Enhance summer stress resilience and photosynthetic efficiency of grapevines through foliar application of silicon and kaolin combination

Abstract Background Climate change is increasingly exposing vineyards to higher temperatures and water deficits, negatively affecting grapevine physiological performance and berry composition. These effects are particularly evident in Mediterranean regions such as the Douro Demarcated Region, where summer stress often limits yield and quality. Materials and methods In this context, this study evaluated four foliar applications combining 2% kaolin (Kl) with silicon (Si) concentrations ranging from 2% to 8% (MiKS) and their effects on physiological and biochemical responses and water status of Vitis vinifera cv. “Touriga Nacional”, with MiKS 1- MiKS 3 evaluated in 2023, while MiKS 2- MiKS 4 were assessed in 2024 to explore the effect of higher silicon concentrations. Results Foliar application of MiKS formulations, especially MiKS 3 (2% Kl + 6% Si), improved grapevine water status during critical phenological stages, as indicated by higher predawn leaf water potential (Ψ pd ). Gas exchange parameters showed a positive response, with increased stomatal conductance ( gs ), transpiration ( E ), and intrinsic water use efficiency ( A/gs ), particularly at midday. MiKS 2 (2% Kl + 4% Si) and MiKS 3 (2% Kl + 6% Si) were especially effective at midday, improving photosynthetic performance and stomatal regulation. Chlorophyll a fluorescence parameters indicated improved photosystem II performance, with higher Φ PSII and qP and lower NPQ , especially in these treatments. Anatomical analysis revealed thicker leaf cuticles in MiKS-treated vines, particularly under higher silicon concentrations, suggesting a structural adaptation to reduce water loss. In addition, MiKS treatments were associated with increased chlorophyll and carotenoid contents. Lower levels of reactive oxygen species (ROS), including hydrogen peroxide (H 2 O 2 ) levels, and lower proline content, suggest that MiKS 2 and MiKS 3 may contribute to mitigating oxidative stress and maintaining photosynthetic machinery integrity. Conclusions These findings highlight the potential of combined KL and Si formulations, particularly MiKS 2 and MiKS 3 (4–6% Si), to alleviate summer stress in grapevines, improving physiological performance, structural protection and productivity, supporting their use as sustainable foliar strategies for Mediterranean viticulture.