We have developed an electrochemical sensor for monitoring the levels of salicylic acid (SA) and abscisic acid (ABA) in the plant stem. The sensor design is devised considering that cowpea possesses a dicot stem, leading to the arrangement of vascular bundles along the periphery. Also, the diameter of cowpea plants ranges from 5mm to 10mm. Consequently, the active region of the sensor penetrates through the vascular bundle, leaving the remaining portion accessible for signal acquisition. The sensor was calibrated for the entire concentration range of SA and ABA typically found in plants. Like animals, plants produce hormones as signaling molecules to control various growth processes. SA and ABA are two primary stress hormones in plants. Progressive variations in their levels have been reported in many drought, salt, and cold/heat-stressed plants.

We have tested the sensors in a growth chamber facility in June 2024 and a greenhouse in November 2024. Cowpea plants were grown under controlled conditions in coir pith, hydroponic solutions, and a mixture of peat and perlite media. We used a completely randomized design, with 3 replications used for each treatment. We observed that plants grown in hydroponics had the highest assimilation, transpiration, stomatal conductance, and CO2 concentration, which indicates high water use. In contrast, plants grown in the peat perlite media demonstrated the lowest levels for these parameters, thus indicating water stress. Meanwhile, plants grown in coir pith media displayed intermediate values for these metrics. These results strongly support our sensor measurements. It was noted that the plants grown in the peat perlite media exhibited the highest concentrations of both SA and ABA, with SA concentration reaching upto 8.1 µM and ABA concentration reaching upto 28 µM, indicating a water stress scenario. In contrast, the plants grown in the hydroponic solution exhibited the lowest concentrations of both SA and ABA, with SA and ABA concentrations peaking 0.195 µM and 4.65 µM, respectively, indicating sufficient water use by the plants. Meanwhile, plants grown in coir pith media displayed intermediate concentrations for both the hormones, which indicates moderate stress. The above and below-ground plant biomass data further corroborates the plant physiological traits and hormonal responses.

Our immediate research plan includes the following:

• Complete statistical analysis correlating plant physiological traits with the hormonal responses. • Complete validating the SA and ABA measurements with the sensors against the values obtained from high-performance liquid chromatography (HPLC).

We have developed a three-electrode based electrochemical sensor for salicylic acid detection in plant sap. The sensor design is devised considering that cowpea possesses a dicot stem, leading to the arrangement of vascular bundles along the periphery. Also, the diameter of cowpea plants ranges from 5mm to 10mm. Consequently, the active region of the sensor penetrates through the vascular bundle, leaving the remaining portion accessible for signal acquisition. The sensor was characterized for different concentrations of salicylic acid (SA) typically found in small plants. Like animals, plants produce hormones as signaling molecules to control various growth processes. Salicylic acid (SA) and abscisic acid (ABA) are two primary stress hormones in plants. Progressive variations in their levels have been reported in many drought, salt, and cold/heat-stressed plants.

The sensor was calibrated with different SA concentrations. Multiple sensors were calibrated and all of them demonstrated similar responses, therefore confirming their repeatability. The sensor was tested for potential cross-interference from other hormones that are typically present in the sap. The results show that the sensor exhibits minimal response to interfering hormones, when compared to even a very low concentration of SA. We also conducted experiments to assess the reproducibility and longevity of the sensor. The sensor demonstrated reproducible response for up to 3 days. Beyond this period, the sensor’s performance began to decline, a phenomenon attributed to the degradation of the immunoassay. To overcome this challenge, our subsequent efforts will focus on substituting the assay with a more durable assay to prolong the sensor’s functional lifespan.

We tested the growth facilities at Texas A&M University and tested the growth of cowpea under controlled growth chamber conditions in coir pith, two types of hydroponic solutions, and a mixture of peat and perlite. We observed that plants grown in hydroponics had the highest stomatal conductance and transpiration, which indicates high water use. The experiments involving the sensors under two CO2 levels and temperature conditions will commence in February 2024.

Our immediate research plan includes the following:

• replace the current immunoassay with a more durable assay to prolong the sensor’s functional lifespan

• develop and optimize an abscisic acid sensor alongside the SA sensor

• test the hormone sensors in cowpea plants under the effects of growth medium and elevated CO2 and temperature levels. These tests to identify the correlations between hormonal variations and plant physiological responses will commence in February 2024. This experiment will use cowpea as the test plant due to its ultra-short duration life cycle and versatility of use. Plants will be grown in a super lightweight media of coir pith and compared against hydroponics and regular potting mix, which will be used as a control.

The sensors will be deployed 20 days after plant germination and plant performance will be monitored. Plants will be grown under ambient CO2 conditions and elevated CO2 conditions in growth chambers located at Texas A&M University. NASA facilities will not be used for the plant growth experiments.