Phenotypic plasticity in root systems is a critical adaptive mechanism enabling rice (Oryza sativa L.) to thrive under varying soil water gradients. This study aims to elucidate the extent and nature of rice root plasticity in response to differential soil moisture levels, from drought to waterlogging. We conducted controlled experiments using a rice cultivar (IR42) and a wild rice species (Oryza longistaminata) subjected to a gradient of soil water conditions. Root histochemical staining, anatomical traits, and ecophysiological responses, including suberin and lignin deposition, aerenchyma formation, cortex to stele ratio (CSR), root diameter, the barrier to radial oxygen loss (ROL), and radial water loss (RWL), were systematically measured and analyzed.

Our findings reveal significant phenotypic and physiological variations in root responses to soil water availability. Under drought conditions, rice roots exhibited increased lignin deposition and aerenchyma formation, inducing barriers to restrict RWL. Similarly, in waterlogged conditions, roots demonstrated increased lignin deposition and aerenchyma formation, inducing barriers to ROL, facilitating oxygen transport. Additionally, rhizomes of O. longistaminata exhibited enhanced tissue diameter and larger gas spaces under soil flooding, while decreasing diameter under water deficit conditions.

These results underscore the adaptive plasticity of rice root systems, which is crucial for maintaining productivity under fluctuating water regimes. Understanding the physiological basis of this plasticity provides valuable insights for breeding programs aimed at developing rice varieties with enhanced resilience to water stress. Our study contributes to the broader knowledge of plant root ecophysiology, offering potential strategies for improving crop performance in the face of climate change-induced water variability.