Manipulating Regular Bearing in ‘Golden Delicious’/M9 Apple Trees using GA₄₊₇ and Ethephon

ABSTRACT

Control of regular cropping in apple is considered critical for fruit growers to ensure their economical sustainability. Irregular flowering can be mitigated by plant growth regulators thanks to their promotion or inhibition effects. In this study, responses of GA₄₊₇ and ethephon on alternate bearing in ‘Golden Delicious’/M9 were examined. GA₄₊₇ and ethephon were applied on the same trees during three consecutive years (2010–12). Flowering, yield, shoot growth, and also some fruit quality parameters were assessed. GA₄₊₇ appeared to be more effectual to regulate alternate bearing. The alternate bearing index calculated with yield in successive years was high in control (0.91), moderate in ethephon (0.71), and low in GA₄₊₇ treated trees (0.41). Modified alternate bearing index, based on cluster number, indicated moderate intensity in GA₄₊₇ (0.53) and high intensity in the others. Although GA₄₊₇ reduced crop density at about 50%, there was little variation in yield per tree and crop efficiency due to increasing of fruit size. GA₄₊₇ did not increase shot length after establishment of equilibrium between vegetative and generative growth in 2012. Ethephon slightly reduced vegetative growth and relatively increased fruit size due to the decrease of fruit set.

KEYWORDS:

• Crop load management
• return bloom
• seed number
• shoot length

Introduction

Standard practices, such as thinning, pruning, nutrition, and irrigation, contribute to the establishment of equilibrium between vegetative and generative growth but generally fall short of modifying the alternate bearing (Schmidt et al., 2009). The irregularities in a flowering cycle can be theoretically reduced since potential effects of plant growth regulators (PGRs) on promotion or inhibition of flowering were considered (Bukovac et al., 2006; Duyvelshoff, 2011; Schmidt et al., 2009).

The gibberellins (GAs) are the most closely related with flowering and have been considered as the main determinants of alternate bearing (Tromp, 2005). While GAs increase the expression of flower-inducing genes in some herbaceous plants (Blazquez et al., 1997), they have inhibitory effects on flower bud formation in many fruit species, such as peach, pear, apricot, plum, cherry, citrus, and apple (Monselise and Goldschmidt, 1982). In fact, the reduction of flower bud formation in apple by GAs provides a different crop load management, which is considered as an indirect thinning method (Garcia-Pallas et al., 2001). External GAs applied especially throughout the flower bud formation significantly reduced flowering in apples (Schmidt et al., 2009). Davis (2002) listed the inhibitory effect of GAs in apples in the following order: GA₇ > GA₃ > GA₄. In contrast to these results, Schmidt (2006) revealed that GA₃, GA₄, GA₇, and GA₄₊₇ have no significant differences from each other. GA₄₊₇ can be used ranging between 100 and 400 ppm (McArtney and Hoover, 2005). The most appropriate application time for gibberellins has been determined as fruits 10 mm in diameter for apples (Schmidt et al., 2009).

Ethephon (2-chloroethylphosphonic acid) decomposes to ethylene in aqueous solutions above pH 5 (Wertheim and Webster, 2005). Ethylene regulates flowering and many other physiological processes in almost all plants (Bukovac et al., 2006; Cline, 2006). However, since ethephon is very sensitive to environmental factors, particularly to the temperature after application, there are some contrary findings regarding its flowering promoting potential (McArtney et al., 2007; Schmidt et al., 2009). Therefore, it is recommended to experiment ethephon in different cultivars and diverse environments to better understand its flowering promoting potential in apples (Schmidt et al., 2009).

‘Golden Delicious’ is undoubtedly one of the most important apple varieties of the world’s apple production. Nonetheless, it has a strong tendency to alternate bearing (Atay et al., 2013). The purpose of the present study is to evaluate the efficiency of ethephon and GA₄₊₇ treatments on reducing irregular flowering in ‘Golden Delicious’ apple trees grafted on M9 rootstock. The treatment was also assessed with respect to their effects on fruit quality, yield, and vegetative growth.

Materials and methods

Experimental procedures and treatments

The trial was conducted in the Fruit Research Station situated in Isparta province (37° 48′ 52.16″ N, 30° 52′ 39.66″ E, 920 m a.s.l.) of Turkey. Experiments were carried out on the adult trees of Golden Delicious/M9 apple cultivar planted with 3.5 × 1 m distances. The experiments were performed on the same trees during 3 consecutive years between 2010 and 2012. The full bloom date of the trees was defined on 19 Apr. 2010, 1 May 2011, and 2 May 2012. The orchard was irrigated by drip irrigation and fertilized according to soil analysis. The treatments employed factorial randomized complete block design with at least five replications including five trees. All trees were sprayed to run-off using a backpack sprayer. Surfactant (Tween 20®, Merck, Schuchardt OHG, Hohenbrunn, Germany) was added in all sprays at 0.1% to increase their efficiency. In the first trial year, selected trees produced high yields in the ‘on-year’. Hand thinning was not done in order to determine actual impact of treatments. The treatments and pertinent details are given below.

• Control trees were not sprayed.

• Ethephon (Efhun, AgroBest Group, Turkey) was applied with 100 + 100 ppm at 3-week intervals. The first spray was performed when fruit size averaged 20 mm (21–26 days after full bloom (DAFB)).

• GA₄₊₇ (Novagib, Fine Agrochemicals Ltd., USA) trees were sprayed with 150 + 150 ppm at 3-week intervals when fruits reached 10 mm (16–21 DAFB).

Data collection

Flower density was assessed as the number of flower clusters per trunk cross-sectional area (TCSA) at pink bloom. Calculations of flowering index (FI) and cluster numbers per branch cross-sectional area (BCSA) were detected on three limbs of uniform size on each tree by Westwood (1995). Fruit set, yield efficiency, and crop density were determined at commercial harvest. Alternate bearing index (ABI) was calculated by the formula of Hoblyn et al. (1936) based on fluctuations of yield in succesive years. As indicated by the formula, ABI of cultivars ranges between 0–1, with 0 = no alternation and 1 = complete alternation. We also assessed modified alternate bearing index (MABI), developed by Racsko (2008), with flowering values instead of yield. Alternation groups of MABI were indicated as: not susceptible, <0.26; medium alternation, 0.26–0.50; susceptible to alternation, 0.51–0.75; and high alternation, >0.75.

The shoot length was measured at randomly selected 10 bourse shoots in each tree after the terminal bud formation. Average fruit weight, fruit firmness, fruit diameter, and length/diameter (L/D) were assessed on 45 fruits in triplicate at the harvest time. After fruit firmness was performed by a hand-held penetrometer with an 11.1-mm tip, seeds in each fruit were also counted. Soluble solids concentration (SSC) was assessed by a digital refractometer HANNA (Hanna Instruments 96801, USA) in fruit juice.

Statistical analysis

The obtained data were subjected to variance analysis by using the software package SAS-JUMP 8.0 (SAS Institute Inc., USA) and means with significant differences were grouped by a least significant difference (LSD) multiple comparison test.

Results

Flowering

In a 2010 trial, the first treatment of GA₄₊₇ was applied at 10-mm fruit diameter. Therefore, flowering measurements in the first year were the initial data. Since all of the selected trees were almost uniform, all measurements except cluster number per BCSA were found to be statistically insignificant (Table 1). In 2011, all trees were in ‘off-year’. However, the highest cluster number per TCSA was obtained with GA₄₊₇ (1.21 cluster cm⁻²), which was followed by ethephon (0.73 cluster cm⁻²) and control (0.25 cluster cm⁻²). The effects of PGRs on all flowering data were statistically significant in 2012 (P ≤ 0.05). GA₄₊₇ significantly reduced flowering values. In 2012, the cluster number per TCSA was the lowest in GA₄₊₇ (4.93 cluster cm⁻²) while the highest was in ethephon (10.69 cluster cm⁻²). Flowering data was also recorded in the 4th year (2013) to obtain more accurate results related to alternate bearing. In 2013, all trees were generally in ‘off-year’ similar to the cycle in 2011. The flowering values of ethephon in this year were the highest (Table 1), which was followed by GA₄₊₇. There was a highly significant interaction between treatment and year for flowering parameter.

Table 1. Effects of GA₄₊₇ and ethephon on flowering in ‘Golden Delicious’/M9 apple trees.

Treatment	Cluster no cm^−2 (TCSA)^z	Cluster no cm^−2 (BCSA)^z	Flowering index
2010
GA4+7	6.23	5.81a^y	0.79
Ethephon	6.80	6.91a	0.80
Control	5.89	3.61b	0.73
LSD (P ≤ 0.05)	NS	2.10	NS
2011
GA4+7	1.21a	0.58	0.13
Ethephon	0.73ab	0.70	0.13
Control	0.25b	0.48	0.05
LSD (P ≤ 0.05)	0.67	NS	NS
2012
GA4+7	4.93b	3.18b	0.68b
Ethephon	10.69a	5.35a	0.96a
Control	9.40a	5.40a	0.95a
LSD (P ≤ 0.05)	2.76	2.12	0.12
2013
GA4+7	1.08b	0.49b	0.14b
Ethephon	2.14a	0.96a	0.29a
Control	0.72b	0.33b	0.08b
LSD (P ≤ 0.05)	0.99	0.44	0.08
Treatment × year (2010 and 2013) interaction
P value	0.0001	0.0062	0.0004

^zTCSA: trunk cross-sectional area; BCSA: branch cross-sectional area.

^yMeans with different letters in each column are significantly different at probability level P ≤ 0.05, according to LSD multiple comparison test. NS: non significant.

Yield

The effects of PGRs on yield were statistically significant in 2010 (P ≤ 0.05). Control trees had the highest yield parameters in 2010 followed by ethephon and GA₄₊₇. There was a significant fruit set reduction in ethephon and GA₄₊₇ compared to control (Table 2). After the ‘on year’, yield values of treated trees significantly (P ≤ 0.05) decreased parallel to flowering values in 2011. The most effective response was obtained by GA₄₊₇ (Table 2). While yield efficiency and crop density were respectively 0.43 and 1.93 in GA₄₊₇, the same values were respectively 0.08 and 0.37 in control. The PGRs had a statistically insignificant effect on fruit set in 2011.

Table 2. Effects of GA₄₊₇ and ethephon on yield parameters in ‘Golden Delicious’/M9 apple trees.

Treatment	Yield efficiency (kg cm^−2)	Crop density (fruit no cm^−2)	Fruit set (%)
2010
GA4+7	1.05b^z	4.57c	17.8b
Ethephon	1.34ab	7.32b	23.5b
Control	1.80a	9.93a	36.8a
LSD (P ≤ 0.05)	0.46	2.16	8.2
2011
GA4+7	0.43a	1.93a	33.9
Ethephon	0.17ab	0.76ab	34.2
Control	0.08b	0.37b	40.6
LSD (P ≤ 0.05)	0.28	1.25	NS
2012
GA4+7	0.85b	5.56b	24.7a
Ethephon	1.01b	6.88b	12.4b
Control	1.40a	11.55a	22.6a
LSD (P ≤ 0.05)	0.27	2.48	7.1
Treatment × year (2010 and 2012) interaction
P value	0.0004	0.0001	0.2253

^zMeans with different letters in each column are significantly different at probability level P ≤ 0.05, according to LSD multiple comparison test. NS: non significant.

In 2012, yield remarkably increased compared to 2011, and all yield values were statistically significant (P ≤ 0.05). Similar to 2010, yield was the highest in control trees while the lowest were obtained from ethephon, and GA₄₊₇ (Table 2). Yield efficiency was recorded as 1.40 kg cm⁻² in control, 1.01 kg cm⁻² in ethephon, and 0.85 in GA₄₊₇. The highest fruit set was observed in GA₄₊₇ in 2012. There was a significant interaction between treatment and year for yield characteristics except fruit set (Table 2).

Alternate bearing

The lowest ABI was recorded in GA₄₊₇ (0.41), which indicated low alternate bearing intensity (Table 3), whereas ethephon was the moderate (0.71). The highest alternate bearing intensity was observed in control trees (0.91). Based on MABI values calculated with cluster number per tree, GA₄₊₇ was associated with moderate alternate bearing intensity (0.53), while ethephon and control showed high alternate bearing intensities.

Table 3. Influence of GA₄₊₇ and ethephon on alternate bearing Index (ABI) and modified alternate bearing index (MABI) in ‘Golden Delicious’/M9 apple trees.

Treatment	ABI	MABI
GA4+7	0.41 (low)^z	0.53 (medium)
Ethephon	0.71 (medium)	0.79 (high)
Control	0.91 (high)	0.90 (high)

^zIndex values were grouped according to Racsko (2008).

Shoot growth

The effects of PGRs on mean shoot length were statistically significant (P ≤ 0.05) during 3 years (Table 4). Ethephon reduced shoot length in 2011 and 2012 relative to the control, although there was no significant effect in 2010. GA₄₊₇ induced shoots elongation compared to control in 2010 and 2011, whereas in 2012 shoot lengths of GA₄₊₇ and control were similar. Treatment × year interaction was significant for shoot length (P ≤ 0.0001).

Table 4. Influence of GA₄₊₇ and ethephon on shoot length in ‘Golden Delicious’/M9 apple trees.

	Shoot length (cm)
Treatment	2010	2011	2012
GA4+7	51.0a^z	60.7a	43.1a
Ethephon	36.9b	45.7c	39.9b
Control	38.2b	54.0b	43.1a
LSD (P ≤ 0.05)	3.2	2.7	2.1
Treatment × year (2010 and 2012) interaction
P value	0.0001

^zMeans with different letters in each column are significantly different at probability level P ≤ 0.05, according to LSD multiple comparison test.

Fruit quality

The heavier and larger sized fruits were obtained from GA₄₊₇-treated trees in each 3 trial years (Table 5), and differences among treatments were statistically significant (P ≤ 0.05). While fruit size was almost similar in control and ethephon in both 2010 and 2011 trials, ethephon-treated trees had the larger sized fruits than control in 2012. The effects of treatments on L/D were a statistically significant exception in 2011, and GA₄₊₇ applications gave the highest L/D. Fruit firmness ranged between 6.2 and 7.4 kg during the 3 years. The SSC values varied depending on applications and years. In the ‘off-year’, SSC values were generally high in all treatments compared to the other 2 years. The treatments affected to seed number (Table 5), and the differences were statistically significant except 2011 (P ≤ 0.05). GA₄₊₇-treated trees had the lowest seed number, while control and ethephon were almost similar. There were significant interactions between treatment and year for fruit weight, diameter, L/D, SSC, and seed number, while treatment × year interaction was not significant for firmness (Table 5).

Table 5. Effects of GA₄₊₇ and ethephon on fruit quality features in ‘Golden Delicious’ apple.

Treatment	Fruit weight (g)	Fruit diameter (mm)	L/D^z	Firmness (kg)	SSC^z (%)	Seed number
2010
GA4+7	263.6a^y	83.2a	1.00a	6.7	14.5a	3.6b
Ethephon	211.1b	78.2b	0.94bc	6.7	13.1b	7.6a
Control	208.3b*	78.7b	0.92c	6.9	12.7b	7.0a
LSD (P ≤ 0.05)	17.1	1.8	0.01	NS	0.7	1.2
2011
GA4+7	234.4a	79.7a	0.94	7.1	14.4	4.8
Ethephon	199.0b	75.7b	0.93	7.3	14.4	5.4
Control	203.6b	77.0b	0.93	7.4	14.7	5.0
LSD (P ≤ 0.05)	14.6	1.7	NS	NS	NS	NS
2012
GA4+7	211.1a	76.3a	1.01a	6.4b	13.7	5.0b
Ethephon	161.1b	71.3b	0.93b	6.2b	13.5	7.3a
Control	141.5c	68.4c	0.94b	6.8a	13.4	6.4a
LSD (P ≤ 0.05)	10.1	1.6	0.01	0.3	NS	1.1
Treatment × year (2010 and 2012) interaction
P value	0.0054	0.0004	0.0001	0.1456	0.0006	0.0008

^zL/D: length/diameter; SSC: soluble solids concentration.

^yMeans with different letters in each column are significantly different at probability level P ≤ 0.05, according to LSD multiple comparison test. NS: non significant.

Discussion

GA₄₊₇ Trial

Luckwill (1974) explained that GAs affect flower bud formation by lengthening the plastochron. Too short or too long durations of plastochron, the time interval between initiation of successive primordia, were reported to induce distortions in meristem (Jackson, 2003), which inhibits the differentiation of vegetative bud to flower bud. On the other hand, plastochron durations and critical nod number are not considered as significant determinants for flowering by some researchers (Verheij, 1996). Even if flowering inhibiting mechanism of GAs cannot be expressed clearly, they are accepted as the most closely related with flowering among all plant hormones known so far.

In the study, trees treated GA₄₊₇ were in the low alternation group in accordence with ABI (0.41), which was performed with yield per tree, while control trees (0.91) were classified as high. Nevertheless, ABI based on only yield can be misleading about alternation. Flowering is generally accepted as an adequate indicator of generative development for many fruit trees. However, fruit set is considered as a different aspect, which can be subject to much more interactive circumstance. Therefore, the index values representing only the yield fluctuation by years were not assumed to be totally precise by Racsko (2008), and a different index, called MABI, was enhanced for describing alternate bearing more exactly based on flowering. We also performed MABI, and GA₄₊₇-treated trees demonstrated moderate alternate bearing intensity (0.53) and control trees had high alternate bearing intensity (0.90). In this respect, GA₄₊₇ was found to be fairly promising for reducing alternate bearing for ‘Golden Delicious’ apples in agreement with former studies (Davis, 2002; Schmidt, 2006). This approach is a practical and effective method of crop load control that could provide growers with an alternative approach to chemical thinning. Indeed, exogenous NAA and BA alone may prove to be effective thinning chemicals, but did nothing or little to influence flowering or return bloom of apples (Davis, 2002; Reyes et al., 2008). The flower thinning has been improved to the fruit quality of the current year rather than flower bud formation (Wright et al., 2006).

In 2012, GA₄₊₇ decreased cluster number per TCSA by 48%, cluster number per BCSA by 41%, and cluster number per BL by 47% in comparison to control. Similarly, GA₄₊₇ reduced FI by 28%. Garcia-Pallas et al. (2001) reported that the reduction of flower bud formation by GAs in apple provides a varied crop load management, and they recommended this application as an indirect thinning to apple growers. In agreement with our results, external GA₄₊₇ treatments were stated to reduce flowering also in ‘Royal Gala’ (McArtney and Hoover, 2005), ‘Fuji’ and ‘Honeycrisp’ (Schmidt, 2006), and ‘Cameo’ (Schmidt et al., 2009).

The inhibiting effect of GA₄₊₇ on flowering became observable particularly in 2012. GA₄₊₇, applied to trees having abundant flowers/fruits in the first trial year (2010), slightly increased to flower bud formation in 2011. The interaction between GA treatments and initial crop load has not been clear in the literature. However, ‘off-year’ trees are stated as more sensitive to external GA than ‘on-year’, and hence they demonstrate more explicit responses to GA (Schmidt et al., 2009). Immature seeds in apple are the main sources of GAs and their distributions are known to exceed in ‘on-year’ (Tromp, 2005). Due to this fact, supplementation of additional GA by an external treatment in ‘on-year’ would lead to an excessive increase in the already existing high internal GA level. Schmidt et al. (2009) claimed that high doses of GA in early development phases may be harmful and stimulate ethylene synthesis. The increases in flowering in 2011 and 2013 can be attributed to stimulating ethylene synthesis or several stress factors in the current study.

The crop density reduced with GA₄₊₇ to about 50% at the end of the 3rd year. On the other hand, there was little variation in yield per tree and crop efficiency. It can be ascribed to the effect of GA₄₊₇ increasing fruit size and decreasing fruit number. As a matter of fact, GAs have been widely used to increase fruit size in many fruit species (Greene, 2000). Wertheim and Webster (2005) reported that GA₄₊₇ did not have any effect on initial fruit set until 40 days after full bloom, but it reduced June drop and resulted in increased fruit number. Although GAs can increase fruit set in apples, there are inconsistent reports about this effect of GAs (Greene, 2000). Also, in the current study GA₄₊₇ significantly reduced fruit set particularly in the first trial year compared to control. However, GA₄₊₇ resulted in higher fruit set in the following years. GAs are generally known to promote vegetative growth depending on cultivar and environmental conditions (Wertheim and Webster, 2005). GA₄₊₇ increased shoot length by about 32% in ‘Fuji’, by about 29–138% in ‘Cameo’ and did not induce a significant increase in ‘Honeycrisp’ (Schmidt et al., 2009). In our study, GA₄₊₇ increased shoot length by about 12–34% in the first 2 years. However, in 2012, average shoot lengths were not statistically significant in GA₄₊₇ and control. This outcome is possibly attributed to the equilibrium between vegetative and generative growth.

Flowering is an important indicator of yield and also affects fruit quality. The increase in flower number leads to reduced flower quality, and hence, reduced fruit quality. Due to this fact, management of flowering is accepted as a standard procedure for improving fruit quality in many species. The competition is stronger for metabolics in the first phase of fruit growth, and adverse growth conditions can reduce fruit size during harvest (Guardiola, 1997).

In the study, GA₄₊₇ had significant advantages for fruit quality. External GA treatments increase fruit size by stimulating cell division and particularly cell growth. Compared to control, GA₄₊₇ increased fruit weight by 27%, 15%, and 49% in 2010, 2011, and 2012, respectively. The rises of fruit diameter were about 6%, 3%, and 12%, respectively. The relatively small increase in fruit size induced by GA₄₊₇ in 2011 can be attributed to trees being in ‘off-year’. High L/D is greatly favored for some apple cultivars in commercial apple production. It was formerly reported that GA₄₊₇ stimulated to elongate fruits (Wertheim and Webster, 2005). This also accords with our findings in which GA₄₊₇ significantly increased L/D in ‘Golden Delicious’ apple.

Fruit firmness of GA₄₊₇-treated apples was relatively lower than control. GA₄₊₇ reduced firmness in ‘Honeycrisp’ (Schmidt, 2006) apples. On the other hand, different cultivars could have varying responses to GAs. GA₄₊₇ had no effect on firmness in ‘Cameo’ (Schmidt, 2006). The partial decrease in fruit firmness induced by GA₄₊₇ in the current study can be attributed to the high dose. Schmidt et al. (2009) claimed that high doses of GA in early development phases may be harmful and stimulate ethylene synthesis. GAs also stimulate cell growth, and larger cells are arranged with larger intercellular spaces. This can reduce specific weight and firmness of fruits (Karaçalı, 2004).

Although its exact mechanism is not clear, parthenocarpy is largely known as deterioration of hormonal balance between GAs and auxins, and external GA₄₊₇ treatments increase parthenocarpic fruit set (Watanabe et al., 2008). The changes of internal hormone levels (especially auxin and GA) due to GA treatments can cause embryo abortion. In the present study, GA₄₊₇ was applied after the post-fertilization; parthenocarpic fruit development was induced through embryo abortion. Because of that, average seed number of GA₄₊₇ fruits was relatively low compared to the others. However, parthenocarpic fruit development is much more important for pears flowering earlier than apples, especially in cases of frost damage or poor pollination.

Ethephon trial

Many researchers suggested that ethephon promotes generative growth (Greene, 1996; Westwood, 1995), whereas there are insignificant findings (McArtney et al., 2007; Schmidt et al., 2009). Ethephon was also found as insignificant in the first three years of our trial. However, in the 4th year (2013), which was an ‘off-year’, ethephon significantly increased flowering compared to the others. The most evident response observed in 2013 possibly resulted in some climatic parameters in the previous treatment year. We recorded average temperatures of June, ethephon applied period, as 18°C in 2010 and 2011 and 23°C in 2012 for the experimental orchard (TSMS, 2013). Ethephon is highly sensitive to environmental factors, such as post-treatment temperatures, and hence, the amount of concentration must be adjusted very carefully. High doses (above 500 ppm) were not recommended for bearing trees due to increasing thinning, fruit drop, and ripening (Byers, 1993). Bukovac et al. (2006) and McArtney et al. (2007) recommended 200 ppm of ethephon to enhance return bloom. However, in our study, ethephon became inefficient, and we think that a trial with higher doses could be more effective in our environment.

Ethephon is generally applied at bloom or petals fall as a thinning agent. We used ethephon due to its promoting potential of flowering year after year. It was recommended that ethephon can be sprayed at 5, 7, and 9 weeks after bloom or until fruit up to 25-mm diameter (Autio and Cowgill, 2009). Therefore, the first ethephon treatment was applied at 20-mm fruit diameter (21–29 DAFB) to promote flowering in bearing apple trees without excessive thinning of the current crop. However, ethephon resulted in remarkably lower fruit set while flowering data of control and ethephon-treated trees were closely similar during the trial.

ABI was moderate in ethephon-treated trees (0.71) and high in control (0.91). MABI values indicated high group in both ethephon (0.79) and control (0.90). As seen from our findings, ethephon was relatively less influential to mitigate alternate bearing. Schmidt et al. (2009) also reported that ethephon was generally ineffective for establishing the equilibrium between vegetative and generative growth compared to GAs.

Ethephon has reduced shoot length by 7–15% except the first trial year. Growth-inhibiting effect of ethephon is less than GA inhibitors (Wertheim and Webster, 2005). Additionally, multiple low doses (100–200 ppm) or a single high dose (500–1000 ppm) treatment is more effective on control of shoot growth (Byers, 1993). However, a high dose can be safely used only in non-bearing trees or mature trees exposed to spring frost damages (Wertheim and Webster, 2005). Multiple low doses were chosen since this research was evaluated on bearing trees. On the other hand, the application at late phase of shoot growth may also be another factor.

Except for the last trial year, similar size fruits were obtained from ethephon and control. In 2012, ethephon significantly increased fruit weight and fruit diameter. This is a possible outcome of reduced number of fruits particularly induced by ethephon in 2012. The effect of crop load management on fruit quality is well known in deciduous fruit trees, such as apples. Ethephon treatments in early phases were revealed not to have a significant effect on fruit quality parameters, such as firmness and SSC, compared to control (Bukovac et al., 2006; Karaçalı, 2004). These results are also in accord with our findings. Ethylene can promote fruit ripening when fruits accumulated sufficient substances and reached physiological maturity. For this reason, young fruits respond as non-climacteric fruits or vegetative tissues (Karaçalı, 2004).

Treatment × year interaction was generally significant in this study. There is a year-to-year effect on flowering, yield, shoot length, and fruit quality. ‘Golden Delicious’ has a strong tendency to alternate bearing (Atay et al., 2013); therefore, flower and fruit density vary depending on ‘off years’ and ‘on years’. In this study, the years 2010 and 2012 were ‘on years’, and flower and yield values were higher in those years.

Conclusions

Our results showed that GA₄₊₇ is an effective and promising PGR on management of regular cropping and also modification of alternate bearing in apples. Compared to GA₄₊₇, ethephon had no effect on alternate bearing. However, forthcoming studies with higher ethephon doses are expected to be more useful in understanding of its effects. Manipulation of the flowering cycle with GA₄₊₇ application could provide an alternative approach to chemical thinning for the growers. The use of reliable and applicable flowering management techniques by improving flower quality, particularly in the abundant flowers years, is also expected to improve fruit quality. GA₄₊₇ notably enhanced fruit size (15–49%) and increased fruit quality.

Funding

This work was supported by the Scientific Research Projects Coordination Unit of Süleyman Demirel University of Turkey [Project No: 2782-D-11].

Additional information

Funding

This work was supported by the Scientific Research Projects Coordination Unit of Süleyman Demirel University of Turkey [Project No: 2782-D-11].