Gibberellins Stimulate Lateral Branch Development in Young Sweet Cherry Trees in the Orchard

Abstract

Previous studies showed that successful stimulation of lateral branch development from 1-year-old wood in young sweet cherry trees without reliance on pruning could be accomplished if suitable cytokinin- or cytokinin/gibberellic acid-containing products were applied to assure penetration into active tissues. The goal of the trials reported here was to determine the potential for stimulating lateral branching by applying gibberellic acid alone under similar conditions. Three commercially available products containing gibberellic acid were evaluated. Treatments included the isomers GA₃, GA₄, and GA₄ combined with GA₇. All gibberellic acid isomer/formulations were similar in their ability to stimulate branching from lateral buds on 1-year-old wood. In one trial, GA₄₊₇ alone was nearly as effective as the cytokinin 6-benzyladenine. Combining the surfactant Pentra-bark with gibberellic acid resulted in successful branch induction without the need to apply gibberellic acid to scoring or nicking cuts in the bark.

Keywords:

• Prunus avium
• plant growth regulator
• bioregulator
• benzyladenine
• gibberellic acid
• auxin
• forchlorfenuron
• CPPU
• Maxcel
• Novagib
• Prestige
• Pro-Gibb
• Pro-Vide
• Promalin
• thidiazuron
• TDZ
• scoring
• cincturing
• nicking
• notching
• bark injury
• vegetative shoot growth
• lateral shoots
• lateral branching

INTRODUCTION

The term “apical dominance” describes the physiological system active in many woody plants in which hormones produced by the shoot tip and new leaves during growth inhibit the development of lateral buds into sylleptic shoots on the newly-developing leafy shoot, as well as the growth of buds into lateral shoots on 1-year-old or older wood (Anderson et al., 2001; Martin, 1987). Buds controlled by apical dominance are said to be in “paradormancy” (Lang, 1987; Lang et al., 1987). Apical dominance is thought to operate as a dynamic balance between auxin (bud growth inhibitor) production and basipetal movement from the tip or apical portion of a shoot and acropetal movement of cytokinin (bud growth promoter) originating from lower stem tissues and/or roots (Cline, 1991, 1997; Faust et al., 1997; Powell, 1987).

Sweet cherry (Prunus avium) trees display strong apical dominance (Elfving and Visser, 2007); for this reason commonly used canopy training systems for sweet cherries rely on pruning as an important method for interruption of apical dominance and stimulation of lateral branching during active growth (Long, 2001, 2005; Long et al., 2005; Nugent et al., 2001).

Elfving and Visser (2007, 2009) reported that cytokinin- or cytokinin/gibberellic-acid-based bioregulators could induce lateral shoot development and improve the distribution of those shoots on unpruned 1-year-old sweet cherry leader branches if penetration through the outermost, papery bark layer on the branches was assured.

Although gibberellic acid (GA) has been implicated in the termination and release of buds from endodormancy (Powell, 1987), little evidence exists for a role for GA (cell elongation promoter) in the apical dominance control system (Hillman, 1984; Martin, 1987). Application of GA can strengthen apical dominance in conifers (Ross et al., 1983; Pharis et al., 1972) and in some other species (Jacobs and Case, 1965; Scott et al., 1967). The research described here was undertaken to determine the extent to which GA alone could encourage lateral buds under the control of apical dominance (i.e., paradormant) on unpruned, vertically-oriented 1-year-old sweet cherry branches to develop into lateral shoots.

MATERIALS AND METHODS

Five experiments were conducted between 2006 and 2009. All trials employed randomized complete-block designs with a minimum of three single-tree replications per treatment. Proprietary formulations of the cytokinins benzyladenine [Maxcel (MX), Valent BioSciences, Walnut Creek, CA, USA], thidiazuron [TDZ, Bayer CropScience, Kansas City, MO, USA], and forchlorfenuron [Prestige (CPPU), Valent BioSciences, Walnut Creek, CA, USA], gibberellins GA₃ [ProGibb 40 (PG), Valent BioSciences, Walnut Creek, CA, USA], GA₄ [Novagib 10L (NG), Fine Americas, Walnut Creek, CA, USA], and GA₄₊₇ [ProVide 10SG (PV), Valent BioSciences, Walnut Creek, CA, USA], and a proprietary combination of benzyladenine and GA₄₊₇ [Promalin (PR), Valent BioSciences, Walnut Creek, CA, USA] were used in the trials. Bioregulator solutions were supplemented with the surfactants Regulaid (Kalo Labs, Overland Park, KS, USA) or Pentra-bark (Quest Products Corp., Louisburg, KS, USA). Bioregulator treatments were applied by hand with a small paintbrush. Bioregulator concentrations are expressed in terms of the active ingredient(s). Supplements are expressed in terms of volume/volume (v/v) ratios.

On upright, unpruned 1-year-old leader branches, scoring cuts were made circumferentially to the cambium layer with a sharp knife either completely around the branch or only halfway around the branch. Scoring cuts were made every 30 cm starting 30 cm below the terminal bud and repeated to the base of the 1-year-old branch. Nicking cuts (Elfving and Visser, 2007) were made with a small knife without regard to bud location, cutting into the phloem tissue every 30 cm starting 30 cm below the terminal bud and repeated to the base of the 1-year-old branch. Treatments were applied at green-tip each year (green-tip defined as when a majority of the buds on 1-year-old wood was showing a point of green tissue at the distal end of the bud).

Experiment 1, 2006, Quincy, Washington

Full scoring cuts were applied to a single, vertical, unpruned 1-year-old leader branch per tree in 8 separate fourth-leaf ‘Skeena’/Mazzard sweet cherry trees in each of five blocks. Following scoring, each of the following treatments was mixed with the surfactant Regulaid (0.1% v/v) and applied to the scoring cuts on a single tree in each block using a small paintbrush: (1) Regulaid (0.1% v/v) only (scored control); (2) PV (5,000 mg•L⁻¹) only; (3) MX (5,000 mg•L⁻¹) only; (4) PR [GA₄₊₇ (5,000 mg•L⁻¹) + benzyladenine (5,000 mg•L⁻¹)]; (5) Prestige (500 mg•L⁻¹) only; (6) Prestige (500 mg•L⁻¹) + PV (5,000 mg•L⁻¹); (7) TDZ (1,000 mg•L⁻¹) only; or (8) TDZ (1,000 mg•L⁻¹) + PV (5,000 mg•L⁻¹).

Treatments were applied at green-tip (April 6, 2006). After shoot growth was completed, each vertical leader branch from which new lateral shoots developed was divided into the approximately 30-cm sections in scored treatments. The length of each leader branch section was measured and all newly developed lateral shoots >10 cm in length in each section were counted and their lengths determined.

Experiment 2, 2007, Quincy, Washington

Half-scoring cuts were applied as described in Experiment 1 to a single, vertical, unpruned 1-year-old leader branch per tree in nine replicate, second-leaf ‘Skeena’/G.6 sweet cherry trees in each of four blocks. A tenth tree in each block was left unscored to serve as a control. All bioregulator solutions were supplemented with Regulaid (0.1% v/v). A single tree in each block received one of the following bioregulator treatments as a band painted over each scoring cut with a small paintbrush immediately following scoring. Those bioregulator treatments included: (1) Control (not scored, untreated); (2) PG (1,000 mg•L⁻¹); (3) PG (2,500 mg•L⁻¹); (4) PG (5,000 mg•L⁻¹); (5) NG (1,000 mg•L⁻¹); (6) NG (2,500 mg•L⁻¹); (7) NG (5,000 mg•L⁻¹); (8) PV (1,000 mg•L⁻¹); (9) PV (2,500 mg•L⁻¹); or (10) PV (5,000 mg•L⁻¹).

Treatments were applied at green-tip (April 4, 2007). Shoot measurements were carried out as described for Experiment 1 except that shoot lengths were not measured.

Experiment 3, 2008, Quincy, Washington

Five randomized complete blocks of nine trees each were selected in a third-leaf ‘Skeena’/G.6 sweet cherry orchard. Nicking cuts were applied to the outside half of a single, vertical, unpruned 1-year-old leader branch per tree in two of the nine trees in each block. All the nicking cuts on a particular tree were then treated either with (1) NG (5,000 mg•L⁻¹) or (2) PV (5,000 mg•L⁻¹) supplemented with Regulaid (0.1% v/v). Each of six additional bioregulator treatments was applied as half bands to the outside portion of a single, vertical, unpruned and non-injured 1-year-old leader branch per tree in each of the five blocks as described in Experiment 1. The ninth tree in each block was left untreated as a control. The banded bioregulator treatments included: (3) NG [(5,000 mg•L⁻¹) + Regulaid (0.1% v/v)]; (4) PV [(5,000 mg•L⁻¹) + Regulaid (0.1% v/v)]; (5) NG [(2,500 mg•L⁻¹) + Pentra-bark (2% v/v)]; (6) NG [(5,000 mg•L⁻¹) + Pentra-bark (2% v/v)]; (7) PV [(2,500 mg•L⁻¹) + Pentra-bark (2% v/v)]; (8) PV [(5,000 mg•L⁻¹) + Pentra-bark (2% v/v)]; or (9) control (untreated). Treatments were applied at “advanced” green-tip (April 10, 2008, buds showing extended green tips but no unrolled leaves). Shoot measurements were carried out as described for Experiment 1 except that shoot lengths were not measured.

Experiment 4, 2008, Quincy, Washington

Full scoring cuts were applied to a single, vertical, unpruned 1-year-old leader branch per tree in one replicate, third-leaf ‘Skeena’/G.6 sweet cherry tree in each of three blocks. Each cut was then treated with Pentra-bark (2% v/v). The other tree in each block was left as an untreated control. Treatments were applied at “advanced” green-tip (April 10, 2008, buds showing extended green tips but no unrolled leaves). Shoot measurements were carried out as described for Experiment 1 except that shoot lengths were not measured.

Experiment 5, 2009, East Wenatchee, Washington

Full scoring cuts were applied to a single, vertical, unpruned 1-year-old leader branch per tree in one replicate, third-leaf ‘Selah’/Mazzard sweet cherry tree in each of three blocks. Each cut was then treated with (1) PV [(5,000 mg•L⁻¹) + Regulaid (0.1% v/v). Each of three additional bioregulator treatments was applied to a single, vertical, unpruned and non-scored 1-year-old leader branch per tree in each of the five blocks as described in Experiment 1. Treatments included (2) PV (5,000 mg•L⁻¹) + Pentra-bark (3% v/v) applied as circumferential bands every 30 cm; (3) PV (5,000 mg•L⁻¹) + Pentra-bark (6% v/v) applied in a similar manner, or (4) PV (5,000 mg•L⁻¹) + Pentra-bark (9% v/v) applied in a similar manner. A fifth tree in each block was left unscored and untreated as a control. Treatments were applied at green-tip (April 6, 2009). Shoot measurements were carried out as described for Experiment 1 except that shoot lengths were not measured.

One-way analyses of variance were used to determine the significance of treatments in four of the five trials. Although Experiment 1 was configured as a 2-way factorial treatment arrangement of cytokinins vs. ± GA₄₊₇, significant interactions necessitated the use of one-way analysis of variance. Experiment 2 was analyzed using a radiating regression model described by Cochran and Cox (1957) and Elfving and Allen (1987). Shoot distribution was assessed as the percentage of total shoots ≥10 cm in length that developed on each 30-cm section of the unpruned leader branch. All percentage values were transformed using the arcsine function prior to analysis. Mean values were separated with a significant F-test alone (single degree-of-freedom comparison) or the Waller-Duncan Bayesian k-ratio test (P ≤ 0.05) following a significant F-test. Statistical analyses were performed using the General Linear Models procedure of the Statistical Analysis System program package (SAS Institute, Cary, NC, USA).

RESULTS

Experiment 1

All bioregulator treatments applied to scoring cuts improved lateral branching compared to the scored-only control (Table 1). Application of GA₄₊₇ alone to scoring cuts was nearly as effective as the cytokinin benzyladenine alone and more effective than either CPPU or TDZ alone in inducing lateral shoot development from 1-year-old leader branches on ‘Skeena’ sweet cherry. Adding GA₄₊₇ to solutions of either CPPU or TDZ produced lateral shoot development equivalent to the GA₄₊₇ application alone. GA₄₊₇ alone or in combination with cytokinins improved vertical shoot distribution as well as the cytokinins alone. Mean shoot length tended to be shorter when lateral branching was increased by bioregulator treatments. Treatment with GA did not increase shoot length. No symptoms of phytotoxicity were observed.

TABLE 1 Effects of Cytokinin and/or Gibberellic Acid Applications to Scoring Cuts on Lateral Shoot Number, Mean Length, and Vertical Distribution in Fourth-Leaf ‘Skeena’/Mazzard Sweet Cherry Trees (Experiment 1, 2006, Quincy, WA)

	Lateral shoots		Lateral shoots in leader branch section(% of total lateral shoots) ^w
Treatment (mg•L^−1) ^z ^, ^y	No./100 cm leader length	Mean length (cm)	1	2	3	4	5	6–7
Control ^x	5.1d	77a	92a	0d	0b	6a	2b	0a
ProVide (GA4+7, 5,000)	18.0b	49c	66b	19a	9a	4a	2b	0a
Maxcel (BA, 5,000)	21.9a	43c	49b	13ab	13a	14a	10a	0a
Promalin [GA4+7 (5,000) + BA (5,000)]	18.4ab	49c	53b	18a	11a	9a	4ab	5a
Prestige (CPPU, 500)	11.5c	62b	66b	7c	7a	11a	9ab	0a
CPPU (500) + GA4+7 (5,000)	19.9ab	49c	59b	15ab	8a	10a	7ab	0a
Thidiazuron (TDZ, 1,000)	11.4c	68ab	58b	10bc	17a	10a	5ab	0a
TDZ (1,000) + GA4+7 (5,000)	19.9ab	49c	60b	16a	11a	9a	3ab	0a
^zApplied by hand with a small paintbrush to 1-year-old vertical, unpruned central-leader branches at green-tip (April 6, 2006). All treatments supplemented with 0.1% v/v Regulaid surfactant.
^yMean separation in columns by Waller-Duncan Bayesian k-ratio test following significant F-test (p ≤ 0.05). All interaction terms significant (p ≤ 0.05).
^xScored every 30 cm; scoring cuts treated with Regulaid only (0.1% v/v).
^wMean leader branch length 141 cm. Leader branch sections ∼30 cm in length; 1 = terminal 30-cm section. Sections 6–7 (basal) may have been less than 30 cm.

Experiment 2

All three isomers of GA were effective for induction of lateral shoot development from 1-year-old leader branches of ‘Skeena’ sweet cherry trees when applied to half-scoring cuts (Table 2). The GA₄ product showed a simple and positive linear relationship between bioregulator concentration and number of shoots induced, while the other two isomers displayed similar but more complex relationships. Treatments producing a change in lateral branching also tended to improve the vertical distribution of that branching as well. No symptoms of phytotoxicity were observed.

TABLE 2 Effects of Various Gibberellic Acid Isomer/Formulation Applications to Half-Scoring Cuts on Lateral Shoot Number and Vertical Distribution in Second-Leaf ‘Skeena’/G.6 Sweet Cherry Trees (Experiment 2, 2007, Quincy, WA)

	Lateral shoots	Lateral shoots in leader branch section (% of total lateral shoots) ^w
Treatment (mg•L^−1) ^z ^, ^y	No./100 cm leader length	1	2	3	4	5	6
Control (untreated)	11.8	84	11	3	2	0	0
PG (1,000)	13.5	70	3	3	9	13	1
PG (2,500)	21.8	53	21	17	9	0	0
PG (5,000)	16.4	60	20	10	7	2	1
NG (1,000)	13.4	76	9	9	1	4	2
NG (2,500)	16.9	72	6	7	9	4	1
NG (5,000)	25.3	55	21	14	7	4	0
PV (1,000)	19.4	60	15	15	9	1	0
PV (2,500)	19.0	53	21	12	9	5	0
PV (5,000)	20.7	57	14	11	10	6	1
Regression significance ^x
Linear
PG	*	**	*	NS	*	NS	NS
NG	****	**	NS	*	NS	NS	NS
PV	**	**	*	*	*	NS	NS
Quadratic
PG	*	*	NS	NS	**	NS	NS
NG	NS	NS	NS	NS	NS	NS	NS
PV	*	**	*	*	NS	NS	NS
^zApplied by hand with a small paintbrush to 1-year-old vertical, unpruned central-leader branches at advanced green-tip (April 4, 2007). All treatments supplemented with 0.1% v/v Regulaid surfactant.
^yPG = ProGibb 40 (Valent BioSciences); NG = Novagib 10L (Fine Americas); PV = ProVide 10SG (Valent Biosciences).
^xSignificance of linear and quadratic regression effects on active ingredient concentration determined by radiating regression analysis. NS—not significant; *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001; ****p ≤ 0.0001.
^wMean leader branch length 137 cm. Leader branch sections ∼30 cm in length; 1 = terminal 30-cm section. Section 6 (basal) may have been less than 30 cm.

Experiment 3

Where the GA formulations NG or PV were mixed with a low concentration of Regulaid and applied as bands to uncut bark of 1-year-old branches on ‘Skeena’ sweet cherry trees, branch induction did not occur and there were only minor changes in shoot distribution (Table 3). Where the same formulation/surfactant combinations were painted on nicking cuts in the bark, both GA products induced lateral branching and altered the vertical distribution of induced shoots. Mixing those same GA formulations at two concentrations with Pentra-bark surfactant at 2% v/v resulted in significant increases in lateral branching when those combinations were applied as bands to uninjured bark of 1-year-old branches. Where treatments increased lateral branching, vertical distribution of shoots was also improved. No symptoms of phytotoxicity were observed.

TABLE 3 Effects of Surfactants and Nicking Cuts on Gibberellic Acid Isomer/Formulation Stimulation of Lateral Shoot Number and Vertical Distribution in Third-Leaf ‘Skeena’/G.6 Sweet Cherry Trees (Experiment 3, 2008, Quincy, WA)

	Lateral shoots	Lateral shoots in leader branch section (% of total lateral shoots) ^x
Treatment (mg•L^−1) ^z ^, ^y	No./100 cm leader length	1	2	3	4	5–6
Control (untreated)	9.0e	97a	0d	1b	1bc	0a
NG (5,000) + Regulaid (0.1% v/v) banded every 30 cm	11.4de	82bc	4bcd	4ab	3abc	7a
NG (5,000) + Regulaid (0.1% v/v) painted on nicking cuts every 30 cm	19.1a	62d	13a	10a	9a	6a
PV (5,000) + Regulaid (0.1% v/v) banded every 30 cm	10.6de	89b	3cd	2b	0c	6a
PV (5,000) + Regulaid (0.1% v/v) painted on nicking cuts every 30 cm	13.0cd	73bcd	14a	4ab	7ab	2a
NG (2,500) + Pentra-bark (2.0% v/v) banded every 30 cm	14.5bc	78bcd	9ab	4ab	4abc	4a
NG (5,000) + Pentra-bark (2.0% v/v) banded every 30 cm	14.2bc	77bcd	11ab	3ab	6ab	3a
PV (2,500) + Pentra-bark (2.0% v/v) banded every 30 cm	14.4bc	70cd	10abc	8ab	5abc	7a
PV (5,000) + Pentra-bark (2.0% v/v) banded every 30 cm	16.6ab	72cd	16a	5ab	4abc	4a
^zApplied by hand with a small paintbrush to 1-year-old vertical, unpruned central-leader branches at advanced green-tip (April 10, 2008). NG = Novagib 10L (Fine Americas); PV = ProVide 10SG (Valent Biosciences).
^yMean separation in columns by Waller-Duncan Bayesian k-ratio test following significant F-test (p ≤ 0.05).
^xMean leader branch length 160 cm. Leader branch sections ∼30 cm in length; 1 = terminal 30-cm section. Section 6 (basal) may have been less than 30 cm.

Experiment 4

Pentra-bark alone (2% v/v) painted on scoring cuts on 1-year-old branches of ‘Skeena’ sweet cherry had no effect on either lateral branching or vertical shoot distribution compared to branch development from untreated control trees (Table 4). No symptoms of phytotoxicity were observed.

TABLE 4 Effects of Pentra-Bark Surfactant Application to Scoring Cuts on 1-Year-Old Branches on Lateral Shoot Number and Vertical Distribution in Third-Leaf ‘Skeena’/G.6 Sweet Cherry Trees (Experiment 4, 2008, Quincy, WA)

	Lateral shoots	Lateral shoots in leader branch section (% of total lateral shoots) ^x
Treatment (mg•L^−1) ^z ^, ^y	No./100 cm leader length	1	2	3	4	5	6
Control (unscored, untreated)	10.4a	82a	0a	0a	10a	6a	2a
Pentra-bark (2% v/v) on scoring cuts	10.1a	87a	0a	5a	6a	2a	0a
^zApplied by hand with a small paintbrush to 1-year-old vertical, unpruned central-leader shoots at green-tip (April 10, 2008).
^yMean separation in columns by F-test (p ≤ 0.05).
^xMean leader branch length 161 cm. Leader branch sections ∼30 cm in length; 1 = terminal 30-cm section. Section 6 (basal) may have been less than 30 cm.

Experiment 5

PV plus Regulaid (0.1% v/v) painted on scoring cuts on 1-year-old wood of ‘Selah’ sweet cherry increased lateral shoot development by about 5-fold (Table 5). PV mixed with Pentra-bark and applied as bands to uninjured bark of similar branches did not produce a significant change in branching until the surfactant concentration reached 9% v/v according to the overall analysis of variance. When the four non-scored treatments were analyzed by regression, however, lateral branching response to Pentra-bark concentration showed a highly significant linear relationship (r ² = 0.79, P ≤ 0.001). Significant increases in lateral branching were accompanied by improved vertical distribution of induced shoots. No symptoms of phytotoxicity were observed.

TABLE 5 Effects of Gibberellic Acid (GA) Applications to Scoring Cuts on 1-Year-Old Branches or GA Combined with Supplemental Surfactant and Applied to Unscored 1-Year-Old Branches on Lateral Shoot Number and Vertical Distribution in Third-Leaf ‘Selah’/Mazzard Sweet Cherry Trees (Experiment 5, 2009, East Wenatchee, WA)

	Lateral shoots	Lateral shoots in leader branch section (% of total lateral shoots) ^x
Treatment (mg•L^−1) ^z ^, ^y	No./100 cm leader length	1	2	3	4	5	6
Control (untreated)	2.9b	100a	0b	0b	0a	0a	0a
PV (5,000) + Regulaid (0.1% v/v) painted on scoring cut every 30 cm	15.0a	47c	24a	15a	6a	6a	2a
PV (5,000) + Pentra-bark (3% v/v) banded every 30 cm	5.4b	97ab	0b	0b	0a	3a	0a
PV (5,000) + Pentra-bark (6% v/v) banded every 30 cm	8.1b	74b	4b	15a	6a	0a	1a
PV (5,000) + Pentra-bark (9% v/v) banded every 30 cm	14.7a	55c	24a	10ab	7a	4a	0a
^zApplied by hand with a small paintbrush to 1-year-old vertical, unpruned central-leader shoots at green-tip (April 6, 2009). PV = ProVide 10SG (Valent Biosciences).
^yMean separation in columns by Waller-Duncan Bayesian k-ratio test following significant F-test (p ≤ 0.05).
^xMean leader branch length 166 cm. Leader branch sections ∼30 cm in length; 1 = terminal 30-cm section. Section 6 (basal) may have been less than 30 cm.

DISCUSSION

Elfving and Visser (2007, 2009) showed that penetration of cytokinin-containing bioregulator products through the outer bark layer into active tissues was a principal factor influencing lateral-branch induction in 1-year-old sweet cherry branches. Each of the three most commonly available GA isomer/formulation products alone showed efficacy similar to that of cytokinin-containing products for inducing lateral branching in 1-year-old branches when the product was able to enter active tissues through the outer bark.

When combined with Pentra-bark (2% v/v), GA was effective for lateral-branch induction in the absence of bark injury. In 2009, increasing the concentration of Pentra-bark mixed with the applied GA improved the branching response, indicating that the Pentra-bark was aiding the passage of the GA through the bark barrier in proportion to its concentration. The highest concentration of Pentra-bark used in conjunction with GA (9% v/v) did not produce phytotoxicity. Pentra-bark applied alone (2% v/v) had no effect on lateral branch induction, even when combined with scoring cuts.

When endodormancy is terminated by the accumulation of sufficient chilling, buds enter ecodormancy, where growth is contingent upon exposure to suitable temperatures. When budbreak begins, most of the vegetative buds on 1-year-old wood of sweet cherry begin to expand, eventually producing the “green-tip” condition that signals the time for application of bioregulators for stimulation of lateral branching. Faust et al. (1997) proposed that the terminal bud (apex) of a shoot, having a lower chilling requirement than lateral buds (Saure, 1985), can quickly re-impose apical control over further lateral bud development (i.e., paradormancy) by resuming production and export of auxin upon initiating growth in the spring (Stafstrom, 1995). In unpruned 1-year-old sweet cherry branches, most lateral buds produce a rosette of leaves and form spurs, but cease further growth unless either pruning or appropriate bioregulators have been applied for suppression of apical dominance.

The mode of action of GA in overcoming apical control of lateral bud growth is unknown. Stimulation of lateral branch formation by applied GA is likely the result of interaction with the apical-dominance control system in the tree, rather than any direct effect on cell elongation, since the mean length of GA-induced shoots was shorter than for shoots from other treatments or controls. In addition, GA applied once in early spring would not be likely to survive long enough in treated tissues to affect cell growth for the several weeks required for lateral shoot development.

It seems likely that the initial expansion of lateral buds in the spring results from their exposure to favorable temperatures (i.e., loss of ecodormancy) as well as to a growth promoter, such as cytokinin. Their failure to continue to grow into shoots may reflect the rapid re-establishment of apical dominance (i.e., paradormancy), manifested as a deficiency of GA required to stimulate cell elongation. Once apical control is interrupted, cell elongation proceeds and a lateral shoot is formed. Describing terminal bud removal, Martin (1987) stated: “It seems that all constituents for growth are present in the nongrowing lateral bud and that its growth is prevented by conditions in the apex.” In 1-year-old sweet cherry branches, the apex certainly imposes conditions that prevent lateral-bud development into shoots, but perhaps “… all constituents …” are not present until apical control is suppressed. Martin (1987) theorized that apical-dominance-based lateral bud growth may be controlled at the stage of RNA transcription. If this is the case, then de novo synthesis or activation of endogenous GA or other chemical compounds is a critical component of the lateral branching response to applied GA, as well as to other bioregulators that overcome paradormancy. Further research is needed to elucidate the mode of action of lateral-branch-inducing bioregulator treatments.

One practical outcome of this research is the potential for bioregulator-based lateral-branch induction to be applied in organic sweet cherry orchards. Several formulations of GA now have OMRI approval for use in organic production.

ACKNOWLEDGMENTS

The authors thank Bayer CropScience, Fine Americas, Kalo Laboratories, Quest Products, Valent BioSciences, the Washington Tree Fruit Research Commission, and the Washington State University Agricultural Research Center for products and/or funds partially supporting these studies.