Application of Plant Stimulants to Slovak Grape Varieties (Vitis vinifera L.) and Their Effect on Selected Physiological Indicators

Grapevine growth and physiological performance are strongly influenced by biotic and abiotic stresses occurring during the growing season. Plant stimulants are increasingly applied in viticulture as management tools aimed at supporting plant physiological processes and improving plant performance under variable environmental conditions; however, cultivar-specific responses to different application strategies remain insufficiently characterized. The aim of this study was to evaluate the effects of foliar plant stimulant application strategies differing in application frequency and phenological timing on selected physiological and canopy-related indicators in Slovak grapevine cultivars (Vitis vinifera L.) under field conditions. The assessed parameters included leaf chlorophyll a and b contents, chlorophyll a/b ratio, leaf area index (LAI), vegetation indices (NDVI and PRI), cluster weight, and basic must composition. Grapevines were subjected to three treatment variants: a control without plant stimulant application, a variant with two foliar applications, and a variant with three foliar applications of commercial biostimulants (Tecamin Max, Tecamin Flower, and Tecamin Brix) performed at key phenological stages during the growing season. Plant stimulant applications were associated with variations in leaf chlorophyll content and LAI values, particularly under repeated application strategies. NDVI and PRI complemented leaf-level measurements by capturing cultivar-dependent differences in canopy condition and photosynthetic regulation throughout the season. Responses of cluster weight and must composition to plant stimulant application were moderate and varied among cultivars, indicating cultivar-specific responses. Although no consistent increase in cluster yield was observed, treated variants showed higher sugar content and lower titratable acidity in several cultivars, indicating differences in grape composition and ripening-related traits. Overall, the results indicate that foliar plant stimulant application strategies can influence physiological and canopy-level grapevine traits in a cultivar-dependent manner. The combined use of leaf-level, canopy-level, and spectral indicators provides a practical framework for evaluating plant stimulant strategies under field conditions and supports their application in sustainable viticulture.