Oxygen Dynamics in the Rhizosphere of Vallisneria spiralis Characterized by a Fluorescent Planar Optode

Oxygen (O2) leakage in macrophyte rhizospheres is an adaptive strategy for hypoxic environments, which is important in lake ecological restoration. In this investigation, the fluorescent planar optode (PO) technique is used for two-dimensional (2D) distribution of dissolved O2 at a submillimeter scale in the rhizosphere of Vallisneria spiralis under various environmental conditions. The spatial heterogeneity in the distribution of oxic microniches is frequently verified in the rhizosphere. The radial oxygen loss (ROL) rate for root systems is characterized by the following sequence: basal root (20.6 ± 5.1–49.6 ± 9.5 nmol m−2 s−1, n = 7) > lateral root (14.1 ± 4.1–36.6 ± 8.3 nmol m−2 s−1, n = 7) > root tip (13.1 ± 4.6–28.8 ± 6.4 nmol m−2 s−1, n = 7). The O2 maximum value on lines transecting each kind of root also obeys the sequence mentioned above. For one typical root, (1) O2 decreases from 131.2 ± 2.4–147.4 ± 3.7 μmol L−1 at the root center to 47.2 ± 1.4–75.9 ± 2.2 μmol L−1 in the rhizosphere fringe due to O2 supply from the root surface and O2 consumption in rhizosphere sediment, and (2) the furthest distance from the aboveground part to the root tip leads to the lowest O2 concentration at the root apex of that root. The light/dark transition and O2 level in overlying water modulate the photosynthetic activity of leaves and the transfer of oxygen in the water column through aerenchyma tissue to the roots. The sequence of the oxygenated area (%), ROL rate, and O2 concentration in rhizosphere sediment under various conditions is demonstrated as: high illumination/high O2 > darkness/high O2 > high illumination/low O2 > darkness/low O2. The effect of O2 in water on the ROL of Vallisneria spiralis is more distinct than illumination. Oxygen storage in roots, and especially O2 diffusion from overlying water, can supplement O2 deficiency in the rhizosphere during the cessation of photosynthesis under darkness. This research advances the understanding of complex interrelationships among O2 dynamics in different root parts, photosynthesis, O2 in overlying water and O2 transfer through plant aerenchyma to the rhizosphere.