Ultrasonic assessment of plant tissues water potential

The main characteristics of the ultrasonic method to determine plants water status (water content or water potential) are:

Measurements are performed in the leaves
Air-coupled ultrasound is used; through transmission configuration
Thickness resonant modes in the leaves are excited and sensed
Magnitude and phase spectra of the thickness resonant modes are analysed
Leaf properties: thickness, density, elastic modulus, elastic damping are extracted form these spectra
Water conent and turgor pressure are inferred from these properties

The technique is based in three main pilars:

The availability of efficient air-coupled ultrasonic transducers. Capable of:.
- Sensitive enough to transmit ultrasonic pulses through plant leaves
- Bandwidth wide enough so that it is possible to measure the full spectrum of, at least, the first thickness resonance.
- So far, the CSIC air-coupled transducers technology have bee used for this purposes as they present the best available sensitivity and widest bandwidth within the frequecy range of interest for this pplication: 0.1-2.0 MHz. LINK
Techniques to solve the inverse problem, of plate thickness resonances, so that it is possible to extract the leaf parameters from the measure spectra
Knowledge of the relationship between ultrasonic properties and plant physiology to extract water content and turgor pressure from the leaf ultrasonic properties. LINK

The main advantages of this technology are:

It is completely non-invasive and non destructive. Unlike most of the other technologies aimed to determine plant water status, that require to cut leaves or other parts of the plant to process the samples.
Equipment is fully portable. Compared with the Scholander pressure chamber, which is the golden standard which is bulky and heavy.
the time to take and process a measurement is very reduced (few seconds), once again compared with other technologies (Pressure chamber) that are quite time consuming

The first demonstration that it is possible to measure thickness resonances in plant leaves using air-coupled ultrasound and to extract leaf prperties from these measurements was published in 2009:

T. E. Gómez Álvarez-Arenas, D. Sancho-Knapik, J. J. Peguero-Pina, and E. Gil-Pelegrín, “Noncontact and noninvasive study of plant leaves using air-coupled ultrasounds,” Applied Physics Letters, vol. 95, no. 19, p. 193702, 2009.

Shortly after that, it was possible to establish a procedure to extract water potential form the ultrasonic measurements

Domingo Sancho-Knapik, Tomás Gómez Álvarez-Arenas, José Javier Peguero-Pina, Eustaquio Gil-Pelegrín, “Air-coupled broadband ultrasonic spectroscopy as a new non-invasive and non-contact method for the determination of leaf water status.”, Journal of Exp. Botany, Oxford University Press, 2010, vol. 61, num. 5, pp. 1385-1391

And to correlate changes in the celular structure of the tissues due to the loss of water with the ultrasonic measurements

Domingo Sancho-Knapik, Tomás Gómez Álvarez-Arenas, José Javier Peguero-Pina, Victoria Fernández, Eustaquio Gil-Pelegrín, “Relationship between ultrasonic properties and structural changes in the mesophyll during leaf dehydration.” Journal of Experimental Botany, 2011, vol. 62, pp. 3637-3645

and hence, to analyse the relationship between ultrasonic leaf properties, leaf elasticity and leaf water content and turgor pressure

Domingo Sancho-Knapik, Hector Calas, Jose Javier Peguero-Pina, Antonio Ramos Fernandez, Eustaquio Gil-Pelegrin, TE Gómez Alvarez-Arenas, “Air-coupled ultrasonic resonant spectroscopy for the study of the relationship between plant leaves’ elasticity and their water content.” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2012, vol. 59, pp. 319-325

and compare this technique with other technologies:

D. Sancho‐Knapik, J.J. Peguero‐Pina, H. Medrano, M.D. Fariñas, T. Gómez Álvarez‐Arenas, E. Gil‐Pelegrín, “The reflectivity in the S‐band and the broadband ultrasonic spectroscopy as new tools for the study of water relations in Vitis vinifera L.” Physiologia plantarum, vol. 148, no. 4, pp. 515-521, Aug. 2013

Then, the applicability of this technology to a broader number of species was studied:

D. Sancho‐Knapik, J.J. Peguero‐Pina, H. Medrano, M.D. Fariñas, T. Gómez Álvarez‐Arenas, E. Gil‐Pelegrín, “Ultrasonic spectroscopy allows a rapid determination of the relative water content at the turgor loss point: a comparison with pressure–volume curves in 13 woody species” Tree Physiology, vol. 33, no. 7, pp. 695-700, July. 2013

and to monitor in vivo plant water status changes under different stimulii

Fariñas, M. D., Knapik, D. S., Pina, J. J. P., Pelegrin, E. G., & Álvarez-Arenas, T. E. G. (2014). Monitoring Plant Response to Environmental Stimuli by Ultrasonic Sensing of the Leaves. Ultrasound in medicine & biology, 40(9), 2183-2194.

More recently we have introduced Machine Learning procedures to process the leaf ultrasonic information and to correlate it with plant water status and water potential