Agar‐Based Optical Sensing System With a Phosphorescent Grid Pattern for Measuring Stress Distributions of Root Growth Using the Sampling Moiré Method

ABSTRACT Plant roots grow through soil to acquire water and nutrients while generating mechanical forces that deform their surroundings; however, clarifying the resulting stress distribution between roots and soil remains challenging. Transparent soil systems enable the optical analysis of root structures. Additionally, methods such as digital image correlation tracking soil particles and X‐ray digital volume correlation have been used to measure stress distribution on such transparent systems. However, long‐term stability and system complexity limit their practicality for extended multi‐day measurements. Herein, we propose an optical measurement system that combines a transparent agar soil with a phosphorescent grid pattern and the sampling moiré (SM) method. The system captures timelapse images of grid pattern deformation, which are analyzed using the SM method to obtain high‐resolution displacement measurements with 20 µm effective resolution. Using a radish sprout, we obtain x and y direction displacement distributions, showing distinct deformation near the root tip. Stress calculation with Hooke's law and a strain‐dependent Young's modulus reveal distinct stress distribution at the root tip, with a maximum of 6.16 kPa consistent with reported values for small roots. This approach provides a practical method for evaluating root–soil mechanical interactions.