Introduction

Grapevine (Vitis vinifera L.), is among the most cultivated perennial fruit species (Food and Agriculture Organization of the United Nations, 1997) with more than 10,000 cultivars, of which some are widely grown around the world and almost all are of local importance (OIV, 2017). Many highly reputed wine grapes are grown under semi-arid climatic conditions in Mediterranean-type areas, where soil, weather, cultivar and farmers can all contribute to generating specific wine traits and, in turn, to pinpointing a terroir (Vaudour, 2002; van Leeuwen and Seguin, 2006). Nowadays, climate change poses a challenge in terms of crop productivity, economic sustainability (which also applies to viticulture, mainly due to an increase in air temperature), short rainy seasons and the increasing frequency of extreme climatic events, such as heatwaves, storms and heavy rains (Intergovernmental Panel on Climate Change - IPCC, 2014).

It has been observed that the phenological stages (e.g., bud-break and veraison) of grapevines are accelerating as a result of the rise in air temperature, thus potentially causing different types of damage (Jones, 2006; Webb et al., 2012); for example, early bud-break may expose the vines to late spring frost, which will have detrimental effects on buds or the vitality of young shoots (Leolini et al., 2018). Earlier veraison may impact the chemical composition of berries and wine quality, because grape maturation may occur during the hottest period of the season (Jones and Davis, 2000; Keller, 2010; Young et al., 2016). Increasing air temperatures throughout the growing season will also influence evapotranspiration demand and soil water availability, which both impact vine water status (Zavaleta et al., 2003; van Leeuwen and Destrac-Irvine, 2017), and in turn several ecophysiological leaf traits (e.g., stomatal conductance, photosynthesis leaf temperature, berry phenols and sugar accumulation) (Flexas and Medrano, 2002; Castellarin et al., 2007) that can collectively and negatively impact yield and its components (Fraga et al., 2012; Gambetta, 2016).

Supplemental irrigation is increasingly used as an adaptive strategy to address drought stress, as can be seen by the increasing proportion of irrigated vineyards: from 4&nbsp% (end of the&nbsp80s) up to ≈50&nbsp% (2011-2015) (Ayuda et al., 2020). However, the adoption of irrigation to overcome drought events can be debated, mainly because of limited freshwater availability, increased management costs and possible negative consequences on grape quality (Chaves et al., 2010, Ayuda et al., 2020; Gambetta et al., 2020). Changes in grape quality traits (and in turn in wine) due to irrigation consequently have an impact on the terroir (van Leeuwen, 2010); therefore, in order to accommodate drought and adverse thermo-radiative conditions, new mitigation/adaptation strategies alternative to irrigation are highly desirable (Fraga et al., 2012; van Leeuwen and Destrac-Irvine, 2017).

In this context, the application of processed mineral particle films (e.g., kaolin and calcium carbonate) on the surface of leaves and fruit can protect them from higher temperatures, especially when directly exposed to solar radiation (Glenn and Puterka, 2004). In grapevine, kaolin has been tested for several purposes, including the control of some pests (Tubajika et al., 2007), improving fruit quality (Ou et al., 2010; Lobos et al., 2015), reducing leaf or fruit surface temperature (Shellie and Glenn, 2008), and increasing water use efficiency (Glenn et al., 2010; Brillante et al., 2016). Calcite particle film (CaPF), however, has received little attention. Processed CaPF fits the criteria proposed by Glenn and Puterka (2004) for a chemical useful for mitigating drought. Briefly, CaPF is chemically inert with a particle diameter of <&nbsp2&nbspµ, and it is formed in such a way so as to create a uniform film on the treated surface that does not interfere with stomata functionality, as well as to modify the radiative budget of the leaf, and to alter plants/insect/pathogen interaction; furthermore, it can be washed away from the fruit's surface (Glenn and Puterka, 2004; Alvarez et al., 2015; Hagagg et al., 2019). Hence CaPF could be a reliable tool in the face of drought stress.

The effect of CaPF on some gas exchange parameters (e.g.,&nbspphotosynthesis) has been previously tested in both well-watered and drought stressed conditions (Attia et al., 2014), as well as in apricot nutrition and fruit quality (Martinez et al., 2010). Moreover, CaPF applications has been reported to be beneficial to several annual or perennial crops, including grapevine, especially under drought conditions (technical data sheet for Megagreen®: https://dokumen.tips/documents/megagreen-study.html, accessed on 21/08/2020). However, to our knowledge, the thermoregulation effect of applying processed CaPF, along with its impact on yield components and grape quality, has not been adequately studied. Improving knowledge in this specific field would support the viticulture industry in mitigating climate change and preserving terroir reputation. Therefore, this study examined the effects of the application to grapevine of CaPF on vine water relations, leaf area, vine transpiration, yield, and berry composition in well-watered and drought stressed grapevines.

Materials and methods

Results

1. Weather condition

During the experiment, the maximum hourly air temperature ranged from ≈ 27.0&nbsp°C to 35.5&nbsp°C with an average value of approximately 32.8&nbsp°C (Figure 1A). The mean maximum value of VPD was approximately 2.6&nbspMPa, and the highest maximum VPD values (> 3.0&nbspkPa) were recorded in the second week of the experiment if DAT 11 is excluded (Figure 1B). During the experiment, the available radiation at noon was close to 900&nbspW&nbspm^-2 on each day, except for the first two days (Figure 1C). The maximum hourly value of reference evapotranspiration was about 0.9&nbspmm/h per day, except for the first two days (Figure&nbsp1D). Daily ET₀ was above 7&nbspmm/d from the third day on.

Figure 1. Diurnal course of some meteorological variables registered by the local weather station from 29th June (DAT 1) to 13th July (DAT 15) 2018.

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3. Leaf temperature and vine transpiration

On the first day of the trial, mean leaf temperature was nearly 26.4&nbsp± 0.1&nbsp°C and 27.0&nbsp± 0.2&nbsp°C in WW and D vines, respectively, and their Tleaf-Tair difference was comparable (Figure&nbsp2A). On the same day, leaves sprayed with CaPF showed a leaf temperature of about 26.4 ± 0.1&nbsp°C while it was about 27.2 ± 0.1&nbsp°C in leaf of unsprayed vines (Figure&nbsp2B). This made the Tleaf-Tair difference of the CaPF significantly lower than that of NS ones (Figure&nbsp2B).

On DAT&nbsp9 and DAT 15, mean air temperature was higher than that recorded on DAT&nbsp1, reaching 32.0 and 33.8&nbsp°C respectively (Figure&nbsp2). In these conditions, the effect of the irrigation factor was statistically significant, having a leaf cooling effect on WW vines; that is, the leaf temperature of WW vines was about 0.7&nbsp°C and 2&nbsp°C below the air temperature on DAT&nbsp9 and DAT&nbsp15 respectively. Meanwhile, the leaf temperature of D vines was 1.5&nbsp°C (DAT&nbsp9) and 0.8&nbsp°C (DAT&nbsp15) higher than air temperature (Figure&nbsp2A).

On DAT&nbsp9, the application of CaPF significantly lowered the Tleaf-Tair difference compared to that of NS vines across all vines independently of their water status (Figure&nbsp2B). On DAT&nbsp15, CaPF induced an overall cooling effect on leaves, which was significantly higher than that of unsprayed vines (Figure&nbsp2B).

The analysis of the CaPF&nbsp× W interaction revealed that a significant cooling effect of CaPF was detected in vines exposed to drought on DAT 1 and 9 (Figure&nbsp2C). For WW vines on DAT 15, the application of CaPF induced a significantly lower Tleaf-Tair difference, which approached 3&nbsp°C (WW+CaPF) and 1.2&nbsp°C (WW-NS) (Figure&nbsp2C).

Figure 2. Average difference between leaf and mean air temperature (Tleaf-Tair) measured around midday in potted Merlot/SO4 vines. (A) Factor 1 “water” in well-watered (WW) and drought-stressed (D) vines; (B) Factor 2 “Thermoregulation” of the calcite-silicon mediated particle film (CaPF) and unsprayed (NS) vines; (C) W x Thermoreg. interactions. DAT = day after treatment.

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Comparing treatments within the same DAT, factor and interaction, the different letters indicate statistically significant differences (p-value&nbsp<&nbsp0.05) (Holm-Sidak multiple comparisons test). Letters were omitted when means were not statistically different. In&nbsp(A) and (B) n&nbsp=&nbsp72, in (C) n&nbsp=&nbsp36.

Cumulated vine transpiration was similar among all treatments during the first week of the study. Interestingly, differences between WW and D vines started 8&nbspdays after the beginning of the experiment and became statistically significant from DAT&nbsp9 till the end of the experiment (Figure&nbsp3). By contrast, the application of CaPF on D&nbspvines did not have any statistically significant effect compared to D-NS (Holm-Sidak multiple comparison test, p&nbsp=&nbsp0.05).

A total of 2,708 molH₂O&nbspm^-2 was transpired in 15 days from WW+CaPF&nbspvines, which was 11&nbsp% and 45&nbsp% higher than that transpired from WW-NS and D+CaPF, respectively.

Figure 3. The effects of processed calcite-silicon mediated film and water treatments on cumulated transpired water in potted Merlot/SO4 vines. DAT = day after treatment.

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WW stands for well-watered vines, D for drought stressed vines, +CaPF indicates that vines were sprayed with the thermoregulation product, NS indicates that vines were not sprayed. Statistical analysis refers to the interaction of factors. When comparing treatments within each DAT, different letters indicate statistically significant differences (n&nbsp=&nbsp4, p-value&nbsp< 0.05, Holm-Sidak multiple comparisons test) .

4. Leaf area

At the beginning of the experiment, leaf area per vine was similar in all treatments at about 1&nbspm² (Figure&nbsp3A,&nbsp3B,&nbsp3C). Thereafter, leaf area decreased slightly. However, leaf area reduction was less pronounced in vines receiving CaPF independently of their water status (Figure&nbsp3A); that is, from veraison to harvest, CaPF sprayed vines lost about 19&nbsp% of leaf area, while non-sprayed vines lost about 29&nbsp% of the initial leaf area.

No significantly different leaf area reduction was detected between well-watered vines and drought stressed vines, even if WW vines showed about 0.1&nbspm² more leaf area per vine than D&nbspvines did (Figure&nbsp3B).

The interaction of the W&nbsp×&nbspT&nbspfactors shows that WW+CaPF vines had the largest leaf area at harvest with 0.94 m² per vine, which was not statistically different from that estimated for the same treatment at veraison (Figure&nbsp3C). In particular, the leaf area of WW+CaPF vines at harvest was 16&nbsp% larger than that of D+CaPF and WW-NS, and 24&nbsp% larger than that of D-NS vines (Figure&nbsp3C).

Figure 4. Leaf area per vine in foliar sprayed processed calcite-silicon mediated film and water treatments in potted Merlot/SO4 vines. (A) main water effects in well-watered (WW) and drought-stressed (D) conditions; (B) main thermoregulation effect in CaPF sprayed and non-sprayed (NS) vines; (C) W x Thermoregulation interactions. DAT = day after treatment.

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CaPF stands for foliar processed calcite-silicon mediated sprayed; NS stands for non-CaPF foliar sprayed vines; WW stands for well-watered vines and D for drought stressed vines. Different capital letters indicate differences between veraison and harvest. Lower case letters indicate differences among treatments within the same phenological stage. Differences were calculated with Holm-Sidak multiple comparisons test at p-value < 0.05 or lower. Each bar represents the average of 24 (A, B) and 12 (C) values.

Discussion

This study mainly examined the leaf thermoregulation effect of CaPF on leaf temperature and leaf area from veraison to harvest, when the yield and berry quality were greatly influenced by environmental conditions (Castellarin et al., 2007; Keller, 2010; Ou et al., 2010). In Southern Italy, as in most Mediterranean-type climates, this period is often the warmest and driest of the year, contributing to an increase in leaf temperature, which rises above that of air in several crops, including grape, even under full irrigation (Sharma et al., 2015).

The application of particle-film-based compounds has been suggested to mitigate the impacts of heat stress on leaf and fruit, because it is able to modify tissue radiative properties by increasing the reflectance of solar radiation, changing the leaf/fruit radiant energy exchange, and in turn influencing leaf and fruit temperature (Glenn and Puterka, 2004; Arkebauer, 2005).

Among these compounds, both CaPF and kaolin are able to increase the reflectance properties of leaves, even though their effect on leaf temperature reduction is still under debate (Glenn et al., 2003; Glenn et al., 2010; Shellie and King, 2013; Attia et al., 2014; Brillante et al., 2016; Tosin et al., 2019). In the present experiment, all sampling data for leaves sprayed with CaPF have shown, independently of the water status, a significantly lower leaf temperature than that of non-sprayed ones (Figures&nbsp2B,&nbsp2C); this was conceivably due to the reduction of the absorbed radiation. Although the leaf-to-air temperature difference in CaPF vines was 0.3&nbsp(D) and 0.8&nbsp°C (WW) lower than that of NS (Figure&nbsp2B), a potential improvement in leaf photosynthesis-related metabolisms (e.g., light and dark photosynthetic reactions) might have occurred (Medrano et al., 2002; Medrano et al., 2003). However, more research effort is required in order to disentangle the thermoregulation effect of CaPF on leaf radiative characteristics and leaf energy budget (Arkebauer, 2005).

As expected, the level of supplied irrigation water resulted in significant differences in soil moisture and vines water status. The WW vines maintained stable soil moisture (≈ 42&nbspm³&nbspm^-3) and midday stem water potential of about -0.46&nbspMPa throughout the experiment (Table&nbsp2), while those in which only 25&nbsp% of the transpired water was returned, showed a decrease in Ψ to a very low value (-2.14&nbspMPa) on DAT&nbsp15 (Table&nbsp2). The values of Ψ measured in this trial were categorised according to van Leeuwen et al. (2010) as: no water stress (> -0.6&nbspMPa), moderate to severe water stress (from&nbsp-1.1 to&nbsp-1.4&nbspMPa) and very severe water stress (<&nbsp-1.4&nbspMPa).

In this study, soil moisture and midday stem water potential were only impacted by Factor&nbsp1 (water) and not by Factor&nbsp2 (thermoregulation). These results are in line with previous experiments testing CaPF application on grapevines under glasshouse conditions (Attia et al., 2014). In most other studies carried out on kaolin particle film for biotic or abiotic stress mitigation in different crops, the effects of particle film treatments on grapevine plant water status was negligible (Shellie and Glenn, 2008; Glenn et al., 2010; Ou et al., 2010; Lobos et al., 2015; Brillante et al., 2016).

It has been shown that in well-watered conditions leaf temperature is well-correlated with many other metabolic processes of the plant, such as photosynthetic rate, stomatal conductance, (Blonder and Michaletz, 2018). This may explain the high amount of transpired water in well-irrigated CaPF sprayed vines (Figure&nbsp3); that is, under well-watered conditions the low leaf temperature of CaPF vines likely increased stomatal opening and in turn water consumption (Figure&nbsp3) (Brillante et al., 2016). By contrast, under water stress conditions, CaPF likely contributed to reduced water consumption when compared with D not-sprayed, although differences were not significant (Figure&nbsp3) (Blonder and Michaletz, 2018).

During the summer, vines can suffer from anticipated defoliation, even under optimal soil moisture, thus reducing their overall photosynthetic capacity (Chaves et al., 2010; Hochberg et al., 2017). In the present study, leaf area decreased in WW vines from veraison to harvest at a similar rate to that reported by Munitz et al. (2016) and Charrier et al. (2018). Interestingly, the application of CaPF to WW vines contributed to retaining a significantly larger leaf area (25&nbsp%) compared to non-sprayed WW vines; this is a potential advantage in terms of overall vine functioning (e.g.,&nbspcarbon gain). The beneficial effect of CaPF on leaf retention was also observed in D vines (Figures&nbsp4B,&nbsp4C), even though it was not statistically significant. Leaf area retention induced by CaPF significantly increased the leaf area/yield ratio compared to that of not-sprayed in both WW and D vines (Table&nbsp3). This beneficial effect might be due to increased ROS, as observed in tobacco (Tran et al., 2020), which, in turn, might have enhanced the vine acclimation response to high air temperature and radiation (Carvalho et al., 2015; Brito et al., 2019).

According to correlative information reported by Kliewer and Dokoozlian (2005), the larger leaf area might then have caused the high concentration of dissolved solids recorded for both WW and D when sprayed with CaPF (Table&nbsp4).

When not sprayed, the WW vines lose leaf area in a similar way to D&nbspvines (Figures&nbsp4A,&nbsp4C). In drought stressed vines, no significant effect of CaPF application on leaf fall was detected, probably because, under severe drought stress, the programmed leaf death which is also triggered by the impairment of the conductive xylem (embolism) (Hochberg et al., 2017; Charrier et al. 2018), can dominate over any benefits of CaPF. Non-irrigated vines (D+CaPF and D-NS) experienced very severe water stress (Ψ <&nbsp-1.4&nbspMPa) from DAT&nbsp9 to DAT&nbsp15, which reduced vine water consumption by about 30&nbsp% compared to WW vines. It has been reported (Charrier et al., 2018) that when under severe water stress, loss of turgor and xylem cavitation may produce embolism in xylem vessels and then a reduction in conductivity that could lead to leaf shedding. In this experiment, after two weeks of reduced irrigation volume, D&nbspvines lost around 29&nbsp% of their leaves (Figure&nbsp4); therefore, in order to avoid more severe defoliation on DAT&nbsp15, full irrigation in the afternoon was restored and maintained till harvest.

It is well known that under high temperature and solar radiation an excess of reactive oxygen species (ROS) are produced in different cellular components, which may induce oxidative stress (Carvalho et al., 2015). Bernardo et al. (2017) have shown a reduction in berry and leaf ROS in kaolin-treated vines compared to untreated ones. Leaf or stem water potential is often employed to identify differences in drought tolerance capability among grapevine cultivars, or to assess the vine response to agronomical practice(s) intended for their drought adaptation (e.g., summer pruning, soil management, etc.) (Schultz, 2003; Charrier et al., 2018).

As expected from many other experiments (see Gambetta et al. 2020, for review), the non-irrigated vines showed a significantly lower cluster (14&nbsp%) and berry weight (7&nbsp%) and higher pH (3&nbsp%) and skin/berry ratio (8&nbsp%) (data not shown) than for WW vines. Differences in number of berries per cluster do not seem to be related to the effects of treatments, because their number was already fixed at the beginning of the trials. These differences may also have an effect on the significant differences in the cluster weight between irrigated and non-irrigated vines. At the rate used, some CaPF residues (white spots) were visually appraised on about 30&nbsp% of berry surfaces, these spots were not present on the unsprayed grapes. As these residues are likely calcium carbonate, they could reduce the acidity of the must during fermentation, potentially influencing aging, as well as some sensorial traits of wine. However, more research is required to test such effects of CaPF. Moreover, the missing statistical differences in total polyphenols may be related to the time and duration of water stress, as highlighted by Gambetta et al. (2020) and Mirás-Avalos and Intrigliolo (2017).

Our results show that foliar application of processed calcite silicon-mediated particle film at veraison has a cooling effect on leaves of potted Vitis vinifera cv. Merlot, both under well-watered and two-week drought stressed conditions. In addition, the CaPF (i) induced high leaf retention during the veraison-harvest interval time, which was beneficial for the °Brix&nbsplevel, and (ii) allowed water to be saved, which collectively improved vine resilience. Hence, the CaPF application could favourably be considered as an adaptation strategy for dealing with adverse environmental summer conditions in Mediterranean-type grapevine producing regions.

Acknowledgements

Davide Amato was supported by a PhD fellowship of the PhD Programme, “Cities and Landscapes: Architecture, Archeology, Cultural Heritage, History and Resources”, at Università degli Studi della Basilicata. This manuscript was partially funded by the 2014–2020 Rural Development Programme for Basilicata Region (Misura&nbsp16.1, PROSIT, CUP H86G18000080002). Authors acknowledge D.&nbspMelfi (ALSIA-CRM Agrobios) for assistance in berry composition determinations.

Agronutrition is acknowledged for providing the CaPF (Turn-on^®).

References

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