1. Introduction
Apple (
Malus domestica Borkh.) red skin coloration is one of the primary determinants of consumer preference and market value [
1,
2]. Therefore, poor red skin coloration is a key factor that can result in downgrading apple fruit as it is associated with poor visual appearance and thus low consumer acceptance [
3]. Furthermore, in recent years, standards for minimum acceptable red skin coloration have increased from 25% to 50% blush across several commercially important cultivars, such as ‘Honeycrisp’ [
4,
5].
Red skin coloration in apples is mainly determined by the content of anthocyanins [
6]. Anthocyanin accumulation is strongly affected by environmental factors such as temperature and light [
7,
8,
9]. The ideal conditions for red color development in apples correspond to bright, clear days with temperatures of 25 °C and cool nights (15 °C) during 3–4 weeks preharvest [
10,
11]. Nevertheless, due to the hot and humid environmental conditions of the mid-Atlantic region of the US, high-value cultivars such as ‘Honeycrisp’ often produce marginal red skin coloration, not meeting the market standards. Regarding light intensity and wavelength, ultraviolet radiation, intrinsic to sunlight, has shown effects on inducing fruit anthocyanin biosynthesis, thus improving apple red skin coloration [
8,
12]. Light penetration levels can vary widely within the canopy, with less light generally reaching the inner and lower third parts of the canopy while more sunlight tends to be intercepted by the upper and outer parts of the canopy [
3]. Furthermore, considerable light is lost to red fruit color development as it is absorbed by the ground between the orchard rows [
13].
Horticultural practices, such as the use of reflective groundcovers, are used to enhance the tree canopy light environment [
3,
14]. The deployment of reflective groundcovers improves the capacity of apple trees to harness sunlight by reflecting the light that would otherwise be absorbed at the ground surface, back into the canopy [
2,
4,
15]. This amplifies the cumulative light reaching the apple fruit surface, particularly fruit located in the lower third and inner part of the canopy [
16]. The deployment of groundcovers around 4 weeks before the anticipated harvest has been reported to increase red skin coloration in apples [
3,
4,
11,
12,
15,
17,
18,
19]. Despite its proven ability to enhance apple coloration, research on reflective groundcovers has not been widely conducted under the environmental conditions of the US mid-Atlantic region.
In addition to the lack of red skin coloration, ‘Honeycrisp’ apples are prone to preharvest fruit drop, which can begin 4 weeks before the anticipated harvest and prior to the fruit reaching horticultural maturity [
20,
21,
22]. The practice of harvesting fruit early to avoid preharvest fruit drop is not feasible, as immature fruit will not achieve acceptable quality and particularly red skin coloration requirements, leading to poor storability and reduced marketability [
23]. Therefore, reducing preharvest fruit drop is of critical importance for extending the harvest window and decreasing economic losses in the commercially important cultivar ‘Honeycrisp’ in the mid-Atlantic.
Ethylene has been widely reported as a key ripening-related hormone affecting fruit quality [
24], as well as a primary driver of preharvest fruit drop in apples [
25]. Fruit quality is determined by multiple irreversible physiological and biochemical modifications that take place as fruit matures [
26]. These include modifications in fruit skin color (background and red surface color), texture (flesh softening), and flavor (increase in sugar contents, decrease in organic acids, and changes in aroma volatiles) [
27,
28,
29]. In addition, regarding preharvest fruit drop, ethylene can promote the degradation of the cell wall and intercellular tissues in the abscission zone of the pedicel, resulting in fruit drop [
30,
31]. Furthermore, apple fruit is classified as climacteric, characterized by an upsurge in respiration rates and internal ethylene concentration (IEC) as it ripens [
32,
33]. Nevertheless, ethylene production in apple fruit can vary amongst cultivars, with cultivars producing a higher EIC, presenting a higher susceptibility to preharvest fruit drop [
34,
35], such as ‘Honeycrisp’ [
21,
22].
Horticultural practices to reduce preharvest fruit drop currently rely mainly on plant growth regulators such as aminoethoxyvinylglycine (AVG), which inhibit the enzyme that catalyzes the rate-limiting step in ethylene biosynthesis, 1-aminocyclopropane-1-carboxylic acid (ACC) [
20,
36]. AVG applications 4 weeks before the anticipated harvest have significantly reduced preharvest fruit drop in different cultivars, such as ‘Honeycrisp’, ‘Gala’, and ‘McIntosh’, grown under different environmental conditions [
3,
20,
22,
37]. In addition to reducing fruit drop, AVG has been reported to delay fruit maturity, impacting several fruit-quality-related attributes, such as reducing fruit softening, starch breakdown, and soluble solids contents; maintaining high acidity, and significantly reducing red skin coloration development in different apple cultivars [
3,
20,
22,
36,
38,
39,
40,
41]. Particularly regarding red skin coloration, endogenous ethylene has been reported to also play a critical role in regulating anthocyanin accumulation [
26,
42,
43,
44], in addition to environmental factors.
Although earlier research has documented the effects of the use of reflective groundcovers and AVG on apples, investigations of the responses to these strategies under the environmental conditions of the US mid-Atlantic region are lacking. Furthermore, to our knowledge, studies on the impacts of the combination of both horticultural practices on ‘Honeycrisp’ apples have not yet been conducted under our environmental conditions. Based on the above, the aim of the present work was three-fold: first, to evaluate the effect of the reflective groundcover Extenday in light interception and reflectance in a commercial ‘Honeycrisp’ orchard grown under US mid-Atlantic environmental conditions; secondly, to characterize and compare differences in fruit drop, internal ethylene concentration, fruit-quality-related physicochemical parameters, and skin coloration of ‘Honeycrisp’ apples submitted to reflective groundcover Extenday and AVG treatment combinations throughout ripening on the tree; and thirdly, to use multivariate data analysis to identify significant correlations amongst all assessed features.
2. Materials and Methods
2.1. Plant Material and Preharvest Orchard Treatments
A 12-year-old commercial ‘Honeycrisp’/‘M9′ apple orchard located in Aspers, PA (39.96° N, 77.28° W), was used for this study. Tree spacing was 1.5 × 4 m and trees were trained to a central leader system. Four treatments composed of different combinations of the reflective groundcover Extenday (Extenday New Zealand, Auckland, New Zealand) and the plant growth regulator AVG (ReTain, Valent Biosciences Corporation, Libertyville, IL, USA) (
Table 1) were established during two consecutive production seasons (2021 and 2022). For Extenday (T1, T2), a 3.5 m wide white woven polyethylene reflective groundcover was deployed adjacent to 50 tree plots on each side of the row 4 weeks before the anticipated commercial harvest date and secured according to manufacturer recommendations. Extenday (T1, T2)- and non-Extenday (T3, T4)-treated plots were separated down the tree row by at least 30 trees and separated by 3 rows of trees on either side to mitigate potential cofounding due to altered light reflection in trees adjacent to those applied with the Extenday treatment. The AVG treatment was applied to 20 tree subplots on Extenday (T1) and non-Extenday treatments (T3) and comprised a full-rate (130 mg L
−1) application, 4 weeks before the anticipated commercial harvest date. All sprays were mixed with 1.0 mL L
−1 Silwet-77 organosilicone surfactant before application, which were made using a pressurized orchard sprayer. Additional trees in each plot were used as buffers to manage the potential drift of AVG treatment. A randomized complete block design with four replications was used.
‘Honeycrisp’ fruit maturity indices were monitored throughout the season each year to harvest fruit in the optimal commercial maturity stage using control fruit (T4) as the reference. Fruits were harvested in three different ripening stages on the tree: at optimal commercial harvest (CH) (corresponding to 3 September 2021 and 4 September 2022), 1 week after CH (CH + 1) (corresponding to 10 September 2021 and 11 September 2022), and 2 weeks after CH (CH + 2) (corresponding to 17 September 2021 and 18 September 2022). On each harvest date, for each of the four replications per treatment, a total of twenty-five fruit were harvested from the lower third of the canopy (1.5 m above the ground). Fruits with uniform size and an absence of visual blemishes, bruises, and/or diseases were chosen. After harvest, fruits were quickly transported to the laboratory. Per replication, five fruits were used for the analysis of internal ethylene concentration, while the rest of the fruits were used to assess quality-related physicochemical properties (described below).
2.2. Light Interception and Reflectance Measurements
Light interception and reflectance by the reflective groundcover Extenday (T2) and by the ground or control (T4) were quantified in the middle of the drive row (mid-row) and within the tree canopy (in-canopy), proximal to solar noon on a sunny, cloud-free day and on a cloudy day in each year of this study. Light data were collected on two mid-row positions and two trees at the center of each 20-tree-plot replication. Measurements were collected 1.5 m above the ground. Intercepted light was determined with sensors oriented towards the sun (sky), while light reflectance was quantified by inverting the sensors (facing the ground or the reflective groundcover). Photosynthetic photon flux density (PPFD; 400–700 nm waveband; µmol m−2 s−1) was evaluated using an LI-COR LI-191R Line Quantum Sensor attached to an LI-250A Light Meter (LI-COR Environmental, Lincoln, NE, USA). Two measurements of interception and reflectance were taken, with the sensor positioned perpendicular to the row, once each on the north and south side of the trunk. In-canopy measurements were carried out with the distal end of the sensor next to the trunk. Ultraviolet light (UV; 250–400 nm; µmol m−2 s−1) was measured with a portable UV meter (FieldScout model 3414F, Spectrum Technologies Inc., Aurora, IL, USA). Four measurements of interception and reflectance were collected for each tree, once at each of the four points around the trunk (north, east, south, and west). In-canopy measurements were performed with the UV meter positioned 15 cm from the trunk.
2.3. Preharvest Fruit Drop Measurements
For each treatment and replication, two weeks before CH, a total of 5 limbs (from different trees) with a total of 20 fruits each were selected and tagged from either side of the trees. Preharvest fruit drop was evaluated by counting the number of tagged fruits weekly starting from one week before CH (1WBCH) to 2 weeks after CH (CH + 2). The percentage of fruit drop was then calculated relative to the initial fruit count per limb.
2.4. Fruit Internal Ethylene Concentration
The internal ethylene concentration (IEC) of each fruit was measured on 1 mL samples of internal gas from the core cavity using a gas chromatograph (GC-2014C, Shimadzu Co., Kyoto, Japan) equipped with an activated alumina column attached to a flame ionization detector as previously described [
28,
45,
46]. Nitrogen (N
2) was used as the carrier gas at a flow rate of 30 mL min
−1, while O
2 and H
2 were used to create the flame of the detector at a flow rate of 300 and 30 mL min
−1, respectively. Injector, detector, and oven temperatures were set at 140, 150, and 80 °C, respectively.
2.5. Fruit-Quality-Related Measurements
Fruit weight, skin color, index of absorbance difference (IAD), skin red blush percentage, flesh firmness, starch pattern index (SPI), soluble solids contents (SSCs), and titratable acidity (TA) were measured. Fruit weight was quantified using an electronic balance (Sartorius, AG Gottingen, Germany). Skin color was assayed on the two opposite sides of each fruit along the equatorial axes, and the red-green (a*) and yellow-blue (b*) values were measured using a colorimeter (Konica Minolta CR400 Chroma Meter, Konica Minolta Sensing, Inc., Osaka, Japan). Hue angle (hue°), representing changes in primary colors, was calculated as h = arctan(a*/b*) [
47]. The index of absorbance difference (I
AD = A
670 − A
720; DA-Meter, TR Turoni, Forli, Italy) was measured on fruit skin by averaging the values recorded on three spots on each apple fruit [
48]. Flesh firmness was measured on the two opposite peeled sides of each fruit using a TA.XT Plus Connect texture analyzer (Texture Technologies Corp., Scarsdale, NY, USA) equipped with a 50 kg loadcell and analyzed with the Exponent TE32 (v6.0, Texture Technologies Corp., Scarsdale, NY, USA) software fitted with an 11.1 mm diameter probe. The SPI of each fruit cut at the equator was assessed using the Cornell generic chart where 1 = 100%-iodine-stained starch and 8 = 0%-stained starch [
49]. To determine SSC and TA, a wedge from each fruit was removed and pooled to create a composite sample of each replication. Juice was extracted from these composite samples with a hand press and filtered through cheesecloth. SSC was determined by using a digital hand-held refractometer (Atago, Tokyo, Japan) and expressed as %, whereas TA was computed by automatic titration (855 Robotic Titrosampler; Metrohm, Riverview, FL, USA) with a 0.1 N sodium hydroxide solution to an end point to pH 8.2, expressed as % malic acid [
29,
32].
2.6. Statistical Analysis
Response variables were modeled using generalized linear mixed models including treatments and evaluation periods as fixed factors, as well as blocks as a random factor to determine the statistical significance of the interactions and main effects (analysis of variance, ANOVA). When the analysis was statistically significant, the separation of means was carried out using Tukey’s HSD test at a significance level of 5%.
Pearson’s correlation coefficients, using mean-centered data, were calculated for each pairwise-combination of evaluated parameters. PCA, which was applied to reduce the dimensionality of the data, was visualized through a ‘biplot’ graph, thus representing the relationships among the variables (preharvest fruit drop, IEC, physicochemical measurements, and fruit skin color) and the assessed treatments and evaluation periods. The Scree test was used to select the number of principal components that captured most of the variation. The software package JMP (ver 15.2, SAS Institute) was used for all the statistical analyses.
4. Discussion
There are numerous factors influencing red skin coloration in apples, including environmental factors such as temperature and light, as well as plant growth regulators such as AVG, among others [
1,
8,
9,
10,
12,
20,
43,
44]. Regarding light, although there are various reports indicating that the reflective groundcover Extenday has improved red skin coloration in apples [
4,
11,
12,
15,
17,
18], studies evaluating its effects on fruit maturity and quality are inconsistent, and have been lacking under the environmental conditions of the mid-Atlantic of the US in ‘Honeycrisp’ apples. Regarding AVG, it has been reported that it negatively affects red skin coloration development but can effectively delay maturity and reduce preharvest fruit drop [
3,
20,
36,
37,
38,
41,
42]. However, there is limited information about the effects of Extenday and AVG treatment combinations on fruit drop, fruit maturity and quality, and particularly the skin coloration of ‘Honeycrisp’ apples grown under the hot and humid weather of the mid-Atlantic. In the present study, there was a general trend for decreased preharvest fruit drop and increased red skin coloration (>50% blush) without the promotion of overripening in fruit harvested from the lower third of the canopy, with the combined use of the reflective groundcover Extenday and the plant growth regulator AVG, consistent throughout two consecutive years.
Ultraviolet radiation is known to improve red skin coloration in fruits, including apples [
8,
12,
50,
51]. In agreement with our results, the use of Extenday in other regions has also shown an increase in reflected UV radiation from the orchard ground back onto the trees as compared to control trees [
4,
12]. These results support the significantly higher red skin coloration attained by Extenday-only-treated fruit in our work, which displayed >75% skin blush in the first assayed ripening stage (CH). Red color development on apple fruits results from the accumulation of anthocyanin pigments, which have been demonstrated to be highly influenced by UV radiation, as most enzymes involved in the anthocyanin biosynthesis pathway are light-inducible [
10,
52]. Consistent with our results, increased reflected PPFD by the reflective groundcover Extenday was also observed in previous studies [
4,
11,
12,
17,
18]. Nevertheless, it is important to mention that excessive PPFF and UV radiation in other apple-producing regions, such as the Pacific Northwest of the US (hot and dry climate), can have negative impacts on ‘Honeycrisp’ fruit, such as the development of sunburn [
15]. In the current study, under our environmental conditions, we did not observe sunburn incidence with the use of the reflective groundcover Extenday in any of the treatments.
Preharvest fruit drop is a major concern in apple production in many regions, including the mid-Atlantic [
20,
21,
22]. Plant growth regulators, such as AVG, are widely used to prevent fruit drop due to their capacity of inhibiting ethylene biosynthesis [
36,
40]. This is supported by the positive correlation obtained between fruit drop and IEC in this work. Consistent with our results, the application of AVG at full-rate three-to-four weeks before harvest was significantly effective at minimizing preharvest fruit drop in ‘Honeycrisp’ [
22], ‘Gala’ [
3,
20], and ‘McIntosh’ [
37]. In ‘McIntosh’ apples, it has been reported that once applied, the onset of action time for AVG to manifest and significantly reduce fruit drop is between 10 and 14 days [
38]. In this study, the application of AVG consistently decreased preharvest fruit drop in ‘Honeycrisp’ apples, independent of the use of the reflective groundcover Extenday, suggesting that the use of the latter does not significantly impact apple fruit drop.
As apple fruit display a climacteric fruit ripening behavior, and thus IEC has been reported to play a key role in controlling fruit maturity [
28,
32,
33], attempts to control fruit drop by reducing ethylene biosynthesis via AVG are expected to delay fruit maturity. In this work, AVG-treated ‘Honeycrisp’ apples displayed the significantly lowest EIC, which affected several quality-related physicochemical attributes. In agreement with previous studies [
3,
20,
22,
38,
39,
41], AVG delayed fruit flesh softening and starch breakdown, reduced soluble solids contents, and maintained the highest acidity values throughout ripening. Consistent with our results, AVG has been reported to have no effect on apple fruit weight [
20,
37,
53], suggesting that this is an ethylene-independent trait. Likewise, regarding the use of the reflective groundcover Extenday, no significant effects on apple fruit weight have been reported [
4,
11], supporting our results. Nevertheless, the use of Extenday has shown inconsistent results in terms of its effect on fruit maturity and quality. In this study, Extenday deployment hastened fruit maturity by increasing ethylene production in ‘Honeycrisp’ apples, decreasing flesh firmness and acidity, but increasing starch degradation as well as soluble solid contents, in accordance with previous findings in peaches [
54] and apples [
55]. However, other studies have reported no effects on fruit-maturity-related attributes due to Extenday deployment [
2,
3,
4,
11,
14,
15]. This variability in findings amongst studies regarding the impact of Extenday on apple fruit maturity and physicochemical characteristics can most likely be attributed to the differences in growing conditions, apple cultivars, management practices, and the time interval in which the reflective groundcover Extenday is in place. Moreover, in this study, when combining Extenday + AVG under US mid-Atlantic environmental conditions, ‘Honeycrisp’ fruit displayed an intermediate fruit maturity, i.e., significantly advanced with respect to AVG-only-treated apples but, at the same time, significantly delayed as compared to control and Extenday-only-treated fruit. The latter is indicative of an interaction between both Extenday and AVG that is impacting fruit quality properties by advancing maturity, but not overly stimulating the ripening process, and thus avoiding overripening as the fruit is left hanging on the tree. This is of key importance as it suggests that the combined Extenday + AVG treatment could be improving subsequent fruit storability and shelf-life capacity, which has not been accounted for in this study, but is currently under investigation.
Fruit red skin coloration is of major importance for fruit quality, as it is directly tied to consumer preference and market value [
1]. As discussed above, apple skin red coloration is a result of the accumulation of anthocyanins, which, in addition to environmental factors, such as UV radiation, is also partially regulated by endogenous ethylene [
26,
42,
43,
44]. This is supported by the positive correlations between IEC and skin blush, as well as the negative correlations between IEC and hue angle, obtained in this study. AVG application at full-rate three-to-four weeks before harvest in ‘Gala’, ‘Red Delicious’, ‘Jonagold’, ‘Honeycrisp’, and ‘Red Chief’ apples has been reported to significantly reduce red skin coloration [
3,
20,
22,
41], consistent with our results, where the required minimum 50% skin blush for fruit to be marketable was only attained in ‘Honeycrisp’ fruit in the last assayed ripening stage (CH + 2). On the other hand, in this work, the deployment of the reflective groundcover Extenday significantly boosted ‘Honeycrisp’ red skin coloration to >75% skin blush in the first assayed ripening stage (CH), in agreement with other studies conducted in different regions and cultivars [
4,
11,
12,
14,
15,
17,
18,
55]. Furthermore, the combined treatment of Extenday + AVG significantly enhanced the red skin coloration of ‘Honeycrisp’ apples, which displayed >50% red blush in CH under mid-Atlantic environmental conditions, suggesting an interaction between both horticultural practices. This is of crucial importance as fruit from the upper sun-exposed third of the canopy are typically redder than fruit located in the lower third of the canopy [
3]. However, the combined treatment of Extenday + AVG could ensure that fruit from the lower canopy actually pack out in premium grades, while not increasing fruit drop or fruit overripening, therefore increasing total crop value and profitability, and justifying the economic investment in the reflective groundcover Extenday.
Additionally, changes from green to yellow in the background color of apples, through a decrease in the values of hue angle as well as a result of chlorophyll disappearance (I
AD), have been shown to be associated with increased fruit maturity [
28,
48]. These results support the significantly negative correlations between I
AD and IEC obtained in this work, and are consistent with other studies in apples [
28,
42,
56] and peaches [
45,
48]. The significantly highest and lowest hue angle and I
AD values for AVG-only- and Extenday-only-treated ‘Honeycrisp’ apples, respectively, are indicative of the delay and advancement of background color changes, respectively. Nonetheless, the combined Extenday + AVG treatment in ‘Honeycrisp’ apples presented an intermediate background color change, similar to what was observed for fruit maturity and quality-related physicochemical parameters, supporting an advancement in fruit maturity that does not translate into fruit overripening.
Particularly in this study, the distribution of each Extenday and AVG treatment combination/evaluation period along component 1 of the PCA is supported by the AVG-only-treated ‘Honeycrisp’ fruit displaying the significantly lowest IEC, most delayed fruit maturity, and reduced fruit drop but a drastically inhibited red skin coloration in all evaluation periods; followed by the Extenday + AVG treatment in ‘Honeycrisp’ fruit exhibiting an intermediate positioning in terms of IEC, fruit maturity (not leading to overripe fruit), and fruit drop, while significantly enhancing red skin coloration above the required marketable minimum (>50% blush) in all evaluation periods; by the control ‘Honeycrisp’ fruit showing a significantly increased IEC, fruit maturity (leading to overripe fruit towards the later assessed evaluation periods), and fruit drop, with a considerably hindered red skin coloration in the first evaluation periods; and finally by the Extenday-only-treated ‘Honeycrisp’ fruit, presenting the significantly highest IEC, most advanced fruit maturity (leading to overripe fruit), increased preharvest fruit drop, yet a promoted red skin coloration in all evaluation periods (>75% blush). However, the results of this study may only be applicable for fruit grown under US mid-Atlantic conditions. Thus, this work needs to be replicated in major production regions of ‘Honeycrisp’ with different environmental conditions, such as the Pacific Northwest, to assess the transferability of these outcomes between regions. Furthermore, these results are specific for ‘Honeycrisp’ fruit and future work is required to include a wide range of cultivars to assess the robustness of these results.