Efficacy Evaluation of Different Mineral Clay Particles on Olive Production Traits and Olive Oil Quality of ‘Koroneiki’ Olive Cultivar Under Rainfed and Irrigated Conditions in Southern Greece

Abstract

Climate crisis in the Mediterranean region has severely affected olive tree cultivation, especially due to the long, dry summers, when temperature often rises above 40 °C. In order to overcome such climate challenges in the olive sector, the particle film technology (PFT) was used, as an environmentally friendly alleviation technique, due mainly to the reflecting properties of clay materials. Three clay materials—attapulgite, talc, and kaolin—were applied foliarly to olive trees (both rainfed and irrigated) in July and August. At harvest, yield and oil production per tree were assessed, alongside olive oil quality and functional properties. Under irrigated conditions, trees treated with kaolin or talc in July exhibited the highest yields, whereas under rainfed conditions, trees treated with attapulgite in August, followed by those treated with talc in August, showed the greatest yields. Oil production exceeded that of controls in rainfed trees across nearly all clay treatments. Oils from irrigated trees treated with talc in August and rainfed trees treated with talc in July exhibited high phenolic content, though antioxidant capacity peaked in oils from trees treated with talc in August. These oils, along with those from trees treated with attapulgite in August, contained the highest concentrations of hydroxytyrosol and oleacein. In rainfed trees, most clay treatments resulted in oils with elevated oleic acid (C18:1) and reduced linoleic acid levels, yielding a high monounsaturated-to-polyunsaturated fatty acid ratio. In irrigated groves, August applications produced oils with distinct differences from controls, whereas in rainfed conditions, these differences were evident regardless of application timing. Clay materials offer a promising approach for mitigating abiotic stress under Mediterranean summer conditions; however, further research is needed to elucidate their mechanisms of action. This study represents the first report of foliar attapulgite application in plants and talc application in olive trees.

1. Introduction

The olive tree is the iconic symbol of the Mediterranean basin, deeply intertwined with the economic, cultural, and historical characteristics of the countries where it is cultivated [1]. As a xerophytic species, it can endure extended periods of water scarcity [2]. Additionally, it exhibits salt tolerance, the extent of which varies depending on the cultivar, and can thrive in a wide range of soil conditions [3,4].

However, climate change has significantly affected olive trees. In many instances, they have been unable to fulfill their chilling requirements, necessary for dormancy release and successful completion of flower bud differentiation [5,6]. High temperatures during anthesis, prolonged dry summers with extreme heat, and intense solar radiation during fruit development have resulted in very low fruit set, reduced yields, and a decline in olive oil quality [7,8]. In recent years, major olive oil-producing Mediterranean countries have seen drastic reductions in yields, sometimes dropping up to 50% of the typical production levels, driving up olive oil prices. In Greece and other Mediterranean countries, summers are characterized by high solar radiation, extreme temperatures, and minimal rainfall—conditions that exacerbate heat stress, solar injury, and tissue necrosis in plants. In extreme cases, prolonged droughts from fruit set to maturation have even jeopardized tree survival.

Olive oil cultivars are predominantly grown under rainfed conditions in many olive-growing regions [1]. However, the shifting of the precipitation patterns—characterized by reduced overall rainfall and its concentration over shorter periods—has intensified water stress for these trees. This imbalance between water availability and the olive tree’s requirements significantly affects both yield and product quality. The issue has become increasingly critical as water reserves decline, with priority often allocated to urban and tourism-related needs rather than agriculture [9]. Consequently, the judicious use of water resources in agriculture is of paramount importance, and strategies or management practices that enhance water conservation or improve water use efficiency are highly significant [10].

One such strategy, explored over the past two decades, is the application of particle film technology (PFT) in agriculture [10,11]. Initially developed to suppress pest infestations in crops—primarily using mineral clay particles—PFT has proven effective against a variety of pests [10,12,13]. Among the most widely used mineral clays is kaolin, which has demonstrated significant efficacy in pest control across multiple crops [9,10,12,14]. Beyond pest suppression, PFT increases the reflection of incoming solar radiation, particularly in the ultraviolet (UV) and infrared (IR) ranges, reducing the risk of heat-induced damage to leaves and fruits [12,13,15,16], while at the same time mitigating heat stress and preserving vegetative and reproductive growth in grapevine, mango, eucalyptus, and other species [17,18,19,20]. The effect on photosynthesis may vary depending on the species and environmental conditions. Yield increases have been also reported in various crops such as olive, grapevine, and tomato [12]. Similarly, positive influences on yield quality attributes after the use of clay particles have been reported in olive (increase in fruit dry matter and oil content, as well as in phenol content) [12,14,15], in apples (increase in fruit post-harvest quality), in grapevine and peach (increase in the content of berry phenolics, anthocyanins, ascorbic acid, and antioxidant capacity) and in apple, mango and tomato, as it increased the red coloring of the fruits [1,12,13].

The use of PFT to mitigate these stress conditions has shown considerable success across various plant species [9,21,22,23]. While most research has focused on kaolin-based particle films and their ability to alleviate water and heat stress, other mineral clays also show promise for reducing environmental stress. For instance, foliar talc treatments—though less commonly used than kaolin—have proven effective in reducing sunburn in pomegranates and apples [24,25] and in controlling pests in grape cultivation [26]. Similarly, attapulgite, another clay mineral, has primarily been employed as a soil amendment to improve water retention, to support soil remediation, and to enhance overall soil fertility [27,28,29]. However, to our knowledge, attapulgite has not previously been applied as a foliar spray to mitigate high solar radiation and heat stress in plants.

This study aimed to evaluate the efficacy of different clay minerals as foliar sprays on ‘Koroneiki’ olive trees (a major olive oil cultivar worldwide) grown under both rainfed and irrigated conditions. Three commercially available products—containing kaolin, talc (tested in olive trees for the first time), or attapulgite (tested as a foliar spray on plants for the first time)—were applied at their recommended dose rates. The study assessed their effects on yield, oil content per unit fruit mass, and the quality and functional attributes of the resulting olive oil to determine their impact on olive tree productivity.

2. Materials and Methods

2.1. Test Site Location—Plant Material

The trial was conducted in Petrina region, Lakonia county, Southern Greece (altitude 32 m) (Figure S1). Preliminary experiments took place (in ‘Koroneiki’ cultivar in Crete Prefecture and in ‘Megaritiki’ cultivar in Sterea Ellada prefecture), to assess the effects of the various treatments (in Sterea Ellada prefecture two out of the three clay materials were used, since the one was not commercially available at that time) on olive yield, oil production, and basic olive oil quality attributes under rainfed conditions (Supplementary Material). The data presented hereafter are the data of the cultivation period of 2021. The mean annual temperature in the region is approximately 16.7 °C, while in July the average temperature ranges from 21 to 35 °C, and in August from 22 to 36 °C with zero rainfall (Figure 1). During the trial period, the maximum temperature in July reached 40 °C, in August 43 °C, and in September 36 °C (data from www.freemeteo.gr). The mean monthly total solar irradiation on the horizontal level in the surrounding area is 222.0 kWh m⁻² in July and 200.9 kWh m⁻² in August (data from the Greek Ministry of Environment and Energy, https://www.helapco.gr/ims/file/installers/totee-klimatika.pdf, accessed on 6 March 2025).

The cultivar used was ‘Koroneiki’, an olive oil cultivar of paramount importance for Greece, as well as for the entire world. The trial was conducted in two monovarietal olive groves, one drip-irrigated (36.82200° N, 22.50896° E) and one rainfed (36.81758° N, 22.51049° E). All trees in each olive grove were selected based on uniform growth and similar expected yield, without visible symptoms of either nutrient deficiencies, pest infestation or disease infection. The trees were 30 years old, open-vase trained, planted at 7 m × 7 m distances and managed according to the cultivation routine of the region, to ensure unhindered tree growth and healthy fruit production. All cultivation practices (i.e., fertilization, pruning, pesticide application) were applied uniformly to all trees, separately at each orchard.

A total of four treatments were applied in each orchard, i.e., the control and three treatments of reflecting clay particles, forming a film on leaf and fruit surfaces. More precisely, the following commercial products were used:

• Surround^® WP (kaolin—aluminum silicate 95% w/w) (Al₂Si₂O₅(OH)₄ 95%) (Tessenderlo Kerley, Inc. Phoenix, AZ, USA), distributed in Greece by Hellafarm SA (Peania, Greece), at the recommended supplier dose rate of 3 Kg 100 L⁻¹;
• Invelop^® White Protect (Talc E553b) (Mg₃Si₄O₂₁H₂₀ 100% w/w) (COMPO EXPERT GmbH, Münster/Westphalia, Germany), distributed in Greece by Compo Hellas (Marousi, Greece), at the recommended supplier dose rate of 3 Kg 100 L⁻¹; and
• Aglev^® SI 300 (attapulgite 100%) (Geohellas SA, Athens, Greece) at the recommended supplier dose rate of 2 Kg 100 L⁻¹. The chemical formula of attapulgite is Mg₅Si₈O₂0(OH)₂(OH₂)44H₂O [28].

Two schemes of spray application took place, i.e., an application in July (14/07) (BBCH crop growth stage: 74–75) and a separate application in August (25/08) (BBCH crop growth stage: 76–77) (Figure S2). In each treatment, care was taken to efficiently cover the tree by the particle film, by applying a mean of almost 1100 L ha⁻¹ of spraying solution (Figure S3). Four trees were used per treatment, with a total of twenty eight (28) trees per olive grove.

In all spray applications, a surfactant (Pinolene EX (di-l-p menthene 96% w/v, Ody Fer Fertilizers, Lakonia, Greece), at the dose rate of 25 mL 100 L⁻¹) was added to the tank mix.

2.2. Olive Oil Extraction Procedure

Fruits were harvested on the 23rd of January in the irrigated olive grove and on the 16th of January in the rainfed grove, at a maturity index ranging from 3 to 3.8 (based on the color of their skin and pulp). The yield (total fruit weight) per tree was recorded and a sample of approximately 2 Kg of healthy, undamaged fruits was carefully packed and transferred to the laboratory for analyses. Approximately 1 Kg of olive fruits, free from leaves and any other debris, was crushed in an Abencor-type olive mill (Callis S.A., Athens, Greece), at 2800 rpm, and 800 g of the paste was malaxed at 50 rpm for 30 min at 25–27 °C. To accurately determine the oil percentage, a sample of 50 g of the malaxed paste was centrifuged at 2500× g for 15 min in a laboratory centrifuge. The remaining paste was centrifuged at 1400× g for 3–4 min using the semi-industrial centrifuge of the Abencor-type olive mill, and the oil was stored in fully filled bottles at 4 ± 2 °C until analysis.

2.3. Olive Oil Analyses

The determination of olive oil free acidity, peroxides number, and ultraviolet absorption at 232 and 270 (K232, K270, respectively), as well as ΔK (Delta Kappa) took place, following the instructions of the European Official Methods of Analysis 2016/1784.

2.4. Determination of Total Phenols

The phenolic compounds of the olive oil were extracted by adding 5 mL of hexane into 2.5 g of olive oil. After careful vortexing, 5 mL of methanol (high-performance liquid chromatography—HPLC grade)–water (HPLC grade) solution (60:40) was added and the mixture was shaken for another 2 min in a horizontal shaker (IKA, Litecn Lab Equipment Athens, Athens, Greece). The sample was then centrifuged at 2500 g for 10 min at room temperature and the upper phase was discarded. The total phenols, the total o-diphenols, and the total flavonoids were determined in the lower phase, as described by Roussos et al. [30] and expressed as mg gallic acid equivalents (GAEs) Kg⁻¹ olive oil, as mg caffeic acid equivalents (CAEs) Kg⁻¹ olive oil, and as mg catechin equivalents (CtEs) Kg⁻¹ olive oil, respectively.

Individual phenolic compounds were detected in the same phenolic extract by HPLC (Shimadzu Nexera X2—Shimadzu Europa GmbH, Albert-Hahn-Str. 6-10, 47,269 Duisburg, Germany), using a diode array detector operating at 260 nm, 280 nm, 325 nm, and 360 nm. A sample of 40 μL was injected in a Luna LC column 250 × 4.6 mm, 5 μm, C18 (2) 100 A (Phenomenex, 411 Madrid Avenue, Torrance, CA, USA) operating at 20 °C. The gradient program used for the elution of the phenolic compounds consisted of two HPLC grade solutions: (A) methanol:acetonitrile (1:1) and (B) 1% v/v acetic acid in water. The flow rate was adjusted at 1 mL min⁻¹ and the gradient program was as follows: at time 0 min solvent A at 10% until time 5 min, at 10 min and until 35 min A at 20%, at 45 and until 55 min A at 25%, at 70 min A at 30%, at 100 min A at 40%, and at 106 min until 123 min A at 70%. A total of nine (9) phenolic compounds were detected based on their retention time and spectra similarity with the corresponding standards. The phenolic compounds detected were hydroxytyrosol, tyrosol, vanillin, p-coumaric acid, ferulic acid, oleacein, oleocanthal, luteolin, and apigenin. The standards were purchased from Sigma-Aldrich (Sigma-Aldrich, Inc., PO Box 14508, St. Louis, MO, USA) and their concentration was calculated based on a five-point calibration curve. Their concentration in the oil samples was expressed in mg Kg⁻¹ olive oil.

2.5. Antioxidant Capacity

The antioxidant capacity of the oils was determined in the same extract of phenolic compounds, using the diphenyl picryl hydrazyl (DPPH) and the ferric reducing antioxidant power (FRAP) assays, according to Roussos et al. [30], using a five-point calibration curve of Trolox. The antioxidant capacity of the oil samples was expressed as μmol Trolox equivalent Kg⁻¹ olive oil.

2.6. Fatty Acids and Squalene Determination

The fatty acid composition was expressed as a percentage of corresponding methyl esters (FAMEs). For their determination, 0.5 g of olive oil was diluted in 2 mL of GC-grade hexane and vigorously shaken, followed by the addition of 1 mL 0.2 N solution of KOH in methanol. The methyl-esters were analyzed by gas chromatography (Shimadzu GC-2030) (Shimadzu Europa GmbH, Albert-Hahn-Str. 6-10, Duisburg, Germany) equipped with an autosampler (AOC-20i+s), and a flame ionization detector (FID). The analysis was performed using a fused silica capillary column (Supelco SP^®-2340 60 × 0.32 mm × 0.2 μm film thickness) (Sigma-Aldrich, Inc., PO Box 14508, St. Louis, MO, USA). A split ratio of 80.0 was used and the injector temperature was set at 249 °C, while the detector temperature was set at 249 °C. Then, 10 µL of the sample was injected at an initial column temperature of 150 °C, which was held for 18 min. The temperature was then increased at a rate of 2 °C min⁻¹ to 175 °C and held for 5 min and was then raised at a rate of 5 °C min⁻¹ to a final temperature of 200 °C, which was held for 22 min. The following free fatty acids were determined: palmitic (C16:0), palmitoleic (C16:1), heptadecanoic (C17), heptadecenoic (C17:1), stearic (C18:0), oleic (C18:1), linoleic (C18:2), linolenic (C18:3), arachidic (C20:0), gadoleic (C20:1), behenic (C22), and lignoceric (C24) acids. Squalene (Sigma-Aldrich, Inc., PO Box 14508, St. Louis, MO, USA) concentration was determined by a five-point standard curve of the corresponding analytical standard and expressed as mg 100 g⁻¹ of olive oil.

2.7. a-Tocopherol Determination

In 1 g of olive oil, 2 mL of hexane was added and the mixture was vortexed. The a-tocopherol was detected in this solution by the HPLC described above, using a normal phase column (Lichrosorb 250 × 4.6 mm Si 60 5 μm, Merck SA Hellas, Marousi, Greece) working at 20 °C, using as mobile phase a mixture of 99% hexane (HPLC grade) and 1% 2-propanol (HPLC grade). The flow rate was adjusted at 0.8 mL min⁻¹ and a-tocopherol was detected by a fluorescence detector (HP 1046A, Agilent Technologies, Santa Clara, CA, USA) at 290 nm excitation wavelength and 325 nm emission wavelength. Identification and quantification were performed using a five-point calibration curve of an authentic standard purchased by Sigma-Aldrich Inc (St. Louis, MO, USA).

2.8. Statistical Analysis

The trial followed the completely randomized design with four replicates of one tree, in each orchard, i.e., four trees per treatment. The raw data from each olive grove were separately analyzed as an one-way ANOVA, since differences regarding cultivation practices (fertilization, pruning, harvest time) existed between the two groves, which were located in the same region but not in the same farm. Significant differences among treatments under the same irrigation practice were determined based on Tukey’s HSD test, at a = 0.05 after checking the normal distribution of the raw data using standard skewness, standard kurtosis, and the homogeneity of variances. When necessary, the suitable transformation of the raw data was performed in order to obtain a normal distribution. Dunnett’s test was also performed to examine any significant differences between each individual treatment and control only. Hierarchical cluster analysis (Ward method) of raw data (olive oil quality indexes, FAMEs content, phenolic compounds concentration, squalene and tocopherol concentration, and antioxidant capacity as well as yield, oil percentage per fruit and oil quantity per tree) was performed to produce a dense, descriptive information on the effects of the various treatments. Constellation plots were also constructed to graphically present possible similarities of the various treatments based on the measured variables. The statistical software JMP 13.0 (SAS Institute) and Statgraphics Centurion XV (Statgraphics Technologies, Inc., The Plains, VA, USA) were used for the aforementioned analyses.

3. Results

3.1. Effects of the Treatments on Yield Components

The application of kaolin and talc in July resulted in a significant yield increase compared to the control under irrigated conditions (

Table 1. Effects of the various treatments on olive fruit yield, olive oil percentage, and oil produced per tree cv. ‘Koroneiki’ under irrigated and rainfed conditions.

Treatment	Yield (kg/Tree)	Oil Percentage Per Fruit	Oil Per Tree (Kg)	Yield Change (%)	Oil Per Tree Change (%)
	Irrigated olive orchard
Control	40.4 b	17.75 a	7.20 a	100.0 a	100.0 a
At-J	48.3 ab	14.89 a	7.14 a	119.5 a	99.1 a
At-A	44.9 ab	16.90 a	7.57 a	111.1 a	105.1 a
K-J	51.5 a *	16.32 a	8.43 a	127.3 a	117.0 a
K-A	43.0 ab	16.87 a	7.42 a	106.4 a	103.1 a
T-J	49.9 a *	17.18 a	8.76 a	123.6 a	121.6 a
T-A	39.9 b	18.33 a	7.34 a	98.9 a	101.9 a
	Rainfed olive orchard
Control	21.3 b	13.46 b	2.87 c	100.0 b	100.0 c
At-J	24.7 ab	16.89 b	4.12 ab *	115.7 ab	144.1 ab *
At-A	29.0 a *	15.46 b	4.47 a *	136.1 a *	156.6 a *
K-J	23.4 ab	18.04 b*	4.30 a *	109.8 ab	150.7 a *
K-A	23.8 ab	13.46 b	3.25 bc	111.9 ab	113.0 bc
T-J	21.3 b	24.05 a *	5.12 a *	100.0 b	178.5 a *
T-A	27.0 ab *	16.61 b	4.42 a *	126.7 ab *	154.4 a *

Means within the same column and for the same irrigation practice followed by the same letter do not differ significantly based on Tukey’s HSD multiple range test at a = 0.05. Abbreviations: At, attapulgite; K, kaolin; T, talc; J, application in July; A, application in August. The asterisk indicates significant differences of the specific treatment from its corresponding control (irrigated or rainfed) based on Dunnett’s test.

). Nearly all clay particle treatments increased yield (except for talc in August), while oil percentage per fruit mass and oil production per tree remained similar across all treatments (Table 1). Under rainfed conditions, trees treated with attapulgite in August exhibited the highest yield, significantly different from the control, followed by those treated with talc in August (which also showed a significant difference from the control based on Dunnett’s test). The highest oil percentage per fruit was observed in olives treated with talc in July, with a significant difference from all other treatments. Consequently, these trees also recorded the highest oil production per tree (5.12 kg), followed by trees treated with attapulgite in August, both showing a significant difference from the control. Notably, all clay particle treatments (except for kaolin in August) resulted in significantly higher oil production per tree compared to the control. This trend was further supported by increases in both yield and oil production per tree across all treatments. The greatest yield increase occurred with talc spraying in July, achieving oil production per tree that is nearly 1.8 times higher than the control. Oil production per tree increased by approximately 1.44–1.57 times with attapulgite application, 1.13–1.51 times with kaolin, and 1.54–1.8 times with talc spraying.

3.2. Effects of the Treatments on Olive Oil Quality Characteristics

Olive oil free acidity and peroxide values were not affected by any treatments under irrigated conditions (

Table 2. Effects of the various treatments on oil free acidity, peroxide index, K232, K270, and ΔK cv. ‘Koroneiki’ under irrigated and rainfed conditions.

Treatment	Free Acidity (g Oleic Acid 100 g−1)	Peroxides (meq O2 Kg−1)	K232	K270	ΔK
	Irrigated olive orchard
Control	0.30 a	9.75 a	1.46 a	0.107 bc	−0.03 a
At-J	0.25 a	11.0 a	1.50 a	0.140 ab	−0.01 a
At-A	0.25 a	9.87 a	1.55 a	0.122 abc	−0.02 a
K-J	0.26 a	10.00 a	1.31 ab	0.105 bc	−0.03 a
K-A	0.23 a	11.00 a	1.54 a	0.152 a *	−0.01 a
T-J	0.26 a	8.63 a	1.04 b *	0.085 c	−0.06 a
T-A	0.21 a	11.00 a	1.47 a	0.120 abc	−0.02 a
	Rainfed olive orchard
Control	0.31 ab	12.50 a	1.65 a	0.150 a	0.020 a
At-J	0.29 ab	12.50 a	1.52 ab *	0.135 ab	−0.030 ab
At-A	0.33 a	12.37 a	1.30 bc	0.122 ab	−0.010 ab
K-J	0.26 ab	11.25 a	1.43 ab *	0.067 cd *	−0.072 b *
K-A	0.30 ab	15.00 a	1.34 bc	0.050 d *	−0.067 b *
T-J	0.23 b *	9.87 a	1.54 ab *	0.122 ab	−0.005 ab
T-A	0.26 ab	9.87 a	1.14 c	0.095 bc *	−0.072 b *

Means within the same column and for the same irrigation practice followed by the same letter do not differ significantly based on Tukey’s HSD multiple range test at a = 0.05. Abbreviations: At, attapulgite; K, kaolin; T, talc; J, application in July; A, application in August. The asterisk indicates significant differences of the specific treatment from its corresponding control (irrigated or rainfed) based on Dunnett’s test.

). However, UV absorbance indices were significantly impacted, with the lowest values observed in oils produced from trees treated with talc in July.

Under rainfed conditions, the application of clay particles significantly influenced nearly all quality attributes presented in Table 2. Free acidity was lowest in oils from trees treated with talc in July, showing a significant difference compared to oils produced from trees treated with attapulgite in August. Oils from control conditions exhibited high K232 and K270 values, exceeding those measured in oils from trees treated with clay particles in August (specifically for K232).

The application of talc and kaolin in August resulted in oils with high total phenolic compound content under irrigated conditions (

Table 3. Effects of the various treatments on olive total phenols, o-diphenols, and flavonoids concentration and antioxidant capacity cv. ‘Koroneiki’ under irrigated and rainfed conditions.

Treatment	Total Phenols (mg GAE Kg−1)	o-Diphenols (mg CAE/Kg−1)	Total Flavonoids (mg CtE Kg−1)	FRAP (μmol Trolox Equiv. Kg−1)	DPPH (μmol Trolox Equiv. Kg−1)
	Irrigated olive orchard
Control	60.72 c	19.91 ab	52.49 ab	1036.4 a	319.6 a
At-J	77.13 abc	15.96 b	47.74 b	1454.0 a	322.2 a
At-A	73.26 abc	20.67 ab	63.07 ab	1168.0 a	323.5 a
K-J	63.51 c	24.96 ab	58.40 ab	1471.6 a	305.3 a
K-A	98.33 ab *	24.79 ab	77.98 a	1968.0 a *	472.1 a
T-J	66.29 bc	28.32 ab	57.40 ab	1555.6 a	301.4 a
T-A	105.29 a *	32.43 a *	73.23 ab	1913.7 a	426.5 a
	Rainfed olive orchard
Control	69.08 b	25.69 a	53.07 a	1073.2 b	266.2 c
At-J	54.69 b	20.33 a	49.41 a	1201.9 ab	666.2 ab *
At-A	53.29 b	25.21 a	57.48 a	1499.6 ab	667.5 ab *
K-J	64.91 b	20.16 a	52.90 a	1243.0 ab	718.3 ab *
K-A	37.51 b	16.97 a	51.40 a	1277.2 ab	507.3 bc
T-J	120.62 a *	20.17 a	72.31 a	1250.0 ab	651.9 ab *
T-A	63.04 b	13.53 a *	60.98 a	1700.1 a *	933.3 a *

Means within the same column and for the same irrigation practice followed by the same letter do not differ significantly based on Tukey’s HSD multiple range test at a = 0.05. Abbreviations: At, attapulgite; K, kaolin; T, talc; J, application in July; A, application in August. The asterisk indicates significant differences of the specific treatment from its corresponding control (irrigated or rainfed) based on Dunnett’s test.

). In contrast, oils from control trees exhibited low phenolic compound levels. The o-diphenol concentration in olive oil was elevated under the influence of talc applied in August, which was significantly higher than that in oils from control conditions (based on Dunnett’s test). Kaolin applied in August produced oils rich in total flavonoids and with high antioxidant capacity (based on the FRAP assay), exceeding levels observed in oils from the control treatment (based on Dunnett’s test).

Under rainfed conditions, talc application in July yielded oils rich in total phenolic compounds, with a significant difference from all other treatments. However, talc application in August enhanced the antioxidant capacity of the produced oils, which was significantly higher than that of oils from the control treatment, as determined by both DPPH and FRAP assays. Nearly all clay particle treatments (except for kaolin in August) resulted in oils with greater antioxidant capacity (based on the DPPH assay) compared to the control, according to Dunnett’s test.

The application of clay particles to irrigated olive trees had a minor effect on the individual phenolic compounds in the oil produced (

Table 4. Effects of the various treatments on the concentration of individual phenolic compounds detected in olive oils in mg Kg ⁻¹ cv. ‘Koroneiki’ under irrigated and rainfed conditions.

Treatment	Hydroxytyrosol	Tyrosol	Vanillin	p-coumaric Acid	Ferulic Acid	Oleacein	Oleocanthal	Luteolin	Apigenin	a-Tocopherol
	Irrigated olive orchard
Control	0.95 a	5.84 ab	0.12 a	0.42 a	0.08 a	23.99 a	72.48 a	7.32 a	1.56 a	31.26 a
At-J	0.39 a	3.97 ab	0.15 a	0.42 a	0.08 a	26.47 a	76.36 a	6.55 a	1.27 a	33.62 a
At-A	0.19 a	2.83 b *	0.17 a *	0.39 a	0.07 a	28.98 a	80.71 a	6.51 a	1.05 a	33.30 a
K-J	0.86 a	4.17 ab	0.14 a	0.41 a	0.08 a	25.79 a	72.89 a	6.50 a	1.45 a	34.79 a
K-A	0.51 a	3.94 ab	0.13 a	0.32 a	0.08 a	30.39 a *	90.65 a	8.26 a	1.21 a	30.65 a
T-J	1.28 a	6.95 a	0.14 a	0.44 a	0.09 a	27.40 a	75.19 a	6.80 a	1.57 a	35.34 a
T-A	0.78 a	4.84 ab	0.15 a	0.34 a	0.08 a	29.96 a	90.00 a	7.50 a	1.25 a	39.37 a
	Rainfed olive orchard
Control	0.13 b	9.73 a	0.13 b	0.38 a	0.06 a	21.97 c	101.78 b	11.06 ab	1.32 a	44.10 a
At-J	0.27 b	4.62 ab	0.14 b	0.32 ab	0.06 a	25.20 bc	120.93 ab	11.43 ab	1.61 a	46.60 a
At-A	1.00 a *	4.42 ab *	0.14 b	0.27 b *	0.06 a	39.49 a *	123.82 ab	8.66 b	1.65 a	37.66 a
K-J	0.35 ab	5.41 ab	0.20 a *	0.33 ab	0.06 a	25.54 bc	160.56 a *	14.85 a	1.50 a	35.92 a
K-A	0.31 b	3.55 b *	0.16 ab	0.29 ab	0.06 a	31.89 ab *	143.81 ab *	8.38 b	1.80 a	42.41 a
T-J	0.36 ab	9.95 a	0.14 b	0.35 ab	0.04 a	22.23 c	111.90 b	13.02 ab	1.01 a	42.23 a
T-A	1.01 a *	5.99 ab	0.16 ab	0.29 ab *	0.06 a	36.77 a *	137.10 ab	12.08 ab	1.99 a	43.04 a

Means within the same column and for the same irrigation practice followed by the same letter do not differ significantly based on Tukey’s HSD multiple range test at a = 0.05. Abbreviations: At, attapulgite; K, kaolin; T, talc; J, application in July; A, application in August. The asterisk indicates significant differences of the specific treatment from its corresponding control (irrigated or rainfed) based on Dunnett’s test.

). Attapulgite applied in August resulted in oils with a low tyrosol concentration compared to either the talc application in July or the control (based on Dunnett’s test). In contrast, vanillin levels were higher in oils from trees treated with attapulgite in August compared to the control (based on Dunnett’s test). Similarly, oils from trees sprayed with kaolin in August exhibited a higher oleacein concentration than oils from control trees (based on Dunnett’s test). No other significant differences were detected in the oils produced from the irrigated olive grove.

Conversely, the application of clay particles had a substantial effect on the phenolic compound concentrations in oils produced under rainfed conditions. Hydroxytyrosol levels were elevated in oils from trees treated with attapulgite and talc in August, exceeding those in oils from control conditions. Oils from trees treated with kaolin in August had a lower tyrosol concentration than those from the control. Vanillin, however, was detected at a higher concentration in oils from trees treated with kaolin in July, while p-coumaric acid was found at its lowest concentration in oils from trees treated with attapulgite in August. Oleacein concentration was high in oils from trees treated with all clay particles applied in August. Kaolin, regardless of the application month, resulted in oils with elevated oleocanthal levels, higher than those in oils from the control treatment (based on Dunnett’s test). No significant differences were observed among treatments in luteolin, apigenin, or α-tocopherol concentrations in the oils produced.

The foliar application of clay particles on olive trees significantly impacted the fatty acid composition of the oils produced (

Table 5. Effects of the various treatments on the content of free fatty acids (determined as FAMEs—%) in the oil of cv. ‘Koroneiki’ under irrigated and rainfed conditions.

Treatment	C16	C16:1	C17	C17:1	C18	C18:1	C18:2	C20	C18:3	C20:1	C22	C24
	Irrigated olive orchard
Control	8.54 a	0.85 a	0.03 a	0.06 a	2.47 b	80.39 a	5.83 ab	0.47 a	0.76 d	0.37 a	0.16 a	0.04 a
At-J	7.90 ab	0.84 a	0.03 a	0.06 a	2.58 ab	80.79 a	5.90 ab	0.49 a	0.82 a–d *	0.37 a	0.16 a	0.05 a
At-A	8.10 a	0.83 a	0.03 a	0.07 a	2.61 ab *	80.17 a	6.29 a	0.49 a	0.84 abc *	0.37 a	0.16 a	0.05 a
K-J	8.19 a	0.84 a	0.03 a	0.07 a	2.53 ab	80.81 a	5.68 b	0.48 a	0.78 bcd	0.37 a	0.16 a	0.05 a
K-A	7.02 b *	0.84 a	0.03 a	0.07 a	2.57 ab	81.18 a	6.37 a *	0.48 a	0.87 a *	0.37 a	0.15 a	0.05 a
T-J	8.06 a	0.86 a	0.03 a	0.07 a	2.48 ab	80.83 a	5.84 ab	0.47 a	0.77 cd	0.38 a	0.16 a	0.05 a
T-A	7.57 b *	0.84 a	0.03 a	0.07 a	2.64 a *	80.77 a	6.16 ab	0.49 a	0.84 ab *	0.36 a	0.16 a	0.05 a
Rainfed olive orchard
Control	8.47 b	0.86 a	0.06 a	0.12 a	2.52 ab	77.41 b	8.69 a	0.44 a	0.82 a	0.37 ab	0.15 a	0.04 a
At-J	8.68 b	0.94 a *	0.04 b *	0.08 b *	2.47 abc	79.61 a *	6.35 b *	0.45 a	0.84 a	0.35 abc	0.16 a	0.04 a
At-A	8.08 b	0.88 a	0.03 b *	0.08 b *	2.31 bc *	80.67 a *	6.07 b *	0.43 a	0.87 a	0.37 ab	0.15 a	0.04 a
K-J	8.78 b	0.88 a	0.03 b *	0.07 b *	2.64 a	79.48 a *	6.31 b *	0.46 a	0.73 a	0.33 bc *	0.15 a	0.04 a
K-A	7.91 b	0.91 a	0.04 b *	0.08 b *	2.36 bc	80.68 a *	6.10 b *	0.45 a	0.91 a	0.38 a	0.15 a	0.04 a
T-J	10.13 a *	0.93 a	0.03 b *	0.07 b *	2.67 a	77.94 b	6.58 b *	0.45 a	0.69 b *	0.32 c *	0.14 a	0.04 a
T-A	8.40 b	0.89 a	0.04 b *	0.08 b *	2.24 c *	80.85 a *	5.78 b *	0.41 a	0.82 a	0.34 abc	0.14 a	0.03 a

Means within the same column and for the same irrigation practice followed by the same letter do not differ significantly based on Tukey’s HSD multiple range test at a = 0.05. Abbreviations: At, attapulgite; K, kaolin; T, talc; J, application in July; A, application in August. The asterisk indicates significant differences of the specific treatment from its corresponding control (irrigated or rainfed) based on Dunnett’s test.

). Under irrigated conditions, trees treated with kaolin or talc in August produced oils with low C16 content, lower than that of the control. Conversely, oils from trees treated with talc in August exhibited high C18 content, exceeding that of oils from control conditions. Kaolin-treated trees in August produced oils rich in C18:2 and C18:3, with significant differences from control oils (based on Dunnett’s test). Similarly, a high C18:3 content was observed in oils from trees treated with attapulgite (applied in either July or August) and talc in August, with significant differences from the control (based on Dunnett’s test).

Under rainfed conditions, the C16 content was high in oils from trees treated with talc in July, while the C16:1 content was higher than the control in oils from trees treated with attapulgite in July (based on Dunnett’s test). All clay particle treatments, regardless of application timing, resulted in significantly lower C17:1 and C18:2 contents compared to oils from control conditions. Attapulgite and talc applied in August produced oils with low C18 content (based on Dunnett’s test). All clay particles, irrespective of application timing (except for talc in July), enhanced the C18:1 content in the oils produced compared to the control. Kaolin and talc applied in July resulted in oils with low C20:1 content, lower than that of control oils (based on Dunnett’s test).

Under irrigated conditions, the saturated fatty acid (SFA) content in oils produced from trees treated with kaolin in August was lower than that in oils from control trees (

Table 6. Effects of the various treatments on the content of various groups of free fatty acids (determined as FAMEs—%) and squalene concentration in the oil of cv. ‘Koroneiki’ under irrigated and rainfed conditions.

Treatment	SFAs	MUFAs	PUFAs	UFAs	MUFAs/PUFAs	SFA s/UFAs	C18:1/C18:2	Squalene (mg/100 g)
	Irrigated olive orchard
Control	11.72 a	81.67 a	6.59 bc	88.27 b	12.40 ab	0.132 a	13.82 ab	499.3 a
At-J	11.20 ab	82.07 a	6.72 abc	88.74 ab	12.21 ab	0.127 ab	13.70 ab	518.9 a
At-A	11.44 a	81.43 a	7.12 ab	88.56 b	11.45 b	0.130 a	12.78 b	502.9 a
K-J	11.44 a	82.10 a	6.46 c	88.56 b	12.72 a	0.130 a	14.24 a	525.3 a
K-A	10.30 b *	82.46 a	7.23 a*	89.70 a *	11.40 b	0.112 b	12.75 b	504.3 a
T-J	11.26 ab	82.13 a	6.61 bc	88.74 ab	12.43 ab	0.127 ab	13.85 ab	524.2 a
T-A	10.94 ab	82.05 a	7.00 abc	89.05 ab	11.73 ab	0.122 ab	12.75 b	504.2 a
Rainfed olive orchard
Control	11.69 bc	78.76 c	9.54 a	88.30 ab	8.36 b	0.132 bc	9.06 c	412.9 b
At-J	11.82 bc	80.98 a *	7.19 b *	88.17 ab	11.26 a *	0.135 bc	12.54 ab *	529.4 ab
At-A	11.04 c	82.00 a *	6.95 b *	88.95 a	11.81 a *	0.125 bc	13.29 ab *	557.6 ab *
* K-J	12.09 b	80.77 ab *	7.14 b *	87.90 b	11.33 a *	0.140 ab	12.63 ab *	519.8 ab
K-A	10.94 c	82.04 a *	7.01 b *	89.06 a	11.69 a *	0.122 c	13.21 ab *	579.4 a *
T-J	13.46 a *	79.26 bc	7.28 b *	86.54 c *	10.92 a *	0.155 a *	11.89 b *	508.3 ab
T-A	11.25 bc	82.15 a *	6.61 b *	88.76 ab	12.47 a *	0.127 bc	14.03 a *	568.8 a *

Means within the same column and for the same irrigation practice followed by the same letter do not differ significantly based on Tukey’s HSD multiple range test at a = 0.05. Abbreviations: At, attapulgite; K, kaolin; T, talc; J, application in July; A, application in August. The asterisk indicates significant differences of the specific treatment from its corresponding control (irrigated or rainfed) based on Dunnett’s test.

). However, these same oils exhibited higher contents of both polyunsaturated fatty acids (PUFAs) and total unsaturated fatty acids (UFAs) compared to oils from control trees. The ratio of monounsaturated fatty acids (MUFAs) to PUFAs was elevated in oils from trees treated with kaolin in July, surpassing that of oils from trees treated with attapulgite in August. Additionally, oils from kaolin-treated trees in August were characterized by low SFA/UFA and C18:1/C18:2 ratios. Similarly, low C18:1/C18:2 ratios were observed in oils from trees sprayed with either attapulgite or talc in August, significantly lower than the ratio in oils from trees treated with kaolin in July.

Under rainfed conditions, the talc application in July resulted in oils with a higher SFA content than those from the control treatment. Applications of attapulgite and kaolin (regardless of timing) and talc in August produced oils rich in MUFAs, with significant differences from control oils. All clay particle treatments resulted in oils with lower PUFA content than the control, but with higher MUFAs/PUFAs and C18:1/C18:2 ratios. The lowest UFA content was observed in oils from trees treated with talc in July, which also exhibited a high SFA/UFA ratio. Squalene concentration was highest in oils from trees treated with kaolin or talc in August, showing a significant difference from control oils. Oils from trees treated with attapulgite in August also had a higher squalene concentration compared to the control (based on Dunnett’s test).

3.3. Hierarchical Cluster Analysis Results Regarding Olive Yield and Olive Oil Quality Characteristics

3.3.1. Irrigated Olive Grove

The hierarchical cluster analysis and the constellation plot revealed that the application of talc and kaolin exhibited similar results, close to that of the control, under irrigated conditions (Figure 2). Similarly, the two clay minerals applied in August gave similar results, while attapulgite seemed to be differentiated.

This was further supported by the hierarchical cluster analysis of the raw data regarding the products used for the foliar applications, which clearly distinguished all three mineral clay particles from the control, demonstrating their efficacy with respect to the measured variables in the irrigated orchard (Figure 3). Control trees were characterized by low yield and oil production per tree, as well as low contents of MUFAs, PUFAs, C18:1, total flavonoids, total phenols, oleocanthal, oleacein, and squalene in the produced oil. Conversely, oils from control trees were rich in tyrosol, hydroxytyrosol, luteolin, apigenin, and SFAs. Trees treated with attapulgite exhibited high yield, oil production per tree, and PUFA content, while the kaolin application resulted in oils rich in total flavonoids, C18:1 content, and high antioxidant capacity. The talc application, on the other hand, produced oils rich in total phenolic compounds, o-diphenols, oleocanthal, squalene, and oleacein.

The application of mineral clay particles in August was distinctly separated from both the control and applications in July, as shown by the hierarchical cluster analysis (Figure 4). Applying clay particles in August resulted in oils rich in total phenolic compounds, o-diphenols, total flavonoids, oleacein, oleocanthal, and PUFAs, as well as exhibiting high antioxidant capacity. In contrast, the application of clay particles in July led to a high yield per tree, elevated oil production per tree, and oils with high contents of squalene, MUFAs, and α-tocopherol.

3.3.2. Rainfed Olive Grove

Under rainfed conditions, the application of kaolin in August and attapulgite in July presented similar efficacy (Figure 5), while the talc application in July was clearly separated from any of the rest of the mineral clay particles’ applications and located close to the control.

As observed previously under irrigated conditions, control trees exhibited distinct behavior compared to those treated with mineral clay particles, with respect to the measured variables (Figure 6). Control trees displayed low yield and oil production per tree, a low oil percentage per fruit, and reduced contents of MUFAs, oleacein, and antioxidant capacity. In contrast, attapulgite-treated trees showed high fruit yield, elevated contents of C16:1, C18:1, MUFAs, oleacein, and squalene. Talc-treated trees produced fruits with a high oil percentage and yielded high oil production per tree, with oils exhibiting high antioxidant capacity. Kaolin application, on the other hand, resulted in oils rich in C18:1, MUFAs, and squalene.

Treatment with any of the three clay particles, regardless of the month applied, had a significant impact on tree yield and oil characteristics (Figure 7). Trees treated in August exhibited high yield and produced oils with elevated contents of C18:1 and MUFAs, as well as high antioxidant capacity and high concentrations of hydroxytyrosol, oleacein, oleocanthal, and squalene. In contrast, the application of clay particles in July resulted in a high oil percentage per fruit mass, high oil production per tree, and oils rich in total phenolic compounds, total flavonoids, and SFAs. Control (unsprayed) trees produced oils rich in o-diphenols, tyrosol, tocopherol, and PUFAs.

4. Discussion

The hierarchical cluster analyses clearly showed that clay particles as well as the time they are applied on olive canopy have a great impact on the yield characteristics as well as on oil attributes. Furthermore, the magnitude of their efficacy greatly depends on the irrigation status of the grove, since rainfed trees responded differently than the corresponding irrigated ones.

In most cases, the yield increased after mineral clay particle foliar application and this was clearly shown under the July application of kaolin and talc in irrigated grove and August application of attapulgite as well as of talc in the rainfed orchard. There are not many data in the literature on the effect of foliar spray of either talc or attapulgite (in fact, to our knowledge, this is the first report on the effect of foliar application of attapulgite in plants and the first of foliar application of talc in olive), while there are plenty of reports on the effects of kaolin application, which will be mainly used here. Increases in yield after clay particles application have been reported in a number of species, including olive [1,10,11,12,13,31,32,33,34,35,36]. The efficacy of mineral clay particles is primarily attributed to their ability to alleviate heat stress through their reflective properties, which lower leaf temperature [2,21,22,24,37] and protect chlorophyll from degradation [24,31]. Additionally, particle films help maintain high leaf hydration under conditions of elevated transpiration rates [2,14,16,32,36,38], such as those observed during this trial (summer in Greece). Furthermore, kaolin foliar application on olive trees under high irradiance has been shown to promote shoot growth and increase leaf area per plant [34,38] and similarly on mango [39], grapevine [18], and eucalyptus [19]. Considering these factors, an elevated photosynthesis rate at the whole-tree level appears plausible [23,32,36,40], thereby supporting fruit development and enhancing oil biosynthesis and accumulation. Furthermore, the increased scattering of light within the canopy and a reduction in root respiration have also been proposed as factors responsible for the increase in biomass [19]. Nearly all treatments increased oil accumulation and production per tree under rainfed conditions, consistent with previous reports [33], though their effects were less pronounced under irrigated conditions, as similarly observed in almond following kaolin application [41]. According to Tommaso et al. [18], under severe water stress, kaolin was very effective at protecting the photosynthetic machinery in grapevine. In this trial, the oil production per tree increased by up to 1.8 times compared to the rainfed control, aligning with findings from other studies [33,40].

Under irrigated conditions, none of the treatments significantly affected free acidity or peroxide values. However, specific absorbance values (K232 and K270) were influenced, with talc application in July resulting in low values for both, showing a significant difference from the control (for K232 only) and some other treatments. The same treatment (talc in July) also produced the lowest free acidity in oils under rainfed conditions, with a significant difference only from attapulgite applied in August. The lowest K232 value was detected in oils from trees treated with talc in August, while the lowest K270 value was observed in oils from trees treated with kaolin in August. Peroxide values did not differ among treatments, regardless of irrigation status, consistent with findings from other researchers [14]. However, some studies report that kaolin clay particle application reduced olive oil peroxide values [15,32,33]. Issaoui et al. [42] noted that oils produced under warm conditions exhibit higher peroxide values than those from cooler conditions. Based on this, it could be hypothesized that clay particles did not sufficiently lower canopy and fruit temperatures to significantly affect peroxide values. Nevertheless, under all treatments, the oils produced met the criteria for extra virgin olive oil based on the measured variables.

The literature presents varied results concerning these oil quality indices. Some researchers report significant reductions in oil acidity with kaolin clay particle applications [15,32], while others observe no significant changes [1,14]. Free acidity in olive oil reflects the hydrolytic breakdown of triglycerides into di- and monoglycerides, yielding free fatty acids [15], a process accelerated by heat [42,43]. Enzymatic activity (e.g., lipases) and oxygen exposure also increase oil acidity, as well as olive fruit infestation (e.g., by the olive fruit fly) and the harvest of overripe fruits. In this experiment, the olive fruits were healthy, not overripe, and the oils were analyzed promptly, ruling out post-harvest factors. Several authors suggest that high fruit surface temperatures combined with water deprivation may increase oil acidity [15,42,44]. Given that no significant differences in acidity were observed under irrigated conditions, it could be inferred that fruit water status plays a critical role. While this aligns with findings from some authors [15,42,44], other studies report minimal or no effects of plant water status [45,46]. Although clay particles likely reduced fruit surface temperature, this reduction was apparently insufficient to significantly impact oil acidity. Even so, acidity levels under all clay particle treatments were lower (though not significantly) than the control.

Conversely, under rainfed conditions—where evaporative cooling from tissue water is limited—clay particle application positively affected oil acidity, consistent with other reports [32,33]. This effect may stem from a combined reduction in water loss and fruit temperatures compared to the control. Under such conditions, the oxidative stress caused by limited water supply and excessive heat was likely mitigated, resulting in reduced oil acidity [32].

Stress conditions like excessive heat load, drought and heat stress lead to stomata closure, reduce CO₂ diffusion and carboxylation efficiency (stomatal and non-stomata limitations of photosynthesis), and at the end, compromise the photosynthetic capacity of the leaf and entire canopy and increase oxidative phenomena [1,36]. Under such conditions, the plant is forced to activate signaling pathways that upregulate antioxidant proteins, resulting in an adaptive stress response that is only possible under subtoxic conditions [40], meaning that under low to moderate stress conditions plants are still capable of increasing their antioxidant defenses. Such an investment on secondary metabolites may lead though to reserves depletion and fruit growth and oil accumulation cessation [36].

Phenolic compounds belong to the non-enzymatic arsenal of the plant, and are able to a certain extent to counteract the negative impacts of oxidative molecules (ROS) before they start to decrease due to excessive degradation-oxidation. Phenolics also protect olive oil from oxidative degradation, while they offer significant health benefits [47,48]. Nonetheless, the concentration and composition of phenolic compounds depend on many factors, both pre- and post-harvest ones, and their complex interactions. Among those factors, the genotype, environmental conditions, fruit ripening stage, agronomic practices, and of course the olive yield are the most important ones.

In this experiment, their concentration was significantly influenced by the applied treatments, regardless of the orchard’s irrigation status. Clay particle applications resulted in significantly higher total phenolic concentrations in the oils produced—specifically with kaolin and talc applied in August under irrigated conditions and talc in July under rainfed conditions—consistent with the literature [14,38]. Significant differences among treatments were also observed in the concentrations of o-diphenols and flavonoids in the oil, which translated to notable variations in antioxidant capacity. Alleviating products are expected to lower the need for upregulating the biosynthesis of phenolic compounds under stress. Nonetheless, as they do not completely alleviate stress impacts, they offer the plant the chance to increase antioxidants production, without the risk of surpassing the point where their production is being limited by the excessively reduced photosynthesis and the balance is being in favor of ROS production against scavenging [12,36]. Additionally, no one has investigated the effects of clay particles on protein synthesis in olive fruit flesh and pit, as changes in enzyme activities during the pressing and malaxation steps in the olive mill, may also alter phenolic compounds composition and concentration [1]. Considering that kaolin, talc, and attapulgite clay particles are characterized by different properties (size, shape, adhesion capability, reflecting properties, etc.) [13,49], it would not be unexpected for each one of them to have a distinct and different effect on these enzymes [31], which alter the oil phenolic composition and concentration during fruit processing, exhibiting a non-chemical elicitor response [14]. Furthermore, the transfer of phenolic compounds from olive fruit to oil is a complex process influenced by multiple factors, particularly fruit moisture [1], which negatively affects this transfer. Given the higher fruit moisture likely present in the irrigated orchard, a lower phenolic concentration in the oil might be expected [50] compared to the rainfed orchard.

Conversely, under rainfed conditions in groves in Southern Greece, olive trees likely experienced severe drought and heat stress, promoting the generation of oxidative molecules. Under such conditions, phenolic compounds serve as part of the tree’s antioxidant defense system [16]. Consequently, their depletion is anticipated once degradation surpasses regeneration [23], partially explaining the lower concentrations observed in oils under rainfed conditions. Particularly interesting was the fact that differences were observed concerning the efficacy of the same clay material depending on the time of application. As already mentioned above, the concentration and biosynthesis of phenolic compound are influenced by a number of factors and their interactions [1]. During the trial, excessively high temperatures occurred (in early August) between the two spray applications, which may have affected the efficacy of the products. The talc application in July resulted in lower phenolic concentration in the oil under irrigated conditions, while in higher concentration under rainfed ones. Although at first this seems peculiar, it must be noted that the yield of the trees followed the opposite pattern. This alone could explain the differences observed in the concentration of phenolic compounds found in the oil.

Talc applied in July or August under rainfed conditions performed effectively, as evidenced by the elevated concentrations of total phenols and o-diphenols, as well as the enhanced antioxidant capacity, similar to that reported by Dinis et al. and Conde et al. in grapevine under the kaolin application [17,51]. In pomegranates, kaolin has been shown to increase the activity of phenylalanine ammonia-lyase, a key enzyme in phenolic compound biosynthesis [52]. In grapevine, increases in anthocyanins, total flavonoid, and total phenol concentration were attributed to the increased sugar partitioning to the fruit, the increased availability of water, and the increased gene expression involved in the biosynthesis of such compounds [51]. Similarly, Conde et al. attributed the increase in phenolic content in grape berries under kaolin clay particles to a general stimulation of the biosynthetic pathway of phenylpropanoids, flavonoids, and anthocyanins at the gene expression and/or protein activity levels [17]. Brito et al. [12] suggested that microclimatic conditions have a greater impact on olive oil phenolic content than kaolin, yet in this experiment, talc (primarily) and kaolin (secondarily) increased phenolic concentrations in oils under both irrigated and, especially, rainfed conditions. According to Khaleghi et al. [15], kaolin boosts total phenol content in oil due to its UV-reflective properties and ability to reduce leaf and fruit temperatures. Similarly, Saur and Makee [33] report that the kaolin application on olive canopies alters the microclimate, enhancing photosynthesis and, consequently, antioxidant synthesis in olive fruits. Additionally, oils from olive trees grown in cooler climates reportedly have higher phenolic content than those from warmer climates [23,42]. If so, this supports the present trial’s findings, suggesting that clay particles reduced canopy temperature [2], creating a cooler microenvironment, particularly under rainfed conditions.

Nevertheless, the phenolic compound concentrations detected here are relatively low compared to those reported for oils from the Koroneiki olive cultivar in the literature, likely due to the late harvest, which may have negatively impacted their levels in the fruit [47].

The HPLC analysis of individual phenolic compounds in the olive oil identified oleocanthal and oleacein—two key secoiridoids—as major phenolic constituents, consistent with the literature [35,41,53,54,55,56], followed by tyrosol, luteolin, and apigenin, with concentrations within previously reported ranges [57,58]. Under irrigated conditions, the treatments had a minor impact on the concentrations of individual phenolic compounds. Only tyrosol concentration was significantly affected, being higher in oils from trees treated with talc in July compared to those treated with attapulgite in August. This suggests that the influence of clay particles on oil phenolic compounds diminishes when stress conditions (e.g., high heat load and irradiance) are partially mitigated by irrigation. As noted earlier, phenolic compounds play a critical role in the tree’s antioxidant defense under stress and are expected to accumulate in the oil [59], though this depends on various pre- and post-harvest factors. Irrigation likely reduced heat stress on the olive canopy, diminishing the additional protective effect of clay particles, and thus limiting their impact on phenolic concentrations.

In contrast, under rainfed conditions—where the heat load on the tree canopy is expected to be high—the alleviation potential of clay particles was substantial. The concentrations of nearly all detected phenolic compounds were significantly influenced by the treatments, highlighting the pronounced effect of clay particles under such conditions.

Under such conditions (heat and drought stress), the composition of the fatty acids in the oil may change, as well [15,31]. It has been reported that their composition changes between warm and cool climates, with oleic acid, UFAs/SFAs, MUFAs/PUFAs, and MUFAs being in higher concentration in oils produced from cooler regions, while the opposite has been reported regarding PUFAs and palmitic acid [15,31,43]. Considering that clay particles lower leaf and canopy temperatures, changing the tree’s microclimate, the results of the present trial are justified. Slight increases in oleic acid and MUFAs were determined in oils produced under the effect of kaolin and attapulgite under both irrigated and rainfed conditions, while increased MUFAs/PUFAs ratio and lower PUFAs and C18:1/C18:2 ratio were determined under rainfed ones similar to that reported earlier [15,23]. According to several authors, these differences in fatty acid composition could be partly explained by the differences in enzyme activities involved in the synthesis of triacylglycerides [15]. Oleatedesaturase activity, for example, under warm conditions caused the conversion of oleic acid to linoleic acid, changing their ratio [15]. Genes’ expression of fatty acid desaturases (FADs) family is also influenced by temperature [60]. These genes are involved in the biosynthesis of oleatedesaturase as well as of other enzymes involved in fatty acid biosynthesis such as b-ketoacyl-ACP synthases I, II, III, and stearoyl-ACPD9-desaturase [60]. As the application of clay particles lowers plant surface temperature, it may also lower enzyme activity, thus preserving oleic acid content and other mono-unsaturated fatty acids [23,31].

5. Conclusions

Particle film technology in horticulture represents an innovative approach with significant potential, offering applications ranging from water conservation and abiotic stress alleviation to biotic stress management. In this context, the use of three different mineral clay particles as foliar sprays on olive trees is highly relevant, particularly given the impacts of the climate crisis on this sector. The present trial demonstrates that there is no universal prescription for optimizing PFT efficacy, as its effectiveness depends on multiple factors. These include pedoclimatic conditions, agronomic practices (e.g., the presence or absence of irrigation in this study), and the crop and fruit growth stages at the time of application, among others. Nevertheless, in nearly all cases—especially under rainfed conditions—clay particle applications enhanced fruit production and olive oil quality, though the extent of improvement varied depending on the clay material and timing of application. From an agronomic point of view, trees treated with clay particles seem to better withstand summer’s harsh conditions, a period where floral induction, new shoot growth, and fruit growth are taking place, affecting both this year’s production as well as the one of the following year. Oil quality, in most cases, improved under clay particle application, in terms of oxidative stability due to the higher concentration of both phenolic compounds and MUFAs, ensuring long-term storage and health benefits. Nonetheless, further research is needed to determine the optimal clay material and application timing for each olive cultivar and grove, thereby maximizing production and profitability.