Airflow-Induced Modal Competition Reveals Optimal Vibration Modes for Non-Contact Pollination of Greenhouse Tomato Plants

Tomato flowers require vibrational stimulation for pollen release, but natural pollination is often limited in greenhouse environments. Airflow-assisted pollination offers a non-contact alternative; however, the vibration response of tomato plants under airflow excitation and its relationship with pollen release remain insufficiently understood. This study investigated the aeroelastic vibration characteristics of greenhouse tomato plants and their relevance to pollination efficiency. Mechanical properties of tomato stems and flower stalks were measured using a texture analyzer. Axial-flow fans were used to excite plant vibration, while high-speed imaging, Welch power spectral density analysis, and coherence analysis were applied to identify vibration modes. Pollen-shedding experiments were conducted to identify pollination-related frequency bands, and a functional band-weighted response index, Γ(U), was established from normalized integrated displacement-PSD proportions. Results showed that the main stem elastic modulus increased from 10.58 MPa at the apex to 35.69 MPa at the base, while flower-stalk bending stiffness ranged from 16.74 to 52.15 N·mm2. Three vibration bands were identified: Plant band (2.1 Hz), Stem band (8.0 Hz), and Flower band (14.0 Hz). Pollen shedding was more effective in the Stem and Flower bands, and Γ(U) reached its maximum among the seven tested velocities at 5.08 m/s. These findings demonstrate that pollination efficiency under airflow is governed by the spectral distribution of vibration response among frequency bands—rather than by wind speed magnitude alone—and that Γ(U) provides a candidate quantitative indicator for guiding airflow parameter selection.