Integrated metabolomic response of grapevine fruit to foliar silicon and kaolin applications under summer stress

Climate change is intensifying drought and heat stress in Mediterranean vineyards, compromising grape composition and wine quality. Foliar applications of silicon (Si) and kaolin (Kl) have emerged as short-term sustainable strategies to enhance vine resilience; however, their combined effects on berry metabolomic profiles remain underexplored. This field study, conducted in Vitis vinifera L. cv. Touriga Franca, evaluated the impact of three Si + Kl formulations (MiKS 2, 3, and 4) on primary metabolites (amino and organic acids) and secondary metabolites (procyanidins, anthocyanins, flavonols, and stilbenes) under Douro Valley summer conditions. Targeted UPLC–MS analysis revealed that MiKS 4 was associated with higher concentrations of proline and of most procyanidins, key anthocyanins (delphinidin-3- O -glucoside, malvidin-3- O -(6-acetyl)-glucoside), and flavonols (myricetin hexoside, quercetin-3- O -glucoside), consistent with metabolic adjustments associated with stress-related responses, while a general depletion of aminoacids was observed. Conversely, MiKS 3 consistently yielded the lowest procyanidin levels, particularly procyanidin B2, suggesting a lower accumulation of flavonoid-related metabolites. MiKS-treated plants generally accumulated fewer amino and organic acids, particularly phenylalanine, tyrosine, citric, and gallic acids. These results indicate that specific Si + Kl combinations can differentially modulate grapevine metabolism. MiKS 4 was associated with higher concentrations of several phenolic compounds, whereas MiKS 3 produced contrasting responses, highlighting the importance of formulation-specific effects.