Biostimulanten in de wijnbouw: Chemische karakterisering en functionele evaluatie van biostimulanten voor de groei van wijnstokken en de ontwikkeling van druivenbessen onder verschillende klimaatomstandigheden

Modern viticulture is facing increasing pressure to maintain grape yield and fruit quality under changing climatic conditions while reducing reliance on resource-intensive agricultural inputs. Rising temperatures, drought, and increasingly erratic weather patterns are altering grapevine development and berry ripening across many wine-producing regions. In warmer climates, accelerated ripening can disrupt the balance between sugar accumulation and organic acid degradation, while cooler-climate regions are increasingly exposed to inconsistent vegetative development and ripening. Conventional responses such as increased irrigation and fertilisation are often unsustainable and may further disrupt vine balance and long-term resilience. Plant biostimulants have emerged as a promising tool to support grapevine performance under these conditions. These products, often derived from natural sources such as seaweeds, protein hydrolysates, or humic substances, are promoted for their ability to improve plant growth and stress tolerance while reducing dependence on synthetic inputs. However, despite increasing commercial adoption, their effects in vineyards remain inconsistent and poorly understood. Differences in product composition, environmental conditions, and vineyard management practices all contribute to highly variable responses. This PhD research investigated the potential of organic biostimulants to influence grapevine growth, ripening dynamics, and berry composition under contrasting cool- and warm-climate conditions. The work combined controlled laboratory screening with vineyard validation in Belgium and South Africa using Chardonnay (Vitis vinifera L.) as a model cultivar. The first part of the research evaluated nine commercial biostimulants belonging to three major categories: humic substances, protein hydrolysates, and seaweed extracts. Although the products could broadly be grouped according to their chemical composition, plant responses were highly product-specific rather than consistent within a biostimulant class. Some seaweed extracts promoted shoot growth, while certain humic substances stimulated root development. In contrast, some protein hydrolysates reduced growth despite high amino acid content, highlighting the importance of formulation characteristics beyond nutrient composition alone. Overall, these findings demonstrated that broad categories such as "seaweed extracts" or "protein hydrolysates" are insufficient for predicting biological responses. Based on these results, two commercial seaweed extracts derived from Ascophyllum nodosum and Ecklonia maxima were selected for vineyard evaluation. In Belgium, the A. nodosum extract was associated with larger leaf area, improved photosynthetic indicators, and increased berry size and sugar accumulation under certain seasonal conditions. In contrast, E. maxima showed fewer vegetative effects but was associated with increased yield between seasons. In South Africa, responses differed strongly between vineyard sites, with the clearest effects observed under warmer and higher atmospheric-demand conditions where E. maxima increased total amino acid concentrations in grape must. These findings suggest that biostimulant efficacy is strongly influenced by environmental conditions and application timing. A central outcome of this thesis is that biostimulant responses in grapevine are highly context-dependent. The efficacy of seaweed extracts depended not only on the seaweed species used, but also on climate, vineyard environment, phenological timing, and vine physiological status. Early-season applications were primarily associated with vegetative and physiological responses, whereas later-season applications were more closely linked to ripening dynamics and berry chemistry. Importantly, these responses could not be explained solely by mineral nutrition, as nutrient-matched fertiliser treatments did not consistently reproduce the same effects. Together, the results support a more targeted and evidence-based approach to biostimulant use in viticulture. Rather than functioning as universal plant growth enhancers, biostimulants appear to act as context-sensitive tools whose efficacy depends on matching the correct formulation and application strategy to a specific physiological objective and environmental condition. As climate pressures continue to intensify across wine-producing regions, improving our understanding of how biostimulants interact with grapevine physiology and vineyard environments may contribute to more resilient and sustainable vineyard management practices.