Preharvest Sound Waves Enhance Yield and Nutritional Quality of Greenhouse Vegetables in a Frequency-Dependent Manner

This study evaluated whether preharvest ultrasonic sound waves can simultaneously improve yield and nutritional quality of three short-cycle vegetables with contrasting edible organs: carrot (root), tomato (fruit) and Chinese cabbage (leaf). Plants were grown in a greenhouse and exposed for 20 days before harvest to four acoustic regimes: no sound (CK), low-frequency (LF, ~200 Hz), medium- frequency (MF, ~1 kHz) and high-frequency (HF, ~8 kHz) continuous sine waves at 90 ± 2 dB for 2 h per day. MF consistently produced the highest marketable yield, increasing area-based yield by about 6–7% in cabbage (from ~7.1 to 7.5–7.6 kg/m2) and by 8–12% in carrot and tomato relative to CK, whereas HF failed to improve yield and reduced cabbage yield to 6.6 kg/m2. MF also enhanced edible quality: vitamin C increased by 5–10% in cabbage and 10–15% in carrot and tomato; total phenolics increased in all species, while HF often drove phenolics below control values. MF raised total soluble solids by 0.3–0.5 °Brix in carrot and tomato, slightly reduced moisture, and increased dry matter, carbohydrate and protein contents, indicating denser, nutrient-rich tissues. Correlation and clustering analyses showed that, especially under MF, yield was strongly and positively associated with total phenolics, protein, carbohydrates and dry matter (r up to 0.97) and negatively with moisture, and principal component analysis grouped MF-treated cabbage and carrot in the “high-yield–high-quality” sector of the trait space.