Introduction

The amino acids of must are key compounds for the growth and development of yeast during the alcoholic fermentation and for bacteria in the course of malolactic fermentation [1]. Their content can affect the kinetic of the fermentation [2]. Furthermore, some of amino acids are precursors of volatile compounds such as higher alcohols, aldehydes, ketones and esters [3]. The amino acid content of grapes changes significantly during ripening and these changes can influence grape and wine quality. Likewise, the initial nitrogen (N) pool in grapes can affect a large number of metabolites that contribute to wine’s quality. At berry set commence grape N accumulation [4]. The total amino acid grape content increases from veraison to harvest. However, sometimes the total amino acid content achieves a peak before harvest, after which it stabilized and/or decrease slowly until harvest [5]. Furthermore, the content of amino acids and its profile in grapes can be influenced by different factors such as viticultural practices, environmental conditions, and grapes variety [6]. The amino acid profile of grapes is generally similar from year to year for each variety, whereas the amino acid concentration can vary broadly [7]. However, climatic change is modifying the develop of grapes and therefore, grape composition and flavour. Berry ripening is accelerated under high temperatures, achieving a high content of sugars versus a faster breakdown of acids in the grape, which leads to higher alcohol and lower acidity in the resulting wine. These effects may appear by modification of secondary metabolites such as flavonoids, amino acids and carotenoids, affecting aroma and wine color [8]. For this reason, it is interesting to study how the evolution of amino acids in grapes during ripening is develo**, in the current climatic change scenario. To mitigate the climatic change effects several approaches have been studied in last years, foliar application of biostimulants to grapevines is one of them [9,10,11,12]. Among them, stand out the use of elicitors and nitrogen compounds as foliar treatments to grapevines.

Methyl jasmonate (MeJA) is one of the elicitors more used, is a phytohormone present in several plant tissues and, acts as inductor of secondary metabolites in plants [13]. Its foliar application to vineyard increases the phenolic content in grapes, mainly anthocyanins and stilbenes [14, 15], presumably because MeJA can activate the phenylalanine ammonia-lyase (PAL), enzyme which catalyses the first step in the phenolic biosynthesis pathway [16]. However, its effect can be influenced by grape variety, season, or climate conditions [17,18,19]. The effect of MeJA foliar application in other fruits also has been studied. In sweet cherry fruits, MeJA treatments were effective in maintaining of fruit firmness, although a decrease on total phenolics, antioxidant capacity and total monomeric anthocyanin values was observed [20]. Another study focused on kiwi concluded that treatments with MeJA can be used as an efficient postharvest tool to reduce weight loss and minimize losses in vitamin C, total phenolics, and total flavonoids [21]. Regarding the effect on amino acids content in grapes, MeJA foliar application to grapevines presents an unclear effect. In this way Garde-Cerdán et al. [22], observed an increase in the content of some amino acids in the must from ‘Tempranillo’ grapes, whereas Gutiérrez-Gamboa et al. [23] showed a decrease on the must amino acids content. Recently, Garde-Cerdán et al. [24], in their study about the effect of MeJA and MeJA-dopped nanoparticles on nitrogen composition of ‘Tempranillo’ grapes, have observed a different effect of these foliar treatments depending on the vintage.

Foliar fertilization is another practice that researchers carry out to improve grapes quality. Hannam et al. [25] showed that nitrogen foliar application at veraison-time to vineyard is an effective method of improve YAN (yeast assimilable nitrogen) content in must and it produces changes in amino acid profiles of must. Among the different nitrogen sources, foliar application of urea (Ur) is widespread due to its small molecular size, higher water solubility and low cost [11, 26]. Previous studies reported an increase on the concentration of several amino acids in must of grapes coming from grapevines foliar treated with urea [25, 27]. Nevertheless, Gutiérrez-Gamboa et al. [28] showed a decrease on the concentration of some amino acids and, an increase on the proline content in must from ‘Cabernet Sauvignon’ grapes foliar treated with urea.

There are previous works [29, 30] which analyze the effect of foliar application of MeJA and MeJA combined with Ur on the phenolic, aromatic and nitrogen composition of ‘Tempranillo’ wines and phenolic grape composition. Authors concluded that foliar treatments were season dependent and the effect of MeJA + Ur foliar treatment was greater than the effect of MeJA improving the wine chemical composition. However, there is only a recent publication on the effect of MeJA + Ur foliar treatment on the amino acids composition of grapes at the harvest date. This study concluded that, in the first vintage examined, the foliar application of MeJA and MeJA + Ur increased ammonium nitrogen, amino nitrogen, and yeast assimilable nitrogen in ‘Tempranillo’ grapes compared to the control grapes, with the combined treatment exhibiting a more pronounced effect. However, in the second year of the study, these treatments did not significantly impact nitrogen parameters, suggesting a season-dependent influence, possibly attributed to environmental conditions and variations in grapevine nitrogen content [31].Taking into account the aforementioned, we wonder about the impact of foliar application of MeJA and MeJA + Ur on the amino acids content of grapes during ripening. Based on the hypothesis that both treatments will increase amino acids content in grapes, although, MeJA + Ur will probably have a greater effect on amino acid content compared to the application of MeJA alone. Hence, the aim of this work was to study, for the first time, the evolution (from 1 day before the first foliar application to 15 days after harvest) of the content of the different amino acids on grapes coming from ‘Tempranillo’ grapevines foliar treated with MeJA and MeJA + Ur over two vintages.

Materials and methods

Vineyard site and experimental layout

This work was conducted in the 2019 and 2020 vintages with grapes from ‘Tempranillo’ (Vitis vinifera L.) variety grown in the experimental vineyard of Finca La Grajera. This vineyard was located in Logroño, La Rioja (Spain) (Lat: 42º26′25.36′′ North; Long: 2º30′56.41′′ West; 456 m above sea level). Vines were planted in 1997, were trained to a vertical shoot positioned (VSP) trellis system with a grapevine spacing of 2.80 m × 1.25 m and grafted onto a R-110 rootstock. For this trial, three foliar applications were carried out to vineyard: (i) control (sprayed with aqueous solution of Tween 80 alone), (ii) methyl jasmonate (MeJA, 10 mM of methyl jasmonate) and (iii) methyl jasmonate plus urea (MeJA + Ur, 10 mM of methyl jasmonate and a dose of 6 kg N/ha of urea).

The products employed to foliar applications were dissolved in water (the concentration of treatments was decided following previous works [11, 22, 32], and Tween 80 (1 mL/L) were used as wetting agent. Treatments were carried out twice, at veraison and 1 week later. For each treatment, 200 mL of solution was sprayed over leaves. The treatments were performed in triplicate and the experimental layout was arranged in a complete randomized block design along the vineyard. 10 vines were sprayed for each replication and treatment.

Grapes were hand-harvested at five different timing. Fol1: 1 day before the first foliar application; Fol2: 1 day before second foliar application; Preharvest: 15 days after the second foliar application; Harvest: the day of harvest [when grapes reached their optimum technological maturity, i.e., the weight of 100 berries remained constant and the probable alcohol reached 13 (% v/v)]; and Postharvest: 15 days after harvest. For each time of sampling, 150 berries per replicate and treatment was collected haphazard and frozen at – 20 °C until the analyses of amino acids were carried out.

Analysis of amino acids in the musts by HPLC–DAD

The amino acids analysis was carried out following the method described by Garde-Cerdán et al. [33]. In brief, a derivatization of amino acids was performed by reaction of 1.75 mL of borate buffer 1 M (pH 9), 750 μL of methanol (Merck, Darmstadt, Germany), 1 mL of sample (previously filtered), 20 μL of internal standard (2-aminoadipic acid, 1 g/L) (Sigma–Aldrich, Madrid, Spain) and 30 μL of derivatization reagent diethyl ethoxymethylenemalonate (DEEMM) (Sigma–Aldrich). In a screw-cap test tube was done the reaction of derivatization over 30 min in an ultrasound bath (DU-100 ARGO Lab, Modena, Italy). Then, the samples were heated at 70–80 °C in an incubator (INC 120 plus ARGO Lab) for 2 h to complete the degradation of excess DEEMM and reagent by-products.

The analyses were carried out on an Agilent 1260 Infinity II chromatograph (Palo Alto, USA), with a diode array detector (DAD). An ACE HPLC column (C18-HL) (Aberdeen, Scotland) particle size 5 μm (250 mm × 4.6 mm) was employed for the chromatographic separation. Amino acids were eluted following the conditions described by Garde-Cerdán et al. [2]. Phase A, 25 mM acetate buffer, pH 5.8, with 0.4 g of sodium azide; phase B, 80:20 (v/v) mixture of acetonitrile and methanol (Merck). DAD was used for the detection, and was monitored at 280, 269 and 300 nm. The volume of injection was 50 μL. The identification of the target compounds was performed according to the retention times and the UV–Vis spectral characteristics of corresponding standards (Sigma-Aldrich) derivatizated. Quantification was performed using the calibration graphs of each standard in 0.1 N HCl (R2 ≥ 0.97), which underwent the same process of derivatization that the samples.

The treatments in vineyard were carried out in triplicate, so the results of free amino acids correspond to the average of 3 analyses (n = 3).

Statistical analysis

The SPSS Version 21.0 statistical package for Windows (SPSS, Chicago, USA) was employed to perform the statistical analysis of the data. The differences among the means of nitrogen compounds data were processed using the variance analysis (ANOVA) (p ≤ 0.05) and a post hoc Duncan´s multiple range test was carried out. The effect of foliar treatments, time of sampling, seasons and their interaction were analyzed using a multifactor analysis (MANOVA).

Results and discussion

Influence of the foliar MeJA and MeJA + Ur treatments on amino acids content in each time of sampling in grape musts

Table 1 shows the results of must amino acids content from control and treated vines with methyl jasmonate (MeJA) and with methyl jasmonate plus urea (MeJA + Ur), in 2019 season for each time of sampling (Fol1, Fol2, Preharvest, Harvest, and Postharvest). The amino acid present in a higher content in all samples was arginine, except for MeJA + Ur treatment from Preharvest samples, in which glutamine was the predominant. This result is consistent with observations made by Hernández-Orte et al. [7] on the ‘Tempranillo’ grape variety. Arginine contains four nitrogen atoms in its molecule, making it the most effective nitrogen source for yeasts. Glutamine, Ɣ-aminobutyric acid (GABA), and the sum of threonine and citrulline were found in greater proportion in grapes across all samples. In both Harvest and Postharvest samples, histidine reached similar levels as GABA and the sum of threonine and citrulline. Hernández-Orte et al. [7] studied the amino acid profile of grapes from four varieties over a 3-year period and showed that arginine, proline, histidine, and glutamine were the most prevalent amino acids across all four varieties. In addition, it is noteworthy that arginine, along with ammonium, serves as the main nitrogen sources for yeast through alcoholic fermentation [2]. Valine, isoleucine, leucine and phenylalanine are amino acids that acts as precursors of higher alcohols in alcoholic fermentation [2]. Their representation in grapes, as shown in Fol1, was less than 5% of the total amino acids content. In Fol2, this group of amino acids accounted for approximately 5% in both control and MeJA + Ur grapes, whereas in MeJA grapes, it represented about 8%. In Preharvest samples, it constituted roughly 7% in both control and MeJA + Ur samples, and 13.5% in MeJA grapes. At Harvest, these amino acids accounted for 6% in both control and MeJA + Ur grapes, and around 9% in MeJA grapes. Finally, in Postharvest samples, the content in control grapes was around 8%, in MeJA grapes it was 11%, and in MeJA + Ur samples, it was 10%. Therefore, foliar application of MeJA increased the content of amino acid precursors of higher alcohols with respect to control and MeJA + Ur grapes during grape ripening. Excluding the aforementioned, the amino acids present in grapes in a lower concentration were ornithine, methionine, glycine, lysine and tyrosine. In all samples collected a different times, their content was lower than 3% of the total amino acids content, except for MeJA grapes from Preharvest (3.3%) and Postharvest (3.1%). Lysine, glycine, and methionine were characterized as minor amino acids in grapes, specifically in certain grape varieties such as ‘Monastrell’, ‘Merlot’, and ‘Petit Verdot’. A previous work highlighted that glycine and lysine are not a good nitrogen sources for Saccharomyces cerevisiae yeast, but they are suitable for non-Saccharomyces [2]. Amino acids can be categorized based on the trends observed during the ripening period. Hernández-Orte et al. [7] described that most amino acids exhibited varying development patterns during the ripening stages in different years of their study, with amino acids reaching their highest content before the harvest. Aspartic acid, phenylalanine, ornithine, and lysine demonstrated an increase in their content in grapes until the Preharvest stage, followed by a decrease until Postharvest (Table 1). On the other hand, glutamic acid, histidine, glycine, alanine, valine, methionine, leucine, isoleucine, and tryptophan showed an increase in their content up to the Preharvest stage, followed by a decrease until Harvest and subsequently an increase again until the Postharvest stage. Asparagine was the only amino acid which presented a decrease in its content from Fol1 to Postharvest (Table 1). Proline, for its part, underwent a general increase until Postharvest sample. Given the observed trends in these two amino acids, they could be considered suitable parameters for monitoring grape ripening. The trends observed for glutamine, tyrosine, and arginine depended on the sample (control, MeJA and MeJA + Ur) being studied. Serine and GABA exhibited an increase in their content in grapes up to Preharvest stage, after which their content remained relatively constant (Table 1). The increase in the concentration of free amino acids as the fruit ripens could be due to a decrease in the demand for these metabolites as the growth process progresses through ripening [7]. The range of concentrations measured for all samples of the amino acids at Harvest was consistent with those described by Beel &Henschke [5] except for tryptophan, which was found in higher concentration (38–54 mg/L). MeJA and MeJA + Ur treatments increased the content of several amino acids in Harvest and Postharvest samples. These amino acids included aspartic acid, glutamic acid, asparagine, serine, histidine, the sum of threonine and citrulline, alanine (treatments also increased its content at Preharvest, and in the case of MeJA, in Fol1), and tyrosine (MeJA treatment also increased its content in Fol1). MeJA treatment increased the glutamine content at Harvest and Postharvest, whereas MeJA + Ur only increased glutamine concentration at Harvest, in comparison to the content of control grapes (Table 1). Grapes from grapevines treated only with MeJA showed a higher content of glycine than MeJA + Ur grapes, which also had higher content than the control grapes in Postharvest samples. On the other hand, MeJA and MeJA + Ur foliar treatments increased the arginine content at Preharvest and Postharvest samples, with MeJA + Ur additionally raising the arginine content at Harvest time compared to control samples. Furthermore, treatments rose GABA concentration at Preharvest when compared with control samples (Table 1). However, foliar treatments did not affect the content of proline in any of the samples studied. Valine, methionine, isoleucine, and leucine underwent a similar pattern; treatments jumped their concentration from Preharvest to Postharvest, and MeJA also increased their content in Fol2, compared to their content in control grapes (Table 1). Grapes from grapevines treated with MeJA showed a high content of tryptophan from Fol2 to Postharvest, whereas MeJA + Ur treatment increased its concentration in Preharvest and Postharvest samples. The content of phenylalanine and ornithine was increased from Fol2 to Postharvest by both treatments studied. Finally, the lysine content rose for treatments from Preharvest to Postharvest samples (Table 1).

Table 1 Amino acids concentration (mg/L) in grapes from control, methyl jasmonate (MeJA) and MeJA + Urea (MeJA + Ur) treatments in 2019
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In summary, all amino acids underwent an increase in their concentrations at any sampling time, except for proline, due to the effect of foliar treatments compared to control grapes. Therefore, both MeJA and MeJA + Ur treatments affected the biosynthesis of amino acids in grapes during the 2019 season. Garde-Cerdán et al. [24] also observed an enhance of the synthesis of most amino acids during the first season of their study, attributed to MeJA foliar application.

Table 2 presents the results of must amino acids content from control and treated vines with methyl jasmonate (MeJA) and with methyl jasmonate plus urea (MeJA + Ur), in 2020 season for each time of sampling (Fol1, Fol2, Preharvest, Harvest, and Postharvest). The amino acids present in a higher content across all samples were glutamine or arginine, following by alanine, GABA, glutamic acid and histidine. The amino acids content that act as precursors for higher alcohols was: in Fol1, it accounted for 9% in the control sample, 7% in MeJA, and 5% in MeJA + Ur of the total amino acids content; in Fol2, it represented 7% in the control sample, 9.7% in MeJA, and 6% in MeJA + Ur of the total amino acids content; in the Preharvest samples these amino acids accounted for 6.6% in control, 8.4% in MeJA, and 5.8% in MeJA + Ur samples of the total amino acids content; at Harvest, this group of amino acids constituted 10% in control samples, 11% in MeJA, and 9% in MeJA + Ur samples of the total amino acids content; and finally, at Postharvest, these amino acids accounted for 12% in the control, 9.6% in MeJA, and, 9.5% in MeJA + Ur samples. It was again observed that foliar application of MeJA increased the content of amino acid precursors of higher alcohols, this time from Fol1 to Harvest. The amino acids with lower content in all samples were glycine, the sum of threonine and citrulline, ornithine, and lysine. Their content represented less than 3% of the total amino acids content in all samples across various treatments and sampling times. Aspartic and glutamic acids, GABA, histidine, glycine, methionine, and tyrosine showed a more or less pronounced increase in their concentration from Fol1 to Postharvest (Table 2). A similar trend was observed for the following amino acids: serine, glutamine, citrulline + threonine, leucine, valine, isoleucine + tryptophan, phenylalanine and lysine, which presented a minimal or no increase in their concentration from Fol1 to Preharvest, followed by an increase in their content until Postharvest (Table 2). In the 2020 season, asparagine was the only amino acid which displayed a decrease in its content in grapes from Fol1 to Post-harvest, consistent with the evolution observed in 2019. Therefore, as mentioned above, it seems that asparagine could be a suitable amino acid for monitoring grape ripening, since its content decreased from Fol1 to Postharvest in both vintages studied. Arginine and proline exhibited an increase in their content from Fol1 to Harvest, followed by a decrease until Postharvest. Alanine increased its content in grapes from Fol1 to Fol2, underwent a decrease until Preharvest and then slightly increased its content until Postharvest (Table 2). Ornithine demonstrated a decrease from Fol1 to Fol2 and then, an increase until Postharvest. Overall, the foliar treatments did not significantly affect the content of amino acids in grapes during the ripening process in this second season, with some cases indicating a slight decrease. All amino acids presented a concentration range at Harvest that aligned with those previously described by Bell &Henschke [5], except for tyrosine, which showed in control grapes a content higher than 33 mg/L.

Table 2 Amino acids concentration (mg/L) in grapes from control, methyl jasmonate (MeJA) and MeJA + Urea (MeJA + Ur) treatments in 2020
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Figure 1 shows the total amino acids content, with and without proline, throughout grape ripening for control, MeJA and MeJA + Ur samples in both vintages (2019 and 2020). In Fig. 1a, b, it can be observed that in 2019 season, MeJA and MeJA + Ur treatments increased the total amino acids and the total amino acids without proline content from Fol2 to Postharvest stages. Stand out the notable effect of MeJA + Ur foliar treatment at Preharvest moment; however, the MeJA treatment also led to an increase in both total amino acids and total amino acids without proline, in comparison to the control grapes. However, during the 2020 season, the effect of foliar treatments was totally different. Both MeJA and MeJA + Ur produced a decrease in total amino acids content and total amino acids content without proline (Fig. 1c, d) regarding to the amino acids content in control grapes, observed at Fol1, Fol2 and Postharvest stages. Furthermore, no significant differences were observed between the treated grapes and the control grapes at the Preharvest and Harvest stages.

Fig. 1
figure 1

Amino acids concentration (mg/L) in grapes from control and treated vineyards with foliar application, methyl jasmonate (MeJA) and methyl jasmonate plus urea (MeJA + Ur): a total amino acids concentration from 2019 season, b total amino acids concentration without proline from 2019 season, c total amino acids concentration from 2020 season, d total amino acids concentration without proline from 2020 season. Different lowercase letters indicate significant differences between treatments at each time of maturation (p ≤ 0.05). Uppercase letters indicate differences among time of ripening for each treatment (p ≤ 0.05). Absence of letters indicates no significant differences

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The effect of foliar treatments was different between the two study seasons, suggesting that foliar applications show a dependence on the season in which they are applied. This dependence has already been observed by other authors previously [24]. Mainly, the different effect of foliar treatments observed could be explained by differences on the pre harvest rainfall recorded among seasons. In 2020 season, the preharvest rainfalls were higher (32.9 l/m2) than in 2019 vintage (11.5 l/m2). In addition, a previous study reported data on nitrogen compound content in grapes at harvest [24]. In the 2020 season, the nitrogen content in the control grapes was approximately twice as high as that in 2019. Thus, the impact of foliar treatments was less pronounced when the grapes had a higher content of nitrogen compounds.

It should be noted the differences on the total content (with and without Pro, Fig. 1) of amino acids in musts between the two years of the study. In 2019 at harvest moment, total amino acids content of control must was around 2070 mg/L, whereas in 2020 this content was around 3215 mg/L, which can be explained by climatological conditions, since they play a key role in the amino acid content of the must [7].

Overall, the variation in amino acids evolution during ripening differed between seasons. In 2019, amino acids attained their peak content in preharvest or harvest samples, aligning with findings by Hernández-Orte et al. [7]. In contrast, during the 2020 season, the highest concentrations were observed at post-harvest, a notable deviation from the previous vintage, potentially attributed to climatic change.

Multifactor analysis of variance of amino acids in musts

Tables 3 and 4 show the results of the multifactor analysis of variance of amino acids content, during the 2019 and 2020 seasons, considering the two factors under investigation: treatment and sampling time. In 2019 (Table 3), the treatments influenced the content of all individual amino acids, except for asparagine and proline in MeJA treatment. Additionally, the total amino acids content, both with and without proline, was affected by the treatments, with the MeJA + Ur treatment showing a more substantial impact compared to the MeJA treatment (Table 3). The “sampling time” factor also significantly affected the content of all individual amino acids at various sampling points, as well as the total amino acid content with and without proline, reaching the highest content at Preharvest for several amino acids (Table 3). The interaction between the two factors was statistically significant for all individual amino acids, except for proline, and also affected the total amino acids content (Table 3). In 2020 season (Table 4), the studied foliar treatments impacted the individual content of several amino acids, leading to a decrease in their content in grapes when compared with control grapes. The sampling time also influenced the grape’s content of both individual and total amino acids. However, in this vintage, the maximum concentration values were observed either at Harvest or Postharvest stage (Table 4). In this vintage, the interaction between both factors was significant for all amino acids, except for aspartic and glutamic acids, threonine + citrulline, arginine, alanine, GABA, proline, tyrosine and lysine. These findings further confirm the dependence of the effect of foliar application’s effect on the vineyard in relation to the specific season.

Table 3 Multifactor analysis of variance of amino acids content in 2019 season, with the two factors studied: treatment (Control, MeJA, MeJA + Ur) and time of sampling (Fol1, Fol2, Preharvest, Harvest, Postharvest) and their interaction (treatment × time of sampling)
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Table 4 Multifactor analysis of variance of amino acids content in 2020 season, with the two factors studied: treatment (Control, MeJA, MeJA + Ur) and time of sampling (Fol1, Fol2, Preharvest, Harvest, Postharvest) and their interaction (treatment × time of sampling)
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Table 5 shows the percentage of variance attributed to each factor (season, sampling time, and treatment), and their interactions. The main source of variability was the sampling time, which is logical considering the changes in amino acid content during grape ripening. The season also showed a significant influence on specific amino acids, such as aspartic acid, the sum of threonine and citrulline, alanine and tyrosine (Table 5). However, the effect of the treatments was minor, producing an effect lower than 5% in all amino acids. Overall, the interaction effect among factors influenced the concentration of amino acids in grapes, with the most substantial impact observed only for glutamine.

Table 5 Percentage of variance attributable to season, time of sampling, treatment and their interactions
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Conclusions

The influence of foliar treatments with MeJA and MeJA + Ur on ‘Tempranillo’ grapes, applied at veraison and 1 week later, during ripening was studied in this research. The evolution of the different amino acids varied between vintages. Overall, during the 2019 season, amino acids reached their highest concentration in grapes at Preharvest moment, whereas in 2020, this maximum was achieved at Postharvest stage. Moreover, the season dependence of the treatments is evident, as the effect of both foliar treatments differed significantly depending on the vintage. In the first season, foliar treatments increased the content of several amino acids in grapes, while no such improvement was observed in 2020 season. The asparagine content in grapes could be used to follow the ripening of grapes, as it decreased from Fol1 to the Postharvest stage in the two vintages studied. As well as the MeJA foliar application increased the content of amino acid precursors of higher alcohols in both seasons. In conclusion, further in-depth research is needed to comprehend the impact of foliar treatments on the amino acid content of grapes, to develop an effective tool for enhancing the nitrogen quality of grapes.