Fresh water is a key natural resource for food production, sanitation and industrial uses and has a high environmental value. The largest water use worldwide (~70%) corresponds to irrigation in agriculture, where use of water is becoming essential to maintain productivity. Efficient irrigation control largely depends on having access to reliable information about the actual plant water needs. Therefore, fast, portable and non-invasive sensing techniques able to measure water requirements directly on the plant are essential to face the huge challenge posed by the extensive water use in agriculture, the increasing water shortage and the impact of climate change. Non-contact resonant ultrasonic spectroscopy (NC-RUS) in the frequency range 0.1–1.2 MHz has revealed as an efficient and powerful non-destructive, non-invasive and in vivo sensing technique for leaves of different plant species. In particular, NC-RUS allows determining surface mass, thickness and elastic modulus of the leaves. Hence, valuable information can be obtained about water content and turgor pressure. This work analyzes and reviews the main requirements for sensors, electronics, signal processing and data analysis in order to develop a fast, portable, robust and non-invasive NC-RUS system to monitor variations in leaves water content or turgor pressure. A sensing prototype is proposed, described and, as application example, used to study two different species: Vitis vinifera and Coffea arabica, whose leaves present thickness resonances in two different frequency bands (400–900 kHz and 200–400 kHz, respectively), These species are representative of two different climates and are related to two high-added value agricultural products where efficient irrigation management can be critical. Moreover, the technique can also be applied to other species and similar results can be obtained.

In this work, dispersive wave propagation in plant leaves is studied using a long distance laser-based ultrasonic method. Guided waves traveling through and along the midrib, as well as in the lamina, are measured and analyzed via wave attenuation, characteristic diagram and dispersion curves. Additionally, an estimation of the leaf elastic properties using the Rayleigh–Lamb model, supported by thickness-resonance frequency constraints, is proposed. This study paves the way for the development of acoustic fingerprints for the identification of leaves and their non-invasive and fairly non-destructive mechanical characterization, which is highly related to the plant hydraulics and photosynthetic activity.

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The study of vegetable tissue is essential in plant growth analysis. The elasticity is an important characteristic when firmness is studied in fruits or when turgidity is tested in plant leaves. This property gives information about the ripeness of fruits, the water content in leaves, and, in general, the morphological and physiological state of plants. Hence, the propagation of elastic waves in these media is a useful tool that gives information about the developmental stage of the plant. In this sense, laser ultrasonics is a promising method in the characterization of composite materials. In a certain way, vegetable tissues can be thought of as a kind of biological composite abstracted as a stratified plate of several layers. In this work, as a first approximation, we propose a methodology to generate guided waves in vegetable tissues by using laser ultrasonics techniques. We implement a pulsed laser system to induce ultrasonic waves without damage on the samples. Besides, we develop a methodology for processing the propagated acoustic waves. This analysis allows us the assessment of dispersive properties, a useful tool when determining important descriptors such as firmness index, water content, and provide useful information to feed biophysical models, among others.