Synergistic Effects of Gibberellic Acid, Forchlorfenuron, Thidiazuron, and Brassinosteroid Combinations on Seedless Berry Development and Quality Enhancement in ‘Shine Muscat’ and ‘Red Muscat of Alexandria’ Grapes

Plant growth regulators (PGRs) are widely used to enhance table grape quality, yet cultivar-specific responses and microstructural outcomes remain insufficiently resolved. We evaluated five PGRs-gibberellic acid (GA₃), 24-epibrassinolide (EBR), forchlorfenuron (CPPU), thidiazuron (TDZ), and streptomycin (SM)-in 'Shine Muscat' and 'Red Muscat of Alexandria'. Twelve combinations were applied at full bloom (D0) and 14 days post-anthesis (D14), and morphological, physiological, and histological traits were monitored over 104 days. In 'Shine Muscat', GA₃ + TDZ and GA₃ + CPPU achieved nearly 100% seedlessness, while CPPU and TDZ markedly increased berry weight and size via cortical expansion. GA₃ increased berry weight but caused rachis thickening and reduced total soluble solids (TSS). EBR at concentrations ≥ 0.2 mg L^-1 inhibited berry growth, and SM reduced berry size and weight. Histology showed that TDZ and CPPU enlarged cortical area and epidermal thickness, potentially strengthening pericarp robustness, whereas GA₃ enlarged vascular bundle area. 'Red Muscat of Alexandria' showed generally weaker responses, reflecting differences between <i>Vitis vinifera</i> and hybrid backgrounds. These results demonstrate that combinatorial PGR strategies can be tailored to optimize berry development and quality through coordinated regulation of growth, metabolism, and microstructure. TDZ and CPPU exhibit strong production potential for enlargement, whereas GA₃ and EBR require careful dose optimization to balance benefits and quality risks.