RATIONAL MAGNESIUM APPLICATION IN VINEYARDS AND GRAPE QUALITY: EXPERIENCES IN NORTHEAST ITALY

Soil fertilization stands as one of the most crucial viticultural techniques, significantly impacting plant productivity and grape quality; imbalanced applications or excessive inputs may lead to adverse effects on yield and fruit quality (Zhang et al., 2016). The importance of magnesium is verified due to its involvement in numerous plant physiological functions (Lošák et al., 2017). Magnesium availability depends on various factors such as soil texture, cation exchange capacity, climatic and anthropogenic factors specific to the location, agronomic practices management, and cultivated species (Blume et al., 2002; Mikkelsen, 2010). Soil microbial biomass is also influenced by short-term magnesium sulphate application, acting as a regulator in the nutrient cycle. This study evaluated the effect of a specific fertilization regime based on magnesium sulphate heptahydrate on the quality of different white and red grape varieties cultivated in Northeast Italy. Field fertilization trials, in collaboration with BluAgri - EVER were conducted in triplicate on homogeneous plots using a randomized block design for each variety. Sampled grapes were assessed for berry weight, average diameter, sugar content, pH, acidity, polyphenolic substances, skin extractability index, and skin break resistance using a penetrometer. All determinations were performed at various ripening stages to obtain ripening kinetics. The tested varieties included Sauvignon Blanc, Tocai Friulano, Pinot grigio, Cabernet sauvignon, and Corvina. Magnesium application was administered twice: the first fertilization at planting, with a dose of 18 kg/ha, and the second application at pre-flowering, also with a dose of 18 kg/ha. The control treatment did not receive magnesium supplementation The sugar content of grapes depended on both the variety and the applied fertilization. This was further confirmed by the analysis of grape must during vinification, which showed generally higher sugar concentrations in samples from magnesium sulphate fertilization, potentially enhancing plant photosynthetic efficiency in some instances. Acidity and pH did not show significant differences between treatments. Polyphenol extractability from the skins was consistently higher in grapes subjected to magnesium sulphate fertilization. This result is noteworthy and should be considered and managed in winemaking, as there are different expected direct and indirect effects between red and white grapes. The accumulation of proteins in grapes was found to be variety-dependent, as confirmed by wine protein stability tests. Nonetheless, winemaking techniques allow for managing any potential protein instability in wines while preserving the aromatic characteristics of each variety. Significant correlations were observed between penetrometer measurements on individual berries and cellular extractability in white grapes. This initial finding could pave the way for refining a rapid field system to evaluate skin component extractability without resorting to laboratory analyses. Magnesium sulphate-based fertilization demonstrated significant effects on some grape analytical parameters, particularly on polyphenols. However, the variability in environmental and vineyard management factors makes generalizing the results challenging. Further studies on a wide statistical basis will be necessary to understand and delve into the effects of magnesium supplementation, aiming to identify the best conditions based on the cultivar and specific winemaking objectives.