**Background:** Climate change is altering vine physiology and phenolic-technological maturity, prompting interest in elicitors like methyl jasmonate (MeJ) to improve grape composition. MeJ activates plant defense mechanisms and has been shown to affect phenolic, nitrogen, and volatile compounds in red grape varieties, but data on white varieties—especially Tempranillo Blanco—are scarce. Tempranillo Blanco is a white mutation of Tempranillo authorized in D.O.Ca. Rioja in 2008, with high organic acid and phenolic content. This study aimed to evaluate the effect of MeJ foliar application at two phenological stages (veraison and post-veraison) on the phenolic, aromatic, and nitrogen composition of Tempranillo Blanco grapes.
**Methods:** The experiment was conducted in the 2020 season at Finca La Grajera, Logroño, La Rioja, Spain. Grapevines planted in 2002 (110-Richter rootstock, 3.00 m × 1.10 m spacing) were treated with aqueous solutions of 10 mM MeJ plus Tween 80 (1 mL/L) at veraison (MeJ-Ver) or post-veraison (MeJ-Post), with two applications one week apart (200 mL/plant sprayed over leaves). Control plants received Tween 80 solution only. Treatments were in triplicate (3 vines/replicate) in a randomized complete block design. Grapes were harvested at optimum technological maturity (~13% v/v potential alcohol). General must parameters were analyzed by OIV official methods and enzymatic instruments. Volatile compounds were determined by HS-SPME-GC-MS (DVB/CAR/PDMS fiber, 60°C extraction, 105 min). Phenolic compounds were extracted from grapes (methanol/water/formic acid) and analyzed by HPLC-DAD. Amino acids were analyzed by HPLC-DAD-FLD after OPA/FMOC derivatization. Statistical analysis used ANOVA (p ≤ 0.05).
**Key Results:** MeJ application did not significantly affect berry weight, °Brix, pH, total acidity, or total phenols, but glucose was lower in MeJ-Post (103.31 g/L) vs. control (118.34 g/L). Ammonium nitrogen increased with MeJ-Ver (127.92 mg N/L) vs. control (93.34 mg N/L), and amino nitrogen increased with both MeJ-Ver (228.21 mg N/L) and MeJ-Post (220.00 mg N/L) vs. control (164.85 mg N/L). YAN was higher in MeJ-Ver (356.13 mg N/L) vs. control (258.19 mg N/L). For volatile compounds, total terpenoids, C13 norisoprenoids, benzenoids, alcohols, and carbonyl compounds were highest in MeJ-Post samples. For example, 2-phenylethanol increased from 1.44 μg/L (control) to 21.88 μg/L (MeJ-Post), and linalool, citronellol, geraniol, and geranyl acetone all increased with MeJ treatment. C6 compounds increased with MeJ-Ver but not MeJ-Post. For phenolic compounds, kaempferol-3-gal, isorhamnetin-3-glc, catechin, epicatechin, gallic acid, p-coumaric acid, and trans-fertaric acid increased with MeJ regardless of timing. trans-Caftaric and trans+cis-coutaric acids increased only with MeJ-Post. Total flavonols increased only with MeJ-Post; total flavanols and total stilbenes increased with both treatments. For nitrogen compounds, aspartic acid, glutamic acid, valine, tryptophan, phenylalanine, isoleucine, and leucine increased with MeJ regardless of timing. Asparagine increased most with MeJ-Ver (7.62 mg/L vs. 3.29 mg/L control), histidine increased most with MeJ-Post (17.47 mg/L vs. 7.63 mg/L control). Total amino acids (with and without proline) increased significantly with both MeJ treatments vs. control. Arginine was the most abundant amino acid (674.83–793.42 mg/L), and the Pro/Arg ratio was <1, confirming Tempranillo Blanco as an arginine-accumulating variety.
**Clinical Implications:** While this is not a clinical study, the findings have implications for human nutrition and health. MeJ-treated grapes showed increased concentrations of bioactive phenolic compounds (flavanols, stilbenes, hydroxycinnamic acids) with known antioxidant, neuroprotective, and antitumor properties. The enhanced amino acid content (especially precursors of fermentative volatile compounds) may improve wine quality and ensure adequate yeast assimilable nitrogen for fermentation. The post-veraison application timing generally yielded the best results across compound classes, suggesting an optimal window for elicitor application. Further multi-year studies are needed to confirm these findings.