Divergence of root system plasticity in soybean between modern breeding lines and diverse germplasm accessions

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Abstract Roots are critical for supporting basic plant functions such as anchoring in various substrates, uptake of water and nutrients, and hosting symbiotic relationships. In crops, indirect changes to root system architecture (RSA) have occurred largely as a result of selection for yield or other related aboveground traits. In cultivated soybean ( Glycine max (L.) Merr.), evidence of changes to RSA resulting from breeding for crop performance has been inconsistent, with some studies supporting an overall decrease in performance‐related trait values, such as root length and density, and other work showing the opposite. The current study sets out to ask whether there is any systematic differentiation in RSA between a set of elite breeding lines ( n = 8) of soybean developed for the Midwest United States and a group of diversity lines from the USDA Soybean Germplasm Collection ( n = 16). Groups are compared across three distinct developmental stages (V2–V6, V7–R2, and R3–R7) and two contrasting soil environments. In total, 432 root systems were phenotyped for 12 structural traits derived from two‐dimensional images along with root and shoot biomass. A new three‐dimensional root modeling approach leveraging photogrammetry‐derived pointclouds is additionally tested on a subset of 30 contrasting root systems. Results indicate that the diversity lines had smaller root systems overall but greater phenotypic plasticity in response to soil environment as compared to breeding lines. Plants grown in clay loam soil had reduced taproot length (14.2%), root biomass (18%), root volume (22.9%), root spread (22.7%), and average root diameter (7.6%) compared to sandy loam soil. In addition, root traits showed generally low heritabilities. Overall mean heritabilities were found to be highest in the earlier timepoint and declined over time. Maximum taproot diameter ( H 2 = 0.37 and h 2 = 0.21) and maximum lateral branch length ( H 2 = 0.22 and h 2 = 0.13) were the most heritable traits. Furthermore, the study finds evidence for trade‐offs between aboveground and belowground trait plasticity.

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