Effect of Foliar Application of Calcium and Salicylic Acid on Fruit Quality and Antioxidant Capacity of Sweet Pepper (Capsicum annuum L.) in Hydroponic Cultivation

Abstract

The aim of this study was to investigate the effect of foliar application of calcium and salicylic acid on improving the physicochemical quality, sensory quality and antioxidant potential of pepper fruits grown hydroponically in mineral wool substrate. Two sweet pepper varieties with red fruit type were used: block Aifos and elongated Palermo. Fruit quality was tested from four plant treatment combinations: (1) water (control), (2) calcium nitrate 0.7% (Ca), (3) salicylic acid 0.03% (SA), (4) calcium nitrate and salicylic acid combined (Ca+SA). Fruits of both varieties showed high concentrations of health-promoting constituents, including potassium, phosphorus, vitamin C (over 47 mg 100 g⁻¹ of FW (fresh weight)), and carotenoids, with capsanthin being the most abundant (more than 1226 μg 100 g⁻¹ of FW). The results of the sensory evaluation demonstrated that the attributes tested scores above 7 out of 10, indicating a high sensory quality. The antioxidant activity of pepper fruits was determined by three different methods: DPPH (method for measuring the antioxidant activity of DPPH), ABTS (method for measuring the antioxidant activity of ABTS) and TPC (total polyphenol content) and averaged more than 86%, 78% RSC (radical scavenging capacity) and almost 54 mg CE (catechin) 100 g⁻¹ of FW for both cultivars, respectively. Fruit quality results were analysed using PCA (principal component analysis). The first two principal components (PC1 and PC2) explained almost 54% of the variation, highlighting the strong correlations of PC1 with dry matter content, soluble sugars, potassium, acidity and sensory characteristics of pepper fruit such as skin hardness and flesh firmness. The application of SA to peppers resulted in an increase in the carotenoid content of the fruit. Furthermore, a notable positive correlation was detected between total sugars and the sugar/acid ratio when Ca+SA was combined in both cultivars. Palermo fruit showed better quality parameters and higher antioxidant activity, making this sweet pepper variety particularly valuable in a health-promoting context.

1. Introduction

Peppers are a very popular ingredient, both as a vegetable and as a spice. Pepper fruits are characterised by their high degree of flavour and health-promoting properties [1,2]. The incorporation of pepper fruits in the diet is strongly recommended due to the presence of bioactive compounds in these fruits that act as antioxidants [3]. Pepper fruits are an excellent source of antioxidants such as polyphenolic compounds, carotenoids, vitamins C, A, E, flavonoids and are rich in minerals, making them an important source of nutrients in the human diet [4,5,6]. Their solvent extracts exhibit potent antifungal, antibacterial and chemotherapeutic activities [7]. A diversity of morphological characteristics, fruit size, and quality are observed among the numerous varieties of pepper. The quality and chemical composition of pepper fruits are also influenced by agronomic and environmental factors [6]. Red and orange pepper fruits are valued for the presence of carotene and capsanthin, while β-cryptoxanthin, zeaxanthin, lutein and β-carotene are responsible for the yellowish–orange colour of peppers [8]. In contrast, green peppers represent a significant source of chlorophylls [9]. The researchers have observed that the levels of ascorbic acid, along with the colouring and ripening of the fruit, have either decreased or remained unchanged in some pepper varieties, while in other varieties there has been an increase. In contrast, provitamin A content increased with progressive fruit colouration in most pepper cultivars, with the exception of yellow cultivars. Brown peppers showed the highest provitamin A activity of 10–15% RDA (retinol activity equivalents) per 100 g⁻¹ of FW, compared to other varieties with different fruit colours [10]. The properties of hot and semi-spicy peppers are mainly due to the presence of capsaicinoids in their fruits, a complex of alkaloids that give them their characteristic spicy taste and biological activity. Capsaicinoids are mainly accumulated in the cells of the placental epidermis and in the seed sepals of the fruit [11]. Sweet peppers are distinguished by their high vitamin C content and the absence of large amounts of capsaicinoids, including capsaicin. The concentration of these substances in sweet pepper fruit is below 0.02% of dry weight [12,13]. The interest in natural antioxidants in food and other biological materials is due to their safety, as well as their potential nutritional and therapeutic value. Bioactive compounds, including polyphenols, carotenoids (α-carotene, β-carotene, lycopene and lutein) and vitamins A, B, C and E, which are present in fruits and vegetables, have been found to exhibit protective effects against cellular oxidation [14,15]. The regular consumption of these bioactive compounds with food in adequate quantities represents an important factor in the prevention of noncommunicable diseases, including cancer and cardiovascular disease [16,17,18,19]. The growing interest in natural antioxidants has prompted a surge in research activity aimed at assessing the antioxidant properties of plant-based products [20,21,22]. The antioxidant capacity of foods depends on the synergistic action of various antioxidant compounds. In addition to other reasons, the determination of the antioxidant capacity of foods in vitro demands a combination of more than one analytical method [23]. The determination of antioxidant activity frequently employs methods that involve the scavenging of free, stable radicals, such as the use of ABTS+ and DPPH [24,25]. It is frequently the case that antioxidant potential is also expressed in terms of total polyphenol content [26].

In addition to varietal traits, environmental factors also affect the nutritional profile of pepper fruit. Growing vegetables under cover allows for a high yield with stable quality. Nowadays, the majority of greenhouse cultivation is based on soilless technology, which is characterised by high efficiency and the use of a variety of substrates [27,28]. The soilless cultivation system (SCS) is regarded as a particularly promising solution, combining increased crop yield with minimal impact on the supporting ecosystem [29,30]. Soilless cultivation techniques are an effective solution in cases of water scarcity and low soil fertility [31]. Furthermore, they support plant growth under abiotic stress conditions, primarily salinity and drought [32]. However, soilless cultivation requires control of the root system in a reduced root zone volume, in comparison to the techniques employed in soil-based cultivation [33]. In hydroponic cultivation of key vegetable species such as tomatoes, peppers and cucumbers, mineral wool is the substrate most often used [34,35]. This inert substrate allows for control of root system parameters and high predictable yields [36,37]. On the other hand, physiological disorders are a common problem in greenhouse pepper production, which often employs the hydroponic method. These are usually caused by various types of abiotic stresses. A disorder known as dry rot, abbreviated blossom-end rot (BER), is a common cause of reduced quality in pepper fruit. The occurrence of BER is attributable to various factors. One such factor is water stress, which has the effect of inhibiting calcium transport in the plant [38,39,40].

Calcium is a crucial nutrient for plants, promoting optimal growth and development. It is essential for maintaining cell wall stability and integrity and improving fruit quality [41,42,43]. It is involved in intercellular signalling, affecting plant metabolism and cell growth [44]. Calcium is considered a secondary messenger molecule that balances the presence of stored intracellular Ca. In response to a variety of stimuli, calcium levels are elevated. When plants are subjected to stress, intracellular signalling events are triggered in the cell. Calcium signalling begins with sensors that recognise elevated calcium ion levels and activate protein kinases. The activated kinases regulate a number of genes, which in turn lead to phenotypic responses associated with stress tolerance [39,45].

The majority of the research concentrates on the utilisation of formulations in agricultural crops with the objective of reducing the incidence of adverse stress effects in plants. This includes the use of salicylic acid in the cultivation of certain vegetable species. Salicylic acid (SA) is one of the phenolic compounds produced by plants from the hydroxyl group or derivatives. Plant phenols are commonly described as specialised metabolites that perform vital functions, including the biosynthesis of lignin and allelopathic compounds that regulate plant responses to external stimuli [46], thermoregulation [47] and defence signalling activity in plants [46]. SA also stimulates morphological, physiological and biochemical pathways of general plant defence [48,49]. By inducing disease tolerance in plants (Arabidopsis, Nicotiana benthamiana and tomato), SA also controls ion uptake and antioxidant defence [50]. According to Sobczak et al. [51], foliar application of salicylic acid to pepper plants at a concentration of 0.03% positively influences the growth and yield of peppers in hydroponic cultivation and on pepper fruit quality. The treatment of pepper plants with SA resulted in a reduced proportion of fruit manifesting symptoms of BER [51]. Additionally, the application of SA in aeroponic cultivation has been observed to promote favourable growth and development in pepper plants. Peppers grown at high nutrient solution concentrations (EC (electrical conductivity) about 7 dS m⁻¹) showed symptoms of oxidative stress. The application of SA was found to reduce both the number and weight of fruits showing symptoms of calcium deficiency. The results obtained by Sobczak et al. [52] confirm the effect of SA in alleviating high substrate EC stress in pepper, increasing the photosynthetic activity of pepper leaves by increasing, among other things, overall PSII-PI viability and leaf SPAD index. According to Sobczak et al. [52], high EC disrupts the oxidative balance, while exogenous SA activates CAT (catalase), which detoxifies large amounts of ROS (reactive oxygen species) and alleviates the stress response. Furthermore, Amin et al. [53] report that spraying pepper plants with salicylic acid has the effect of significantly increasing peroxidase activity while maintaining catalase activity at a level comparable to that of the untreated control. SA applied before harvest induced a significant increase in polyphenol oxidase and peroxidase activity in pepper fruit [53]. As reported by Jiankang et al. [54], pre- and post-harvest application of SA resulted in better control over the pathogens affecting pears and cherries. This was achieved by inducing the defence immune system [54] and stimulating the activity of antioxidant enzymes [55]. At present, there is a considerable volume of research examining the efficacy of employing diverse eustressors in crop cultivation, the way they are applied, the impact of concentration levels or the amount of doses [56,57,58]. In order to reduce the negative effects of stresses occurring in pepper cultivation, effective solutions are constantly being sought for commodity sub-production, including those involving soilless cultivation techniques. At the same time, a great deal of attention is paid to factors increasing the quality of vegetables, especially with regard to health-promoting components. Therefore, the aim of this study was to evaluate the effectiveness of foliar application of calcium (Ca) to sweet pepper plants in comparison to treatment with an SA solution and a simultaneous application of Ca and SA, in order to determine the impact on the physicochemical quality, sensory quality and antioxidant capacity of sweet pepper fruits grown hydroponically.

2. Materials and Methods

2.1. Location of Research

The research was carried out at the Greenhouse Experimental Centre of Warsaw University of Life Sciences (21° E, 51°15′ N), within the cultivation chambers of the Department of Vegetable and Medicinal Plants. The greenhouse is covered with glass (ridge height: 6.70 m, sidewall height of 3.50 m).

2.2. Plant Material and Experimental Design

Two sweet pepper varieties with red fruit were selected for the study: Aifos F1 (Seminis, Bayer) block type and Palermo F1 (Rijk Zwaan) Dulce Italiano type. The study was conducted in 2019 (term 1) and 2020 (term 2), using a hydroponic system with mineral wool substrate for the cultivation of peppers. Pepper seedlings were planted on growing mats in the experimental chamber on 13 May in both 2019 and 2020 (0 DAP–day after planting). The seedlings were planted on 28 DAS (days after sowing) into Grotop Master mats (Grodan), with three plants per mat, at a density of 2.5 plants/m² (1.2 × 0.55 m). Plants were cultivated on two shoots and a nutrient solution of a specific composition was supplied by capillaries. The average daytime temperature was 25.5–25.8 °C and nighttime temperature was 20.8–20.9 °C. The radiation totals were 192.11 kJ cm⁻² (term 1) and 209.41 kJ cm⁻² (term 2). The cultivation details are outlined in Sobczak et al. [51].

Experimental Factors

Aifos and Palermo pepper plants were sprayed once a week between 7 and 151 DAP (day after planting). The following treatments were applied to the plants: (1) control plants were sprayed with water (control), (2) calcium nitrate at a concentration of 0.7% (Ca), (3) salicylic acid at a concentration of 0.03% (SA) and (4) calcium nitrate in a combination with with salicylic acid at a concentration of 0.7% and 0.03% (Ca+SA), respectively. Oleate 85 EC (rapeseed oil) Danmar was used as a surfactant. Tests for each cultivar and combination (Ca; SA; Ca+SA and control) were performed in 4 replicates. A repetition was a randomly selected experimental plot with two mineral wool growing mats, i.e., 6 plants. The experiment was conducted on both dates using the randomised block method.

2.3. Evaluated Parameters

Physicochemical analyses of the fruit, evaluation of the antioxidant activity of the fruit and sensory evaluation of the fruit were carried out immediately after fruit harvest, during term 1 and term 2 on two occasions: 84 DAP (5 August) and 140 DAP (30 September). Fruits that had reached physiological maturity and full colouration were harvested for subsequent analysis. Five fruits from each combination were randomly selected for physicochemical analyses. Fruits without seeds or seed sepals were cut into cubes of approximately 4 mm × 4 mm. Following the preparation of the mixed fruit sample, triplicate samples were then taken for analysis.

2.3.1. Physicochemical Analysis of the Fruit

Dry matter content (DW)

The dry matter content of the fruit was determined using the dryer-weight method at 105 °C, as described in detail by Sobczak et al. [51].

Ascorbic acid (AA)

Total vitamin C content was determined in accordance with the method of Odriozola-Serrano et al. [59] and Grobelna et al. [60]. Ascorbic acid content was determined by high-performance liquid chromatography (HPLC) with the use of the UV–vis SPD-10A VP detector (Shimadzu, Kyoto, Japan), LC-10AT pump (Shimadzu, Kyoto, Japan), CTO-10AS VP oven (Shimadzu, Kyoto, Japan), DEGASSEX model D-4400 (Shimadzu, Kyoto, Japan) degasser by Shimadzu, using the software for collecting LC Solution data (Shimadzu, Kyoto, Japan, version 1.21 SP1). An OnyxMonolithic C18 column, 100 × 4.6 mm (Phenomenex) was used. The mobile phase was the H₃PO₄ solution. The results were recorded at 254 nm. The content of L-ascorbic acid is expressed as milligram content per 100 g sample.

Total Soluble Solids (TSS)

In order to determine the concentration of dissolved components in the cell juice, the juice was extracted from finely chopped pepper fruits. Subsequently, the sugar extract content was measured using an Atago refractometer, with the result expressed in °Brix. The TSS/TA ratio was calculated by dividing the TSS score by the TA (total acidity) score.

Total sugars (TS)

The total sugar content of the tested fruits was determined using the Luff-Schoorl method [60]. The ratio of the total sugar content to the acidity of the pepper fruits was determined as the quotient TS:TA.

Total acidity (TA)

The acidity of the pepper fruits was determined by a titration potentiometric method [61].

Nitrates

Nitrate concentration (NO₃) was determined spectrophotometrically using a FIAstar instrument (Foss Tecator AB, Hoeganaes, Sweden) at 440 nm, and P concentration using a colorimetric assay. K and Ca concentrations were determined using the flame method.

Carotenoids

The content of β-carotene, β-cryptoxanthin, lutein, ascorbic acid, neoxanthin, zeaxanthin, violaxanthin and capsanthin in fruits was quantified by HPLC using a Shimadzu LC-20 system (Kyoto, Japan). To prepare the samples, 2 g of Na₂SO₄ per 100 g⁻¹ was added to each pepper fruit sample and then homogenised. Then, 5 g of this homogenised material was taken, a pinch of quartz sand was added and ground in a mortar with acetone at a temperature of 4 °C. The extract was transferred to 50 mL volumetric flasks and made up with the same acetone. The supernatant obtained after centrifugation (1500 rpm) was filtered using a 25 mm filter with a pore size of 0.22 µm (Supelco IsoDisc™ PTFE syringe filter) into vials. The vials were placed on a thermostatted tray (4 °C) and 5 µL was applied to a column (Phenomenex, Kinetex 2.6 µm, C18, 100 mm × 4.6 mm) thermostatted at 40 °C using an autosampler. Methanol in isocratic elution was used as the mobile phase. Data for β-carotene were collected at 450 nm, 430 nm for chlorophyll a and 470 nm for chlorophyll b.

2.3.2. Evaluation of Antioxidant Potential

Preparation of fruit extracts

The extracts were prepared from the pericarp pulp with peel. They were obtained by ultrasonic-assisted extraction with methanol (25 mL 1 g⁻¹ of raw material), for a period of 60 min. The extraction process was performed at room temperature. The extracts were filtered through a tissue paper filter and then centrifuged. The filtrate was collected in a clean tube. The samples were stored at 4 °C until the point of analysis.

DPPH Assay

The assay involved the colorimetric measurement of the extent to which a known amount of DPPH (2,2-diphenyl-1-picrylhydrazyl) was reduced by the extract. DPPH is a stable cation radical that has an unpaired electron on the valence shell. It forms a nitrogen bridge on one of the nitrogen atoms. When reacting with the test substance, it releases a hydrogen atom, transforming into the reduced form of DPPH. This reaction is accompanied by a colour change of the solution from violet to pale yellow, which is quantitatively measured spectrophotometrically at 517 nm. The result is given in % DPPH inhibition [62].

Preparation of DPPH radical solution

First, 0.012 g of DPPH was weighed and quantitatively transferred to a 100 mL volumetric flask. It was then made up to the mark with pure methanol and the reagent was dissolved using an ultrasonic bath for approximately 60 min. The reagent was stored in a dark glass bottle.

Preparation of reagent blank

We added 1 mL of distilled water to 3 mL of methanol and 1 mL of DPPH solution. The whole mixture was mixed well and, 10 min after the last component was added, the absorbance was measured at λ = 517 nm.

Preparation of test solutions

We took 0.5 mL of extract and transferred it to a 10 mL volumetric flask and made it up to 10 mL with methanol. Then, 0.2 mL of test sample, 1 mL, 3 mL of methanol and 1 mL of DPPH radical solution were measured into the tube. Ten minutes after the last reagent was added, the absorbance of the mixture against methanol was measured at λ = 517 nm.

The percentage of DPPH inhibition was calculated according to the formula:

% DPPH = [(AB − AA)/AB] × 100

where

AA is bsorbance of test solution (t = 10 min);

AB is absorbance of the blank (t = 0 min) [63];

The antioxidant activity of the tested extracts was given as the percentage of DPPH radical scavenging capacity (RSC, %).

ABTS Assay

Assessment of antioxidant activity by the ABTS (2,2′-azobis(3-ethylbenzothiazoline-6-sulfonate) method involves the reduction of the colourful, intensely blue–green cation radical ABTS+ (the oxidised form) to the colourless ABTS, which is the reduced form. The radical is reduced by antioxidants present in the test sample [64].

Preparation of the ABTS solution

A 4.9 mM potassium persulphate (K₂S₂O₈) solution was prepared, with a stability period of only one day. To prepare the solution, 0.132 g of K₂S₂O₈ was weighed and thoroughly dissolved in 100 mL of redistilled water. The PSB phosphate buffer, pH 7.4, was then prepared. This solution was prepared from two PBS tablets dissolved in 400 mL of redistilled water.

ABTS solution was prepared by dissolving one ABTS tablet in 1.3 mL potassium persulphate solution and 1.3 mL distilled water. The solution was stored in a dark location at room temperature for a period of 16 h. After this time, a working solution of ABTS was prepared by diluting it with PBS so that its absorbance at 734 nm was A = 070.

Sample Measurement

One by one, 100 μL of the test extract and 1 mL of ABTS were added to the cuvette. Six minutes after the addition of the last component, the measurement was performed in a spectrophotometer at 734 nm. The drift capacity of the stable ABTS radical was calculated according to the formula

% ABTS = [(AB − AA)/AB] × 100

where

AA is absorbance of test solution (t = 6 min);

AB is absorbance of the blank (t = 0 min) [65].

The antioxidant activity of the tested extracts was given as the percentage of ABTS radical scavenging capacity (RSC, %).

TPC Assay

The determination was performed by the colorimetric method using the Folin–Ciocalteu reagent [66].

Preparation of extracts

The analysis was conducted on 5% methanolic extracts prepared from pericarp with peel, obtained through ultrasound-assisted extraction for 60 min. The extraction was performed at room temperature.

Preparation of the calibration curve

A calibration curve was prepared from a standard solution of (+)catechin in methanol (catechin concentration 50 mg 100 cm³). We added 0.5, 1.25, 2.5, 3.75 and 5 cm³ of the catechin solution to 25 cm³ flasks successively. The flasks were topped up with methanol 25 cm³. From each flask, 2.5 cm³ of the solution was taken into 25 cm³ flasks and topped up with redistilled water. A volume of 5 cm³ of these solutions was taken and 0.25 cm³ of Folin–Ciocalteau reagent (diluted 1:1 with redistilled water and 0.5 cm³ of 7% Na₂CO₃) was added. After thorough mixing, the solutions were left to stand in the dark. After 30 min, the absorbance of the solutions was measured in a spectrophotometer at 760 nm. A calibration curve was plotted from the results obtained [67].

Specific measurement

Dilutions of the initial extract were prepared, 2-, 5- and 10-fold with methane. For the analysis, 2.5 cm³ of each sample was popped and topped up with water to 25 cm³. Then, 5 cm³ each was taken from the solutions and 0.25 cm³ of Folin–Ciocalteu reagent and 0.5 cm³ of 7% Na₂CO₃ were added. The samples were mixed thoroughly and set aside in a dark place. After 30 min, the absorbance was measured in a spectrophotometer at 760 nm. The result was expressed as mg catechin (CE) 100 g⁻¹ of FW (mg CE 100 g⁻¹ of FW).

2.3.3. Sensory Evaluation of Pepper Fruit Quality

The QDA method (quantitative description analysis) was used for sensory quality testing of pepper fruits evaluation, which is described in Sobczak et al. [51].

2.4. Statistical Methods

The results were statistically processed using multivariate factor analysis. Based on the average values of traits for each experimental setup, divided by pepper variety, a multivariate factor analysis was conducted. The principal component analysis (PCA) method was used to reduce the number of variables and capture the main sources of variance in the data. To maximise the variance of loadings between factors and minimise their variance within the new factor, Varimax rotation was applied. Based on the obtained components, in the form of correlation coefficients of the studied variables with the components and the correlation of factor objects divided by variety, the interdependencies were illustrated using a biplot. Analyses were performed using IBM SPSS Statistics version 29.

3. Results

3.1. Physicochemical and Sensory Characteristics, Yield and Antioxidant Potential of Pepper Fruits

The detailed yield characteristics of Aifos and Palermo peppers grown hydroponically, as well as the dry matter content of the fruit and the distinguishing characteristics of the sensory evaluation of the pepper fruits, are described in Sobczak et al. [51]. Data for the treatment of pepper plants with Ca, SA and their combination are published in the article Sobczak et al. [51]. High levels of solar radiation and high air temperature were recorded during the pepper yielding period [51]. However, the commercial fruits of peppers of both cultivars obtained at date 1 and date 2 were of high quality. On the other hand, a high proportion of BER fruit was found in the total pepper yield, especially in the cultivar Palermo (

Table 1. Average values with standard deviation of the examined physicochemical parameters of fruit quality, sensory quality, yield and antioxidant activity of pepper fruits for cultivars Aifos and Palermo. The table indicates the codes for the parameters used in the processing of the results of principal component analysis, PCA.

Parameter	Unit	Code	Cultivar		Average
			Aifos	Palermo
Physicochemical composition (average of 48 values)
Dry weight	%	DW	8.36 ± 0.50	9.91 ± 1.69	9.14 ± 1.47
Total soluble solids	°Brix	TSS	8.40 ± 0.58	9.63 ± 0.99	9.01 ± 1.01
Total sugars	g 100 g−1 FW	TS	3.83 ± 1.68	4.35 ± 1.90	4.09 ± 1.80
Titratable acidity		TA	0.21 ± 0.02	0.17 ± 0.04	0.188 ± 0.03
TSS/TS ratio	i.u.	TSS/TA	40.70 ± 3.99	58.82 ± 10.87	49.761 ± 12.22
TS/TA ratio	i.u.	TS/TA	18.59 ± 8.39	26.01 ± 10.91	22.301 ± 10.373
Phosphorus	mg kg−1 FW	P	223.92 ± 25.38	214.83 ± 49.60	219.38 ± 39.46
Nitrates		NO3	108.14 ± 4.32	110.00 ± 3.81	109.07 ± 4.16
Calcium		Ca	23.02 ± 4.08	24.09 ± 4.30	23.56 ± 4.20
Potassium		K	2294.48 ± 122.82	2180.94 ± 216.45	2237.71 ± 184.12
Ascorbic acid	mg 100 g−1 FW	AA	48.15 ± 10.07	47.15 ± 5.97	47.65 ± 8.25
Neoxanthin	μg 100 g−1 FW	Neox.	39.23 ± 27.05	36.53 ± 12.31	37.88 ± 20.95
Lutein		Lut.	234.36 ± 172.64	184.80 ± 74.89	209.58 ± 134.68
Zeaxanthin		Zeax.	24.68 ± 12.97	19.50 ± 4.10	22.09 ± 9.91
Violaxanthin		Violax.	6.46 ± 5.16	3.55 ± 2.98	5.00 ± 4.44
Capsanthin		Caps.	1478.43 ± 1309.22	974.31 ± 528.73	1226.37 ± 1024.95
β-cryptoxanthin		Crypt.	100.63 ± 56.74	76.11 ± 21.86	88.37 ± 44.51
α-carotene		α-carot.	198.25 ± 121.44	138.09 ± 101.78	168.17 ± 115.48
β-carotene		β-carot.	820.81 ± 458.92	650.26 ± 417.85	735.53 ± 444.89
Sensory quality (average of 48 values)
Odour of fresh pepper	0–10 point’s scale	Odor-fre.p.	5.01 ± 1.93	4.78 ± 2.43	4.89 ± 2.19
Skin hardness		Skin-hard.	5.97 ± 2.08	5.00 ± 2.59	5.49 ± 2.39
Flesh fibrousness		Flesh-fibr.	6.14 ± 1.88	5.24 ± 1.83	5.69 ± 1.91
Flesh juiciness		Flesh-juic.	6.18 ± 1.90	4.86 ± 2.05	5.52 ± 2.081
Flesh firmness		Flesh-firmn.	6.84 ± 1.98	3.02 ± 1.37	4.93 ± 2.555
Typical pepper taste		Taste-p.	6.46 ± 1.27	6.65 ± 1.58	6.55 ± 1.43
Sour taste		Taste-so.	3.74 ± 2.14	2.21 ± 1.66	2.98 ± 2.06
Sweet taste		Taste-swe.	3.79 ± 2.11	6.40 ± 2.12	5.09 ± 2.48
Bitter taste		Taste-bit.	0.51 ± 0.89	0.36 ± 1.08	0.44 ± 0.99
Pungent flavour		Flav.pung.	0.97 ± 1.31	0.80 ± 1.02	0.88 ± 1.17
Off-flavor		Flav.of.	0 ± 0	0 ± 0	0 ± 0
Overall quality		OQ	7.18 ± 0.95	6.766 ± 1.22	6.973 ± 1.11
Overall sensory preference		Over.sense.pref.	6.73 ± 1.40	7.29 ± 1.43	7.009 ± 1.44
Yield (average of 48 values)
Total yield	g plant−1	Tyield	2844.07 ± 469.92	2955.05 ± 925.76	2899.56 ± 730.41
Fruit with BER		BER	738.48 ± 266.25	1714.22 ± 353.67	1226.35 ± 581.57
Marketable yield		MYield	2105.58 ± 553.39	1240.82 ± 714.44	1673.20 ± 769.27
No. of Fruit Total yield	No. plant−1	No.fr.Tyield	17.69 ± 3.97	45.04 ± 11.63	31.37 ± 16.26
Fruit with BER		No.fr.BER	6.51 ± 2.92	30.70 ± 9.62	18.60 ± 14.09
Marketable yield		No.fr.Myield	11.18 ± 3.47	14.34 ± 7.70	12.76 ± 6.13
Antioxidant activity of pepper fruits (average of 48 values)
In vitro antioxidant DPPH assay	% RSC	DPPH	64.12 ± 2.49	73.30 ± 2.92	68.71 ± 5.35
ABTS assay		ABTS	72.86 ± 2.47	83.43 ± 4.17	78.22 ± 6.32
Total phenolic content	mg CE 100 g−1 FW	TPC	47.10 ± 3.50	60.80 ± 5.60	53.91 ± 8.31

). Pepper plants treated with both Ca and SA alone and with both Ca and SA showed less BER on fruit than plants in the control [51]. Commercial sweet pepper fruits from plants grown hydroponically in a mineral wool substrate were characterised by high health-promoting quality. High concentrations of the mineral salts potassium and phosphorus, vitamin C averaging over 47 mg 100 g⁻¹ of FW, carotenoids including lutein averaging more than 209 μg 100 g⁻¹ of FW, capsanthin more than 1226 μg 100 g⁻¹ of FW, β-cryptoxanthin more than 88 μg 100 g⁻¹ of FW and α and β carotene more than 168 and 735 μg 100 g⁻¹ of FW, respectively, were found in the fruit of both cultivars (Table 1). In the evaluation of sensory quality, the peppers obtained an overall sensory preference score for the cultivars of more than 7 points out of a maximum of 10, which indicates their high sensory quality [51]. Fruits of both cultivars were characterised by high antioxidant activity measured by DPPH, ABTS and TPC methods obtaining, on average, levels of oxidative activity in each method of more than 86%; more than 78% RSC and almost 54 mg CE 100 g⁻¹ of FW, respectively (Table 1). Large differences were found between the heights of individual parameters in pepper fruits depending on the cultivar. The fruits of the Aifos cultivar accumulated more potassium ions, bioactive compounds such as lutein, capsanthin, β-cryptoxanthin and α and β- carotene. At the same time, in the sensory evaluation, they were considered to be fruits with a tougher skin than those of the Palermo variety. They were rated higher than Palermo variety fruit in terms of most distinguishing characteristics such as flesh fibrousness, flesh juiciness, flesh firmness, sour taste, bitter taste, pungent flavour and overall quality of pepper fruit but these differences were not statistically significant. On the other hand, sweetness was significantly higher for Palermo than for Aifos. Palermo fruit was characterised by higher dry matter content, TSS and sugar/acid ratio TSS/TA and TS/TA than Aifos fruit and tended to receive higher overall sensory preference scores than Aifos fruit in the consumer evaluation (Table 1). In addition, the Palermo fruit received higher scores for antioxidant activity, regardless of the method used in this study to measure the activity of the fruit in vitro (Table 1).

3.2. PCA Analysis for Physicochemical and Sensory Quality Parameters of Peppers Considering Foliar Treatment of Peppers with Calcium (Ca), Salicylic Acid (SA) and Calcium Combined with Salicylic Acid (Ca+SA)

Table 1 shows the mean values of the physicochemical and sensory quality parameters, as well as the yield characteristics and antioxidant activity of the fruit of the two pepper cultivars Aifos and Palermo. Each parameter studied was identified by a code (Table 1), which was used in the graphs (PCA) principal component analysis.

A PCA factor analysis performed for fruit physicochemical and sensory quality parameters depending on foliar treatment of peppers with calcium (Ca), salicylic acid (SA) and calcium combined with salicylic acid (Ca+SA), showed more than 95% variability in the five principal components (

Table 2. Eigenvalues and proportion of the total variance in 8 experimental combinations (cultivar: Aifos and Palermo x treatment: control; Ca; SA; Ca+SA), as explained by the first five principal components for the physicochemical and sensory quality parameters of pepper fruit and the correlation coefficients between these parameters and the first five PCs.

Parameter a	Principal Components (PCs)
	PC1	PC2	PC3	PC4	PC5
DW	−0.661 b	−0.499	−0.521	0.036	−0.097
TSS	−0.655	−0.638	−0.282	0.076	0.135
TS	−0.031	−0.086	−0.905	0.165	−0.283
TS/TA	−0.472	−0.176	−0.816	0.138	−0.153
TA	0.935	0.203	0.175	0.015	−0.123
TSS/TA	−0.867	−0.373	−0.191	−0.047	0.171
P	0.202	0.000	0.225	−0.050	0.916
NO3	−0.306	−0.557	−0.114	−0.420	0.617
Ca	−0.399	−0.072	0.400	−0.241	0.782
K	0.915	−0.119	0.099	0.303	−0.028
AA	0.177	0.054	0.676	0.195	−0.671
Neox.	0.026	0.574	0.275	−0.216	0.720
Lut.	0.326	0.657	0.436	−0.495	0.030
Zeax.	0.294	0.596	0.723	0.014	0.183
Violax.	0.337	0.792	0.384	0.246	−0.118
Caps.	0.538	0.453	0.613	−0.149	0.157
Crypt.	0.434	0.713	0.469	−0.008	−0.053
α-carot.	0.368	0.823	0.082	0.334	−0.197
β-carot.	0.359	0.800	−0.072	0.315	0.190
Odor-fre.p.	0.176	0.256	0.111	0.912	−0.233
Skin-hard.	0.789	0.386	0.025	0.017	0.089
Flesh-fibr.	0.518	0.685	0.035	−0.46	0.123
Flesh-juic.	0.413	0.598	0.086	−0.529	0.341
Flesh-firmn.	0.710	0.559	0.335	−0.248	0.068
Taste-p.	−0.115	−0.139	−0.042	0.900	−0.105
Taste-so.	0.892	0.313	0.092	0.194	−0.110
Taste-swe.	−0.863	−0.434	−0.234	−0.083	0.041
Taste-bit.	0.055	−0.068	0.513	−0.768	0.162
Flav.pung.	0.428	0.173	−0.098	0.876	−0.008
OQ	0.779	0.609	−0.009	−0.055	−0.061
Over.sense.pref.	−0.855	−0.306	−0.307	0.128	−0.183
Total variance explained-rotation sums of squared loadings
Total	9.598	7.138	4.704	4.624	3.410
% of Variance	30.961	23.026	15.174	14.916	11.001
Cumulative %	30.961	53.986	69.161	84.077	95.078

a—Parameters described in Table 1; b—Bold values indicate the maximum correlation coefficient of the variable with the obtained components.

).

The first and third principal components (PC1, PC3) explain more than 46% of the variation and the first and second (PC1, PC2) almost 54% of the variation (Table 2, Figure 1 and Figure 2). The PC1 component is most strongly correlated with fruit quality parameters (Table 2) such as dry matter content (DW), soluble sugars (TSS), potassium, total acidity per citric acid (TA), sugar/acid ratio (TSS/TA) and variables described in the sensory analysis of pepper fruits such as skin hardness (Skin-hard.), flesh firmness (Flesh-firmn.), sour taste (Taste-so.), sweet taste (Taste-swe.), overall quality (OQ) and also overall sensory preference (Over.sens.pref.). Figure 1 and Figure 2 show clear varietal differences in terms of the above characteristics. In the case of the cultivar Aifos, the values for control are to the right of the intersection of the X axis. The factor used in the experiment, such as the addition of Ca, resulted in a higher correlation with traits positively related to PC1. The total factor applied Ca+SA additionally showed a high correlation with the PC3 component and consequently with inter-variables such as total sugars (TS) and sugar/acid ratio (TS/TA) in pepper fruits (Figure 1). The cultivar Palermo is on the opposite side of the intersection of the X-axis with respect to the characteristics correlated with PC1; i.e., this cultivar was strongly positively correlated with variables such as dry matter content of the fruit (DW), soluble components in cell juice (TSS) and their proportion to acid concentration in the fruit (TSS/TA), as well as with the sensory evaluation discriminator sweet taste (Taste-swe.), whereas it was inversely correlated with variables with which the cultivar Aifos was positively correlated (Figure 1). Both cultivars responded similarly to the Ca+SA factor, showing a high positive correlation with total sugars in fruit (TS) and sugar/acid ratio (TS/TA) (Figure 1).

The PC2 component is most strongly correlated with parameters relating to the concentration of pigments such as lutein, violaxanthin, α-carotene and β-carotene in pepper fruits (Table 2). The use of SA in the experiment shifted these objects up the y-axis relative to the control, thus increasing the parameters positively correlated with PC2 (Figure 2). The situation was similar in the case of the Ca+SA experimental system. It can be concluded that the application of SA had a favourable impact on the accumulation of pigments in the pepper fruits. In the context of Ca factor experiments, a significant positive effect was observed for the cultivar Palermo, while the Ca factor for the cultivar Aifos showed an inverse correlation with the PC2 component (Figure 2).

3.3. PCA Analysis for Physicochemical Parameters and Antioxidant Activity of Pepper Fruits Considering Foliar Treatment of Plants with Calcium (Ca), Salicylic Acid (SA) and Calcium Combined with Salicylic Acid (Ca+SA)

For PCA analysis of the physicochemical parameters and antioxidant activity of pepper fruits, the four main components explain almost 94% of the variation (

Table 3. Eigenvalues and proportion of the total variance in 8 experimental combinations (cultivar: Aifos and Palermo x treatment: control; Ca; SA; Ca+SA), as explained by the first four principal components for the physicochemical parameters and antioxidant properties of pepper fruit and the correlation coefficients between these parameters and the first four PCs.

Parameter a	Principal Components (PCs)
	PC1	PC2	PC3	PC4
DW	0.640 b	−0.537	−0.516	−0.134
TSS	0.611	−0.69	−0.282	0.035
TS	0.049	−0.003	−0.962	−0.206
TS/TA	0.470	−0.152	−0.852	−0.108
TA	−0.918	0.314	0.123	−0.066
TSS/TA	0.842	−0.479	−0.142	0.108
P	−0.218	−0.018	0.233	0.909
NO3	0.304	−0.662	−0.011	0.636
Ca	0.394	−0.183	0.473	0.753
K	−0.949	−0.003	0.017	−0.097
AA	−0.223	0.122	0.57	−0.739
Neox.	0.034	0.487	0.387	0.762
Lut.	−0.239	0.608	0.547	0.183
Zeax.	−0.265	0.585	0.741	0.154
Violax.	−0.279	0.819	0.389	−0.168
Caps.	−0.486	0.473	0.632	0.184
Crypt.	−0.406	0.733	0.472	0.003
α-carot.	−0.286	0.886	0.071	−0.236
β-carot.	−0.266	0.879	−0.097	0.180
DPPH	0.757	−0.436	−0.461	0.000
ABTS	0.830	−0.478	−0.239	0.030
TPC	0.829	−0.336	−0.372	0.060
Total variance explained: rotation sums of squared loadings
Total	6.511	6.073	4.831	3.236
% of Variance	29.597	27.606	21.959	14.708
Cumulative %	29.597	57.203	79.162	93.87

a—Parameters described in Table 1; b—Bold values indicate the maximum correlation coefficient of the variable with the obtained components.

).

The PC1 component was most strongly correlated with such variables from the chemical analysis as dry matter (DW), potassium (K), cell sap soluble components (TSS), total acidity (TA) and TSS/TA ratio in pepper fruits, and with variables characterising the antioxidant capacity of pepper fruits; i.e., antioxidant DPPH and ABTS assay and total phenolic content (TPC), among others. Figure 3 clearly shows varietal differences in terms of the above characteristics. For the cultivar Aifos, the values for control are to the left of the intersection of the X axis. The factor used in the experiment, for example, spraying the pepper plants with Ca, resulted in a higher correlation with traits negatively associated with PC1. A positive correlation has been identified between the PC2 component and the concentration of carotenoids, including lutein (Lut.), violaxanthin (Violax.), β-cryptoxanthin (Crypt.), α-carotene (α-carot.) and β-carotene (β-carot.), in fruit. The use of the SA factor in the experiment resulted in a shift of these objects relative to the control up the Y-axis, thereby leading to an increase in the parameter values that showed a positive correlation with PC2. The situation was similar in the case of the Ca+SA experimental system. The application of the Ca factor demonstrated a significant positive effect on the cultivar Palermo, while the inverse correlation between the Ca factor and the PC2 component was evident in the cultivar Aifos. In conclusion, the addition of SA had a positive effect on the increase of pigments in the pepper fruits tested (Figure 3).

Furthermore, the factor Ca+SA demonstrated a strong correlation with the PC3 component, indicating a significant relationship with inter-variables such as the total sugar content of the pepper fruit (TS) and the sugar/acid ratio (TS/TA) (Figure 4). With regard to the traits associated with PC1, the cultivar Palermo lies on the opposite side of the intersection of the X axis. Consequently, this cultivar showed a strong positive correlation with variables such as DW, TSS, TSS/TA, DPPH, ABTS and TPC and an inverse correlation with variables with which the cultivar Aifos was positively correlated. Both varieties responded similarly to the Ca+SA factor showing high positive spiking with the variable TS, TS/TA (Figure 5).

3.4. PCA Analysis for Physicochemical Parameters of Pepper Fruit and Yield Parameters Considering Foliar Treatment of Peppers with Calcium (Ca), Salicylic Acid (SA) and Calcium Combined with Salicylic Acid (Ca+SA)

For PCA analysis of physical and chemical parameters and yield of pepper fruit parameters, the four principal components explained almost 93% of the variability (

Table 4. Eigenvalues and proportion of the total variance in 8 experimental combinations (cultivar: Aifos and Palermo x treatment: control; Ca; SA; Ca+SA), as explained by the first four principal components for the physicochemical parameters of pepper fruit and yield parameters and the correlation coefficients between these parameters and the first four PCs.

Parameter a	Principal Components (PCs)
	PC1	PC2	PC3	PC4
DW	−0.571	0.594 b	−0.525	0.146
TSS	−0.719	0.578	−0.276	0.012
TS	−0.021	0.012	−0.968	0.209
TS/TA	−0.199	0.435	−0.859	0.119
TA	0.374	−0.910	0.127	0.054
TSS/TA	−0.530	0.820	−0.147	−0.091
P	−0.041	−0.200	0.254	−0.839
NO3	−0.685	0.227	−0.045	−0.649
Ca	−0.216	0.383	0.453	−0.744
K	0.049	−0.896	0.086	0.190
AA	0.179	−0.197	0.575	0.711
Neox.	0.456	0.071	0.376	−0.767
Lut.	0.630	−0.258	0.480	−0.317
Zeax.	0.609	−0.202	0.742	−0.163
Violax.	0.846	−0.187	0.405	0.176
Caps.	0.526	−0.476	0.596	−0.237
Crypt.	0.745	−0.344	0.481	−0.030
α-carot.	0.916	−0.205	0.076	0.244
β-carot.	0.897	−0.210	−0.110	−0.161
Tyield	−0.186	0.184	−0.057	0.885
BER	−0.642	0.651	−0.186	0.319
MYield	0.658	−0.669	0.190	−0.072
No.fr.Tyield	−0.636	0.594	−0.216	0.395
No.fr.BER	−0.677	0.560	−0.165	0.388
No.fr.Myield	−0.207	0.711	−0.537	0.355
Total variance explained: rotation sums of squared loadings
Total	7.806	6.111	4.8	4.522
% of Variance	31.222	24.444	19.198	18.09
Cumulative %	31.222	55.666	74.864	92.954

a—Parameters described in Table 1; b—Bold values indicate the maximum correlation coefficient of the variable with the obtained components.

).

The components PC1 and PC2 and PC3, in addition to strong correlations with chemical traits of fruit and pigment content, showed correlations with yield and quality traits such as weight of marketable yield of pepper fruit (MYield), number of fruit of total and marketable yield (No. fr. Tyield, and No. fr. Myield) and weight and number of fruit with BER (BER and No. fr. BER). The experimental systems, in which SA was applied, were situated above the control systems and, as a result, demonstrated a positive correlation with PC2-related traits. The introduction of SA may have contributed to the quantity and quality of produce, particularly with regards to the number of healthy fruit, as well as the overall marketable yield. As evidenced in the correlation analysis, this yield was most closely associated with PC1, which is reflected in the graphical representation (Figure 6). The layouts of the SA-added experiments were thus shifted to the right and upwards in the graph relative to the control. At the same time, Figure 7 confirms the clear varietal differentiation in terms of value level and in terms of similar correlations.

4. Discussion

4.1. Quality Characteristics of Pepper Fruits Grown Hydroponically

Peppers grown hydroponically in mineral wool substrate produce fruit with high biological value. The fruits of both cultivars tested were characterised by high concentrations of vitamins, carotenoids, mineral salts, sugars and organic acids. Peppers harvested at physiological maturity from plants grown in mineral wool contained on average more than 1226 μg 100 g⁻¹ FW of capsanthin. According to Hassan et al. [68], the level of capsanthin is significantly increased during the ripening of red peppers. It is the principal pigment that accounts for almost 80% of the total carotenoids, ranging from 230 to 848 μg 100 g⁻¹ of fresh weight in capsicum. The main carotenoids of capsicum include capsanthin, capsorubin, β-carotene, zeaxanthin, violaxanthin, lutein and antheraxanthin, the concentration of which varies according to the ripeness of the fruit [69,70]. Carotenoids are a family of natural pigments covering a spectrum from yellow to red, characterised by strong antioxidant properties. Among these compounds, capsanthin stands out as a valuable carotenoid pigment, not only for its role in determining the degree of colour and fruit maturity of red pepper varieties but also for its high antioxidant properties and potential health-promoting effects [71,72,73]. Among the labelled carotenoids in ripe Aifos and Palermo fruit, capsanthin had the highest proportion. Among the labelled carotenoids in ripe Aifos and Palermo fruit, capsanthin had the highest contribution. The lutein content of the fruits tested averaged more than 209 μg 100 g⁻¹ of FW, β-cryptoxanthin more than 88 μg 100 g⁻¹ of FW and α and β carotene more than 168 and 735 μg 100 g⁻¹ of FW, respectively. Vitamin C levels averaged more than 47 mg 100 g⁻¹ of FW. Vitamin C is thermolabile and its degradation in the product depends on water activity, light access, oxygen content and the presence of heavy metals (e.g., Cu, Fe), the type of raw material, its maturity, and pretreatment [74]. These compounds are responsible for the antioxidant activity observed in peppers [75]. At the same time, the methanolic extracts of Aifos and Palermo fruits tested showed high antioxidant potential. A study by Guil-Guerrero et al. [4] showed that for 10 Spanish pepper varieties, the main carotenoids were lutein and its isomer, accounting for more than 60% of all carotenoids. The content of lycopene as well as the other components in pepper fruits is influenced by various factors. For instance, the analysis of pepper fruits harvested in September revealed higher concentrations of lycopene and β-carotene in comparison to those harvested in July or August [76]. Certainly, the high temperature occurring during the study period affected the concentration of the different components and the quality of the pepper yield of both varieties. According to Oh and Koh [77], the temperature range of 20–25 °C is conducive to vegetative growth and fruit development in sweet peppers, and the total sugar content of ripe fruit increases significantly at 20–25 °C, while the capsaicinoid content of ripe fruit increases with increasing temperature in the range of 15–30 °C. These results indicate that the temperature range of 20–25 °C is beneficial for vegetative growth, fruit development and fruit quality of peppers [78]. The physicochemical and biological properties of pepper fruits are also determined by their maturity stage at harvest. Pepper fruit ripening is the characteristic change in fruit colour from green to red, yellow, orange or purple depending on the variety. This process involves, among other things, the breakdown of chlorophyll and the synthesis of new carotenoids and anthocyanins, the emission of organic volatiles, the synthesis of new proteins and the cleavage of existing ones and the softening of cell walls [79,80,81]. Significant differences between the transcriptomes of unripe and ripe pepper fruits, involving thousands of genes, have also been reported [82]. From a redox point of view, fruit ripening has also been found to affect the metabolism of reactive oxygen species (ROS), leading to major changes in the total soluble reducing equivalents and antioxidant capacity of the fruit. The profile of major non-enzymatic antioxidants, including ascorbate, glutathione, carotenoids and polyphenols, has been monitored throughout the ripening process of pepper fruit [83], but less is known about how enzymatic antioxidants evolve during this physiological process. In their study, the in vitro DPPH antioxidant assay and in vitro ABTS tests were at 68.71% RSC and 78.22% RSC, respectively, while the total phenolic content was 53.91 mg CE 100 g⁻¹ of FW. A study by Papathanasiou et al. [84] proved that the sweet pepper cultivars Dolma F1, Yahoo F1 and Florinis NS 700 harvested at full maturity (80 days after flowering) are a rich source of antioxidant compounds, including polyphenols and ascorbic acid, and have been found to have the highest antioxidant activity. The tested cultivars Aifos and Palermo differed in terms of the content of individual bioactive components, while on average, fully coloured fruit yielded ascorbic acid of about 48 mg 100 g⁻¹ of FW and total phenolic compounds of about 54 mg CE 100 g⁻¹ of FW. In contrast, antioxidant activity assessed by the DPPH method averaged about 68% RSC and ABTS method 78% RSC. In a study by Hamed et al. [85], the ranges of total phenolic compounds and flavonoids in peppers were in green fruit 2096 to 7689 μg/g⁻¹ of FW and red/yellow ripe fruit 204 to 962 μg/g⁻¹ of FW. Ascorbic acid levels in peppers ranged from 223 to 1025 mg/100 g of dry weight (DW). In contrast, both raw and roasted peppers showed strong antioxidant activity by DPPH, with values ranging from 61% to 87%, and by ABTS, with values ranging from 73 μmol TE/g to 159 μmol TE/g of a freeze-dried sample. As reported by Chilczuk et al. [86], the highest antioxidant activity was observed in sweet pepper fruit extracts prepared with 40% methanol. The highest total content of phenolic compounds was obtained in an analogous extract derived from hot pepper fruits, which also demonstrated the most potent cytotoxic activity against the PC-3 cancer line. Medina-Juáre et al. [87] report that the highest percentage of oxidation inhibition for the DPPH radical is associated with the highest levels of gallic acid, chlorogenic acid, epicatechin, rutin, luteolin, resveratrol (r ≥ 0.85) and ascorbic acid in pepper fruits. Of the varieties tested, Caribe and Bell had the highest antioxidant capacity, which correlated with the highest levels of phenols and total flavonoids. Positive correlations were shown between individual phenols and antioxidant activity, confirming the results obtained in the experiment. Additionally, the researchers demonstrated a positive correlation between individual phenols and antioxidant activity, with the highest correlation observed for catechin, epicatechin, rutin and resveratrol.

The fruit of both pepper varieties examined in the experiment also scored highly in terms of sensory quality and consumer desirability. The observed differences in the heights of individual fruit quality parameters were dependent on the variety of pepper. PCA analysis showed significant differences between the yield quality characteristics of the Aifos and Palermo cultivars. Fruits of the Palermo cultivar contain significantly more, inter alia, dry matter (DW), soluble sugars (TSS) and potassium and show a higher total acidity (TA) and a higher sugar/acid ratio (TSS/TA). These characteristics influence the higher evaluation of the fruits of this cultivar in terms of sensory quality, particularly in terms of sweet taste and overall sensory preference. Other studies have also shown a significant correlation between the content of chemical attributes of fruit quality and individual determinants of sensory evaluation [88,89].

4.2. Increase in Yield, Quality, and Antioxidant Potential of Pepper Fruit as a Result of CA and SA Application

The Ca and SA spray treatments applied to the crop, and the combination of these compounds in a single spray of Ca combined with SA (Ca+SA), resulted in a beneficial impact on the quality of the pepper crop. This included an increase in the proportion of marketable yield, while the non-marketable crop had a BER symptom prevalence of 99%. Peppers grown under covers, like other vegetables, are exposed to high temperature stress, which occurs during periods of high solar radiation. Depending on the susceptibility of the pepper cultivar to the physiological disorder caused, among other things, by inhibition of transpiration and Ca transport to the fruit tip cells, the proportion of fruit affected by BER in the pepper yield may vary [40,51].

Spraying the plants with Ca increased the total acidity in the pepper fruit. It also improved fruit sensory quality, firmness and increased the antioxidant potential of pepper fruit. Similar results were observed in tomato, where calcium spraying had a beneficial impact on maintaining high fruit firmness, reducing fruit weight loss and increasing soluble solids and total acidity. These treatments also showed positive correlations with total sugars and overall quality during tomato fruit storage, indicating a consistently beneficial effect of calcium on fruit quality parameters [90]. Both pepper cultivars responded similarly to foliar treatment with simultaneous calcium and salicylic acid, i.e., a Ca+SA combination, showing high positive correlations with total sugars in fruit (TS) and total sugars/acids ratio (TS/TA).

The application of SA in the experiment increased compared to the control the concentration of pigments such as lutein, violaxanthin, β-cryptoxanthin, α-carotene and β-carotene in the pepper fruit. A comparable context can be observed with the simultaneous application of calcium and salicylic acid (Ca+SA) to peppers. It can be concluded that the application of SA, either as a standalone treatment or in combination with Ca, has a favourable impact on the pigmentation of pepper fruits. In the context of treating peppers with Ca alone, the cultivar Palermo showed a significant positive effect on the fruit’s bioactive components, while the opposite was observed for these pigments in the fruit of the cultivar Aifos following the application of Ca. The factor used in the experiment, for example spraying the pepper plants with Ca, resulted in a higher correlation with total acidity and K content in Aifos fruit. Among others, Khalid et al. [91] found a positive effect of foliar treatment of SA peppers in mitigating the negative effects of salinity stress in peppers. The Ge et al. [92] study revealed that combined TSP (phosphate) + SA treatment of peppers mitigated chilling injury by increasing water retention in the pepper fruit.

Furthermore, the study confirmed that the antioxidant activity of the fruit differed between the two varieties, with the Palermo variety proving to have a higher antioxidant capacity. This was confirmed by three methods, DPPH, ABTS and total polyphenol content of TPC fruit, expressed in mg CE 100 g⁻¹ of fresh weight of pepper fruit. According to Dobón-Suárez et al. [93], foliar application of 0.5 mM SA to pepper plants has a significant effect on increasing fruit quality parameters and antioxidant capacity at harvest and after 21 days of storage at 7 °C. Yang et al. [94] proved that exogenous 3 mmol L-1 SA delayed the decrease in firmness, colour, TA and TSS in winter jujube fruits (Ziziphus jujuba Mill cv. Dongzao) during their shelf life. At the same time, SA increased the activity of antioxidant enzymes (superoxide dismutase, peroxidase, catalase, ascorbate peroxidase) and the content of ascorbic acid, total phenols, total flavonoids and glutathione in the fruit, which improved the antioxidant properties of the fruit.

The analysis of the results obtained confirmed that the use of SA in peppers can lead to an increase in the number of healthy fruits and the marketable yield. The arrangements of experiments involving the addition of SA confirm the clear varietal differentiation in terms of the level of values and confirm similar correlations. Foliar calcium feeding of sweet pepper plants significantly increased fruit yield in the field crop and resulted in a reduction in the number of fruits with BER symptoms compared to the control. The application of calcium in the form of Ca(NO₃)₂ had a positive effect on vitamin C and carotenoid accumulation compared to other calcium fertilisers [95].

Among others, Khazaei and Estaji [96] report that salicylic acid provides improved drought stress tolerance in peppers through its effects on vegetative, biochemical and physiological traits. The foliar application of SA, inducing an antioxidant system in pepper seedlings, resulted in a reduction of the harmful effects of drought conditions and an improvement in plant growth [96]. In contrast, Khalid et al. [97] found that application of SA reduced Hg toxicity in sweet pepper seedlings. SA reduced Hg accumulation in sweet pepper roots and leaves and impeded Hg translocation from roots to fruit. The researchers report the need for continued studies to identify SA signalling pathways, with the aim of providing further insight into these mechanisms in pepper plants. In a study conducted by Dobón-Suárez et al. [98], the application of salicylic acid at a concentration of 0.5 mM to pepper plants prior to the harvesting of green fruit resulted in increased yields (kg per plant, number of fruits and average fruit weight) and increased quality parameters (firmness, greenness and total acidity). Moreover, the content of phenolic compounds and the total antioxidant content both demonstrated a notable increase. The results obtained by Dobón-Suárez et al. [98] suggest that pre-harvest application of SA on pepper plants can be an effective method for increasing yield and improving fruit quality parameters at harvest, while also maintaining post-harvest quality.

5. Conclusions

The hydroponic cultivation of peppers using mineral wool and Ca and SA as nutrients results in fruit with high biological value and excellent sensory qualities. Fruits of the Aifos and Palermo varieties show high concentrations of nutrients and antioxidants, with Palermo having a higher dry matter content, soluble sugars, potassium and a higher sensory score.

Further research should concentrate on optimising hydroponic growing conditions and establishing the mechanisms by which different factors affect yield quality and fruit antioxidant potential. It is also important to extend the research by clarifying the effects of SA application on the flowering biology of pepper plants, the uptake and transport of individual components to the fruit and as well as their health-promoting effects.

The results indicate that foliar treatment of peppers with Ca and SA is even more beneficial in the Ca+SA combination for increasing fruit quality and antioxidant capacity. Therefore, prophylactic spraying of peppers with the Ca+SA mixture can be recommended in hydroponic production. However, the determination of the most effective concentrations of both components requires further research.