1. Introduction
Blueberry consumption has increased globally over the last years, mainly due to its high antioxidant potential because of its richness in phenolic compounds [
1]. North America is a traditional market in which more than half of the fresh blueberries produced is consumed and their demand has considerably increased in new markets, such as Europe and China [
2]. Based on this, traditional and new growth areas across the globe have increased blueberry production [
3]. The highbush blueberry (
Vaccinium corymbosum L.) presents a high fruit quality for fresh markets and a wide adaptation to subtropical and tropical production areas. This blueberry species is the most widely planted blueberry worldwide and is grown in different countries such as the United States, Canada, Perú, Mexico, Chile, Argentina, South Africa, Australia, New Zealand, Japan, China, and several European countries [
4,
5,
6].
Most of the blueberry producing areas are characterized by significantly higher temperatures and solar radiation than the natural habitats in which blueberries grow [
7]. Today, in the Mediterranean zones, there is a need to produce berries earlier in the season, which has forced growers to produce blueberries under high temperatures and solar radiation [
8]. Blueberries are commercially harvested weekly for three to five or more weeks [
9], which coincides with the period of greatest environmental stress in the season. It is important to highlight that erythemal UV irradiance in the southern hemisphere is up to 40% higher than in the northern hemisphere in summer [
10]. Moreover, the expansion of highbush blueberry cultivation into warmer regions will be challenged in the future by global warming [
5], making it necessary to adopt short-term adaption strategies to face these unfavourable productive conditions. Recently, the use of photo-selective color shading nets has been studied in blueberry production, allowing to mitigate the environmental stress conditions in open production orchards [
7,
8]. Shading nets have been shown to affect fruit yield and quality of blueberries, changing the harvest date towards cooler conditions [
7,
8,
11,
12,
13].
Antitranspirants and particle-film sprays have been used in several horticultural crops to mitigate environmental stress conditions, reducing sunburn, and improve fruit color [
14,
15,
16]. However, to our knowledge, there are few studies about this subject in blueberries and much fewer regarding the application of dicarboxylic acids. Dicarboxylic acids and their derivatives are known to possess therapeutic properties and have been used in the treatment of a variety of skin disorders [
17]. The application of these compounds in agriculture has allowed for an increases in their tolerance to biotic and abiotic stress [
18,
19]. Mono and disubstituted esters of dicarboxylic acids delayed plant growth and chlorophyll degradation [
18]. Similar results have been found by Todorov et al. [
19], who showed that monoesters of carboxylic acids retard plant growth and delay senescence processes by inhibiting chlorophyllase and peroxidase activities. In addition, these authors showed that dicarboxylic acids prolonged carbon assimilative activity and enhanced leaf water-use efficiency in treated plants. In addition, dicarboxylic acid esters and salts have been used in plant disease prevention, retarding the infection of tomato leaves against different phytopathogens [
20]. Therefore, the aim of this study was to evaluate the foliar application of a sunscreen elaborated by dicarboxylic acid salts on plant water status, gas exchange, productivity, and berry soluble solids in blueberries (
V.
corymbosum L. ‘Duke’, ‘Star’) plants, growing under Mediterranean conditions.
4. Discussion
As expected, the decrease in irrigation frequency in water stress treatments decreased the water applied by 50% compared to the control. In addition, this allowed for a reduction in stem-water potential (Ψs) on the days closest to harvest in both varieties and decreased stomatal conductance (g
s) in blueberry plants (
Table 3,
Table 4 and
Table 5). After the irrigation treatments had a statistically significant influence, Duke plants without irrigation restriction presented Ψs values from −0.6 to −0.7 MPa and the stressed plants reached Ψs from −0.8 to −1.0 MPa. Under these same conditions, Ψs ranged from −0.6 to −0,9 in non-stressed plants and from −0.7 to −1.1 MPa in stressed plants in the Star cultivar. Different authors have reported similar results of Ψs after applying regulated deficit irrigation. Lobos et al. [
25] reported −0.70 and −1.20 MPa in Brigitta plants for fully irrigated plants and severe water deficits, respectively. Keen and Slavich [
26] described a range from −0.60 to −1.50 MPa in Star plants. Estrada et al. [
27] measured −0.64 to −1.29 MPa as a mean for Bluegold, Elliott and Liberty cultivars. In this regard, some authors suggested that threshold values of Ψs < 1.0 MPa can be used to avoid water stress, significant reductions in yield and berry quality in blueberries plants [
25,
27,
28].
Looking at the results, sunscreen based on dicarboxylic acid salts applications to blueberries (
Vaccinium corymbosum L.) plants scarcely affected the Ψs in Duke and Star, but considerably maintained g
s in Star plants managed under water stress conditions at the same level as irrigated plants (
Table 5). Thus, concerning Ψs and g
s, there was no difference in the response of both cultivars. Severe water stress results in a low photosynthesis rate because stomatal conductance limits CO
2 diffusion from the atmosphere to the leaves [
25,
29]. Regulated deficit irrigation treatments applied to Brigitta plants (50% ETc) altered stomatal conductance and transpiration, reducing photosynthetic rate, stomatal conductance, and transpiration by 20 to 80% compared to control (100% ETc) [
25]. Midday stomatal conductance in different highbush blueberry cultivars decreased as the leaf water potential reached −0.6 to −0.8 MPa [
30,
31]. Net photosynthesis and stomatal conductance reduced by 30% due to water stress resulting from no irrigation for three weeks at the onset of shoot growth compared to daily irrigation in Bluecrop blueberries [
32]. In this fashion, sunscreen based on dicarboxylic acid salts applied to Star stressed plants allows for a maintenance of stomatal conductance levels such as in the fully irrigated plants.
Thermal and water stresses are increasingly frequent in the Mediterranean production area during summer, limiting productivity and berry quality [
8,
33,
34]. The exogenous foliar application of radiation-reflecting products has proven effective in mitigating the negative impacts of these abiotic stresses in grapevine and other fruit crops [
15,
34,
35,
36,
37]. However, to our knowledge, there is little available information about sunscreen foliar applications to mitigate thermal and water stress conditions in blueberries. Some authors have proposed that radiation-reflecting products could be used as additional tools to save water in several species, such as olives, clementine, tomatoes, and grapefruits [
37,
38,
39,
40,
41]. However, there is an ongoing debate on its effects on gas exchanges, and mechanisms of action are not yet completely understood [
35]. Some authors reported no effect or even an increase in net assimilation and stomatal conductance, while other researchers observed a reduction [
42,
43]. Based on this, the effectiveness of radiation-reflecting products in improving leaf resilience when subjected to severe environmental conditions, mainly through gas-exchange assessment, can be significantly changed according to the variety, water status, and stress severity [
37]. The effects of these products suggest the modulation of indole-3-acetic acid (IAA), salicylic acid (SA), and abscisic acid (ABA) levels in crops cultivated in Mediterranean-type climates [
37,
44,
45]. ABA is a growth regulator that controls stomatal closure, transpiration, and the plant’s response to water stress and has been foliar-applied on apples with inconclusive results [
46,
47]. In this fashion, Frioni et al. [
48] showed that leaves covered with kaolin (a radiation-reflecting inert product) had higher violaxanthin (Vx) + antheraxanthin (Ax) + zeaxanthin (Zx) pool and a significantly lower neoxanthin (Nx) content (VAZ) when water deficit became severe. Thus, kaolin allowed for the prevention of the ABA biosynthesis by avoiding the deviation of the VAZ epoxidation/de-epoxidation cycle into the ABA precursor biosynthetic direction. These authors suggested that the preservation of the active VAZ cycle and transpiration led to an improve in the dissipation of exceeding electrons, which explained the higher resilience of canopy functionality expressed by the application of kaolin.
A sunscreen based on dicarboxylic acid salts applications to Duke plants with managed under-water stress conditions produced berries with higher total soluble solids than those from non-stressed plants (
Table 8). No different response of the two cultivars to treatments for yield and fruit weight was observed, and total soluble solids were slightly higher in plants subjected to water stress and sunscreen applications, probably because the yield was a little lower for this treatment than in the rest of the applications. To our knowledge, there is a scarce availability of information about the effects of dicarboxylic acid salts in horticulture. In general, dicarboxylic acids applications to crops allows for the hindering of plant growth and delays senescence processes by inhibiting chlorophyllase and peroxidase activities [
19]. However, these results did not coincide those found in this manuscript. Currently, the study of the applications of radiation-reflecting products, mainly of kaolin, has increased, especially in viticulture and vegetable production [
37,
42]. Kaolin foliar applications to grapevines decreased berry sugar content without affecting malic and tartaric acid levels and reactive oxygen species accumulation throughout berry ripening [
37]. Most of the authors agreed that kaolin exerts a delaying effect in triggering ripening-related processes under severe summer stress conditions [
33,
37,
49]. However, the application in tomato may or may not improve the accumulation of total soluble solids under salinity and water stress conditions, improving water use efficiency [
42,
50]. Therefore, under water stress conditions, sunscreen based on dicarboxylic acid salts applications to Duke increased berry total soluble solids in comparison to its application to non-stressed plants. Moreover, this treatment, when applied to Star blueberries, maintained stomatal conductance at the same level as irrigated plants.
Diurnal changes in gs, Ψs and transpiration have been reported to be closely related in blueberries [
31]. Regardless of cultivar, gs decreases quickly as Ψs reaches close to −0.6 MPa [
51]. Byers et al. [
52] showed that the root system of highbush blueberries is inefficient in water uptake, even if soils water soil levels are adequate. There is a scarce availability of information in the scientific literature regarding the response to drought or heat stress by blueberries plants. ‘Duke’ maintained lower Ψs and higher gs than ‘Elliott’ and ‘Bluecrop’ as soil water was depleted [
51]. This may indicate that this cultivar has the highest tolerance for short-term soil water deficits [
51]. In this fashion, Bryla and Strik [
31] suggested that ‘Duke’ might require a lower irrigation frequency than the other cultivars since it develops a deep-root system and extracts more water at depths below 0.6 m. No further evidence was found in the literature about the response of Star to drought or heat stress.
Based on these preliminary results, it is necessary to perform long-term studies accounting for more factors and variables, such as CO2 assimilation and water use efficiency, among others, to establish accurate conclusions related to the application of dicarboxylic acid salts to blueberries.