Plasticity of Root Architecture and ROS–Auxin Regulation in Paeonia ostii Under Root-Zone Restriction

Root zone restriction (RZR) technology optimizes plant growth and quality. However, the fleshy root system of <i>Paeonia ostii</i> exhibits sensitivity to spatial constraints, and research on the plasticity of its root architecture and adaptation mechanisms remains inadequate. This study provides a functional analysis of biomass allocation and root architectural responses to the root-zone restriction (RZR) in <i>P. ostii</i>, comparing three container volumes (8.5, 17, and 34 L). While the total biomass increased with root zone volume (e.g., shoot biomass rose from 9.30 g to 59.94 g), RZR induced a 44.8% increase in root-to-shoot ratio, indicating carbon reallocation to enhance belowground resource acquisition. The principal component analysis identified root biomass, volume, and surface area as key plasticity drivers. Optimal root efficiency occurred at 26.09-28.23 L, where root length and tip/fork numbers peaked. Mechanistically, RZR elevated superoxide dismutase (SOD) activity by 49.74% but reduced catalase (CAT) by 74.24%, disrupting H₂O₂ homeostasis. Concurrently, auxin transporter genes (<i>PIN1</i>, <i>AUX1</i>) were upregulated, promoting root elongation and lateral branching through auxin redistribution. We hypothesize that ROS-auxin crosstalk mediates architectural reconfiguration to mitigate spatial stress, with thickened roots enhancing structural stability in restricted environments. The study underscores the need to optimize root zone volume in woody species cultivation, providing thresholds (e.g., >28 L for mature plants) to balance biomass yield and physiological costs in horticultural management.