Water Stress Assessment through Gaseous Emissions Monitoring: A Case of Study in Tomato Fields
Authors: Fabbri Barbara , Valt Matteo , Gherardi Sandro , Gaiardo Andrea , Malagù Cesare , Guidi Vincenzo 
Published: July 2021

Abstract

The innovative contribution of this research lies in designing, developing and validating a simplified technology platform consisting of a hardware for monitoring of gaseous emissions, such as Volatile Organic Compounds (VOCs) from the soil-plant-atmosphere system of crops, i.e., tomato. We analyzed experimental data acquired in situ by portable sensing units based on Metal-OXide (MOX) gas sensors, thus comparing the results with meteo-sat data and farming operations. A dependence of gaseous emissions on the hydric/metabolic status of the plants together with a correlation between collected sensor signals and significant events for the crops were proved. Finally, for each crop, we identified and calibrated one sensor to assess their water state.

Introduction

Although the technological science provides several tools and analysis techniques for the remote sensing, a well-structured system has not been widely documented for the parallel evaluation of the soil moisture status and the monitoring of emissions variability of the crops over a whole growing season [1]. Plants emit a broad range of VOCs. These emissions are affected by internal (genetic and biochemical) and external (abiotic and biotic) factors. Nonetheless, hydric stress is not yet considered as an input in plant emissions, maybe because it affects them in different ways. Therefore, it is necessary to clarify the relationship between the soil water availability and variability and the potential need of water by plants [2]. For the estimation of VOCs emitted from soil and plants equipment should be cheap and not bulky, whereas analysis techniques should be easy to perform and not invasive. In this perspective, electronic nose is a potential technology, which may comply with these requirements allowing real-time acquisition, providing a fast response without a direct contact with soil or plants, but it needs a complex signals deconvolution system.

Method

Starting from this consideration, we designed and prepared a portable unit for monitoring gaseous emissions in tomato crop. By means of a proper simple system, we were able to collect data in situ from the sowing to the harvest. After a preliminary lab-test with target gases identified by literature, we selected 4 MOX gas sensors to integrate in the dedicated hardware [3]: SnO2 + Au 2%, SnO2 + Pt 2%, WO3, and SnO2 + Pd 2%, each employed at its best operating temperature, ranging from 350 to 450 °C.
The experimental activities were organized as follow: 1) on-field experimental measurements of gaseous emissions in the crops, with an array of chemoresistive sensors, the sensor response was calculated as (Gcrop-Gbase)/Gbase
where Gcrop
is the sensor conductance during the crop growth and Gbase
is the conductance at the crop sowing; 2) comparison of sensors response to meteo-sat data and farming operations, 3) lab-tests of the selected devices with target gases (ethylene, ethanol) at proper concentrations, deduced by literature, to identify sensors calibration parameters (allometric fit y=axb
, with y sensor response, x gas concentration, a and b fit parameters); 4) field-lab results comparison to select one sensor suitable to monitor water stress for each crop: application of allometric fit parameters, considering as y’ the sensors response on-field, to calculate x’ gas concentration on-field.

Results and Conclusions

Signal analysis allowed to identify a strong dependence of gaseous emissions on significant events for the crop, such as irrigation and rain, or for its growth evolution. In the last case, it was possible to select one sensing material, which correlates the emissions with the tomato growth. Indeed, SnO2 + Pt 2% showed a decrease of response up to a constant value when the tomato plants development reached its culmination, after which only the fruits continued their growth (Fig.1). From a technological point of view, the highlighted information may provide irrigation advices about the time of intervention, whereas from a biological point of view it could be possible to investigate the correlation between morphological changes in plants and their water stress, getting at the root cause.

References

[1] M. Ruiz-Altisent,et al., Sensors for product characterization and quality of specialty crops-A review, Computers and Electronics in Agriculture 74, 2010, pp. 176-194.
[2] E. Ormeño, et al., Water deficit stress induces different monoterpene and sesquiterpene emission changes in Mediterranean species. Relationship between terpene emissions and plant water potential, Chemosphere 67, 2007, pp. 276-284.
[3] B. Fabbri, M. Valt, C. Parretta, S. Gherardi, A. Gaiardo, C. Malagù, F. Mantovani, V. Strati, V. Guidi, Correlation of gaseous emissions to water stress in tomato and maize crops: From field to laboratory and back, Sensors & Actuators: B. Chemical 303 (2020) 127227.
Figure 1. SnO2 + Pt 2% response during two months of tomato on-field monitoring.

Figure 1