Exogenous Gibberellins Affect the Setting, Development, and Quality of ‘Golden Delicious’ Apple Fruits

Abstract

The aim of this study was to investigate the effect of gibberellins on the setting and quality of parthenocarpic apples (Malus × domestica Borkh.). The experiment was conducted in 2021 on the ‘Golden Delicious’ cultivar in the Warsaw University of Life Sciences experimental orchard. During the trial, we compared the effect of various gibberellins, such as GA₃, GA₄₊₇, and a mixture of GA₃ + GA₄₊₇. These gibberellins were administered to both intact and mechanically injured flowers (damaged by emasculation and style removal) at the pink bud stage. The results clearly demonstrate that gibberellins applied during blooming supported the induction of parthenocarpic fruit setting in Golden Delicious apples; however, fruitlet retention remained significantly lower than in natural pollinated flowers. The most efficient treatment among emasculated flowers was the mixture of GAs, resulting in a final fruit retention of 23.6%. Fruit size and morphology differed across treatments: GA₃ applied on intact flowers resulted in the largest parthenocarpic fruits, while the GA₄₊₇ and GAs mixture promoted a more elongated fruit shape. Moreover, gibberellin treatment affected other fruit quality traits. Almost all GA treatments led to a higher soluble solids content in fruits. In addition, apples derived from intact flowers treated with GA₃ showed reduced firmness. Overall, our findings indicate that gibberellins can support fruit set, even when flowers are injured, and to lower extent modify fruit quality, but the results depend on flower condi-tions and the type of GA used.

1. Introduction

One of the more commonly used groups of phytohormones in fruit production is gibberellins, which are involved in growth regulation (elongation growth), dormancy, seed germination, flowering, and fruit setting [1,2]. Several types of gibberellins have been described in higher plants, which differ mainly in their level of activity due to their structure. According to [3], the most biologically active forms include GA₃, as well as GA₁, GA₄, and GA₂; among these forms, GA₃ and GA₄ (the latter as a mixture of GA₄₊₇) are most commonly used in fruit production.

In fruit production, gibberellins are primarily used to improve fruit quality [4,5]. Several studies have shown the positive effect of using GA₄₊₇ in reducing the occurrence of russeting in ‘Golden Delicious’ apples [6,7,8,9,10] and preventing other skin defects such as the formation of cracks in the calyx cavity in ‘Pink Lady’ apples [11,12] or whole-fruit cracking in the ‘Stayman’ variety [13]. According to previous studies, the beneficial effect on fruit skin quality may be due to an increase in the number of epidermal cells alongside a simultaneous reduction in their size, as well as an increase in the amount and density of cuticles on fruit treated with gibberellin preparations [14,15,16,17]. Such changes result from an increase in skin tissue flexibility and mechanical strength and may reduce the formation of microcracks, the risk of which is particularly high during rapid fruitlet growth [18].

Gibberellins are also commonly used to improve the shape of apples. According to Marcelle [19], to obtain high-quality ‘Golden Delicious’ apples, the ratio of fruit height to diameter should be at least 0.95. GA₄₊₇, in particular, is used in combination with benzyl adenine to achieve this ratio. Moreover, fruits from trees treated with gibberellins become more slender. In the case of varieties with pronounced ribbing around the calyx (varieties of the ‘Red Delicious’ group), this feature of the fruit also becomes significantly more pronounced, increasing their attractiveness [20].

Gibberellins have been used for many years to improve fruit setting [21]. In practice, they are commonly used in pear cultivation to spray plants (flowers) immediately before and during flowering to enhance parthenocarpy. Such stimulatory parthenocarpy leads to the development of ovaries and other flower elements and, as a result, to the formation of seedless or seed-limited fruit. The induction of parthenocarpic fruit formation is widely used not only in pear trees but also in vines, and has also been confirmed and described in apple trees.

One way to exploit this phenomenon in apple crops is to improve fruit setting when adverse weather conditions occur during flowering. Spring frosts are one of the main causes of yield losses in orchard crops [22], where the temperature drops below critical values for the crop during flowering [23,24]. The effectiveness of growth regulators, including gibberellins, is confirmed by the research of Goldwin [25]. A comparison of the efficacy of GA₃ and GA₄₊₇ treatment, carried out by the aforementioned author, on the cultivar ‘Cox’s Orange Pippin’ showed that both forms improve setting but that GA₄₊₇ showed higher efficacy. The possibility of stimulating parthenocarpic fruit setting in apple trees was also confirmed by Galimba et al. [26] using, among others, GA₃ on the cultivar ‘Honeycrisp’. This well-known outcome is particularly relevant in the context of developing strategies to mitigate the negative effects of spring frosts on apple yield using hormonal treatments. Since individual floral organs differ in their susceptibility to low-temperature injury during the bloom period, some tissues (including ovary structures or gametophytes) may remain physiologically functional even after exposure to spring frosts [27,28,29]. This observation suggests that when the flower is only partially damaged, stimulation with hormones may lead to fruit development even if the style/stigma has been injured.

The aim of this work was to assess the impact of exogenous gibberellins on parthenocarpic fruit formation from partially damaged apple flowers. Using the ‘Golden Delicious’ cultivar—the most widely cultivated apple in Europe—increases the potential application of the results to a broader range of commercial orchards. The outcome of this study contributes to the development of sustainable agrotechnical practices that can support consistent production under challenging environmental conditions, especially in changing climatic conditions.

2. Materials and Methods

2.1. Experimental Site, Conditions, and Design

The trial was set up in the spring of 2021 in an experimental orchard of Warsaw University of Life Sciences. The experimental site is located in the suburbs of Warsaw in the east-central part of Poland (N 52°9′36.1′′, E 21°5′58.2′′). Weather conditions at the experimental site in 2021 were monitored with a Davis Vantage Pro 7 weather station installed in the orchard and are presented in Figure 1.

The plant material used for our hormone studies consisted of ten-year-old uniformly sized apple (Malus × domestica Borkh.) trees of the ‘Golden Delicious’ cultivar grafted on M.9 and trained in slender spindle canopy. The experimental block was located on silty loam alluvial soil, and trees were grown with 3.5 × 1 m spacing between them. There was no irrigation provided during the trial. Other agrotechnical treatments were made in line with local standards of Integrated Production rules.

During the trial, 8 different combinations were compared as follows:

1.

Open pollination of intact flowers as a control treatment (IFOP);

2.

Open pollination of flowers previously emasculated with a removed style (EFOP);

3.

Intact flowers protected from pollination, followed by spraying with GA₃ (IFGA3);

4.

Emasculated flowers with a removed style, protected from pollination, followed by spraying with GA₃ (EFGA3);

5.

Intact flowers, protected from pollination, followed by spraying with GA_4/7 (IFGA4/7);

6.

Emasculated flowers with a removed style, protected from pollination, followed by spraying with GA_4/7 (EFGA4/7);

7.

Intact flowers protected from pollination, sprayed with a GA3 and GA4/7 mixture (IFGAmix);

8.

Emasculated flowers with the style removed, protected from pollination and sprayed with GA₃ and GA₄/₇ (EFGAmix).

Each combination was represented by 3 replicates of 200 flower clusters, in which a central flower and one side flower were left at the pink bud stage and permanently marked so that they could be identified later. Flower clusters were chosen on the east side of the row, in the middle part of the crown at a height of approximately 150 cm on twenty trees for each replication. Flowers to be sprayed with gibberellins were isolated with white agro-tissue to prevent pollination. Spraying with gibberellin solution was performed twice—directly before the beginning of flowering (12 May) and after 7 days. Depending on the combination, a solution of gibberellin or a mixture of gibberellins at a concentration of 2.9 mM [26] was used for spraying.

2.2. Fruitlet Retention and Growth

To determine the retention and growth of fruitlets/fruits, they were counted at two-week intervals and measured using calipers from the time of flowering until one week before harvest. The data presented in this work account for the results from every other measurement date (presented as days after treatment, DAT) and are expressed as the share of the initial number of fruitlets still present on the tree given in % and the diameter of fruitlets given in mm.

2.3. Fruit Morphology and Quality

Fruit parameters were determined at harvest (160 DAT) on apples remaining on the tree. Apples were harvested from each combination represented by three replications. Ten fruits per replication were tested, excluding the combinations IFGA4/7, EFGA3, and EFOP as the final retention rate was low, and thus, we were only able to collect six fruits per repetition.

Selected characteristics of fruit morphology such as fruit height and width, the depth of the stalk cavity, and the eye basin’s depth (the latter two measured after sectioning the fruit) were measured using a digital caliper and given in mm. After initial evaluation of individual fruit morphology, seeds were divided into two groups—well-developed (represented by properly shaped and plump seeds) and undeveloped—counted, and weighed on a digital scale.

Fruit firmness was determined using an Instron 5540-type firmness meter (Instron, Norwood, MA, USA). A narrow peel strip was peeled from the fruit on the blush (if was present) and opposite sides, exposing the flesh. The measurement was made in the peeled areas directly in the flesh, with an 11 mm diameter stylus penetrating the flesh to a depth of 10 mm. Measurement results were expressed in Newtons [N].

Soluble solids content (SSC) was determined in apple juice made using a juicer. The juice was dripped on a refractometer (Atago, Palette PR-32, Atago, Co., Ltd., Tokyo, Japan) and the measurements were performed directly after evaluating fruit firmness. The results were expressed in Brix degrees (°Brix).

Titratable acidity (TA) was measured using a solution composed of apple juice and distilled water (10 mL of juice per 100 mL of water). Then, the solution was titrated with 0.1-molic NaOH to an end point pH = 8.1. TA was measured using an automatic titrator (TitroLine 5000, Xylem Analytics Germany GmbH, Weilheim, Germany) and the results were expressed as the percent of malic acid.

The starch index was measured on the cross sections of apples by dipping their surface in an iodine solution in potassium iodide. The color of fruit flesh was assessed from 1 to 10 according to standard tables of starch degradation, where a score of 1 indicates unripe fruits (initial stage of starch degradation) and a score of 10 corresponds to fully ripe fruits.

Considering the flesh firmness, SSC, and starch index value, the Streif Index was calculated according to the following formula:

SI = F/(R × S)

where SI is the Streif Index, F is the firmness (recalculated and expressed in kgs), R is the soluble solids content (°Bx), and S is the starch test result (scale 1–10).

2.4. Statistical Analysis

To analyze the data gathered during the trial, one-way ANOVA was used. All tests were made in the Statistica 13.3 software package. Regarding fruitlet retention and growth, only data gathered on the last date of assessment (154 DAT) were analyzed. The means were separated according to the Newman–Keuls post hoc test at a significance level of p ≤ 0.05.

3. Results

3.1. Fruitlet Retention and Growth

According to the gathered data, in all treatments, a gradual decline in fruitlet retention was observed (Figure 2). In particular, a rapid decrease was noted up to 42 DAT, which corresponds to physiological fruitlet abscission known as June drop in apple. Following this period, the observed values stabilized, with only minor changes, except for IFGA3, where a slight decline continued up to 70 DAT. The final retention rates differed significantly. The highest values were observed for the IFOP combination, where almost 50% of fruitlets were retained until harvest. A significantly lower rate was noted for EFGAmix, where values reached 23.6%, followed by even lower rates for EFGA4/7, IFGA3, and IFGAmix. The lowest fruitlet retention rates were observed for IFGA4/7, EFGA3, and EFOP, varying between 3.1 and 5.9%.

In our experiment, fruitlet growth exhibited a gradual change, with relatively high growth rates noted up to 84 DAT, after which all growth curves start to flatten steadily. Flowers treated with the IFOP combination showed the least pronounced plateau in fruitlet growth (Figure 3). As such, higher values were noted for IFOP when compared with other treatments, excluding IFGA3, at the final point of observation.

3.2. Fruit Mass and Shape

The results indicate that applying gibberellins significantly affected fruit mass (

Table 1. The size and shape of the fruit depending on treatment.

Treatment	Fruit Mass [g]	Fruit Height (h) [mm]	Fruit Width (w) [mm]	Depth of Stalk Cavity (sc) [mm]	Depth of Eye Basin (eb) [mm]	Ratios of Chosen Fruit Dimensions [No Units]
						h/w	h/sc	h/eb
IFOP	156 b	74.0 a	72.7 a	15.0 b	6.47 a	1.02 ab	4.92 ab	11.4 a
EFOP	136 ab	56.0 a	65.3 a	13.3 ab	5.88 a	0.86 a	4.24 a	9.61 a
IFGA3	152 b	72.0 a	68.2 a	13.6 ab	6.95 a	1.06 ab	5.36 ab	10.6 a
EFGA3	125 ab	67.1 a	63.5 a	11.3 a	6.41 a	1.06 ab	5.91 ab	10.6 a
IFGA4/7	137 ab	67.8 a	64.5 a	11.2 a	5.25 a	1.05 ab	6.13 ab	13.0 a
EFGA4/7	125 ab	70.4 a	61.6 a	11.2 a	6.46 a	1.14 b	6.29 b	11.1 a
IFGAmix	118 a	70.3 a	62.1 a	11.3 a	5.65 a	1.13 b	6.31 b	12.5 a
EFGAmix	109 a	73.9 a	60.6 a	12.0 a	6.93 a	1.22 b	6.14 ab	10.7 a

Note: Means followed by the same letter in columns do not differ statistically according to the Newman–Keuls post hoc test at p ≤ 0.05.

). A lower final mass was noted for fruits treated with the IFGAmix combination and EFGAmix when compared with IFOP and IFGA3. The results obtained for the remaining combinations showed intermediate values.

Regarding apple shape, gibberellin treatment did not significantly affect the fruit’s height and width, the depth of the eye basin, and the ratio of height to depth of the eye basin (h/eb). A clear effect was observed for the height-to-width ratio (h/w): higher values were observed for fruits treated with the EFGA4/7, IFGAmix, and EFGAmix combinations in comparison with the EFOP combination. The remaining treatments resulted in intermediate values that did not differ statistically from other tested combinations. A significant influence was also noted on the height-to-stalk cavity depth ratio (h/sc). Higher ratios were observed for the EFGA4/7 and IFGAmix combinations and differed significantly compared with apples grown under EFOP treatment.

The results indicate that the application of gibberellins had a significant effect on the number of properly formed seeds in the seed chambers of the fruits. The fruits treated with the IFOP combination had a higher number of seeds in the seed socket compared with apples treated with all other combinations (Figure 4).

The treatments tested in our experiment affected the number of well-developed seeds. The lowest value was recorded for combinations in which GA₄₊₇ or GA₃ + GA₄₊₇ spraying was used, regardless of whether the flowers were emasculated or not. A significantly higher number of properly formed seeds was observed for the EFGA3, IFGA3, and EFOP treatments; however, the values were still significantly lower than those obtained for fruits developed under the IFOP combination. No undeveloped seeds were found for the IFOP, EFOP, IFGA4/7, and EFGA4/7 combinations.

The use of gibberellins had a significant effect on average seed mass. The highest 100-seed mass was noted for the IFOP control combination; significantly smaller seeds were produced under the EFOP, IFGA3, and IFGA4/7 combinations, with the smallest observed for EFGAmix.

3.3. Fruit Quality

The combinations used in the experiment affected fruit quality (

Table 2. Fruit quality parameters depending on treatment.

Treatment	Flesh Firmness [N]	Soluble Solids Content [°Brix]	Titratable Acidity [% Malic Acid]	SSC/TA Ratio [-]	Starch Index [-]	Streif Index [-]
IFOP	67.2 ab	11.6 a	0.47 a	24.7 a	8.2 a	0.072 b
EFOP	74.1 b	12.2 ab	0.50 a	24.3 a	8.3 a	0.075 b
IFGA3	64.4 a	12.7 b	0.50 a	25.7 a	8.3 a	0.062 a
EFGA3	75.5 b	12.3 ab	0.48 a	25.6 a	8.4 a	0.074 b
IFGA4/7	72.5 b	13.1 b	0.51 a	25.8 a	8.0 a	0.071 b
EFGA4/7	75.5 b	12.7 b	0.46 a	27.4 a	8.1 a	0.075 b
IFGAmix	75.3 b	13.1 b	0.51 a	25.7 a	8.2 a	0.072 b
EFGAmix	66.2 ab	13.2 b	0.47 a	28.1 a	8.1 a	0.063 a

Note: Means followed by the same letter in columns do not differ statistically according to the Newman–Keuls post hoc test at p ≤ 0.05.

). Apples treated with the IFGA3 combination showed reduced flesh firmness compared with the other combinations, excluding fruits treated with the IFOP and EFGAmix combinations, for which intermediate values were obtained.

Analysis of soluble solids content showed that parthenocarpic apples exhibited relatively high values. Almost all combinations in which gibberellins were used (excluding EFGA3) yielded apples with significantly higher soluble solids content compared with the control combination. On the other hand, there was no significant variation in the titratable acidity of apples nor in the ratio of soluble solids content to the titratable acidity of apples.

The application of gibberellins did not significantly affect the results of the starch test. Fruits treated with the different combinations exhibited similar values. On the other hand, the results of the Streif index were affected by treatment. A lower Streif index was noted for fruits treated with IFGA3 and EFGAmix combinations.

4. Discussion

The main goal of this study was to evaluate the efficacy of exogenous gibberellins in inducing parthenocarpic fruit setting in ‘Golden Delicious’ apple trees and to determine how flower damage (emasculation and stigma removal) modifies the response to GA treatment. The obtained results confirm that gibberellins applied during flowering can induce parthenocarpy, but the efficiency of fruitlet retention was significantly lower than under natural pollination conditions. Reduced fruitlet retention results from fundamental physiological effects, whereby in the absence of fertilization, the developing ovary does not receive the endogenous hormonal signals produced by seeds—primarily gibberellins and auxins—which are essential for sustaining young fruit growth and preventing their drop [30,31,32]. As a result, fruitlet abscission was most intense immediately after flowering, especially in non-pollinated flowers and those treated with GA only. Nevertheless, GA combinations showed a level of flower setting that, especially in conditions of regular blooming, may be of practical importance in fruit production.

A significant mechanism revealed in our study involves the interaction between exogenous gibberellin usage and the presence or absence of developing seeds. Emasculated flowers showed a more effective response to GA₄₊₇ than intact flowers, most likely due to the significantly reduced level of endogenous GA produced in the seedless ovary, which increases the sensitivity of this organ to externally derived hormonal signals. A similar phenomenon has been described in pear trees, where increased GA₄ biosynthesis intensified parthenocarpy, particularly in unpollinated flowers [33,34]. In contrast, intact flowers, in which the embryo develops, received a natural amount of gibberellins and auxins produced by the seeds, which limited the relative effect of externally applied GAs. This difference could explain the higher retention rate and better growth stimulation observed in emasculated flowers.

The unexpected presence of normal seeds in fruits developed from emasculated flowers may result from imprecise emasculation and accidental pollination [35]. Alternatively, seed development could be the result of facultative apomixis, as is observed in some Malus species, but it needs to be emphasized that this phenomenon has not been clearly confirmed in commercial cultivars [36]. However, the presence of seeds in single fruits does not completely undermine the general conclusion that flowers without generative organs are more sensitive to gibberellins.

Differences in fruit size and morphology demonstrate the different roles of endogenous and exogenous signals. Fruits from open pollination were relatively large (excluding comparison with the IFGA3 combination), emphasizing the importance of the seed signal for cell division and pericarp development. Among the gibberellin treatments, GA₃ applied to intact flowers resulted in the formation of relatively large parthenocarpic fruits, probably due to strong stimulation of cell division in early development stages. In contrast, the mixture of GAs induced the formation of smaller but more elongated fruits (high h/w ratio), partially consistent with the well-known effect of GA₄₊₇ promoting cell elongation more than lateral growth. Changes in fruit shape were also noted by Galimba et al. [26] in GA₃-induced parthenocarpic fruits of ‘Honeycrisp’, which showed reduced ovary width and modified fruit geometry. These differences may have practical implications for cultivars where elongation of fruit shape is desired.

Recent studies have shown that exogenous GAs can increase apple weight and modify the firmness and external quality of apples, depending on the dose and timing of application [36,37]. In our experiment, gibberellin treatments affected fruit quality, assessed directly after harvest. In our study, fruits derived from intact flowers treated with GA₃ showed relatively low firmness, while other combinations, excluding IFOP and EFGAmix, exhibited higher values. In this case, the observed differences may be attributed to variations in fruit size, which is crucial determinant of fruit firmness. Larger apples typically showed reduced firmness due to the lower density of parenchyma cells, increased cell volume, and higher rate of intercellular spaces, which can collectively weaken the mechanical resistance of the tissues [38]. The higher total soluble solids content (SSC) observed in most GA combinations suggests improved carbohydrate mobilization and starch degradation, which can relate to more advanced stages of fruit ripening. However, considering the Streif index values observed in our experiment, this phenomenon cannot be confirmed. Meanwhile, the results obtained for this parameter suggest that fruits treated with only IFGA3 and EFGAmix show a more advanced ripening stage.

Despite the clear trends observed in this study, further research is needed to better understand the underlying interactions of hormones, especially the balance between exogenous GA supply and seed-produced signals. Multi-year trials and dose–response studies are crucial for providing clear recommendations for commercial apple growers and ensuring consistent results across different environmental conditions.

5. Conclusions

Our study demonstrated that exogenous gibberellins can effectively induce parthenocarpy in ‘Golden Delicious’ apple trees, but the effectiveness of applied hormones is highly dependent on the type of GA used and flower quality. The highest level of fruit setting among GA treatments was observed after application of the GA₃ + GA₄₊₇ mixture to flowers previously subjected to emasculation (EFGAmix, 23.6%). However, the efficiency of fruitlet retention was significantly lower compared with natural pollination. Moreover, GA treatments, to a lower extent, affected fruit quality traits. These results show that gibberellins may have a practical role in improving fruit setting in apple under conditions of poor pollination or after slight spring frost damage to flowers during blooming. This study was limited to a period of one vegetation season and lacks analysis of physiological changes in hormonal pathways, which will be addressed in future studies. In further research, we aim to assess optimal doses, application timing, and the possibility of combining gibberellins with other technological applications to improve fruit setting in modern orchards.