Redox homeostasis under summer stress: targeted ROS modulation by foliar kaolin-silicon mixtures in Vitis vinifera L

Climate change-driven increases in heat and drought intensity pose major challenges to grapevine performance, largely through the exacerbation of oxidative stress. This study investigated the effects of combined foliar applications of kaolin (Kl) and silicon (Si) on oxidative stress regulation in Vitis vinifera L. cv. Touriga Franca grafted onto 1103P under field conditions in the Douro Demarcated Region during the 2023 and 2024 growing seasons.

Grapevines were treated with different Kl + Si formulations (MiKS), consisting of 2% Kl combined with 2-8% Si, and evaluated at veraison (E-L35) and harvest (E-L38). Leaf proline content, total and specific reactive oxygen species (ROS, H₂O₂ and O₂-), antioxidant enzyme activities (superoxide dismutase- SOD and catalase- CAT), and the expression of key oxidative stress-related genes (FeSOD and CAT) were analyzed.

MiKS formulations significantly modulated grapevine redox metabolism, although responses depended on season, phenological stage, and Si concentration. Intermediate formulations, particularly MiKS 2 (2% Kl + 4% Si) and MiKS 3 (2% Kl + 6% Si), consistently reduced total ROS accumulation by up to 61.7% and decreased H2O2 levels under summer stress conditions. These responses were associated with transient increases in proline accumulation at veraison, enhanced CAT activity, and strong upregulation of FeSOD and CAT expression, indicating coordinated activation of antioxidant and osmoprotective mechanisms. Although O2•- levels increased in some treatments, this pattern likely reflected controlled redox signaling rather than oxidative damage, as ROS detoxification remained effective. Correlation analysis further supported the close association between ROS modulation and antioxidant regulation.

The combined application of kaolin and silicon, especially at intermediate Si concentrations, promoted a more balanced redox state and improved oxidative stress management under Mediterranean summer conditions. These findings provide mechanistic evidence supporting MiKS formulations as sustainable, low-impact, climate-adaptive tools to enhance grapevine resilience under increasingly hot and dry viticultural scenarios.