Non-destructive imaging of root system growth is required to understand the influence of water and nutrient availability, diseases, and stress on the productivity of soybean plants. Limitations in the current root imaging and monitoring approaches have hindered the development of stress-resistant soybean varieties with optimized root systems. The current state-of-art methods for measuring root growth are either invasive or too complex and expensive, restricting their application in soybean research. Therefore, there is a need for a low-cost and non-invasive approach to monitoring root growth and development in soybean plants. To address the need, we propose a new approach that utilizes electrochemical impedance measurements for creating a 4DMap of the root system to visualize the temporal and spatial changes in its architecture and spread during plant growth.

Our long-term goal is to develop a portable device that can continuously monitor the growth of soybean roots. The root system of soybean plants is essential for their capacity to absorb water and nutrients and maintain overall stability but imaging the roots in the soil is challenging. We propose a new approach that utilizes electrochemical impedance measurements to monitor plant roots' spatial and temporal changes. We can monitor the changes in root spatial distribution and growth by installing a miniature electrode on the plant stem and deploying an array of miniature electrodes in the soil surrounding the plant roots. The impedance of plant tissue, like roots and stems, depends on their size, shape, and ionic content, but it is significantly lower than soil impedance. Thus, the impedance between the electrodes in the plant stem and soil will depend on the proximity of the root to the electrode location. Monitoring the impedance between the stem and different locations in the soil will allow us to monitor the spatial distribution and growth of the root system.

1. Utilize the impedance reader and electrodes to build a device capable of monitoring impedance changes between a single stem electrode and 27 different electrodes distributed in a three-dimensional grid in a rhizotron.
2. Utilize the device to monitor the root growth in several soybean varieties grown in rhizotrons and validate the root distribution measurements through comparison with root measurements at different time intervals.
3. Investigate the influence of thermal stress on root growth and development in soybean varieties.

In year 1, we will develop the prototype of the instrument in a rhizotron for validation experiments. The prototype will include 27 needle electrodes distributed in a three-dimensional grid in the rhizotron. The electrodes will be connected to potentiostat devices through a multiplexer, and instrumentation software will be developed to monitor the impedance between the stem-mounted electrode and the grid electrodes. We will develop computer models that interpret the measured spatial distribution of impedances to create a spatial map of the root. Plants will be extracted, and their root architecture will be compared to the model predictions to validate and improve the root imaging.

In year 2, we will utilize the instrument prototype and developed software to monitor the temporal changes in the root architecture during plant growth. Soybean varieties with different root architectures will be grown in instrumented rhizotrons. The root development will be monitored, and plants will be extracted at different stages of growth to compare the instrument predictions of root growth with the physical root measurements. This will allow us to tune the instrument for monitoring plants with various root architectures.

In year 3, we will utilize the root monitoring instrument to study the plant root development under different thermal stresses. Soybean plants of different varieties will be grown under different levels of thermal stress, and their root growth will be monitored. The in-situ measurement of root growth will allow us to monitor the influence of thermal stress on plant root growth.

The successful completion of the research will provide an opportunity to rapidly develop low-cost sensors for monitoring root growth. The research will also provide insights into the relationship between impedance parameters and root morphology and the impact of stress factors on root growth. We aim to complete the following milestones to demonstrate the efficacy of the proposed instrument in monitoring root growth:
Year 1 Milestone: Development of instrumented rhizotron and computer models capable of creating spatial and temporal (4DMap) of the plant root. Validation of instrument measurement with measurement of soybean plant of a single variety.
Year 2 Milestone: 4DMaps of the root development in five different soybean varieties with different root architectures.
Year 3 Milestone: 4DMaps of the root development in two different varieties of soybean plants subjected to thermal stresses. Design of the instrument prototype that is amenable for deployment in the field.

Update:
We have made the following progress towards the year 1 goal:
1. Identified a computational approach to identify the distribution of current sources in a medium from the local measurement of potentials.
2. Developed an instrumented rhizotron for measuring potential at different points.
3. Identified two different varieties of soybean seeds for root mapping
4. Demonstrated that the selected soybean varieties can grow well in the instrumented rhizotron
5. Developed a method to apply electrical stimulation to the plant stem for root mapping

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Developing root growth monitoring instruments will significantly affect soybean farmers and the industry. Researchers will be able to identify soybean varieties with greater resistance to biotic stresses, leading to increased crop protection and higher yields. Additionally, the instruments will aid in identifying varieties that can maintain high yields under future climate scenarios characterized by higher temperatures and carbon dioxide content. The ability to select varieties with optimized root architecture will benefit farmers and the soybean industry and contribute to global food security.