Stele tissues of chickpea roots show higher metabolic activity than the cortex, independent of external O₂ supply

Variations in external O₂ supply, tissue diffusivity, and O₂ consumption rates determine internal O₂ gradients in roots. We tested the hypothesis that root metabolism differs between cortex and stele tissues in response to external partial pressure of O₂ (pO₂). Using O₂ microsensors, we measured in vivo O₂ dynamics in Cicer arietinum, chickpea, primary roots of contrasting diameters exposed to different external pO₂ levels. Respiratory O₂ consumption was assessed in the isolated cortex (plus epidermis) and stele tissues. Additionally, activity profiles for 23 key enzymes of carbohydrate and antioxidant metabolism were determined under aerobic, hypoxic, and severely hypoxic conditions. The stele of thick roots (1900 μm) became severely hypoxic/anoxic (≤ 0.3 kPa) even under external air equilibrium, whereas thinner roots (900 μm) remained oxic-to-hypoxic (c. 11.9 kPa). The stele had a 3.3-fold higher O₂ consumption rate than the cortex. In 1400 μm roots, severe hypoxia occurred in the stele at 8.9 kPa external pO₂ and in the cortex at 4.5 kPa. Activities of carbohydrates and antioxidant enzymes were consistently higher in the stele than in the cortex, regardless of external pO₂, indicating that tissue type, in addition to external O₂ availability, plays a key role in determining root metabolic activity.