Background 

Precision agriculture can provide farmers with valuable information on a range of aspects of their crops, such as nutrient levels, presence of pests and diseases, and abiotic stress. This data allows them to apply appropriate management strategies, helping to enhance crop yield. Although in the past few years remote sensing technologies have been advancing and becoming widespread, current methods for monitoring plant biomarkers directly are still very scarce. Thinking about the potential of these methods, Antonio Ruiz, Research Associate in the Haseloff Lab, at the Department of Plant Sciences, developed a low-cost electrochemical sensor to monitor pH of xylem sap in plants. “We focused on pH as it is quite a well reported biomarker, especially for abiotic stress. Sensors that measure moisture in soil are widely available, but it might not necessarily reflect on the plant status.
The plant might have the ability to endure a bit longer
in the soil that is dry, and you might be able to save some water, which is in high need in some places,”
said Antonio. 

Function

It is an electrochemical sensor designed to monitor the pH in xylem sap continuously. 

Development process

The first step in the sensor development was the material choice to ensure that the sensor could be used for long-term measurements and would be compatible with the plant organism. Antonio and his colleagues decided to deposit ruthenium oxide-based nanoparticles on top of the copper electrodes, as this material has been shown to be more stable and biocompatible than other options available. Also, the presence of ruthenium oxide is responsible for the H+ sensitivity, the ion measured to determine pH. When the first tests were performed with the sensor, the data generated had high noise rates, resulting in inaccuracies on the measured pH. To improve the stability and reduce the noise, the developers applied a cellulose-based coating to the electrodes. The final device was able to monitor pH in the xylem sap of tomato plants for at least 5 hours, with low noise. 

Target user

Researchers and farmers aiming to assess plant abiotic stress in vivo directly.

Comparison to other technologies

Lab-bench pH meters with the reference probe suspended in a solution are common and well tested pieces of equipment. However, there are not many studies reporting devices to measure pH directly in the plant xylem. Antonio explains that most of the publications available use very destructive processes, extracting the sap, and then measuring the pH afterwards with a common pH meter. “We took the precision process of the sensors, which is normally done with benchmark equipment, to an easy setting that I managed to assemble at home, during lockdown. I don’t think it’s reported elsewhere of any other low-cost devices which are similar to ours,” explains Antonio. Another new aspect that this work has highlighted was the healing process that takes place between the sensor and the plant tissue, a response that is not well reported in the literature, and it is one of the major challenges in the field of implanted biosensors.