Non-Resonant Air-Coupled Ultrasound for Quantifying Leaf Hydration Dynamics and Turgor Loss in Potato Leaves

This study investigates the use of non-resonant air-coupled ultrasound technique as a non-destructive tool for quantifyinghydration dynamics in potato (Solanum tuberosum) leaves. Conventional methods for measuring leaf water status, such asrelative water content or pressure–volume (P–V) curves, are destructive and labor-intensive, whereas ultrasound can probeinternal tissue structure through mechanical wave propagation. In this work, freshly detached potato leaves were graduallydehydrated under controlled laboratory conditions while ultrasonic transmission signals were recorded using broadbandair-coupled transducers operating between 250 kHz and 900 kHz. Time-domain and frequency-domain features, includingpeak-to-peak amplitude, time-of-flight (ToF), spectral amplitude, and transmission coefficient, were extracted and analyzed as afunction of relative water content (RWC). The normalized peak-to-peak (pk–pk) amplitude curve exhibited a clear correlation tothe P–V curve, with ultrasonic turgor loss point (TLP) candidate (TLPc = 0.973) that closely matched the physiological turgorloss point (TLP) obtained from P–V analysis (RCWTLP = 0.978). ToF showed a characteristic biphasic response with a slightinitial decrease at high RWC followed by a sharp decrease around the dehydration threshold (TLPc ≈ 0.956), reflecting theprogressive loss of cell-wall tension and increasing mesophyll deformability that reduce acoustic velocity during dehydration.Spectral amplitude and transmission coefficient exhibited frequency-dependent sensitivity that peaked near 650 kHz. Allultrasonic parameters produced TLP candidates within 2.5% of the physiological reference, confirming that acoustic propagationis governed by turgor-driven structural transitions in the mesophyll. These results establish that non-resonant ultrasoundtechnique can characterize hydration status in leaves that lack thickness resonances, extending the scope of ultrasonichydration sensing beyond resonant techniques. The high physiological TLP observed for potato confirms that turgor is lost veryearly during dehydration, reflecting the species limited osmotic adjustment and drought tolerance. The limitation of this study isthat measurements were performed on detached leaves under controlled conditions; future in situ validation on intact plantsunder varying microclimates is required.Future work will also focus on deriving attenuation coefficients to separate intrinsicviscoelastic attenuation from absorption mechanisms. Overall in this study, pk–pk amplitude emerged as the most sensitive,computationally efficient, and practical parameter for real-time ultrasonic monitoring of leaf hydration and early stress detection