Hort. Sci. (Prague), X:X | DOI: 10.17221/109/2025-HORTSCI

Impact of thinning level and timing on yield, maturity, and quality of Merlot grapevines under the Big T trellis systemOriginal Paper

Ilknur Korkutal ORCID...1, Elman Bahar ORCID...1, Gamze Uysal Seckin ORCID...2, Derya Akinci ORCID...3
1 Horticulture Department, Agricultural Faculty, University of Tekirdağ Namik Kemal, Tekirdağ, Türkiye
2 Department of Food Technologies, Tekirdağ Viticulture Research Institute, General Directorate of Agricultural Research and Policies, Republic of Turkey Ministry of Agriculture and Forestry, Tekirdağ, Türkiy
Bozcaada District Directorate of Agriculture and Forestry, Çanakkale Provincial Directorate of Agriculture and Forestry, Republic of Turkey Ministry of Agriculture and Forestry,
3 Çanakkale, Türkiye


This study evaluated the effects of cluster thinning on yield and quality of 12-year-old Merlot/41B grapevines over the 2019–2020 and 2020–2021 growing seasons. Six phenological stages-before flowering, setting, bunch closure, veraison, half maturity, and before maturity-and three thinning levels (0%, 25%, and 50%) were assessed. Key parameters included total soluble solids (TSS), total acidity (TA), pH, sugar accumulation, maturity indices (TSS/TA, pH2 × °Brix), secondary metabolites (total anthocyanins, total tannins, total phenolic content, total polyphenol index), and yield. Cluster thinning significantly enhanced most quality traits, with veraison emerging as the optimal stage for improving phytochemical composition. The highest TSS was observed at setting, while maximum total anthocyanins, tannins, total polyphenol index (TPI), and maturity indices occurred at veraison under 50% thinning. Sugar accumulation was optimised at 25% thinning. Yield decreased proportionally with thinning intensity, with Control vines producing the highest yield and 50% thinning the lowest. Overall, 25% cluster thinning at veraison achieved a balance between improved grape quality and commercial yield, providing a practical management strategy for Merlot production.

Keywords: Merlot; cluster thinning; phenological stages; grape quality; yield; secondary metabolites

Received: September 25, 2025; Revised: November 11, 2025; Accepted: November 24, 2025; Prepublished online: July 23, 2026 

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