3.1.1. Agrometeorological Data
The meteorological data were collected from the agrometeorological station located near the experimental field and the data are shown in
Figure 1. In particular, the amount of rainfall was 374 mm during the 2019 season, higher than the rainfall amount reported in the 2020 season (321 mm). The first season (2019) showed non-uniformity of the rainfall water spells, in which more than 70% of the total seasonal rainfall amount occurred during the months of January, May, July, and September, with more than 50 mm in each month (
Figure 1). In the season, a larger amount of rainfall over the deficit irrigation period was reported. In 2020, the rainfall events experienced some uniformity during this season, however, with less than 50 mm monthly. In 2019, the irrigation began on the DOY 158. During this year, there was rainfall during the flowering phase (May and June) and during the fruit setting phase (July) (
Figure 1). Similarly, in the 2020 season, there were spells of rainfall during the flowering and fruit setting phases; however, to a lesser extent. The cumulated annual value of the reference evapotranspiration (ET
o) for the two experimental seasons was 1130 mm and 1001 mm during the 2019 and 2020 seasons, respectively (
Figure 1). In both seasons, the ET
o was higher during the fruit setting phase (July, with the highest rate being reported in the 2020 season) and lower during winter (November–January). Considering that the vapor pressure deficit (VPD) can influence water stress in grapevines depending on the crop’s phenology [
39], the saturated VPD was estimated in this study throughout both irrigation seasons. The VPD data (
Figure 1) tends to follow the same trend of the ET
o in both seasons. The highest VPD value was reported during August, while the lowest value was reported in January in both seasons.
3.1.2. Soil Water Balance
The application of deficit irrigation (DI) in the vineyard was performed at later plant growth stages as a common practice in the area in order to save water without affecting the crop yield. In fact, several studies found that deficit irrigation in vineyards during specific phenological phases such as fruit setting [
40,
41] and pre- and post-veraison [
42] can maintain yield and improve crop quality. In this study, the DI regime consisted of two different irrigation interventions which were applied to keep the soil moisture at 30% with an RP between 30–40%. The first deficit irrigation was applied during the fruit setting phase, in which every time the sensors detected a reduction in the humidity up to 21.00% θ (RP 30%), the vineyard was irrigated to bring the soil moisture back to 30% θ. The second deficit irrigation was instead applied during the post-veraison phase, in which every time the sensors detected a reduction in the humidity up to 19.50% θ (RP 35%), the vineyard was irrigated to bring the soil moisture back to 30% θ.
In both the 2019 and 2020 irrigation seasons, the soil water content in the root zone (0–60 cm) fluctuated greatly during both full irrigation and deficit irrigation periods (
Figure 2) due to the fluctuation in rainfall events; however, the VWC values were within the range previously set by the calibration of the probe. There were statistically significant differences between the global mean of the VWC of each treatment at both depths; the interaction between year × treatment was significant per each depth too.
We believe that the VWC values out of the identified soil moisture range was due to heavy spells of rainfall, as in the case of July 2019 during the fruit setting period (
Figure 1). Similarly, the VWC values out of the range of the threshold used for DI applied during the fruit setting and the post-veraison phase were due to the lack of water from rainfall; moreover, a wide temporal variability in the soil water content during the irrigation season might be due to soil structural changes, for example as a result of shrinkage cracks [
43]. As mentioned above, the rainfall data (
Figure 1) experienced a non-uniform distribution throughout the seasons, which was more evident during the 2019 season, and especially during the DI period (
Figure 2). During the 2020 seasons, the relatively heavy, however, homogeneous distribution of rainfall throughout the season during the fruit setting phase allowed for the maintenance of the VWC within the defined range, especially during the DI period. During the FI period, however, the VWC reported higher values (out of the range) due to heavy rainfall events.
3.1.3. Plant Water Status
In this study, we measured the SWP throughout the development stages of the vines. The measurement time was expressed as days of the year (DOY) and SWP measurements were taken 12 times throughout the 2019 irrigation season, while, in 2020, the SWP measurements were taken 11 times due to differences in the irrigation season length between the two seasons (in 2020, the irrigation season started later and ended a bit earlier compared to the 2019 season (see
Section 2.2.
Irrigation Treatments and the Experimental Design)). In particular, in the 2019 season, the SWP was measured on the 163rd, 171st, 178th, 185th, 192nd, 199th, 204th, 213th, 217th, 233rd, 239th, 246th, 254th, 260th, 268th, and 277th DOY in each irrigation treatment and replicate. In the 2020 season, instead, the SWP was measured on the 178th, 185th, 196th, 204th, 220th, 237th, 244th, 246th, 260th, 290th, and 300th DOY in each irrigation treatment and replicate.
The vine water status is believed to be an important factor that determines crop quality in viticulture [
44]. The analysis of the SWP results indicates that vines under the DSS treatments showed lower water stress levels compared with farm irrigation in both years. The seasonal and inter-seasonal variations in the midday SWP (mPa) in both irrigation seasons are reported in
Figure 3. In the 2019 season, the plants in both irrigation treatments did not show a high level of water stress compared to the stress threshold value (SWP > −1.2 mPa). Nevertheless, the plants irrigated with the DSS were less stressed and in some growth stages (DOY), the differences were significantly in favor of the DSS when compared to the farm irrigation system. In particular, on DOY 178, the vines irrigated by DSS were significantly less stressed than the vines irrigated using the farm irrigation system (SWP-DSS = −0.77 mPa vs. SWP-farm irrigation = −0.96 mPa). Similarly, in DOY 213, the SWP of the plants irrigated using the DSS was significantly higher than those irrigated using the farm irrigation system (SWP-DSS = −0.95 mPa vs. SWP-farm irrigation = −1.03 mPa) (
Figure 3). On DOY 192, the plant stress levels were much lower in both treatments (SWP-DSS = −0.33 mPa vs. SWP-farm irrigation = −0.31 mPa) mainly due to heavy rainfall events. On the contrary, during the veraison phase, the lack of rainfall events (
Figure 1) led to a significant decrease in the SWP values (below the threshold level) under both irrigation systems, particularly on the 239th, 254th, 260th, and 268th DOY. In particular, on DOY 239, the SWP values were lowered far beyond the stress threshold value (SWP < −1.2 mPa) in both treatments; however, the SWP value was higher for the plants irrigated using the DSS, but the difference was not significant as compared to the SWP for the plants irrigated using a farm irrigation system. In September (260th and 268th DOY), during the pre-ripening phase, the SWP values were significantly lower in the plants irrigated using the DSS as compared to the plants irrigated using the farm irrigation system (
Figure 3).
In 2020, irrigation began on DOY 172. During this year, the rainfall was distributed more homogenously compared to the 2019 season, with good rainfall amounts reported in August (during the veraison phase) (
Figure 1). We believe that the rainfall distribution in the 2020 season directly and/or indirectly influenced the SWP values, in which the values dropped under the stress threshold value on just two occasions (in four occasions, the SWP values were reported under the stress threshold values in the 2019 season), specifically, on the 237th and 260th DOY, and the plants irrigated using the DSS were less stressed (
Figure 3). In general, there were no significant differences in the SWP values between the two irrigation treatments except for the 220th DOY, on which the plants under the DSS were significantly less stressed (SWP-DSS = −0.99 mPa vs. SWP-farm irrigation = −1.11 mPa).
In addition, the water stress integral (WSI) values can be used as a quality indicator [
46]; therefore, it was calculated using the data on the seasonal variations in the midday SWP (mPa) in both irrigation seasons and under both irrigation treatments. In both years of the experiment, no significant differences were found between the DSS and farm irrigation treatments; despite this, the WSI values tended to be higher for the farm irrigation vines in both years than the DSS vines (
Table 2).
Concerning the leaf gas exchange results, it is widely accepted that the optimum stomatal behavior should occur when the opening of the stomata during the day allows for minimum transpiration and maximum photosynthesis, and their ratio remains constant [
47]. In this study, stomatal conductance tended to be higher for the plants subjected to the DSS irrigation treatment.
Figure 4 shows that a statistically significant difference was observed on DOY 217 in 2019, during the pre-veraison phase (DSS: 0.14 mmol m
−2 s
−1; farm irrigation: 0.09 mmol m
−2 s
−1). On the same DOY, the SWP values indicate that the vines irrigated according to the DSS were less stressed (SWP-DSS = −0.92 mPa vs. SWP-farm irrigation = −1.12 mPa). In fact, the 2020 season was drier than the 2019 season (in the irrigation season, the amount of rainfall was 216.3 mm in 2019 and was 153.2 mm in 2020); therefore, the stomatal conductance was lower in response to drier soil conditions (
Figure 4) as was found by Beis and Patakas [
48]. Similarly, net assimilation followed the trend of stomatal conductance. As shown in
Figure 4, net assimilation tended to be higher for plants subjected to DSS treatment in the 2019 season; however, there were no statistically significant differences between the two seasons, despite the deficit irrigation that was applied during the post-veraison phase in vines under the DSS irrigation treatment. Zufferey et al. [
49] reported that water stress influences the net assimilation. In this study, the net assimilation was higher during the fruit set phase in the 2020 season than in the 2019 season for both irrigation treatments (
Figure 4).
In the 2019 season, on DOY 182, there were statistically significant differences between the two treatments, with a greater intrinsic water use efficiency (P
n/g
s) for the vines irrigated using the DSS probably due to improved irrigation management, with an optimal evaluation of the soil water status by the probes. On the contrary, on DOY 217, there were significant differences in the (P
n/g
s) value, with a higher intrinsic water use efficiency reported for the vines irrigated using the farm irrigation system. The significantly lower intrinsic water use efficiency value on the 217 DOY for the vines irrigated using the DSS was probably due to a significantly higher stomatal conductance (
Figure 4) (in August 2019, there was no rain and irrigation using the DSS probably allowed the plants to maintain a higher stomatal conductance). In the 2020 season, there were no statistically significant differences between the two irrigation treatments regarding the intrinsic water use efficiency, probably thanks to a better distribution of meteoric inputs throughout the summer under the DSS. In 2019, the WUEi followed the trend of the intrinsic water use efficiency for both treatments; however, different from the intrinsic water use efficiency, the WUEi showed no statistically significant differences only in the last measurement of the irrigation season; in 2020, the WUEi decreased less markedly than the intrinsic water use efficiency in the first part of the irrigation season. No statistically significant differences were detected between the treatments in 2020 (
Figure 4).