Biomechanical analysis reveals hormonal crosstalk in inflorescence bending of cut stock flower

In cut stock (Matthiola incana L.), inflorescence bending is primarily caused by the bending moment generated by the weight of the floral structure. This phenomenon results in drooping of inflorescence onto neighboring plants, diminishing ornamental quality and increased space requirements among plants. Reduction of the bending moment increases the inflorescence's curvature radius, limits apical deviation and enables more compact plant spacing. This study investigated the individual and interactive effects of three auxins (IAA, NAA and IBA) and three cytokinins (KIN, BA and ZT), each applied three times beginning at the 10-leaf stage and repeated at 7-day intervals at the concentrations of 0, 10 and 100 ppm. Treatments were evaluated through a series of nine factorial experiments. Of the 81 auxin-cytokinin combinations tested, 15 treatments completely prevented inflorescence bending by producing upright stems. The effective treatments significantly reduced the length-to-diameter ratio (LDR) (p < 0.05), which was strongly correlated with both apical deviation (δ) and bending stress. LDR had a greater influence on δ than the stem's Young's modulus (E), suggesting that hormonal crosstalk primarily affected stem geometry rather than material stiffness. Notably, combinations of IBA with either KIN or ZT improved biomechanical stability by lowering the center of gravity, and shortening the torque arm, thereby reducing bending stress below critical levels. Treatments containing BA were associated with higher E and mechanical resistance index (MRI), indicating enhanced stem stiffness. Among the most effective combinations, IAA 10 ppm + BA 10 ppm yielded the most visually desirable inflorescences-long, straight stems with ideal floret arrangement.