Summary
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.
