited by auxin (Table 4). Large swelling buds belowthe point of decapitation were manually removed. Thepercentage of sprouting lateral buds was much higher interminal stems than in side stems. When 1% NAA wasapplied to decapitated terminal stems that still had manylarge lateral buds on the verge of sprouting, the inhibitoryand somewhat toxic effect of the NAA was limited to thetopmost one or two lateral buds only 1–2 cm from thesite of NAA application, whereas the remaining large lat-


TABLE 2. Greenhouse studies on decapitation promotion of outgrowthof second-order (current) lateral buds and auxin effects at varioustimes after decapitation of elongating primary shoots of 1-yr-oldseedlings of white/green ash and red oak. See Fig. 2.

Numberof buds

% of budsgrowing

out

Mean (6 1 SD)bud outgrowth

(mm)

% auxininhibition

of out-growth

White ash (two shoots/seedling with two lateral buds/shoot, 14 d afterdecapitation)

Control1% IAA

2020

8045

47.4 6 36.37.9 6 8.4

—83

Control1% NAA

1818

610

23.4 6 24.70

—100

Green ash (two shoots/seedling with two lateral buds/shoot, 14 d afterdecapitation)

Control1% IAA

1212

9242

46.8 6 36.120.7 6 22.8

—56

Control1% NAA

1010

420

11.8 6 5.40

—100

Number ofseedlings

% ofseedlings

with lateralbuds growing

Mean (6 1 SD)number of lateral

buds/seedlinggrowing out

Red oak (Single branch seedlings with one lateral bud/branch, 13 dafter decapitation)

Control1% IAA

43

750

1.5 6 1.00

Double branch seedlings with one lateral bud/branch, 28 d after decap-itation

Control1% IAA

77

430

0.4 6 0.80

Double branch seedlings with one lateral bud/branch, 35 d after decap-itation

Control1% IAA

66

500

0.7 6 0.80

Fig. 4. Winter shoot (left) and flushing spring shoot (right) of ma-ture white ash. The point of decapitation on the spring shoot was belowthe 1996 nonrepressible flushing lateral shoots and above the repressiblebuds in the 1995 wood.

TABLE 3. Effects of decapitation and 1% NAA on lateral bud and budscale bud outgrowth of twigs of white ash and green ash grownoutdoors. Decapitation was carried out between the first and thefourth nodes down from the terminal bud, leaving at least one nodefor potential bud outgrowth. N 5 number of twigs observed.

% oftwigswithbuds

growingout

Mean (6 1 SD)number of

buds/twig ofthese plants

with growinglateral buds

Mean (6 1 SD)length (cm)of growing

buds

White ashLateral buds

Decapitation (N 5 27)Decapitation 1 1% NAA

(N 5 14)

44

7

1.4 6 0.5

1 6 0

15.5 6 9.6

1.3 6 0

Bud scale budsDecapitation (N 5 27)Decapitation 1 1% NAA

(N 5 14)

44

7

2.3 6 1.0

1 6 0

9.7 6 4.6

1 6 0

Green ashLateral buds

Decapitation (N 5 27)Decapitation 1 1% NAA

(N 5 14)

96

29

1.6 6 0.5

1.3 6 0.5

3.2 6 1.0

—

Bud scale budsDecapitation (N 5 27)Decapitation 1 1% NAA

(N 5 14)

4

0

2.0

0

0.2

0

eral buds, lower on the shoot, appeared to sprout nor-mally (data not shown). BA spray had no effects.

DISCUSSION

In all three species, both with seedlings and with oldertrees, decapitation induced growth of the repressiblebuds, which in white ash also included the bud-scale buds(Fig. 5) at the base of the 1995 wood. These releaseswere repressible by auxin application to the decapitatedshoot in all cases except for red oak seedlings where therepression was negligible. Perhaps the considerable dis-tance between the site of auxin application and the po-sitions of some of the widely scattered lateral buds pre-vented the response. Second-order branching after decap-itation of the first-order branches, repressible by exoge-nous auxin, was observed in the seedlings to some degreein all three species (Fig. 2). In contrast to the cytokinin-promotive effects that a number of investigators havefound (Williams and Stahly, 1968; Yang and Read, 1991),no BA enhancement effect was observed on lateral budgrowth here.

This investigation has clearly elucidated some signifi-cant differences between the execution of the auxin re-placement experiment with herbaceous vs. woody spe-cies. Decapitation of a vigorously growing herbaceousplant with relatively strong apical dominance such as pea

will at any time almost immediately trigger the initiationof the outgrowth of some of the lower lateral buds (Staf-strom, 1995; Cline, 1996). This release of apical domi-nance can be repressed by the application of auxin on thestump of the decapitated stem. However, with the three


Fig. 5. Bud-scale buds growing out in decapitated twigs of white ash.

TABLE 4. Effects of decapitation and 1% NAA on lateral bud out-growth of twigs of red oak trees grown outdoors. Decapitation (29–30 April) was done partway down the 1995 growth wood belowthe large buds and above the medium/small sized buds. N 5 num-ber of twigs observed. Measurements were done on 29 August.


lateralbuds/twig

left on afterdecapitation


lateralbuds/twiggrown out

%lateralbuds

grownout

Mean (6 1 SD)length (cm)

of budsgrown out

DecapitationN

1.8 6 0.620

1.5 6 0.620

8320

1.5 6 0.531

Decapitation 6 1%NAA in lanolin

N1.4 6 0.6

160.1 6 0.3

164

160.51

woody species studied here, the time of the execution ofthe experiment was essentially restricted to spring flush.

Auxin replacement experiments cannot be done if de-capitation does not release the lateral buds in the controls.Rapid bud growth such as occurs during the flush periodprovides optimal conditions for the detection of auxinrepression in decapitated shoots. This is particularly truefor older trees in field conditions. In the present study,decapitation of the shoot apex during this period usuallyreleased the lateral buds within a week or so (data notshown), and the repressive effects of applied auxin couldbe clearly observed. If the bud release in the decapitatedcontrol shoots is slow, i.e., over a period of many weeksor months, then it is possible that complicating physio-logical and environmental factors may obscure the inter-pretation of the auxin effects. When white ash shootswere decapitated later in the growing season, the out-growth of the lateral buds was very limited (data notshown) and unsuitable for auxin replacement experi-ments.

There are two further restrictions. The first is that thedecapitation must be made in the previous season’s over-

wintered growth wood except for seedlings. It is reportedelsewhere (Cline and Deppong, 1999) for these three spe-cies (in older trees) that decapitation in the present sea-son’s growth wood during the flush period does not re-lease the current lateral buds, suggesting the existence ofsome inhibitory influence other than apical dominance.The second restriction is that the point of decapitationmust be below the larger upper irrepressible lateral buds,which grow out synchronously with the terminal bud dur-ing the spring flush, and above the smaller, lower repress-ible buds, which will not grow out without decapitation(Fig. 4). These latter proleptic buds were those that re-sponded to the auxin replacement test. Whether auxin isthe natural repressor of these latent buds remains to beconfirmed, but the evidence is suggestive.

Since the large upper irrepressible lateral buds exhibitlittle or no inhibition in their vigorous outgrowth duringspring flush, it was deemed unfruitful to attempt auxinreplacement experiments to test auxin inhibitory effectsin these buds. As mentioned in the Results section witholder red oaks, exogenous auxin had little effect on theselarge buds (data not shown). Brown (1970) has pointedout that the larger upper irrepressible lateral buds ofwoody plants are not under auxin inhibition of the ter-minal bud nor under the control of apical dominance dur-ing flushing. However, this is not to say that there maynot be, under certain conditions, some kind of apical con-trol by the elongating terminal bud in the subsequent out-growth of these irrepressible lateral buds. The occasionallack of such control in summer flushing of douglas-firappears to cause an anomalous branching and forkingresponse in the shoot apex.

The lack of uniformity observed here in the bud dis-tribution of red oak and the fact that some buds weredamaged or missing has also been reported by otherworkers (Ward, 1964; Harmer, 1991). Harmer points outthat in Quercus petraea the buds are spirally arrangedwith the pitch of the spiral being much smaller near the


shoot apex with greatly increased bud density. Likewisethe fact that decapitation was followed by the outgrowthof lateral buds closest to the cutting point was also foundby Wilson and Kelty in black oak (1994). Furthermore,their finding that the removal of all large buds in blackoak enhanced the outgrowth of epicormic buds also wassupported in the present study with red oak, although aseparation between normal lateral and epicormic shootswas not made. NAA inhibited the outgrowth of bothkinds of shoots. Similarly, in the present study of redoaks, decapitation (and removal of large buds) released94% of the remaining lower buds. Many of the lateralbuds abscised. Harmer and Baker (1995) determined thatdecapitation of terminal buds in Quercus petraea wasmost effective in promoting branching when carried outduring the second flush.

Remphrey and Davidson (1992) have pointed out thatthe outgrowth of epicormic shoots is often thought to bedue to the sudden exposure to light because of pruningor thinning and the subsequent photodestruction of auxin.They further indicate that the work of Wignall andBrowning (1988) suggests that it is not the decrease inauxin but the counteractive effect of cytokinin on auxinthat promotes outgrowth. However, as Remphrey and Da-vidson (1992) explain, the pruning and thinning may sim-ply result in a larger supply of nutrients, water, and cy-tokinin, which promote the outgrowth. Auxin replace-ment studies with apical dominance in herbaceous plantssuggest indirect auxin action with cytokinin effects(Cline, 1994; Cline, Wessel, and Iwamura, 1997).

As far as can be determined, the present study providesthe first report of an apical dominance auxin replacementexperiment having been carried out on the stem of anintact branch of a hardwood tree. Although the experi-ment was made to work in white ash, green ash, and redoak under the particular noted conditions, the foregoingresults and interpretation of data do not necessarily applyuniversally, given the wide spectrum of physiological re-sponses present in different woody species under differ-ent growing conditions (Brown and Sommer, 1992).

The results of these auxin replacement experiments onintact trees generally agree with those on excised shootsreported in the literature. Where apical dominance occursin normal development, auxin plays a primary role. How-ever, in spite of the considerable research which has beendone on the production, content, and transport of endog-enous auxin in woody tissues, its precise role in apicaldominance has yet to be elucidated (Wareing and Saun-ders, 1971; Rinne, Tuominen, and Sundberg, 1993;Tuominen et al., 1995). Although the use of the auxinreplacement experiment with woody plants has certainlimitations, it is still a powerful tool for investigating therole of auxin in apical dominance and has generatedmuch valuable data for providing a framework for phys-iological, genetic, and molecular studies of the involvedmechanisms.

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