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Article

The Effect of Organic and Amino Acid Biostimulants on Actinidia deliciosa ‘Hayward’ Cultivation: Evaluation of Growth, Metabolism, and Kiwifruit Postharvest Performance

by
Vasileios Papantzikos
*,
Vasileios Stournaras
,
Paraskevi Mpeza
and
Georgios Patakioutas
*
Department of Agriculture, University of Ioannina, Arta Campus, 47100 Arta, Greece
*
Authors to whom correspondence should be addressed.
Appl. Biosci. 2024, 3(3), 360-377; https://doi.org/10.3390/applbiosci3030024
Submission received: 28 July 2024 / Revised: 17 August 2024 / Accepted: 20 August 2024 / Published: 21 August 2024
Versions Notes

Abstract

The commercial value of kiwifruit is determined mostly by its nutritional composition and antioxidant content. The enrichment of these traits in an era where climate change affects kiwi growth conditions is worth investigating via the application of biostimulants that enhance plant growth. In this work, we evaluated the effects of three commercial biostimulants on the metabolism and growth characteristics of the Actinidia deliciosa cultivar ‘Hayward’: (I) a humic and fulvic-based biostimulant, (II) a glycine–betaine–proline-based biostimulant, and (III) a vegetal amino acid-based biostimulant. A biostimulant-free treatment was used as a control. In the two-year experiment (2022 and 2023), the growth characteristics of kiwi trees were measured, such as stem length, the number of leaves, leaf area, and fresh and dry biomass at the end of each growing season. The leaves’ total chlorophyll, total phenolics, and proline content were detected during the two seasons in order to obtain more insights into plant metabolism. Κiwifruit qualities and antioxidant traits, such as total soluble solids, titratable acidity, firmness, fresh and dry biomass, DPPH, and ascorbic acid (vitamin C), were recorded during the postharvest life for each season. Data analysis illustrated the beneficial impact of some biostimulants on several of the previously mentioned parameters, such as antioxidant kiwifruit traits, especially in the case of glycine–betaine–proline-based acid-based biostimulants.

1. Introduction

Actinidia sp. (Ericales: Actinidiaceae) is a crucial part of the agricultural economy in many regions of the world due to the high economic value of kiwifruits [1] and their nutritional properties [2]. The production of kiwifruit has quite a significant economic value in Greece, as in 2020 it had the fourth place in the global rankings of the kiwifruit trade with 307.44 Kt [3]. However, its cultivation faces significant problems due to environmental stress, linked to climate changes, plant pests, and diseases [4].
There are a plethora of climate change factors, such as drought [5], rising temperatures [6], altered rainfall patterns, and soil water content [7]. All these changes are causing an increased impact of abiotic stress on kiwifruit production. Kiwifruit development, yield, and quality have been influenced by these biotic stressors [8,9]. To lessen environmental stress in kiwifruit cultivation, more integrated responses are required [10]. Using resistant cultivars, soil, and water management and implementing disease-preventive and control strategies may be significant coping strategies [11,12].
The use of organic fertilizers and the utilization of efficient irrigation systems can improve plant resistance and reduce environmental stress in kiwifruit cultivation [13]. The utilization of biostimulants plays an alternative supporting role in kiwifruit cultivation [14]. Biostimulants are natural or synthetic substances that affect plant growth and physiology in a multitude of ways, including root promotion [15], germination, photosynthesis, and stress resistance [16]. The application of biostimulants in kiwifruit cultivation may have multiple benefits, including increasing fruit production and quality; improving resistance to abiotic factors, such as environmental stress [17,18]; or other applications, such as the bud-break [19].
In kiwifruit cultivation, the soil application of humic and fulvic compounds has been suggested as a potential strategy to improve the physiology of kiwi orchards [20]. Humic and fulvic acids may promote plant growth, enhancing plant primary metabolism in terms of nutrient absorption, especially through the induction of carbon and nitrogen metabolism [21]. Many authors document the stimulation induced by different humic substances (HS) on glutamine synthetase (GS), glutamate dehydrogenase (GDH), and nitrate reductase (NR), which are enzymes linked to N assimilation pathways [22,23]. The secondary metabolism of plants can be significantly impacted by HS [24]. Research has shown that humic compounds increase the expression of a protein called phenylalanine (tyrosine) ammonia-lyase (PAL/TAL). This protein is essential for the first step of phenolic biosynthesis because it converts tyrosine into p-coumaric acid and phenylalanine into trans-cinnamic acid. Research evidence shows a correlation between increased phenolic buildup in leaves and the increased expression of PAL/TAL [25]. HS impacts plants’ secondary metabolism, providing a novel way to study how plants react to stress. HS promotes the plasma membrane synthesis of H+-ATPase; thus, root growth can be achieved through ATP hydrolysis and H+ transport across the cell membrane [21,26,27]. This makes the outer body more acidic, which loosens the cell wall. The loosened cell wall, in turn, lengthens the cell [28,29]. In this way, plant nutrition is more effective because of the enhanced electrochemical proton gradient, responsible for driving ion transport across the cellular membrane [26,27]. In field experiments with the application of humic and fulvic substances to tomato leaves, a significant enhancement of PAL activity was displayed, which resulted in reduced infection with Phytophthora infestans [30]. Similar data regarding PAL activity have been documented in lettuce [31].
Amino acid biostimulants containing glycine–betaine and proline act as important factors, promoting the synthesis of biological molecules that are essential for plant growth [32]. They promote photosynthesis and the transport of nutrients in plants by enhancing plant growth and fruit production [33]. The addition of amino acids to the cultivation of kiwi may also present potential benefits for the fruits [34]. Among the pathways activated by amino acid-based biostimulants, jasmonic acid and ethylene, obtained by the accumulation of salicylic acid, are the strongest options for inducing the induced systemic resistance (ISR) mechanism, important for resisting a wide range of diseases [35]. Glycine–betaine (GB) protects plants, mostly against oxidative stress, and this occurs when the equilibrium between the production of reactive oxygen species (ROS) and the quenching activity is disturbed by an external stress factor [36]. Proline has a multifunctional role in plant stress metabolism [37,38,39]. On the one hand, it acts as an osmolyte, regulating osmotic adjustment upon the activation of metabolic pathways in response to conditions, such as irrigation deficits [40], to regulate cellular damage. Proline accumulates in high concentrations [41], without causing any metabolic harm. On the other hand, it is a scavenger of the reactive oxygen species (ROS), helping sub-cellular structures to stabilize and controlling cells’ redox homeostasis [3,42].
Biostimulants promise a substantial contribution to overcoming abiotic challenges. For this reason, we decided to investigate the application of organic and amino acid-based biostimulant formulations in kiwi cultivation as a complex way of compensating for the possible deficits that may occur due to the environmental stress of climate change. The purpose of this study was to gain insights into the physiology and productivity of kiwi plants, focusing on the most widespread and economically important variety (Hayward) cultivated in Greece.

2. Materials and Methods

2.1. Experimental Design

The experiment was carried out on a 7-year-old kiwifruit Actinidia deliciosa cultivar (cv) ‘Hayward’ orchard, 48 m long and 40 m wide, arranged on a T-bar trellis system containing vines at a 3 m × 5 m planting density. The study lasted for 2 years, running for a total of 577 days from the bud-break of the first year until the harvest point of the second year. The average climatic data of the two growing seasons were obtained from the meteorological station of the University of Ioannina. In the 2022 season, the average temperature was 22.10 °C (min: 10.54 °C, max: 34.58 °C), rainfall was 24.77 mm, and relative humidity (RH) scored 77.21%. In the 2023 season, the average temperature was 20.62 °C (min: 5.84 °C, max: 31.21 °C), rainfall was 30.6 mm, and RH scored 84.27%.
The natural bud-break of the orchard started in the first experimental season on 8 April 2022 and in the second season on 29 March 2023. The fruit harvest took place for each year when the kiwifruit total soluble solids content reached the required maturity degree (>6.2% on the Brix° scale). Ιn the first year, it took place on 22 October 2022 (day 197), and in the second year it took place on 6 November 2023 (day 577). The second year’s harvest was later due to the environmental conditions that prevailed during cultivation. The soil substrate was clay, the basic fertilization was carried out yearly at the end of each February (N14-P8-K18), and organic manure was applied as digested manure to each plot at the beginning of March for each year. Drip irrigation started for the first experimental year at the end of April 2022 and for the second season at the end of May 2023 (because the rainfall was higher at the beginning of Spring of 2023) and was conducted every one to five days based on evapotranspiration data through each season using 4 m3 of water ha−1. Pruning was performed every January, leaving about 14–18 buds on each vine.
The experiment was conducted using a completely randomized design with three biostimulant treatments and one control biostimulant-free treatment (C). The treatments were arranged in the orchard tree lines. Each treatment consisted of 12 trees arranged in blocks of 3, with a buffer tree in between each block. Between each tree line containing treatments, there was also a buffer line of trees. The biostimulants were applied at the onset of each growing season’s flowering time. The following formulations were studied in terms of kiwi tree cultivation: (a) a humic and fulvic acid-based (>25% w/w) biostimulant (HF): BLACKJACK BIO (Sofbey Co., S.A., Mendrisio, Switzerland); (b) a glycine–betaine–proline-based (glycine–betaine 80%, proline 10%, antioxidants, bioflavonoids and ellagic acid 0.5% w/w) biostimulant (GBP): FITOMAAT (Futureco Bioscience Co., S.A., Barcelona, Spain); and (c) a vegetal amino acid-based (Free amino acids 9.0%, total nitrogen 7.2% w/w) biostimulant (AA): CODASTING (SAS Co., S.A.U., Lleida, Spain). The rationale for selecting these biostimulants for study is based on their widespread market availability and their recommendation for use across a diverse array of tree crops. The number of biostimulant applications/intervals between repetitions (days) was determined to be 3/20 based on their label instructions. The quantities of the biostimulants applied were as follows: 2.5 L m−3 ha−1 for HF, 2 L m−3 ha−1 for GBP, and 1 L m−3 ha−1 for AA. The biostimulants were administrated to the kiwi trees via the irrigation system because it is a cost-effective and easy way of application for kiwifruit growers. Thus, the experiment aimed to study the effect of the formulations, sticking as closely as possible to realistic growing conditions.

2.2. Growth Measurements

In the experimental orchard, two young productive shoots were selected per tree in each cardinal direction (north, east, south, and west), with eight shoots per tree in total, and growth measurements were performed on each one. At the end of each growing season, the total length (cm); the number of leaves; the leaf area (cm2) according to the protocol of Bakr, 2005 [43]; and total fresh and dry biomass (g) (leaves and stems) were measured.

2.3. Quantitative Assessments of Plant Metabolism

During the two-season experiment, total chlorophyll content was analyzed in the leaves of the selected shoots for 38, 91, 119, 146, 178, 405, 449, 484, 498, and 536 days after the beginning of the experiment (DABE). We assessed total phenolic and proline content for days 35, 91, 112, 144, 178, 403, 434, 475, 550, and 577, as described in the following Section 2.3.1, Section 2.3.2 and Section 2.3.3.

2.3.1. Determination of Total Chlorophyll Content

The determination of total chlorophyll content (TCHL) was carried out regularly in a non-destructive manner with the SPAD 502 device (Minolta Co., Ltd., Osaka, Japan). To test the accuracy of SPAD measurements, the linear correlation of the device readings with the actual chlorophyll values was performed using the chemical determination method of chlorophyll according to the protocol of Razeto and Valdés, 2006 [44]. Then, 10 mL of 100% acetone was used as an extraction solvent for 0.04 g of homogenized fresh plant tissue of kiwi leaf (2.66 cm2 leaf disc area). The sample was placed in a glass tube and then vortexed and left overnight in the dark at 4°C. The absorption was measured in a spectrophotometer (V630 UV-VIS, Jasco International Co., Ltd., Tokyo, Japan) at 644.8 and 661.6 nm. The result was expressed in µg of fresh leaf (FL) per cm2 of leaf area according to the equations of Lichtenthaler and Buschmann [45], described below:
Ca (μg/mL) = 11.24 × A661.6 – 2.04 × A644.8
Cb (μg/mL) = 20.13 A644.8 – 4.19 A661.6

2.3.2. Determination of Total Phenolic Content

The determination of total phenolic content (TPC) was performed using the protocol described by Katalinic et al., 2013 [46]. Overall, 10 mL of 80% ethanol solvent was used as an extraction solvent for 0.1 g of dry leaf tissue. The sample was centrifuged in 3000× g at 12 °C for 15 min. In total, 250 μL of the supernatant-extracted solution was diluted in a final volume of 10 mL diH2O. Then, 1 mL of the diluted extract solution was added in a new test tube, with 4.5 mL diH2O and 500 μL of Folin–Ciocalteu 2N reagent, and after 3 min 4 mL of dehydrated Na2CO3 solution 7.5% w/v was infused. The final mixture was vortexed and then incubated in a water bath (40 °C) for 20 min and then stored at room temperature in dark conditions. A spectrophotometer (V630 UV-VIS, Jasco International Co., Ltd., Tokyo, Japan) was used to record the absorption at 765 nm compared to the prepared blank. The standard for TPC quantification was Gallic acid, and the results were expressed in mg GAE (Gallic Acid Equivalent) g−1 of dry leaf (DL) weight.

2.3.3. Proline Determination

The determination of proline content was conducted according to the protocol of Carillo and Gibon, 2011 [47], with modifications: 0.1 g of fresh homogenized kiwi leaf tissue was extracted in 4 mL of 70% ethanol, and then centrifuged at 4000× g for 10 min at 4 °C. Then, 2 mL of a freshly prepared acid–ninhydrin solution and 1 mL of the supernatant-extracted solution were placed in a new test tube. The final mixture was vortexed and incubated for 25 min in a water bath at 95 °C. An ice bath was used to cool the reaction mixture, and when room temperature was reached, it was centrifuged at 4000× g for 5 min. Absorbance was spectrophotometrically (Jasco-V630 UV-VIS) assessed at 520 nm. To quantify the proline results, a calibration curve was established using proline solutions (ranging from 0.025 to 0.8 mM) in the same medium as the one used for the extraction, and the data were reported in μmol of proline g−1 of FL weight.

2.4. Postharvest Quality Analysis

Furthermore, to obtain more insights into the potential of biostimulants to alter kiwifruit quality traits, qualitative analyses were performed at 0, 30, 60, and 90 days after harvest (DAH) during cold storage at 1 ± 1 °C for each of the two growing seasons. The extraction protocol used for the 2,2-diphenyl-1-picrylhydrazyl and ascorbic acid determination was based on the freeze-drying method, as the advantage provided by this method is the ability of long storage in powdered sample form. The kiwifruit was peeled with stainless steel knives and pureed in a common pulping machine. Then, the sample was added to a freeze-drying device (Alpha 1-2 LDplus, Christ Co, GmbH, Osterode, Germany) equipped with a 0.45 mm sieve. Then, after 72 h it underwent dry vitrification. The sample was added in a 50 mL plastic falcon with 5 stainless steel round balls (5 mm in diameter), and these were vortexed together for 10 min, resulting in a dry powder. The samples were stored at 80 ± 1 °C to be further used in analytical determinations.

2.4.1. Firmness and Fresh and Dry Weights of Kiwifruit

In order to measure the fruit firmness, an approximately 1/2″–3/4″ mm diameter disc of peel (thick equatorial section) was removed from the fruit’s opposite sides with an inox peeler. Kiwifruit firmness was measured by using a handheld penetrometer (53203 penetrometer FT327, Turoni Co, srl, Forli, Italy), equipped with an 8 mm diameter plunger. The force required to penetrate kiwifruit flesh at a maximum depth of 8 mm was recorded in newtons (N). The fresh weight of the fruit was recorded for each storage period, while the dry weight was taken after 14 days at 80 ± 1 °C following each storage period, using a precision electronic scale (Kern EG-N, Kern & Sohn GmbH, Balingen, Germany).

2.4.2. Determination of Soluble Solids Content

To record the soluble solids content (SSC), approximately 2 drops of kiwi juice from two 10 mm thick slices, one from the calyx end of the fruit and one from the stem, according to the research of Abbate et al., 2021 [48], were entered on a hand-held refractometer (ATC, Atago Co, Ltd., Tokyo, Japan). The result was measured in Βrix° (%).

2.4.3. Titratable Acidity Record

To assess titratable acidity (TA), 5 mL of kiwifruit juice (V) were diluted with 25 mL of diH2O [49] and titrated with 0.1 N NaOH (N), adding 2 drops of 1% C20H14O4 (phenolphthalein) until the juice color turned to an endpoint pink (pH 8.2). The total acidity was recorded based on the consumed volume of 0.1 N NaOH (n) as a percentage of citric acid (64 = citric acid coefficient) using the following equation:
T A = N × n × 100 V × 1000 × 64

2.4.4. Determination of Antioxidant Activity Using the 2,2-Diphenyl-1-picrylhydrazyl (DPPH) Radical Scavenging Method

To analyze the kiwifruit antioxidant capacity, DPPH analysis was performed according to the protocol of Wang et al., 2018 [50], with modifications. Overall, 0.1 g of stored freeze-dried kiwifruit was added to a glass tube with 20 mL of 70% ethanol. The extraction took place overnight in the glass tube, which was stored at 4 °C in the dark. The sample was centrifuged at 4000× g for 20 min at 4 °C and 200 μL of the supernatant-extracted solution was placed in a new test tube with 2 mL of 100 mM DPPH work solution. After vortexing for 15 s, the sample was placed in the dark for 30 min and the absorbance was measured in a spectrophotometer (Jasco-V630 UV-VIS) at 515 nm. The radical scavenging activity was calculated as a percentage of DPPH discoloration using the following equation:
DPPH radical scavenging (%) = [(Acontrol − Asample)/Acontrol] × 100
where Asample is the absorbance of the solution when the extract/reference is added at a particular level, and Acontrol is the absorbance of the DPPH solution without the addition of the extract.

2.4.5. Ascorbic Acid

The ascorbic acid content (vitamin C) of the kiwifruit samples was recorded during the postharvest storage period using the method described by [51]. Overall, 0.5 g of freeze-dried kiwifruit was added in 5 mL diH2O and then diluted in a volume of 10 mL. A second dilution of the previous step took part in a 100 mL flask filled with C2H2O4 0.4% w/v. Overall, 10 mL from the previous step was added in a conical flask containing 15 mL C2H2O4 0.4% w/v. The final sample was titrated with a 50 mg vitamin C solution, which was prepared with NaHCO3 0.02% w/v and 2.6-dichloroindophenol (DCPI) 0.03% w/v. The titration was completed when the color of the sample changed to pale pink for 10–20 s. The results were reported as vitamin C equivalents in mg AA/g DW−1.

2.5. Statistical Analysis

To compare the parameters of the treatments (TR), on each table, statistical analysis was performed using Bonferoni’s post hoc test (p ≤ 0.05) in one-way ANOVA by using the program SPSS v. 25 (IBM-SPSS Statistics, Armonk, NY, USA).

3. Results

3.1. Growth Parameters

In both years of the experiment, growth characteristics were boosted in treatments with biostimulants, especially in GBP. More specifically, at the end of the first year, the total growth length was higher in the GBP (109.8 ± 5.3 cm) and HF (91.8 ± 3.6 cm) treatments compared to the control, C (71.8 ± 2.8 cm) (F = 21.083, df = 3, p ≤ 0.05). This was also valid in the second year (F= 277.003, df = 3, p < 0.001). According to Table 1, a similar picture was observed in the number of leaves of the same treatments in the year 2022 (F = 37.651, df = 3, p < 0.001) and in the year 2023 (F = 26.654, df = 3, p < 0.001). In addition, the GBP treatment presented the largest leaf area at the end of the 2022 growing season (4889.9 ± 106.8 cm2), with a statistically significant difference compared to C (3733.2 ± 68.9 cm2) (F = 34.082, df = 3, p < 0.001), and also at the end of the 2023 growing season ( GBP = 4448.6 ± 233 cm2) compared to C (3234.8 ± 194.2 cm2) (F = 6.328, df = 3, p = 0.023). A similar pattern was observed in the total fresh biomass for the year 2022 (F = 578.787, df = 3, p < 0.05) and 2023 (F = 218.7531, df = 3, p < 0.05).

3.2. Total Chlorophyll Content

The TCHL content of kiwifruit leaves was lower in the control throughout the experiment. The GBP treatment showed the highest TCHL content from day 119 (144.9 ± 2 μg cm2) compared to C (126.6 ± 2.1 μg cm2), with a statistically significant difference (F = 16.324, df = 3, p < 0.001) (Table 2). A similar picture was maintained until day 536 in the second year of the experiment, where GBP (137.4 ± 0.8 μg cm2) presented a statistically significant difference with C (133.5 ± 1 μg cm2) and with the rest of the treatments (F = 5.872, df = 3, p ≤ 0.05).

3.3. Total Phenolics Content

TPC was enriched to a significant extent by AA and GBP treatments (Table 3). During the first growing season, we observed the peak of TPC on day 112 in GBP (25.49 ± 0.63 mg GAE g−1) and AA (25.90 ± 0.92 mg GAE g−1) treatments compared to the control (22.37 ± 0.99 mg GAE g−1), showing a statistically significant difference (F = 3.689, df = 3, p ≤ 0.05). An analogous situation was observed in the second growing season, with a peak on day 475, where GBP (21.75 ± 0.81 mg GAE g−1) and AA (21.44 ± 0.28 mg GAE g−1) recorded higher amounts of TPC than C (18.43 ± 0.37 mg GAE g−1), with a statistically significant difference (F = 4.228, df = 3, p ≤ 0.05).

3.4. Proline Content

In the GBP treatment, a significant increase in proline levels was observed consistently throughout the experiment (Table 4). This held from day 35 (0.365 ± 0.016 µmol g−1) to day 577 (0.301 ± 0.015 µmol g−1). The GBP treatment shows a statistically significant difference with the control in all samplings, with the exception of day 403, which corresponds to the beginning of the second experimental season of 2023 (F = 2.527, df = 3, p > 0.05). The HF treatment did not show a significant difference from C in any of the samplings; however, the AA treatment on days 35 (0.336 ± 0.011 µmol g−1) and 434 (0.380 ± 0.019 µmol g−1) showed a statistically significant difference (F = 10.818–11.613, df = 3, p ≤ 0.05).

3.5. Firmness

Regarding the firmness of kiwifruit, no statistically significant differences were observed between the treatments during their postharvest cold storage in both seasons (Table 5).

3.6. Fresh Weight

At the harvest point (DAH 0), all biostimulant treatments showed a significantly higher fresh weight compared to the control in the 2022 season (F = 22.408, df = 3, p ≤ 0.001) and in 2023 (F = 14.4884, df = 3, p ≤ 0.001). The fresh weight of kiwifruits during their cold storage was increased in the treatments with biostimulants, compared to the control, in almost all samplings. In the first year, after 90 days of storage, the GBP (98.68 ± 0.67 g) and HF (93.63 ± 0.4 g) treatments were heavier, with a statistically significant difference to C (90.22 ± 1.05 g) (F = 23.573, df = 3, p ≤ 0.05), as shown in Table 6. In the second postharvest season, after 90 days of storage, the greatest weight was recorded by treatments the GBP (93.76 ± 1.09 g), HF (92.90 ± 1.16 g), and AA (93.27 ± 0.97 g), with a statistically significant difference to the control (87.87 ± 0.89 g) (F =7.063, df = 3, p ≤ 0.05).

3.7. Dry Weight

The beneficial impact of HF, GBP, and AA was observed to a lesser extent regarding the dry weight of kiwifruit. At the harvest point (DAH 0), all biostimulant treatments presented a higher dry weight compared to the control in 2022 (F = 16.697, df = 3, p ≤ 0.001). On the 90th day of storage in postharvest cold storage during 2022, the GBP (18.00 ± 0.44 g), HF (15.59 ± 0.69 g), and AA (14.74 ± 0.31 g) treatments showed a statistically significant difference with control C (12.59 ± 0.59 g) (F = 18.174, df = 3, p ≤ 0.05), but this was not repeated on the 90th day of the postharvest season of 2023 (F = 2.927, df = 3, p < 0.001) (Table 7).

3.8. Soluble Solids Content

The SSC was lower in all treatments compared to the control, mainly after the 60th day of cold storage, in both 2022 and 2023 (Table 8). The highest variation between biostimulant treatments and C was observed during the 2023 postharvest season storage on day 90, where GBP (12.18 ± 0.19%), AA (12.52 ± 0.18%), and HF (12.61 ± 0.17 %) showed statistically significant differences with C (14.12 ± 0.27%) (F = 17.503, df = 3, p < 0.001).

3.9. Titratable Acidity

The titratable acidity values were observed at the same levels in all the treatments during 2023 postharvest storage at the 60th and 90th days, presenting no significant variations between them (Table 9). On the 90th day of cold storage in 2022, GBP (1.10 ± 0.01%) and AA (1.10 ± 0.01%) presented statistically significant differences with C (0.97 ± 0.01%) (F = 104.306, df = 3, p < 0.001).

3.10. DPPH

As shown in Table 10, the total antioxidant potential of kiwifruits was enhanced in the GBP and AA treatments. At the harvest (DAH 0), GBP and AA performed better compared to the C in the 2022 season (F = 110.536, df = 3, p ≤ 0.003) and in 2023 (F = 133.392, df = 3, p ≤ 0.011). The GBP treatment scored a higher DPPH value on the 60th day of 2022 (80.29 ± 0.34%), as did AA (69.90 ± 0.3 g), on the same day compared to C (61.66 ± 0.66%), presenting a statistically significant difference (F = 305.499, df = 3, p < 0.001).

3.11. Vitamin C

According to Table 11, vitamin C content was higher in GBP treatment, although throughout the postharvest storage it was degraded at some point. At the harvest time of the first season on 0 DAH, GBP treatment showed a higher vitamin C content (0.045 ± 0.011 mg DW−1) compared with all the other treatments (F = 12.614, df = 3, p ≤ 0.05). This picture was repeated in the second experimental season with the higher vitamin C content on GBP (0.045 ± 0.017) (F = 4.651, df = 3, p ≤ 0.05).
Table 1. Growth parameters of A. deliciosa cv. ‘Hayward’ at end of each year (2022–2023): total length (cm), number of leaves, leaf area (cm2), total fresh, and dry biomass (g). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 1. Growth parameters of A. deliciosa cv. ‘Hayward’ at end of each year (2022–2023): total length (cm), number of leaves, leaf area (cm2), total fresh, and dry biomass (g). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TRLengthLeaf NumberLeaf AreaFresh BiomassDry Biomass
2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year
C71.8 ± 2.8 c59.7 ± 1 c23.0 ± 0.7 c26.6 ± 0.7 b3733.2 ± 68.9 c3234.8 ± 194.2 b27.0 ± 4.8 b26.0 ± 2.9 b7.8 ± 1.6 a8.4 ± 0.5 a
HF91.8 ± 3.6 b81.4 ± 0.9 b33.3 ± 0.9 b32.3 ± 1.1 a3968.3 ± 61.3 bc3918.0 ± 181.1 ab32.4 ± 6.9 b30.9 ± 3 b10.0 ± 2.2 a10.7 ± 1.8 a
GBP109.8 ± 5.3 a90.2 ± 0.9 a40.6 ± 1.2 a35.3 ± 0.7 a4889.9 ± 106.8 a4448.6 ± 233 a49.1 ± 3.9 a40.5 ± 0.8 a13.3 ± 0.9 a13.2 ± 0.7 a
AA74.1 ± 3.3 c62.7 ± 0.7 c28.7 ± 1.7 b27.3 ± 0.5 b4204.7 ± 96.6 b3388.8 ± 259 ab30.5 ± 5 b31.2 ± 3 b9.0 ± 1.8 a8.1 ± 1.7 a
Table 2. Total chlorophyll content (μg cm−2 FL) of A. deliciosa cv. ‘Hayward’ during 2022–2023 seasons, 38–536 days after beginning of experiment (DABE). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 2. Total chlorophyll content (μg cm−2 FL) of A. deliciosa cv. ‘Hayward’ during 2022–2023 seasons, 38–536 days after beginning of experiment (DABE). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR38 DABE91 DABE119 DABE146 DABE178 DABE405 DABE449 DABE484 DABE498 DABE536 DABE
C93.1 ± 3.7 a123.8 ± 2.4 b126.6 ± 2.1 c133.6 ± 3.3 b132.6 ± 1.4 b94.2 ± 2.2 a117.1 ± 2.1 a136.9 ± 1.5 b134.5 ± 1.9 b133.5 ± 1 b
HF96.3 ± 2 a128.2 ± 2.1 b136.1 ± 1.6 b137.9 ± 1.3 ab136.6 ± 1.9 ab93.7 ± 2 a118.2 ± 1.5 a136.7 ± 2 b137.7 ± 2 b132.0 ± 0.7 b
GBP94.2 ± 0.6 a136.1 ± 1.6 a144.9 ± 2 a146.6 ± 2.4 a137.9 ± 0.7 a 97.1 ± 1.7 a117.9 ± 2 a143.2 ± 1 a145.9 ± 1.6 a137.4 ± 0.8 a
AA94.5 ± 0.6 a131.5 ± 1.6 ab140.3 ± 2 ab140.6 ± 2.4 ab133.7 ± 0.7 ab96.8 ± 1.5 a120.9 ± 1.9 a136.3 ± 1.6 b135.7 ± 1.9 b133.2 ± 1.3 b
Table 3. Total phenolics content (mg GAE g−1 DL) of A. deliciosa cv. ‘Hayward’ during the 2022–2023 seasons, 38–536 days after beginning of experiment (DABE). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 3. Total phenolics content (mg GAE g−1 DL) of A. deliciosa cv. ‘Hayward’ during the 2022–2023 seasons, 38–536 days after beginning of experiment (DABE). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR35 DABE91 DABE112 DABE144 DABE178 DABE403 DABE434 DABE475 DABE550 DABE577 DABE
C13.65 ± 0.38 b21.34 ± 0.72 b22.37 ± 0.99 b25.28 ± 0.85 a17.91 ± 0.95 a13.65 ± 0.45 a15.94 ± 1.40 ab18.43 ± 0.37 b19.57 ± 1.10 a14.69 ± 1.08 a
HF12.92 ± 0.82 b17.49 ± 0.68 c24.66 ± 0.68 ab24.24 ± 0.63 a14.38 ± 0.75 b12.40 ± 1.10 ab12.92 ± 1.08 b19.78 ± 1.18 ab19.36 ± 1.35 a15.83 ± 0.10 a
GBP13.24 ± 0.90 b21.85 ± 0.45 a25.49 ± 0.63 a24.97 ± 0.45 a16.35 ± 0.48 ab12.92 ± 0.93 ab18.84 ± 0.95 a21.75 ± 0.81 a18.74 ± 1.02 a14.48 ± 0.72 a
AA16.04 ± 0.48 a19.47 ± 0.48 b25.90 ± 0.92 a25.80 ± 0.89 a15.94 ± 0.68 ab10.74 ± 0.36 b13.86 ± 1.60 b21.44 ± 0.28 a17.29 ± 1.56 a15.94 ± 1.35 a
Table 4. Proline content (µmol g−1 FL) of A. deliciosa cv. ‘Hayward’ during the 2022–2023 seasons, 38–536 days after beginning of experiment (DABE). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 4. Proline content (µmol g−1 FL) of A. deliciosa cv. ‘Hayward’ during the 2022–2023 seasons, 38–536 days after beginning of experiment (DABE). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR35 DABE91 DABE112 DABE144 DABE178 DABE403 DABE434 DABE475 DABE550 DABE577 DABE
C0.269 ± 0.003 b0.293 ± 0.02 b0.366 ± 0.026 bc0.334 ± 0.025 b0.185 ± 0.026 b0.252 ± 0.014 a0.268 ± 0.022 b0.334 ± 0.025 b0.313 ± 0.022 b0.201 ± 0.017 b
HF0.307 ± 0.016 ab0.264 ± 0.024 b0.323 ± 0.019 c0.323 ± 0.021 b0.208 ± 0.006 b0.286 ± 0.020 a0.265 ± 0.026 b0.323 ± 0.021 b0.305 ± 0.012 b0.197 ± 0.010 b
GBP0.365 ± 0.016 a0.445 ± 0.018 a0.525 ± 0.018 a0.530 ± 0.016 a0.367 ± 0.013 a0.255 ± 0.009 a0.394 ± 0.014 a0.482 ± 0.016 a0.466 ± 0.015 a0.301 ± 0.015 a
AA0.336 ± 0.011 a0.378 ± 0.035 ab0.459 ± 0.022 ab0.367 ± 0.024 b0.213 ± 0.026 b0.306 ± 0.025 a0.380 ± 0.019 a0.367 ± 0.024 b0.351 ± 0.018 b0.209 ± 0.015 b
Table 5. Firmness (N) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 5. Firmness (N) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR0 DAH30 DAH60 DAH90 DAH
2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year
C77.72 ± 0.89 a77.58 ± 0.71 a67.58 ± 0.89 a69.47 ± 1.07 a57.01 ± 0.99 a56.05 ± 1.07 a39.89 ± 0.78 a43.87 ± 1.46 a
HF75.97 ± 0.77 a76.65 ± 0.73 a66.49 ± 0.95 a67.38 ± 0.86 a57.33 ± 0.71 a55.47 ± 0.81 a40.22 ± 0.88 a41.47 ± 1.34 a
GBP77.28 ± 1.34 a76.07 ± 1.14 a67.25 ± 1.10 a67.22 ± 0.89 a55.05 ± 0.86 a56.02 ± 0.57 a40.00 ± 0.74 a41.88 ± 1.24 a
AA78.48 ± 0.8 a75.12 ± 0.86 a66.82 ± 0.62 a68.08 ± 1 a56.03 ± 1.09 a55.63 ± 1.13 a40.66 ± 0.80 a41.45 ± 1.19 a
Table 6. Fresh weight (g) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 6. Fresh weight (g) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR0 DAH30 DAH60 DAH90 DAH
2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year
C102.88 ± 1.09 b103.19 ± 0.95 b99.68 ± 1.42 c101.44 ± 2.09 a93.72 ± 0.56 c90.81 ± 0.93 b90.22 ± 1.05 c87.87 ± 0.89 b
HF111.97 ± 0.98 a111.71 ± 1.25 a105.54 ± 0.17 b106.52 ± 2.45 a98.98± 0.65 b97.17 ± 0.95 a93.63 ± 0.4 ab92.90 ± 1.16 a
GBP113.39 ± 1.06 a112.03 ± 0.87 a109.99 ± 0.46 a109.18 ± 2.02 a105.38 ± 0.39 a95.76 ± 1.04 a98.68 ± 0.67 a93.76 ± 1.09 a
AA110.21 ± 0.79 a111.80 ± 1.35 a105.29 ± 0.77 b107.70 ± 1.93 a100.30 ± 0.63 b97.98 ± 0.90 a93.03 ± 0.62 bc93.27 ± 0.97 a
Table 7. Dry weight (g) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 7. Dry weight (g) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR0 DAH30 DAH60 DAH90 DAH
2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year
C21.08 ± 1.08 b22.59 ± 1.44 a22.26 ± 1.05 b19.36 ± 0.84 b16.25 ± 0.49 c14.64 ± 0.66 b12.59 ± 0.59 c11.41 ± 0.61 a
HF27.28 ± 0.82 a24.94 ± 1.67 a25.48 ± 0.42 a23.73 ± 0.43 a19.42 ± 0.53 b21.20 ± 0.61 a15.59 ± 0.69 b13.98 ± 0.90 a
GBP27.17 ± 0.42 a25.62 ± 1.29 a26.30 ± 0.27 a22.49 ± 0.75 a23.17 ± 0.34 a20.93 ± 0.71 a18.00 ± 0.44 a14.27 ± 0.87 a
AA26.36 ± 0.26 a24.39 ± 1.98 a24.35 ± 0.33 ab22.34 ± 0.84 a18.35 ± 0.34 b18.73 ± 0.69 a14.74 ± 0.31 b14.03 ± 0.73 a
Table 8. Soluble solids content (%) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 8. Soluble solids content (%) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR0 DAH30 DAH60 DAH90 DAH
2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year
C6.90 ± 0.12 a6.75 ± 0.05 a8.80 ± 0.12 a8.66 ± 0.08 a11.77 ± 0.07 a13.53 ± 0.22 b13.32 ± 0.09 a14.12 ± 0.27 b
HF7.04 ± 0.05 a6.76 ± 0.08 a8.54 ± 0.05 ab8.49 ± 0.06 a10.59 ± 0.07 b11.67 ± 0.23 a12.04 ± 0.07 b12.61 ± 0.17 a
GBP6.70 ± 0.11 a6.67 ± 0.06 a8.38 ± 0.03 b8.70 ± 0.07 a9.87 ± 0.06 c10.96 ± 0.21 a11.49 ± 0.07 c12.18 ± 0.19 a
AA6.86 ± 0.04 a6.67 ± 0.06 a8.46 ± 0.04 b8.58 ± 0.07 a9.99 ± 0.04 c11.68 ± 0.23 a11.43 ± 0.07 c12.52 ± 0.18 a
Table 9. Titratable acidity (%) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 9. Titratable acidity (%) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR0 DAH30 DAH60 DAH90 DAH
2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year
C1.40 ± 0.02 b1.58 ± 0.03 a1.30 ± 0 c1.37 ± 0.03 a1.13 ± 0.01 c1.04 ± 0.02 a0.97 ± 0.01 b0.92 ± 0.02 a
HF1.40 ± 0.01 b1.41 ± 0.03 b1.18 ± 0.01 d1.24 ± 0.03 b1.03 ± 0.01 d1.09 ± 0.03 a0.86 ± 0.01 c0.89 ± 0.01 a
GBP1.45 ± 0.01 b1.43 ± 0.03 b1.39 ± 0.02 b1.22 ± 0.03 b1.28 ± 0.01 a1.04 ± 0.02 a1.10 ± 0.01 a0.92 ± 0.03 a
AA1.53 ± 0.01 a1.39 ± 0.02 b1.45 ± 0.01 a1.22 ± 0.02 b1.22 ± 0.01 b1.05 ± 0.02 a1.08 ± 0.01 a0.94 ± 0.02 a
Table 10. DPPH (%) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 10. DPPH (%) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR0 DAH30 DAH60 DAH90 DAH
2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year
C56.27 ± 0.71 c57.84 ± 0.65 d60.31 ± 0.87 c58.64 ± 0.5 c61.66 ± 0.66 c61.82 ± 0.53 c60.20 ± 0.44 c58.70 ± 0.62 c
HF56.33 ± 0.72 c54.33 ± 0.33 c58.48 ± 0.41 c58.64 ± 0.46 c63.22 ± 0.55 c63.06 ± 0.62 c60.26 ± 0.34 c60.42 ± 0.43 bc
GBP69.52 ± 0.51 a69.84 ± 0.62 a74.04 ± 0.65 a72.27 ± 0.54 a80.29 ± 0.34 a78.89 ± 0.33 a76.52 ± 0.51 a74.47 ± 0.37 a
AA60.90 ± 0.34 b61.87 ± 0.49 b64.62 ± 0.52 b63.17 ± 0.25 b69.90 ± 0.3 b67.96 ± 0.24 b65.16 ± 0.3 b61.82 ± 0.51 b
Table 11. Ascorbic acid content (mg AA/g DW−1) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
Table 11. Ascorbic acid content (mg AA/g DW−1) of A. deliciosa cv. ‘Hayward’ fruit on their postharvest storage during the 2022–2023 seasons, 0–90 days after harvest (DAH). Different letters between treatments (±SE) indicate statistically significant differences according to Bonferroni test (p ≤ 0.05).
TR0 DAH30 DAH60 DAH90 DAH
2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year2022 Year2023 Year
C56.27 ± 0.71 c57.84 ± 0.65 d60.31 ± 0.87 c58.64 ± 0.5 c61.66 ± 0.66 c61.82 ± 0.53 c60.20 ± 0.44 c58.70 ± 0.62 c
HF56.33 ± 0.72 c54.33 ± 0.33 c58.48 ± 0.41 c58.64 ± 0.46 c63.22 ± 0.55 c63.06 ± 0.62 c60.26 ± 0.34 c60.42 ± 0.43 bc
GBP69.52 ± 0.51 a69.84 ± 0.62 a74.04 ± 0.65 a72.27 ± 0.54 a80.29 ± 0.34 a78.89 ± 0.33 a76.52 ± 0.51 a74.47 ± 0.37 a
AA60.90 ± 0.34 b61.87 ± 0.49 b64.62 ± 0.52 b63.17 ± 0.25 b69.90 ± 0.3 b67.96 ± 0.24 b65.16 ± 0.3 b61.82 ± 0.51 b

4. Discussion

Until now, reports on the use of biostimulants in kiwifruit have been limited to some study areas, such as the application of agro-industrial waste extracts (AWEs) [52] to enhance fruit growth and metabolic traits, and the application of organic biostimulants to promote bud-break [53]. Algae-based biostimulants, such as Ascophyllum nodossum [54] and natural plant extract-based biostimulants [55], are regarded as treatments by other reports in the field. Amino acid-based biostimulants have a little reference on kiwi [56]. However, our study showed a beneficial impact on kiwi growth as well as to some metabolic characteristics. Organic biostimulants made with humic and fulvic compounds have been reviewed for their plant growth effectiveness in a wide range of crops, like the apricot Prunus armeniaca [57], peach Prunus persica [58], tomato Solanum lycopersicum [59], and grape Vitis vinifera [60], showing efficiency in several growth parameters. In our study, the growth parameters such as total stem length and leaf area and metabolic traits such as chlorophyll and proline content showed higher enrichment in some biostimulant treatments. Also, during postharvest fruit quality control, the antioxidant capacity was higher in amino acid-based biostimulant treatments.
The increase in the fresh and dry weight of leaves and shoots has been reported in several studies due to the addition of humic and fulvic compounds, such as Impatiens walleriana [61], Geranium L. [62], and yarrow Achillea millefolium L. [63]. These data are in agreement with our study in the case of humic and fulvic acid treatments, which foster certain growth indicators like weight and length. This outcome was also observed in the study of Khalil et al., 2011 [64], where the application of humic and fulvic acids in clay loam soil improved cucumber Cucumis sativus L. vegetation and fruit yield. The greater absorption of nutrients with the addition of HF substances may be a fact that is probably also reflected in our study through the enhanced growth characteristics seen in this treatment. A corresponding efficacy was obtained in our study by the application of a biostimulant containing GBP, which increased the fresh and dry weight of shoots and leaves. An increased growth length was also observed in our experiment for GBP treatment, and this has been noted in other studies on commercially important plant species, such as the maize Zea mays L. [65], soybean Glycine max [66], and wheat Triticum Aestivum L. [67]. The exogenous application of GB on kiwi leads to a better leaf water status [68]. Leaf strengthening by the addition of GBP is a noted fact in other works, being represented by the increase in their number [69] and their surface areas [70,71]. In our study, the leaf area was increased, especially in the case of GBP treatment in both growing seasons, while in the case of HF treatment, this effect lasted for a single growing season.
A plethora of reports concern chlorophyll enhancement by GBP agents [72,73,74,75,76]. Regarding the metabolic parameters of kiwifruit that were assessed, chlorophyll levels remained relatively stable throughout the experiment. But in GBP treatment, it remained higher for some days, and this may be a beneficial indication regarding GBP components in kiwifruit metabolism. This seems to be relevant in other studies as well [65,77]. According to Genard et al., 1991 [78], GB has a protecting role for chloroplasts and thylakoid membranes from oxidative stress damage and may have a cytokinine-like effect, enriching the chlorophyll content [79].
The total phenolic content in our study was better upon GBP treatment. An equivalent status of total phenolics after the implementation of crops with GB seems to correspond to observations in other works [80,81,82], more often in cases concerning their study in plant tissues under abiotic stresses [83,84,85]. The presence of proline was also greater in the treatment of GBP. The exogenous application of proline or biostimulants containing amino acids that promote plant growth can enhance proline content in plant tissues [86], especially in cases where osmotic dysfunctions occur [87]. In conformity with the research of Delfani et al., 2023 [88], the biochemical physiology of orange fruits was enriched by the application of GB, an effect that was also detected in our experiment. In addition, the exogenous application of GBP agents has been observed in many studies to increase proline in many crops. Through the regulation of endogenous proline metabolism, it can improve plants’ tolerance to abiotic stress [89,90]. In the work of Shan et al., 2016 [91], the exogenous addition of GB to P. persica increased the proline levels.
The heightened quantities of metabolic indicators, seen throughout the experiment until its end, motivate us to contemplate the concept of upgraded postharvest performance. The fresh and dry weight of kiwifruit appeared to increase during storage in both HF and GBP cases. This picture is intriguing regarding how these biostimulants may finally act on kiwifruit biomass due to the better performance of plant metabolism in these treatments, shown in the laboratory analyses of our study. An analogous outcome was recorded in the study of Zhang et al., 2022 [92], where the humic and fulvic acids improved the quality of lemon fruit Citrus limon L. by increasing their fresh and dry weight. A similar result was found in the study by Miri Nargesi et al., 2022 [93], where olive fruit quality parameters improved through humic acid treatment. According to the study of Delfani et al., 2022 [88], GBP application improved the quality traits of the orange fruit Citrus sinensis L. We did not detect differences in fruit firmness in the postharvest stage, and the same observation was referred to in the study of Costa et al., 2002 [94], concerning the application of biostimulants to Hayward kiwi. However, contrary to our study, the beneficial impact of soil-applied humic and fulvic components on fruit quality was exhibited in cucumber by increasing the contents of carbohydrates, protein %, nitrogen, phosphorus, and potassium [64]. TA and SSC in our study demonstrated better outcomes in HF treatments, mostly during their cold storage, for both years. Such improvements in SSC and TA were observed for the V. vinifera cv. ‘Askari’ grape in the research of Mohamadineia et al., 2015 [95]. A proportionally upgraded picture of C. limon L. fruit antioxidant traits was detected in the study of Zhang et al., 2022 [92]. On the other hand, the antioxidant indicators of kiwifruit noted a much better performance during storage in the case of GBP. This picture was revealed by the DPPH and vitamin C content, where the values were significantly higher in the GBP treatment. A comparable finding of strong antioxidant activity after GB application was observed in the study of Shafiq et al., 2021 [65], where the ascorbic acid content was high in Z. mays L. GBP elements are vigorous οsmoprotectants that accumulate to prevent cellular damage, thus making plants stronger in terms of coping with environmental stressors. This is accompanied by a higher antioxidant capacity. In the study of Adak 2017 [96], exogenous GB application on the strawberry Fragaria ananassa ‘Albion’ resulted in a higher ascorbic acid concentration. In the same study, a positive effect on total soluble solids and fruit firmness was also recorded, but not in our work.
Additional perspectives arising from the postharvest application of GB-based biostimulants concern the enhancement of chilling tolerance in peaches due to the induction of endogenous GB, GABA, and proline content, preventing membrane damage [91]. A similar effect during cold storage after the postharvest application of GB has been shown in various fruits and vegetables, such as the loquat Eriobotrya japonica (Rosales: Rosaceae) [97], sweet pepper Capsicum annuum L. [98], and banana Musa acuminata [56]. Last but not least, the study of Hoeberichts et al., 2017 [19], uncovered intriguing insights into the A. deliciosa bud-dormancy breaking via its implementation of organic biostimulants. Given the results of our study, the utility of biostimulants in kiwifruit relates to a supporting role, as presented in our experiment by the quality-upgraded kiwifruit. Maximizing the antioxidant potential of kiwifruit is crucial since it increases the fruit’s nutritional worth. Thus, profitability results from sustainable crop management techniques, including the utilization of growth-promoting agents, like some biostimulants. Finally, these practices have a low to zero environmental impact, which makes them suitable for organic farming.

5. Conclusions

Kiwifruit cultivation in Greece faces a plethora of difficulties coming from environmental stress, which is escalated by pests and climate change. Still, growers may alleviate the effects of environmental stresses and assure the sustainability of kiwifruit cultivation by implementing integrated management approaches. The application of biostimulants is a promising strategy with which to improve kiwifruit cultivation. Further research and experimental studies are required to completely understand the modes of action of biostimulants and their optimal dosage and application in kiwifruit cultivation. Humic and fulvic compounds are important tools in kiwi cultivation, offering multiple benefits for plant growth and production; however, in our study, their effect was detected in the partial strengthening of certain growth characteristics, such as total stem length, the number of leaves, and the fresh and dry weight of kiwifruits. The contribution of amino acid-based biostimulants was significant, both in the growth characteristics of kiwifruit (total shoot length, number of leaves, and leaf area), and in certain metabolic parameters such as total chlorophyll, phenolic components, and proline. In addition, the fruit quality traits during cold storage showed enhanced antioxidant potential (DPPH and vitamin C) and fresh and dry fruit weight, especially in the case of GBP. Furthermore, we observed that between the AA and GBP biostimulants, the last one had a stronger performance on plant growth metabolic parameters. This assay, using some organic and amino acid-based biostimulants on kiwifruit, has demonstrated certain benefits, and there is still plenty of unexplored potential for further research. Future studies may concentrate on figuring out the optimal dosage or interval applications of biostimulants for the synergistic use of biostimulants in conjunction with other soil and plant management practices.

Author Contributions

Conceptualization, V.P., V.S. and G.P.; methodology, V.P, P.M., and V.S.; software, V.P.; validation, V.P., V.S. and P.M.; formal analysis, V.P.; investigation, V.P.; resources, V.P.; data curation, V.P.; writing—original draft preparation, V.P.; writing—review and editing, V.P. and V.S.; visualization, V.P.; supervision, V.P. and G.P.; project administration, V.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author, V.P.

Acknowledgments

The authors gratefully acknowledge the Department of Agriculture of the University of Ioannina, for providing the necessary facilities to carry out the experiments, and the Koliou Group Co., S.A., Arta, Greece, for providing an experimental kiwifruit orchard to conduct the experiment.

Conflicts of Interest

The authors declare no conflicts of interest.

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Papantzikos, V.; Stournaras, V.; Mpeza, P.; Patakioutas, G. The Effect of Organic and Amino Acid Biostimulants on Actinidia deliciosa ‘Hayward’ Cultivation: Evaluation of Growth, Metabolism, and Kiwifruit Postharvest Performance. Appl. Biosci. 2024, 3, 360-377. https://doi.org/10.3390/applbiosci3030024

AMA Style

Papantzikos V, Stournaras V, Mpeza P, Patakioutas G. The Effect of Organic and Amino Acid Biostimulants on Actinidia deliciosa ‘Hayward’ Cultivation: Evaluation of Growth, Metabolism, and Kiwifruit Postharvest Performance. Applied Biosciences. 2024; 3(3):360-377. https://doi.org/10.3390/applbiosci3030024

Chicago/Turabian Style

Papantzikos, Vasileios, Vasileios Stournaras, Paraskevi Mpeza, and Georgios Patakioutas. 2024. "The Effect of Organic and Amino Acid Biostimulants on Actinidia deliciosa ‘Hayward’ Cultivation: Evaluation of Growth, Metabolism, and Kiwifruit Postharvest Performance" Applied Biosciences 3, no. 3: 360-377. https://doi.org/10.3390/applbiosci3030024

APA Style

Papantzikos, V., Stournaras, V., Mpeza, P., & Patakioutas, G. (2024). The Effect of Organic and Amino Acid Biostimulants on Actinidia deliciosa ‘Hayward’ Cultivation: Evaluation of Growth, Metabolism, and Kiwifruit Postharvest Performance. Applied Biosciences, 3(3), 360-377. https://doi.org/10.3390/applbiosci3030024

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