3.1. Irrigation Stress, Water Used and Production
Figure 2 shows the daily crop reference evapotranspiration (ET
0) and daily rainfall for the 2018 and 2019 seasons. In the 2018 season, last spring rains occurred at the end of May (day of year, DOY, 146) and autumn rains started on DOY 284 (11 October). In the 2019 season, the dry period was longer, from 24 April to 19 October (DOY 116 and 292, respectively). These climatic conditions are typical of Mediterranean areas, with dry and hot periods from late spring until early autumn. Similar reference evapotranspiration (ET
0) behaviors were observed in both seasons, with the highest ET
0 values reached in July and August.
Crop water status (minimum stem water potential, min
Ψstem and water stress integral,
SI), applied water (
AW) and olive production are summarized in
Table 1. The water used in 2019 to reach the targeted water conditions was higher (484 L m
−2 or mm) compared to that needed (304 L m
−2) in 2018. In both seasons under study, the highest volume of irrigation water was that used in the control treatment, T
0 (mean of the two seasons 753 L m
−2) while the lowest volume was applied in T
2 and T
3 treatments, with 221 L m
−2 and 212 L m
−2, respectively. Minimum
Ψstem values were negatively correlated with the applied water volume (R
2 = 0.6689); this way, the lowest values of
Ψstem were observed in T
2 and T
3 olive trees.
Regarding the applied water, the T2 and T3 treatments (Guadalquivir hydrographic confederation RDI and confederation SDI) resulted in the highest reductions regarding the volume of applied water, reaching values as high as 69.9 and 70.8%, respectively (mean of seasons 2018 and 2019), of the total water applied in the control trees.
The seasonal stress integral (SI total) was not affected by the factor season, but it was affected by the irrigation treatment, as expected. The highest SI value was achieved in T2 trees (179 MPa day−1) followed by those included in T3. Finally, the SI values also depended on the phenological stage, with the highest values being found in the T2 trees, especially at phase II (pit hardening). Moreover, the minimum Ψstem and the SI were negatively correlated, with SI total increasing as the Ψstem decreased (R2 = 0.9337).
3.2. Analytical Parameters of Olive Oil Quality
In general, no important differences were found for the quality parameters (
Table 2); this was expected because all samples were obtained from freshly harvested olives without significant defects. However, in the 2018 season, the peroxide index was higher in the control treatment, T
0 (11.8 meq O
2 kg
−1) compared to that observed in the stressed samples. Similar effects have been previously observed, with lower peroxide index values in samples from water-stressed olive trees [
28].
A similar behavior to that previously described for the peroxide index was also found for the acidity index in the 2018 season, where the T0 value (0.42% oleic acid) was higher than those found for the rest of treatments (mean of 0.18%).
All samples under study were classified as extra virgin olive oil according to the Official Commercial Classification stablished by International Olive Oil Council [
27]. This way, it can be concluded that deficit irrigation treatments did not reduce the quality of the olive oil; similar results were previously reported by Gómez del Campo and García [
29] for the same olive variety, in Toledo (central Spain), despite the water reduction levels.
3.3. Determination of Antioxidant Activity and Quantification of Total Phenolic Content
The irrigation treatment did not have a significant (
p < 0.05) effect on antioxidant activity (AA) as determined by the two methods used (ABTS
+ and DPPH
•) (
Table 3); however, the antioxidant activity was significantly (
p < 0.05) higher for the samples from the 2019 season compared to those determined in the samples from the 2018 season. However, the total phenolic content (TPC) was significantly (
p < 0.05) affected by both factors, the season and the irrigation treatment, with the trees included in the treatments with deficit irrigation (T
1–T
3) showing significantly higher values than those observed in oil samples obtained from the control trees (T
0), and the samples from 2018 had higher values than those from the 2019 season. This experimental positive finding on polyphenol content was possibly due to climatologic conditions.
Sena-Moreno et al. [
30] observed increases in the antioxidant activity of 218.1% and 153.4% when irrigation water was supplied at 20% and 15% of control water dose, respectively. A similar trend to that described for the AA can be observed in the current study but for TPC; this way, the two treatments following the confederation restrictions (T
2 and T
3) reached values of 92.9 and 98.5 mg GAE L
−1 compared to 53.7 mg GAE L
−1 found for the control oil (
Table 3). Those TPC values were lower than those reported in previous studies [
25,
31]. On the other hand, Dag et al. [
32] concluded that an extremely intense irrigation deficit could negatively affect the TPC.
The effects caused by water stress on the values of the TPC were similar to those previously reported in the literature. For example, Ahumada-Orellana et al. [
28] showed that polyphenols increased by 26.8%, 52.2% and 49.3% when minimum
Ψstem was reduced from −1.2 MPA to −3.5 MPA, −5.0 MPA and −6.0 MPA, respectively. In the current study, in the 2018 season, the TPC increased by 55.2%, 56.9% and 77.6% compared to the value of the control treatment when the control min
Ψstem (−2.21 MPa) was reduced to −2.74 MPA, −4.71% and −3.75%, respectively. A similar trend was also observed in the 2019 season, with TPC increasing by 31.3%, 93.8% and 93.8% compared to the control when min
Ψstem decreased from the −1.54 MPa of the control down to −3.15, −5.21 and −3.99 MPa, respectively.
3.4. Fatty Acids Profile
Fifteen fatty acids were identified and quantified in ‘Arbequina’ extra virgin olive oils (
Table 4). In both years, oleic acid (C18:1
cis 9) was the major fatty acid in all olive oil samples (49.6% in 2018 and 44.1% in 2019). Regarding the irrigation treatment, higher oleic acid content was reached by sustained deficit irrigation (T
3, 48.9%). These oleic acid contents were lower than that expected for olive oil samples. For instance, Ahumada-Orellana et al. [
28] and Gómez del Campo and García [
29] observed values close to 70% of total fatty acids for the same variety, although no changes were reported in water-stressed samples. However, Hernández et al. [
33] observed similar effects in the oleic acid content of other ‘Arbequina’ orchards when deficit irrigation was applied. Those differences could be generated by the different locations of the orchards [
34] and edaphoclimatic conditions [
35]. In this sense, the highest concentration of oleic acid was reached in samples obtained from trees subjected to sustained deficit irrigation (T
3, 48.9%). The second most abundant compound was palmitic acid (C16:0), with 14.6% and 18.4% in the 2018 and 2019 season, respectively. This fatty acid did not present significative differences among irrigation treatments. The third predominant compound was linoleic acid (C18:2
cis6), which had a season mean content of 11.6% in 2018 and 17.4 in 2019.
Regarding fatty acid families, monounsaturated fatty acids (MUFAs), with oleic acid as the main compound, were the predominant family (62% in 2018 and 55.6% in 2019), and their content depended on the irrigation treatment, especially higher after T
3 (61.1%). This family is extensively associated with beneficial effects on human health [
36]. Saturated fatty acids (SFAs) were the second most concentrated family, with 17% in 2018 and 24.5% in 2019. Finally, polyunsaturated fatty acids (PUFAs) were lower in the 2018 season than in the 2019 season (12.5% and 19.2%, respectively), and control irrigation treatment reached the highest content (16.9%) and lower PUFA content was reached after T
2, 14.9%.
Regarding the health indicators, thrombogenic index (TI) and atherogenic index (AI) values were higher in the 2019 season (0.541 and 0.249, respectively). On the other hand, irrigation treatments only had an effect on the atherogenic index, with lower values reached when sustained deficit irrigation was applied (T
3, 0.221). Both parameters are associated with correct blood circulation [
37]. Similar values were obtained by Sánchez-Rodríguez et al. [
24] with the same variety and other irrigation treatments.
3.5. Volatile Compound Profile
In general, the concentration of the volatile compounds in olive oil was significantly affected by the irrigation practices, as previously reported by other authors [
38,
39], but not by the factor season, with 18 compounds being identified and quantified in the volatile profile of the super-high-intensive ‘Arbequina’ oils under study (
Table 5). These compounds provide olive oils with notes of alcohol, apple, sweet, sour, bitter, green, pungent, almond, astringent, leaves, earthy, fruity, floral and herbal, among others, all of them positive characteristics of an extra virgin olive oil, EVOO [
25,
40,
41].
No effect caused by the factor season was observed on the volatile profile of these oils and the mean values of the 2018 and 2019 seasons are summarized in
Table 5. The two predominant compounds were
trans-2-hexenal and
trans-2-hexen-1-ol; however, the irrigation treatment only caused significant effects on the content of the first compound, with T
1 and T
3 generating equivalent contents (91.67 and 96.28, respectively). The
trans-2-hexenal compound is one of the main compounds in the volatile fraction of olive oils, especially in extra virgin olive oils [
42]. In this sense, the concentration of this compound significantly affected the final concentration of volatile compounds present in the oils, representing ~46.5%. Oils obtained after T
0 and T
3 had the highest values for total volatile content (188 and 186, respectively). These results agreed with those obtained by Gómez-Rico et al. [
43], who observed that the volatile compounds most affected by irrigation in ‘Cornicabra’ oils were
trans-2-hexenal,
trans-2-hexen-1-ol and the hexen1-ol.
Olive oils also showed significant differences in the contents of ethanol, benzaldehyde, hexyl acetate,
trans-ß-ocimene, benzyl alcohol, acetophenone and benzoic acid. Oil samples obtained after T
1 had the highest values of benzaldehyde and
trans-ß-ocimene (8.14 mg 100 g
−1 olive oil and 2.19 mg 100 g
−1 olive oil, respectively). However, some aldehydes present in olive oils can provide undesirable sensory attributes such as sweaty, musty-damp or vinegary [
42], although this is not the case. On the other hand, the control and confederation SDI treatments led to olive oil with the highest contents of hexyl acetate (3.54 mg 100 g
−1 olive oil and 4.77 mg 100 g
−1 olive oil, respectively). Finally, oil samples from T
0 and T
2 stood out for their contents of benzyl alcohol and benzoic acid.
In previous studies, Sánchez-Rodríguez et al. [
24] and Sánchez-Rodríguez et al. [
25] demonstrated that RDI and SDI led to extra virgin olive oil with similar or even higher amounts of the main volatile. These results agreed well with those obtained in the current study in the sense that both studies demonstrated that the applications of irrigation strategies had no negative effects on the volatile profiles of olive oil.
3.6. Descriptive Analysis
The sensory profiles of the oils under study were established by a trained panel (
Table 6). Olive oils from the 2019 season had higher fruity and pungent intensities than those obtain during the 2018 season. In general, water-stressed trees (T
3 and T
2) led to olive oils with a higher intensity of the fruity, bitter and pungent notes compared to those of the control samples, T
0. Previous studies reported that water stress in olive trees negatively affected the attributes of “bitterness” and “pungent” [
44]; however, this was not the case in the irrigation treatments applied to the super-high-intensive ‘Arbequina’ orchard under study, where water stress led to oils of even higher sensory quality than the control samples. A similar trend has also been reported by Sánchez-Rodríguez et al. [
25].
An important characteristic for the oils to be classified as “extra virgin” is the absence of defects [
27]. Regarding the overall defects presented in the samples under study (rancid, moldy, oxidized, humid and others), none of the studied samples showed significant values of the attribute “defect”.