Search

[Background] Non-contact resonant ultrasound spectroscopy (NC-RUS) has been proven as a reliable technique for the dynamic determination of leaf water status. It has been already tested in more than 50 plant species. In parallel, relative water content (RWC) is highly used in the ecophysiological field to describe the degree of water saturation in plant leaves. Obtaining RWC implies a cumbersome and destructive process that can introduce artefacts and cannot be determined instantaneously., [Results] Here, we present a method for the estimation of RWC in plant leaves from non-contact resonant ultrasound spectroscopy (NC-RUS) data. This technique enables to collect transmission coefficient in a [0.15–1.6] MHz frequency range from plant leaves in a non-invasive, non-destructive and rapid way. Two different approaches for the proposed method are evaluated: convolutional neural networks (CNN) and random forest (RF). While CNN takes the entire ultrasonic spectra acquired from the leaves, RF only uses four relevant parameters resulted from the transmission coefficient data. Both methods were tested successfully in Viburnum tinus leaf samples with Pearson’s correlations between 0.92 and 0.84., [Conclusions] This study showed that the combination of NC-RUS technique with deep learning algorithms is a robust tool for the instantaneous, accurate and non-destructive determination of RWC in plant leaves., This research was supported by Grant (DPI2016-78876-R-AEI/FEDER, UE) from the Spanish State Research Agency (AEI) and the European Regional Development Fund (ERDF/FEDER). It was also partially funded by Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA) Grant number RTA2015-00054-C02-01 and by Gobierno de Aragón H09_17R research group (DOC INIA-CCAA (ESF)). Funding was provided by Conselleria d’Educació, Investigació, Cultura i Esport (APOSTD/2018/203 (ESF)).

Described here is the application of a technique based on the excitation, sensing and spectral analysis of thickness resonances of plant leaves using air-coupled and wide-band ultrasound pulses (150–900 kHz) to monitor variations in leaf properties caused by plant responses to different environmental stimuli, such as a sudden variation in light intensity (from 2000 to 150 μmol m −2 s −1 ), sudden watering after a drought period, and along the diurnal cycle (3–5 days, with continuous variation in light intensity from 150 to 2000 μmol m −2 s −1 and change in temperature of about 5°C). Four different widely available species, both monocots and dicots and evergreen and deciduous, with different leaf features (shape, size, thickness, flatness, vascular structure), were selected to test the technique. After a sudden decrease in light intensity, and depending on the species, there was a relative increase in the thickness resonant frequency from 8% to 12% over a 25- to 50-min period. After sudden watering, the relative increase in the resonant frequency varied from 5% to 30% and the period from 10 to 400 min. Finally, along the diurnal cycle, the measured relative variation is between 4% and 10%. The technique revealed differences in both the amplitude of the frequency oscillations and the kinetics of the leaf response for different species and also within the same species, but for specimens grown under different conditions that present different cell structures at the tissue level. The technique can be equally applied to the leaves of any species that present thickness resonances.

The turgor loss point (TLP), which is considered a threshold for many physiological processes, may be useful in plant-breeding programs or for the selection of reforestation species. Obtaining TLP through the standard pressure–volume (p–v) curve method in a large set of species is highly time-consuming and somewhat subjective. To solve this problem, we present an objective and a less time-consuming technique based on the leaf resonance able to calculate the relative water content (RWC) at TLP (RWCTLP). This method uses air-coupled broadband ultrasonic spectroscopy to obtain the sigmoidal relation between RWC and the standardized resonant frequency (f/fo). For the 13 species measured, the inflexion point of the RWC– f/fo relationship (RWCTLP o ff / ) was not statistically different from the value of RWC at the TLP obtained with the p–v curves (RWCTLP p–v).

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., Work of Domingo Sancho Knapik is supported by a DOC INIA contract co-funded by INIA and ESF. We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI).

Different tissues in Phormium tenax leaves present different morphologies and mechanical properties according to the different roles or functions that they play during the plant life. This is an example of what is known as functional design, a concept which is used in different scientific fields. Four different ultrasonic techniques comprising air-coupling and gel coupling, longitudinal and shear waves, normal and oblique incidence and low (0.2 MHz) and high frequencies (2.25 MHz) have been employed to study these leaves. By changing these experimental conditions it is possible to propagate longitudinal and shear waves in the different tissues present in these leaves (spongy mesophyll, chlorenchyma and sclerenchyma fibres) and in different directions so it is possible to determine their ultrasonic properties (velocity and attenuation) and hence their main elastic mudulii. Additional analysis of microscopic images of the tissues permit to study the correlation between this elastic and ultrasonic tissues properties and main microscopic features like cell size and cell wall thickness, which are determined by the different function of these tissues.

The large water requirements of Vitis vinifera L. together with an increase in temperature and drought events imply the need for irrigation in the driest areas of its distribution range. Generous watering may reduce grape quality so irrigation should be precisely regulated through the development of new methods of accurate irrigation scheduling based on plant 'stress sensing'. Two new methods, the reflectivity in the S-band and the broadband ultrasonic spectroscopy, can be used as non-invasive and reproducible techniques for the study of plant water relations in V. vinifera. On one hand, the measurement of reflectance at frequencies around 2.4 GHz gives an excellent accuracy when the changes in the existing area (S) between two reflectance curves are correlated with the relative water content (RWC). On the other hand, an improvement of the broadband ultrasonic spectroscopy based on the enlargement of the analysis frequency window provides, apart from the determination of the turgor loss point (TLP), additional information about the leaves without additional computational cost or additional leaf information requirements. Before TLP, the frequency associated with the maximum transmittance (f/f(o)), the macroscopic elastic constant of the leaf in the Z direction (c(33)) and, specially, the variation of the attenuation coefficient with the frequency (n), were highly correlated with changes in RWC. Once turgor is lost, a shift in the parameters directly related to the attenuation of the signal was also observed. The use of both techniques allows for a more convincing knowledge of the water status in V. vinifera.

This work presents the application of a technique based on the excitation, sensing and spectral analysis of leaves thickness resonances using air-coupled and wide-band ultrasound to monitor variations in leaves properties due to the plant response along light/dark cycles. The main features of these resonances are determined by the tautness of the cells walls in such a way that small modifications produced by variations in the transpiration rate, stomata aperture or water potential have a direct effect on the thickness resonances that can be measured in a completely non-invasive and contactless way. Results show that it is possible to monitor leaves changes due to variations in light intensity along the diurnal cycle, moreover, the technique reveals differences in the leaf response for different species and also within the same species but for specimens grown under different conditions that present different cell structures at the tissue level. Published 43rd Annual UIA Symposium 23—25 April 2014 CSIC Madrid, Spain