A thermal time framework drives coordinated below- and above-ground development in temperate cereal crops

O_LICereal architecture is underpinned by the coordinated development of modular phytomer units. While above-ground phenology is well characterized by metrics such as the phyllochron, an equivalent framework for root system development is lacking. Because each phytomer node initiates both leaves and adventitious roots, root and shoot development are inherently linked. C_LIO_LIHere, we quantified this coordination in wheat, barley, and rye across contrasting temperature regimes and validated the results under field conditions. We introduce the rhizochron, defined as the thermal time (growing degree-days, {degrees}C d) period between the emergence of nodal roots on successive stem nodes, and the root appearance interval, describing the emergence rate of individual root axes. Root development followed a highly conserved thermal sequence synchronized with shoot phenology. C_LIO_LIAcross species and environments, the rhizochron averaged 146.1{degrees}C d, closely matching the phyllochron (126.6{degrees}C d). We also identified a consistent thermal offset, with nodal roots emerging approximately 185.3{degrees}C d after the corresponding leaf on the same phytomer node. The root appearance interval averaged 45.3{degrees}C d, reflecting continuous root deployment across active nodes. C_LIO_LIBy integrating root phenology into a node-based framework, the rhizochron provides a predictive tool for crop modeling, trait-based breeding, and more target phenotyping aimed at improving resource acquisition and climate resilience. C_LI