Oxygen consumption and diffusional resistance both determine maximum root length in flooded soils

Molecular oxygen (O₂) is essential for aerobic metabolism and tightly linked to cellular activity, growth, and survival. However, its internal distribution in plants is highly uneven because O₂ supply depends on slow diffusion through heterogeneous tissues. Gradients in internal O₂ partial pressure (pO2) arise from the interplay between physical supply via gas-filled spaces and biological demand through tissue respiration. Research has traditionally emphasized structural adaptations, such as the formation of aerenchyma, that enhance internal O₂ diffusion under hypoxic or anoxic conditions. However, the metabolic component, i.e., tissue-specific rates of O₂ consumption, exerts an equally important but often overlooked control on internal aeration. In this Expert View, we propose a conceptual framework that integrates anatomical porosity with metabolic demand to explain how plants maintain aerobic function when O₂ is limiting. Model predictions demonstrate that both enhanced porosity and reduced O₂ consumption extend the aerated path available for root growth in flooded soils, although their effects are not additive. Once gas-phase diffusion becomes efficient, further increases in porosity yield diminishing returns, whereas even modest reductions in respiration markedly increase root length. These findings therefore identify two complementary pathways, greater O2 diffusion or lower O2 consumption, leading to longer roots under flooding.