Can We Develop a Root Ideotype for High Strength Soil Using Bioenergetics, Biomechanics and Root Phenomics?

High soil strength – also referred to as high soil penetration resistance – is the most important physical constraint to root growth and occurs in (i) compacted soil, (ii) naturally dense subsoil and (iii) dry soil. Root growth slows down with increasing penetration resistance. Furthermore, root tips need to exert greater mechanical stresses to successfully penetrate soil of increased strength, which increases the energy turnover and carbon demands for root growth. In turn, water and nutrient accessibility decreases while the metabolic costs of soil exploration increases, and crop yields are reduced eventually. To overcome these adverse effects on crop productivity a root ideotype needs to be developed that (i) enables root elongation and (ii) reduces the energy demands for root growth in high strength soil. To identify specific root traits that define such an ideotype, we combine bioenergetics and root-soil biomechanics with root phenomics. Different wheat genotypes (Triticum aestivum L.) are used as a model system and their growing roots are exposed to different levels of soil penetration resistance. Using isothermal calorimetry the energy requirements, i.e. bioenergetics, for root growth can be quantified in-situ. The mechanical stress that the root tip exerts during growth is modelled based on cavity expansion theory and penetrometer measurements. Finally, the root phenotype is described using root elongation rate, root tip shape and root anatomical properties. The tested genotypes showed significant variation regarding the assessed phenotypic traits. We also showed that under increased soil strength the shape of the root tip governs root elongation rate, the mechanical stress at the root tip and thus the mechanical energy needed for soil penetration. We therefore suggest that the outlined research approach bears large potential to develop a root ideotype for high strength soil, which may be selected for in crop breeding programs.