ABSTRACT: Background and Aims Root soil penetration and path optimization are fundamental for root development in soil. We describe the influence of soil strength on root elongation rate and diameter, response to gravity, and root-structure tortuosity, estimated by average curvature of primary maize roots. Methods Soils with different densities (1·5, 1·6, 1·7?g cm-3), particle sizes (sandy loam; coarse sand mixed with sandy loam) and layering (monolayer, bilayer) were used. In total, five treatments were performed: Mix_low with mixed sand low density (three pots, 12 plants), Mix_medium - mixed sand medium density (three pots, 12 plants), Mix_high - mixed sand high density (three pots, ten plants), Loam_low sandy loam soil low density (four pots, 16 plants), and Bilayer with top layer of sandy loam and bottom layer mixed sand both of low density (four pots, 16 plants). We used non-invasive three-dimensional magnetic resonance imaging to quantify effects of these treatments. Key Results Roots grew more slowly [root growth rate (mm h-1); decreased 50?%] with increased diameters [root diameter (mm); increased 15?%] in denser soils (1·7 vs. 1·5?g cm-3). Root response to gravity decreased 23?% with increased soil compaction, and tortuosity increased 10?% in mixed sand. Response to gravity increased 39?% and tortuosity decreased 3?% in sandy loam. After crossing a bilayered-soil interface, roots grew more slowly, similar to roots grown in soil with a bulk density of 1·64?g cm-3, whereas the actual experimental density was 1·48±0·02?g cm-3. Elongation rate and tortuosity were higher in Mix_low than in Loam_low. Conclusions The present study increases our existing knowledge of the influence of physical soil properties on root growth and presents new assays for studying root growth dynamics in non-transparent media. We found that root tortuosity is indicative of root path selection, because it could result from both mechanical deflection and active root growth in response to touch stimulation and mechanical impedance.
