How do wheat roots improve shoot growth under different local phosphorus supply conditions?

Background and aims To maximize the effectiveness of local phosphorus (P) application, it is necessary to determine the local P supply conditions that improve plant growth and to clarify when, where, and how each root type responds to the conditions and contributes to the overall plasticity of the root system. Methods We grew wheat plants (Triticum aestivum L. 'Norin 61') in transparent rhizoboxes with soil P content varying between patches, but with the same total P in all treatments. We scanned the base of the boxes, segmented the roots in the images using a deep learning-based software, manually traced seminal and nodal roots, and analyzed the spatio-temporal response (root intensity) of each root type (seminal, nodal, and lateral roots) on the surface of the rhizobox to local P supply. Results P uptake and shoot growth improved significantly only in treatments with large differences in P distribution. Among root types, only lateral root intensity significantly increased in P-rich patches but decreased in P-poor patches, despite no difference in total root intensity per plant. The lateral root responses were strongly controlled by local soil available P content shortly after emergence. These responses of lateral roots increased P acquisition efficiency. Conclusion The response of lateral root distribution to local available P content immediately after emergence promoted P uptake and shoot growth only when available P distribution had large differences.