Capturing in-field root system dynamics with RootTracker

Abstract

Optimizing root system architecture offers a promising approach to developing stress tolerant cultivars in the face of climate change, as root systems are critical for water and nutrient uptake as well as mechanical stability. However, breeding for optimal root system architecture has been hindered by the difficulty in measuring root growth in the field. Here, we describe the RootTracker, a technology that employs impedance touch sensors to monitor in-field root growth over time. Configured in a cylindrical, window shutter-like fashion around a planted seed, 264 electrodes are individually charged multiple times over the course of an experiment. Signature changes in the measured capacitance and resistance readings indicate when a root has touched or grown close to an electrode. Using the RootTracker, we have measured root system dynamics of commercial maize (Zea mays) hybrids growing in both typical Midwest field conditions and under different irrigation regimes. We observed rapid responses of root growth to water deficits and found evidence for a "priming response" in which an early water deficit causes more and deeper roots to grow at later time periods. Genotypic variation among hybrid maize lines in their root growth in response to drought indicated a potential to breed for root systems adapted for different environments. Thus, the RootTracker is able to capture changes in root growth over time in response to environmental perturbations.

Figure 1

The RootTracker system. The (A) RootTracker consists of 22 electrodes on each of 12 paddles for detecting roots. B, Hundreds of RootTrackers in a typical field installation. C, Diagram illustrating how multiple RootTrackers in a field communicate raw sensor data via radio transmission to a central base station, which sends the data to cloud-based servers, where the data can be analyzed on local servers for extracting root detections.

Figure 2

Shovelomics comparison in Trial 1. A, Sample photograph of root system excavated from a RootTracker. B, Image segmenting for root pixels. Root pixels located in the region that would interact with RootTracker detectors (red boxes) were counted for each plant. C, Median daily root detection rate time-averaged across the entire trial, $\stackrel{\sim }{R}$, grouped by genotype, versus median shovelomics image root pixels, $\stackrel{\sim }{S}$, grouped by genotype. Correlation between RootTracker detections and shovelomics root characterization: $R^2$ = 0.78.

Figure 3

Trial 1: Response to imposed drought. A, Comparison of root detections between well-watered plants (blue) and drought treatment plants (orange) of smoothed mean daily root detection rate over time, ${\stackrel{-}{r}}_t$. B, Duration, $t_i$, of each irrigation event during the trial for each treatment. C, Comparison of root detections between well-watered plants (blue) and drought treatment plants (orange) of the smoothed mean of the per-electrode-depth daily root detection rate, time-averaged across the drought period (January 4, 2019 to January 19, 2019), ${\stackrel{-}{R}}_d$. All shaded regions indicate ±2 standard errors from the mean. D, Box and whisker distributions of time-averaged daily root detection rate, $R$, during the drought period, separated by genotype and treatment (blue, well-watered; orange, drought). Top and bottom of box indicate 25 and 75 percentiles of RootTrackers; horizontal line in box is median; cross is mean, and whiskers are 9 and 91 percentiles. E, Well-watered (blue) and drought (orange) treatment comparison of ${\stackrel{-}{r}}_t$ (left) and ${\stackrel{-}{R}}_d$ during the drought period (right) for two hybrids (H003, top, and H014, bottom) with variable relative responses to drought.

Figure 4

Trial 2: Priming response to early drought. Comparison between (A) well-watered plants and (B) water-limited plants, of smoothed mean daily root detection rate over time and electrode depth, ${\stackrel{-}{r}}_{td}$. C, Comparison between well-watered plants (blue) and water-limited plants (orange) of smoothed mean daily root detection rate over time, ${\stackrel{-}{r}}_t$, and (c inset) mean cumulative root detections over time, ${\stackrel{-}{c}}_t$. All shaded regions indicate ±2 standard errors from the mean. D, Duration, $t_i$, of each irrigation event during the trial for each treatment.

Figure 5

Trial 3: Priming response to early drought. Comparison between well-watered plants, (blue), single drought plants (orange), and double drought (yellow), of (A) smoothed mean daily root detection rate over time, ${\stackrel{-}{r}}_t$, and (A, inset) mean cumulative root detections over time, ${\stackrel{-}{c}}_t$. B, Duration, $t_i$, of each irrigation event during the trial for each treatment. Comparison between well-watered plants (blue), single drought plants (orange), and double drought (yellow), of the smoothed mean of the time-averaged, per-electrode-depth daily root detection rate, ${\stackrel{-}{R}}_d$, from (C) August 18, 2019 to August 28, 2019 and (D) August 28, 2019 to September 8, 2019. All shaded regions indicate ±2 standard errors from the mean.

Figure 6

Trial 4: Comparison of maize genotypes in Midwest fields. A, Box and whisker distributions of daily root detection rates time-averaged across the entire trial, $R$, separated by genotype. Top and bottom of box indicate 25 and 75 percentile of RootTrackers, horizontal line in box is median, cross is mean, and whiskers are 9 and 91 percentiles. B, Comparison of root detections between plants of genotype H005 (red) and H013 (blue) of smoothed mean daily root detection rate over time, ${\stackrel{-}{r}}_t$. C, Inches of rain for each day received at Aurora, NE by station GHCND:US10hami004 (data provided by the National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center (NCDC) National Centers for Environmental Information, Asheville North Carolina from their website at ncdc.noaa.gov, Menne et al., 2012a). D, Comparison of root detections between plants of genotype H005 (red) and H013 (blue) of smoothed mean of the per-electrode-depth daily root detection rate, time-averaged across the entire trial, ${\stackrel{-}{R}}_d$. Shaded regions indicate ±2 standard errors from the mean.