1. Introduction
The white winegrape variety Glera (
Vitis vinifera L.) is mostly grown in Northeastern Italy, under different pedo-climatic conditions. It was historically known under the name “Prosecco” until 2011 when the establishment of the Prosecco PDO (Protected Denomination by Origin) that disallowed utilizing it for the grape variety. The first records trace back to the 18th century, when historical documents [
1] located the Prosecco variety in “Colli Berici” growing area (Vicenza province, Veneto Region), and to the 19th century on Conegliano hills as described in the Ampelography of Treviso province in 1870. The demand for this grape variety has grown over the years in particular for the production of sparkling wines based on the Martinotti method that was established by the pioneer Antonio Carpenè [
2]. Nowadays, Prosecco bottles amount to about one billion/year worldwide. As a result of the flourishing Prosecco market, the area planted under Glera has increased considerably in recent years exceeding the 40,000 hectares cultivated in the Prosecco production area (Veneto and Friuli Venezia Giulia regions). In the Veneto region, it is now the most widely planted variety and accounts for over a third of the total area under vines.
Glera is known for being a particularly productive variety and for exhibiting quite high vigor. The phenology spreads along extended growing seasons, featuring a fairly early vegetative development and late harvest dates with the production of grapes at middle sugar and acidity level and considered semi-aromatic [
3].
Terroir is a typical concept of the wine world that was defined by OIV (Resolution 333/2010) as follows: “Vitivinicultural terroir is a concept which refers to an area in which collective knowledge of the interactions between the identifiable physical and biological environment and applied vitivinicultural practices develops, providing distinctive characteristics for the products originating from this area. Terroir includes specific soil, topography, climate, characteristics and biodiversity features”. The effects of the terroir are mainly linked to the interaction between the environmental conditions and the cultivation practices that play a clear role in influencing the physiology of the vine therefore the quality of the grapes (i.e., the aromatic characteristics; [
4,
5]) in relation to the varietal behavior and response in term of plasticity. It is well known that grapevine varieties modify their performance in distinct environments, with some varieties (such as Cabernet Sauvignon and Chardonnay) offering more consistency and others (such as Sangiovese, Nebbiolo and Pinot Noir) showing greater variation [
6].
The influence of the terroir factors on Glera grapevines cultivated in the Conegliano Valdobbiadene Denomination area was studied focusing on the effects on the plant physiology and the viticultural and enological parameters [
7,
8,
9,
10], and the role of soil was also dissected [
11]. Molecular approaches have been also applied to characterize the relationship between the geographic origin and the genetic variability of Glera clones [
12]. In recent years, omics approaches have been used to unravel the phenotypic plasticity of grapevine on a broad scale and to dissect the interactions between genotype and environment (i.e., vintage, location) during the course of berry development [
6,
13,
14,
15,
16]. The berry ripening transcriptomic program of Glera was included in the study of five red and five white Italian grapevine varieties, selected from different winegrowing regions to represent diverse agronomic traits and different environmental adaptations [
17]. Transcriptional changes during berry ripening were compared when the 10 varieties were grown under the same environmental conditions creating the opportunity to distinguish the core transcriptomic traits from those dependent on the berry skin color and genotype-specific features. In this study, the investigation context has switched and the transcriptomic changes of the Glera ripening berry were characterized to include the effect of the cultivation site. A similar characterization of the phenotypic plasticity of a white variety was previously performed for cv Garganega, where changes in the berry transcriptome were ascribed to different pedoclimatic conditions at four cultivation sites [
15].
The goal of this survey is to assess the link between viticultural and enological aspects of Glera, enriched with the berry transcriptomic information, and two different terroirs in the Prosecco wine production area.
3. Discussion
The ripening phase is a crucial period that influences the composition of the grapes and therefore of the wine, and during which physical (weight, volume, color, and softness) and chemical (pH, acidity, sugars, phenolic and aromatic composition) changes take place. The environmental and climatic conditions, as well as the agronomic approach, influence the changes that occur within the same grape variety.
The two sites studied in this work differed mainly in altitude, exposure (south and west-facing at CSM and Rauscedo, respectively), water availability (absent at CSM), soil texture, effective soil depth (greater at Rauscedo) and solar radiation (greater at Rauscedo). The variation of these key factors determined the expression of the two terroirs.
Regarding the technological aspects, the grape soluble solids content of both vineyards was in line with the requirements by the respective production regulations to achieve a minimum alcohol level between 9.0 and 9.5% by volume depending on the wine style. Total acidity also exceeded the minimum requirement of 4.5 g/L set by the production regulations.
The estimated crop load varied between the monitored vineyards likely due to the influence of irrigation on the Rauscedo vines that presented greater vigor and higher Ravaz values, especially in 2012. The Ravaz index at CSM in 2012 was slightly lower than the typical value for the area (~6.5) although showing expression of vegetative-productive balance. In both vintages, also the yield was significantly impacted by water availability as lower water potential values were measured at the hilly site (CSM) compared to the plain site of Rauscedo, also remarking the influence of site orography, soil stratigraphy and depth. In 2011, the number of bunches per plant was significantly lower in CSM likely contributing to the drop in production compared to Rauscedo. Overall, the production and quality values determined in this trial at CSM were in line with those highlighted in a previous zoning study in the same area [
18].
Water availability dissimilarities between cultivation sites seemed to have influenced the monoterpenes content, in step with previous studies reporting increased concentrations of monoterpenes such as linalool and geraniol under mild to moderate water stress conditions [
19,
20,
21,
22] likewise at the CSM vineyard. This response was seen associated with increased expression of terpenoid synthase genes [
19,
23].
Although the average temperatures at the two locations were similar throughout the season, Rauscedo recorded consistently higher temperatures than CSM in the five days prior to harvest: the mean and maximum temperature values were respectively augmented 3.5 and 7 °C. This situation could have led to a reduced synthesis of terpene compounds at Rauscedo. In fact, we hypothesize that high temperatures during ripening can reduce the synthesis and accumulation of aromatic compounds, especially the more thermolabile ones: cooler vintages and areas promote a more gradual increase of total terpenes during ripening compared to warm areas, which favors the achievement of greater content at maturity [
24,
25].
At Rauscedo, ambient light availability was always higher than at CSM (+200,000 MJ/m
2) throughout the growing season, including the last days before harvest. This difference may have negatively impacted the final concentration of total terpenes and linalool at Rauscedo, in line with the reports of terpenoids sensitivity to sunlight in Muscat varieties [
26,
27,
28], Sauvignon Blanc and Riesling [
29]. Moreover, Rauscedo vines exhibited narrower leaf walls expositing the bunches to extra sunlight, especially during the period between trimming and canopy recovery (first ten days of June and August), and this condition may also have contributed to a reduced terpenol synthesis and/or an increase in linalool degradation with respect to CSM. On the contrary, the synthesis of kaempferol is regulated by a light-induced transcription factor [
30], and greater levels of this compound were indeed reached in the Rauscedo grapes, which benefited of higher light intensities in the pre-harvest period compared to CSM.
Vintage 2012 showed more pronounced differences in the aroma profiles, in particular the lemon olfactory note that was mentioned for Rauscedo grapes. This could be partially explained by the citrusy lemon note of β-citronellol, whose concentration in the berries was more than double in Rauscedo compared to CSM (14 µg/kg versus 6 µg/kg), although the total terpene content was lower in the grapes grown at Rauscedo than at the CSM hillside site.
To understand the molecular basis of the environmental impact on ripening Glera berries, we explored the transcriptomic changes over vintage 2012 of two vineyards selected to maximize environmental differences (site altitude in particular) and minimize variations in the agricultural practices, such as the training system, row orientation, planting layout, vineyard age, and rootstock. The investigation of transcriptomic data by PCA revealed that the berry ripening program dynamics differed by growing site. The sample distribution explaining the greatest variance evidenced that PC1 described shared molecular changes associated with berry development, as reported in previous transcriptome surveys [
13,
31,
32,
33,
34], whereas PC2 highlighted unique behaviors accounting for the highly plastic responses of Glera berries to the pedoclimatic conditions at the two growing sites. We observed that samples collected from the two vineyards at the MR phase were clearly separated, while mature samples, albeit still distinguishable, were more like each other. Indeed, we found more differentially expressed genes between vineyards at MR than at R, that however confirmed that location influence on the overall grape berry development transcriptomic program as previously reported [
13,
15,
35].
In our investigation we explore genes specifically modulated for each vineyard at both ripening stages. CSM vineyard featured the least number of modulated genes during berry maturation but site-specific, especially at MR. Among these, we found the
Responsive to Dehydration 22 (
RD22, Vitvi04g01872; [
36]) and a heat shock protein (Vitvi10g01990) encoding genes that might be induced in response to the slightly higher temperature registered at CSM compared to Rauscedo at the MR stage.
The 2012 temperature conditions switched around the R stage, when the almost ripe grapes were subjected to significantly higher temperatures at Rauscedo compared to CSM and we speculate that this site-specific environmental variation is connected with the recorded up regulation of many heat shock protein encoding genes.
Besides triggering transcriptional changes related to cellular homeostatic responses, temperature was shown to impact the modulation of genes involved in the biosynthesis of volatile aromas [
37] and, when significantly high, to distort the aromatic pattern by depleting terpenes in favor of benzenoids in post-harvest withering Corvina berries [
38]. Consistently, we here recorded a significantly higher content of benzenoids in Rauscedo vineyard in comparison to that found in CSM at the ripening stage.
In line with metabolomic data showing a higher amount of terpenes in CSM than Rauscedo grapes at the R stage, we detected the up regulation of the gene
1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate synthase (
HDS; Vitvi06g00286), involved in the monoterpenes biosynthetic pathways and recently shown to be upregulated in volatile-reach grapevine varieties [
39]. In line with this, Rauscedo grapes at MR showed positive modulation of genes that were actually found the least expressed in varieties featuring aromatic traits [
39] suggesting the reduced aroma-related transcriptomic layout of Rauscedo grapes, like the
Violaxanthin de-epoxidase (
VDE; Vitvi04g01082) and
Zeaxanthin epoxidase (
ZEP; Vitvi07g01745) encoding genes.
VDE and
ZEP are both involved in the violaxanthin cycle and were shown to increase in expression throughout exocarp development in the white cv Alvarinho [
40], proposing the role of violaxanthin cycle in protecting the photosynthetic apparatus from damage during berry development and ripening. Moreover, the expression of
VDE was shown to increase in light-exposed Sauvignon Blanc berries, compared to shaded ones [
35]. Likewise, Rauscedo grapes responded to the higher radiation detected in this site compared to CSM during the 2012 season by involving the violaxanthin cycle—that we recorded as an increased expression of
VDE and
ZEP in Rauscedo grapes—to likely preserve the bunches from excessive light exposure.
Overall, this survey explored the metabolomic and transcriptomic basis of the plasticity of the cv ‘Glera’. We highlighted different berry ripening dynamics by vineyard site for technological parameters, abundance and composition of phenolic compounds, and grape aroma profile. The genome-wide gene expression analysis of the berries revealed remarkable differences in the ripening transcriptomic program, revealing the differential response of the vines to the pedoclimatic conditions within the typical area of Prosecco wine production.
4. Materials and Methods
4.1. Sites and Climate Description
Grapevine plants of Vitis vinifera cv Glera were grown in Northeastern Italy at two different vineyard sites: the plain territory of the PDO Prosecco in Rauscedo (Pordenone province; 46°02′78″ N; 12°80′87″ E), and the hilly area of the Conegliano Valdobbiadene-Prosecco DOCG district, located in Col San Martino (CSM; Treviso province; 45°89′78″ N; 12°06′22″ E).
The vines in the selected vineyards were 15 years old, grafted onto the Kober 5BB rootstock (Berlandieri × Riparia), and Sylvoz trained with three canes per vine with 10–12 buds each. The permanent cordon was positioned 1.3 m from the ground, and the planting distances were 3.0 m between rows and 1.2 m along the row, with a density of 2777 vines per hectare.
Vineyard management involved native cover crops with mowing during the spring-summer period and two topping operations between the end of flowering and the beginning of veraison. The fertilizer supply provided for the application in post-sprouting and post-harvest of 80 kg/ha of N and K, and 40 kg/ha of P per year. The vineyard in Rauscedo was irrigated with drip irrigation, while in CSM it was rainfed. The trial considers data recorded in 2011 and 2012.
4.2. Soil Analysis and Meteorological Data
Soil samples were taken at the end of the winter (before spring fertilization in 2011) at two different depths (0–30 and 30–60 cm). Five 1-kg subsamples were taken from each vineyard, mixed, and analyzed according to the official methods for soil chemical analysis [
41].
The meteorological data were provided by the Veneto region meteorological services (ARPAV) for the CSM site and by the Friuli Venezia Giulia regional meteorological observatory (ARPA FVG, Pordenone, Italy) for the Rauscedo site.
Huglin’s heliothermal index was calculated using the formula:
where: Tmean = Average daily temperature
Tmax = Maximum daily temperature
K = coefficient of latitude (1.04)
GDD = average daily temperature −10 °C
4.3. Productive Parameters
For both vintages, yield, number of bunches per vine, and average bunch weight were determined at harvest on 15 representative plants. The harvest dates were as follows: 1 September 2011, at Rauscedo—89 days after flowering (DAF); 6 September 2011, at CSM—88 DAF; 11 September 2012, at Rauscedo—102 DAF; 18 September 2012, at CSM—104 DAF. Pruning weights were recorded during the winter, allowing to calculate the Ravaz index (yield/pruning weight).
4.4. Stem Water Potential
Stem water potential was measured with the Scholander pressure chamber (model 600; PMS Instrument Company, Albany, OR, USA) between 18 June and 3 September in 2011 and between 25 June and 24 August in 2012. Measurements were taken between 13:00 and 14:30.
At each site and for each day of measurements, one healthy and undamaged leaf was chosen between the 4th and 6th node after the last cluster of a middle shoot of the fruiting branch of each of ten selected vines [
42].
Prior to the analysis, the leaf was placed in a black aluminum-coated bag for one hour [
43,
44] to promote stomatal closure, stop transpiration and theoretically equilibrate the lymph flow of the petiole with the plant and the water content of the soil. The value of the pressure at which the first drop of lymph flowed out was taken as the value of the water potential.
4.5. Grape Sample Collection
Average berry weights, soluble solids, pH, titratable acidity, malic, and tartaric acids were determined from a pool of 100 berries randomly picked from the harvested grape mass in 2011 and 2012. Three biological replicates were created per sample.
Grape berries for metabolomic and transcriptomic analysis were taken in 2012 at two different ripening stages: at Mid ripening stage (MR) when berries were translucent (BBCH 83), and at harvest (Ripening stage, R; BBCH 89). Aromatic analysis and the determination of flavonols and hydroxycinnamoyl tartaric acids (HCTA) were based on a collection of 80 and 20 berries respectively, whereas 50 were processed for transcriptomic analysis. Three biological replicates were created from both cultivation site and at each stage, collecting berries with pedicels from the top, central, and bottom portions of clusters selected at both row sides (i.e., sun-exposed and shaded). Samples were flash-frozen in liquid nitrogen and then stored at ultra-freezing temperatures (−84 °C) until processing.
4.6. Chemical Analysis
Soluble solids were determined on the must by refractometry at 20 °C with a digital refractometer (ATAGO PR-32) and expressed in Brix degrees, while titratable acidity (g/L) and pH were measured on 20 mL of must at 20 °C with a microtitrator (Crison micro TT 2022—Crison strumenti SPA, Carpi, Modena, Italy), equipped with a pH electrode (Hamilton FlushTrode P/N 238060/08), using a 1 N NaOH titrant solution (SodiumHydroxide ACS reagent Honeywell Fluka 30620).
Malic and tartaric acid content (g/L) was determined using the RP-HPLC method (Agilent 1220 Infinity LC; Agilent Technologies, Santa Clara, CA, USA) on must samples diluted 50 times and filtered following the protocol proposed by Kordi et al. [
45].
The glycosylated aromas (terpenes, norisoprenoids, and benzenoids) were determined according to the method reported by Vrhovsek et al. [
46]. In brief, solid phase extraction (SPE) was performed using ENV+ cartridges 1 g (Biotage, Uppsala, Sweden), the free aromatic compounds were eluted with 30 mL of dichloromethane and the bound aromatic compounds with 30 mL of methanol; the latter was then treated with a AR2000 pectolytic enzyme. GC analysis was performed using a Trace GC Ultra gas chromatograph coupled with a TSQ Quantum Tandem mass spectrometer (Creative Proteomics SUITE 115, Shirley, NY, USA). GC separation was performed on a 30 m VF-WAXms capillary column with an internal diameter of 0.25 mm and a film thickness of 0.25 m (Varian, Inc., Palo Alto, CA, USA).
Flavonols and hydroxycinnamoyl tartaric acids (HCTA) were determined by HPLC analysis on samples of 20 still-frozen berries, according to Di Stefano and Cravero [
47]. Analyses were performed on the supernatant obtained from the pulp and skin samples. Chromatographic separation of HCTA and flavonols from the skins was performed using a ThermoHypersil-Keystone ODS Hypersil RP C-18 column (Thermo Scientific, Waltham, MA, USA) [
48].
4.7. Micro-Vinification and Wine Sensory Analysis
At each site, 150 kg of grapes were collected from three groups of ten vines in different parts of the vineyard and taken to the cellar for micro-vinification. The berries were crushed and pressed using a membrane press operating at 1.2 bar. The must was then mixed with three mg L−1 of pectolytic enzyme and 100 mg L−1 of potassium metabisulphite. After a clarification period of 12 h, the juice and the lees were separated. Alcoholic fermentation took place at 18 °C for 18 to 20 days. At the end of February of the year following the harvest, the wine was filtered and clarified again before being bottled.
The sensory analysis of the wines was carried out each year in June by the same seven trained tasters. The tasting panel was formed by 7 people, all belonging to a sensory group with long experience in wine tasting, composed of oenologists, winemakers, and sommeliers. Seventeen attributes were considered: eleven aroma-related attributes (olfactory intensity, elegance, rose, lemon, apple, pear, banana, pineapple, wisteria/acacia flower, vegetable, fresh vegetable), three mouthfeel attributes (acidity, savouriness, balance) and three final perception characteristics (fruity, floral, pleasantness). The different attributes were quantified using a ten-point intensity scale [
49]. For each sample, the judges received a 30 mL sample served at 18 ± 1 °C in glasses covered to prevent loss of volatiles. The order of presentation was randomized between judges and sessions.
4.8. RNA Extraction and Microarray Analysis
Total RNA was isolated from 200 mg of ground berry pericarp using the Spectrum™ (Spectrum Chemical, New Brunswick, NJ, USA) Plant Total RNA kit with modification according to Fasoli et al. [
50]. The RNA quality was determined using a NanoDrop 2000 spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA) and the quantity was assessed using a Bioanalyzer Chip RNA 7500 series II (Agilent, Hong Kong, China). The cDNA synthesis, labelling, hybridization, and washing steps were conducted according to the NimbleGen Arrays User’s Guide (v3.2). Each hybridization was performed on a NimbleGen microarray 090818 Vitisexp HX12 chip representing 29,549 predicted grapevine genes covering approximately 98.6% of the genes predicted in the V1 annotation of the 12X grapevine genome. Microarray chips were scanned, and images were analyzed as described by Dal Santo et al. [
13]. Reported values in figures and dataset represent the means of three replicates and a Pearson’s correlation analysis was carried out to evaluate the robustness of the replicates. A gene was considered to be expressed if the normalized expression value was higher than the average value of chip negative control present in at least two of the three replicates. Only genes with an average fluorescence value >150 in all the conditions were considered.
4.9. Statistical Analysis
A two-way fixed-effects ANOVA with interaction was performed to evaluate the effects of site and vintage (year) on yield, grape quality parameters, and wine sensorial evaluation, followed by one-way ANOVA (Tukey’s test, p ≤ 0.05) for multiple comparisons of means when a statistically significant interaction was not found, using STATISTICA v.7 software (statSoft Inc., Tulsa, OK, USA). A one-way ANOVA analysis was applied for each individual year. Residual analysis was performed to test for the assumptions of the ANOVA. Normality was assessed using Shapiro-Wilk’s normality test and homogeneity of variances was assessed by Levene’s test. There were no extreme outliers, residuals were normally distributed (p > 0.05) and there was homogeneity of variances (p > 0.05).
Due to the different timing of observation and the different number of sampling points in the two years, the stem water potential data were analyzed with a one-way ANOVA for each individual vintage.
A principal component analysis (PCA) was applied on the standardized and normalized data of the productive and qualitative parameters, and on the transcriptomic dataset, using XLSTAT 2022.1.1 software (statistical and data analysis solution, Paris, France) aiming at exploring the variance of the datasets in unsupervised manner and unveiling hidden connections and trends between variables. Bartlett’s test and Pearson’s correlation were used in advance to check, respectively, the homogeneity of the variances and the significance of the correlations between the variables.
The Kaiser-Meyer-Olkin test (KMO test) was applied to check the adequacy of sampling and variables with value less than 0.5 were rejected. Only variables with a factor/variable correlation value greater than 0.65 were included in the analysis, and the cosine-squared value (>0.5) was considered to interpret the link between the variables and the dimensions (PC1 and PC2).
Differentially expressed genes (DEGs) were determined by performing a
t-test using TMeV software (V4.9.0;
https://webmev.tm4.org/about accessed on 11 January 2024) with a
p-value (
p) of 0.01% and then filtered by applying a fold-change (FC) threshold of >2 or <−2 (
Dataset S1). Genes that were significantly modulated during ripening in the two vineyards and between vineyards at each stage, were extracted.
The Gene Ontology (GO) enrichment analysis was performed by using the ShinyGO v.0.741 software [
51] with a False Discovery Rate (FDR) cutoff of 0.1.