Grapevine (Vitis vinifera L.) is one of the essential fruit crops cultivated globally for its economic and health benefits. The primary product, grapes, are consumed as fresh fruits or juice (table grapes) or processed into wines (wine grapes) [1]. The quality of grape products, especially wine, is influenced mainly by the primary and secondary metabolites of the grapes ^[2][3][2,3]. However, these metabolites are affected by several pests and diseases as well as vine management practices and many other factors (e.g., soil, climate, weather).

The main aim of grape producers in the past was to enhance grape productivity and obtain a good yield to meet the high demand for wines. Therefore, different strategies, such as the use of fungicides and pesticides and other management practices, were employed to prevent any biotic or/and abiotic stresses that could decrease yield ^[4][5][6][4,5,6]. However, the use of fungicides and pesticides has adverse effects on human health and the environment. Excessive usage causes residual buildup in soils, plants, and groundwaters, affecting beneficial soil organisms, humans, and the environment, while continual use leads to pathogen resistance ^[7][8][7,8].

Although it is necessary to prevent grape diseases and infections, adverse effects on fruit yield and quality must be avoided. In addition, there has been more emphasis recently on achieving sustainable quality yields through “green production.” Under this term, the European Commission has recently announced measures aimed at achieving healthy and environmentally friendly food production by 2050 ^[9][10][9,10]. This includes reducing the use of pesticides and fungicides. According to the FAO [11], the world population will grow to 9.7 billion by 2050. To prevent food shortages and ensure the sustainable development of high-quality food, environmentally friendly methods are currently being increasingly used, as opposed to pesticides and fungicides.

Elicitors are stress stimuli capable of inducing similar defense responses in plants as induced by the pathogen infection ^[5][7][5,7]. Elicitors induce plant resistance against pathogens by activating signals that enhance the production of secondary metabolites. Elicitors are of different types; chemical elicitors such as benzothiadiazole or methyl Jasmonate, physical elicitors such as light, salinity, or temperature, and elicitors of biological origin, such as oligosaccharides, yeast derivatives, or protein fragments ^[12][13][12,13]. The use of elicitors as alternatives to agrochemicals in preventing grape diseases and infections also has a great impact on the quality components of grapes ^[14][15][14,15]. Numerous studies intending to improve wine aroma quality have investigated the effects of different elicitors on the volatile compositions of grapes. However, their impact varies depending on several factors such as grape cultivar, type of elicitor, and dose.

2. Grape Composition

Grape quality is primarily assessed by the compositional chemical measures of the grape, such as the pH, sugars, titratable acidity, color (for red grapes), aroma compounds, phenolic compounds, and other volatiles ^[16][17][16,17]. These chemical parameters are influenced by the different vineyard soil conditions, climate conditions, and vine management practices and changes throughout the development period ^[2][17][18][2,17,18]. The credibility of these parameters, especially the sugar content of grapes as a qualifier of “quality” at harvest, is not a point of contention ^[19][20][19,20]. Sugar as a primary metabolite also influences several secondary metabolites, especially the concentrations of aroma compounds ^[21][22][21,22]. According to Rolland et al. [23], soluble sugars also function as signaling molecules aside from their impact on the overall sensory quality of fruits. They modulate genes involved in defense and metabolic processes, thus, affecting fruit maturity and the biosynthesis of secondary metabolites.

2.1. Grape-Derived Aroma Compounds

Aroma is an essential characteristic that varies significantly with grape maturity and ultimately determines the grape and wine quality. The aroma components of wine are an important factor that reflects the nutritional information of the wine and influences consumer liking [24]. Depending on the origin of aroma compounds, they are classified either as primary, secondary, or tertiary aromas [25]. The varietal (primary) aromas are derived from grapes and vary depending on the cultivars, climate conditions, and vineyard practices [4]. Aromas produced during maceration and fermentation are known as secondary aromas, while tertiary aromas are formed during the aging of wine [4]. Grape-derived aromas are found both in the skin and the pulp [26], with a low human detection threshold [27]. Grapes consist of hundreds of volatile compounds, some of which are present in free odor-active forms, and the majority are found in glycosylated form, serving as potential aroma reservoirs [28].

2.1.1. Terpenoids

Terpenoids, among the various classes of grape-derived aromas, are the most studied volatile compounds. Terpenoids are grouped according to their carbon numbers into hemiterpenes (C₅), monoterpenes (C₁₀), sesquiterpenes (C₁₅), diterpenes (C₂₀), and tetraterpenes (C₄₀), with monoterpenes (C₁₀) as the dominant class ^[4][29][30][4,29,30]. Grapes are categorized into Muscat, non-Muscat aromatic, and neutral varieties based on their monoterpene concentration levels [24]. Monoterpenes are synthesized through the mevalonic acid (MVA) pathway and the methylerythritol phosphate (MEP) pathway from isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Thereafter, through the activity of terpene synthases (TPS), monoterpenes are formed from 2-(E)-geranyl diphosphate (GPP) [31]. However, among the two biosynthetic routes, the MEP pathway is said to be the prime route for the formation of terpenoids in grapes [32]. Terpenoids are stored as free and bound volatiles, mainly in the grape skin, with trace concentrations in the pulp ^[4][30][4,30]. Climate, management practices such as grape shading, elicitation, and many other factors (e.g., pruning, irrigation, fertilization) influence the concentrations of terpenes, as reported in the literature ^[33][34][35][33,34,35]. For instance, concentrations of monoterpenes in Sauvignon blanc grapes decreased with high canopy density [36], while the concentration of these molecules increased when Sauvignon blanc grapes were exposed directly to the sun after leaf removal [37].

2.1.2. Norisoprenoids

Norisoprenoids are volatile compounds of 9, 10, 11, or 13 carbon cyclic chemical structures derived from carotenoids ^[4][38][39][4,38,39]. Carotenoids are pigments produced in the chloroplast and decline during grape ripening due to the unavailability of the chloroplast ^[40][41][42][40,41,42]. Hence, decreasing the norisoprenoids synthesized. Norisoprenoids are formed through the conversion of biodegraded carotenoids by enzymes to the aroma precursor and subsequently to the aroma-active compound by the acid-catalyzed conversion ^{[4][30][38][40][41]}[4,30,38,40,41]. Norisoprenoids are grouped into megastigmane and non-megastigmane forms, with most norisoprenoids in the megastigmane form differing based on the position of the oxygen functional group ^[4][38][4,38]. C₁₃-norisoprenoids are the abundant norisoprenoids in grapes, with β-ionone, β-damascenone, vitispirane, actinidiol, 1,1,6-trimethyl-1,2- dihydro naphthalene (TDN), and 2,2,6-trimethylcyclohexanone (TCH) as the most prevalent compounds conferring fruity and floral notes ^[4][30][4,30]. Grape-derived norisoprenoids are affected by vineyard management practices such as leaf removal, cover cropping, irrigation, and many other factors (e.g., fertilization, grape shading) ^[39][43][44][39,43,44].

2.1.3. Methoxypyrazines (MPs)

Nitrogen-containing grape-derived volatiles, 3-Alkyl-2-methoxypyrazines (MPs), are found abundantly in the stems (79.2%) rather than in the berries (20.8%) [45]. The precise biosynthesis pathway of MPs is still unclear, although they are suggested to be derived from the metabolism of amino acids ^[4][30][4,30]. However, the last step in the synthesis of MPs (O-methyltransferases (OMT) methylation of hydroxypyrazine precursors to methoxypyrazines) is explicit, as several identified genes correlated positively with the precursors ^{[46][47][48][49]}[46,47,48,49]. The most important MPs, 2-methoxy-3-isobutylpyrazine (IBMP), 2-methoxy-3-sec-butylpyrazine (SBMP), and 2- methoxy-3-isopropylpyrazine (IPMP), out of the seven detected in grapes, impact grassy, herbal, bell pepper, leafy, and asparagus-like odorants in several wines such as Cabernet Sauvignon, Sauvignon Blanc, Chardonnay, Cabernet franc, Carmènere, and Merlot ^{[33][49][50][51][52]}[33,49,50,51,52]. The most abundant among the three important MPs is IBMP, mostly found in the grape skin ^[4][45][4,45]. Koch et al. [53] studied the accumulation of IBMP in 29 different grapes and reported high levels of IBMP in some cultivars compared to trace levels or undetected IBMP in other cultivars. Several studies have shown that grape variety and other factors such as maturity, climate, leaf removal, and light exposure ^{[39][50][54][55][56]}[39,50,54,55,56] influence the accumulation and concentrations of MPs.

2.1.4. Fatty Acids Derivatives

Fatty acid-derived volatiles, including alcohols, aldehydes, ketones, lactones, esters, and acids, constitute the majority of volatile compounds in grapes ^[38][42][38,42]. These compounds are synthesized through the α-oxidation, β-oxidation, or lipoxygenase pathways [42]. C₆ aldehydes and alcohols are the most abundant compounds among these derivatives. The C₆ compounds are produced from linoleic and linolenic acids enzymatically by lipoxygenase (LOX), hydroperoxide lyase (HPL), (3Z), (2E)-enal isomerase, and alcohol dehydrogenase (ADH) thru the LOX pathway in damaged and crushed grape tissues ^[42][57][42,57]. C₆ compounds are partly responsible for the green, herbaceous odorant in grapes and grape products. The concentrations of C₆ compounds are varietal dependent ^[58][59][58,59] and also influenced by maturity ^[39][59][60][39,59,60] and season ^[59][61][59,61]. The concentrations of the C₆ compounds in most of these studies were high during the pre-veraison and veraison stages but started to decline after veraison. However, this was not the case for all the studies. For example, in the study reported by Salifu et al. [60], they observed decreasing concentrations of all C₆ aldehydes and alcohols from the pre-veraison to maturity stages, except for 1-hexanol, which observed higher concentrations during the pre-veraison and maturity stages. Likewise, the study on Pinot noir grapes by Yuan and Qian [39] reported continuous decreasing concentrations of C₆ alcohols after the veraison stage. These observations affirm that grape variety influences the concentrations of C₆ compounds.

2.2. Grape Amino Acids

Amino acids are vital not only for the synthesis of proteins but also as precursors for the production of aroma compounds ^[62][63][62,63], signaling molecules [64], and triggering defenses against biotic and abiotic stresses ^[65][66][65,66]. Amino acids are the main nitrogenous compounds in grapes (approximately 25–30%) amassed in the skin, seeds, and pulp ^[67][68][69][67,68,69]. The composition and concentration of amino acids vary with vintage, grape variety, level of maturity, and soil fertility ^{[70][71][72][73][74]}[70,71,72,73,74]. In relation to the cultivar, previous works ^[74][75][76][74,75,76] observed that total amino acids concentration in white grapes was higher than total amino acids concentration in red grapes, and within the red grape varieties, those with relatively high chroma (measure of anthocyanins) had low total amino acids concentration compared to varieties with low chroma. According to Guan et al. [77], the inverse relation of the color index and concentrations of amino acids from a metabolic viewpoint hypothesized that the high color index could be at the expense of amino acid precursors (C-skeleton). Furthermore, the nutrient status of the vine, especially the nitrogen level, greatly impacts the composition of grape amino acids, as observed by these authors ^{[70][71][72][73][78]}[70,71,72,73,78].