Node of origin matters: comparative analysis of soil water limitation effects on nodal root anatomy in maize (Zea mays L.).

Background and aim: Root anatomy, determining the composition and organization of root tissues, has implications for water uptake and transport, and potential for enhancing crop resilience amid changing environmental conditions and erratic water supply. While our understanding of the functional relationship between root anatomical traits and soil resource acquisition continues to improve, anatomical traits are commonly investigated on adventitious roots emerging from a single node or averaged across nodes. We test the hypothesis that drought adaptations of anatomical and hydraulic phenes are specific to the nodal origin of the root.

Methods: We grew four maize (Zea mays L.) genotypes in the field under control and drought conditions, imposed by rainout shelters. Subsequently, we investigated the effect of soil drought on crown root anatomical phenes between consecutive shoot nodes. Based on these phenotypes, we inferred root cross-sectional hydraulic properties by integrating simulations of root anatomical networks via the GRANAR model and translating the outputs into hydraulic properties using the MECHA model.

Key results: At the individual node level, drought-induced changes in root anatomical and hydraulic phenes were neither consistently significant nor unidirectional across nodes or genotypes. Notably, only second node crown roots consistently exhibited significant changes in response to drought. However, we observed distinct treatment differences in the development of phenes between consecutive shoot nodes. Most root anatomical and hydraulic phenes showed a (hyper)allometric relationship with increasing root cross-sectional area from older to younger roots. However, under drought, those allometric trajectories shifted. Specifically, the root cross-sectional area and the areas of stele, cortex, metaxylem, and aerenchyma, as well as cortical cell size and the axial hydraulic conductance increased more strongly from older to younger roots under drought. In contrast, metaxylem number increased more strongly under controlled conditions.

Conclusion: Our findings suggest that examining the drought response of root anatomical phenes at a single node may not provide a comprehensive understanding of root system responses to the environment.