Quantitative imaging of radial oxygen loss from Valisneria spiralis roots with a fluorescent planar optode

Abstract

Oxygen (O2) availability within the sediment-root interface is critical to the survival of macrophytes in O2-deficient sediment; however, our knowledge of the fine-scale impact of macrophyte roots upon the spatiotemporal dynamics of O2 is relatively limited. In this study, a non-invasive imaging technology was utilized to map O2 micro-distribution around Vallisneria spiralis. Long-term imaging results gathered during a 36day-period revealed an abundance of O2 spatiotemporal patterns ranging from 0 to 250μmolL(-1). The root-induced O2 leakage and consequent oxygenated area were stronger in the vicinity of the basal root compared to that found in the root tip. The O2 images revealed V. spiralis exhibited radial O2 loss (ROL) along the entire root, and the O2 distribution along the root length showed a high degree of small-scale spatial heterogeneity decreasing from 80% at the basal root surface to 10% at the root tip. The oxygenated zone area around the roots increased as O2 levels increased with root growth and irradiance intensities ranging from 0 to 216μmol photons m(-2)s(-1). A weak ROL measuring <20% air saturation around the basal root surface was maintained in darkness, which was presumably attributed to the O2 supply from overlying water via plant aerenchyma. The estimated total O2 release to the rhizosphere of V. spiralis was determined to range from 8.80±7.32 to 30.34±17.71nmolm(-2)s(-1), which is much higher than many other macrophyte species. This O2 release may be an important contribution to the high-capacity of V. spiralis for quickly colonizing anaerobic sediment.

Spatial heterogeneity and short‐term oxygen dynamics in the rhizosphere of Vallisneria spiralis: Implications for nutrient cycling

“…9.5–10.8 μmol O 2 g DW root −1 hr −1 were similar to the one reported by Han et al. () i.e. 5.2–7.7 μmol O 2 g DW root −1 hr −1 .…”

“…Rates of ROL from single roots in our study (range, 58.3–658 μmol m −2 hr −1 ) are comparable to rates measured in rice plants ( Oryza sativa ) i.e. 19.2–441 μmol m −2 hr −1 (Larsen, Santner, Oburger, Wenzel, & Glud, ), which are among the highest rates reported in the literature (Han et al., and references therein). Rates of ROL from single roots of V. spiralis measured by Han et al.…”

“…To our knowledge, the work of Han et al. () and the present study provided the first direct estimates of ROL in V. spiralis under conditions close to those met in situ. Rates of ROL from single roots in our study (range, 58.3–658 μmol m −2 hr −1 ) are comparable to rates measured in rice plants ( Oryza sativa ) i.e.…”

“…Rates of ROL from single roots of V. spiralis measured by Han et al. () via planar optode imaging ranged between 32 and 109 μmol m −2 hr −1 . Despite the difference in ROL from single roots between the two available studies on V. spiralis , our total ROL rate normalised for the root biomass (calculated from total ROL/whole roots dry weight) i.e.…”

“…The lower concentration of CH 4 and metal ions (Fe 2+ and Mn 2+ ) and the stimulated coupling between denitrification and nitrification in vegetated versus non‐vegetated sediments further supported the hypothesis of rhizosphere‐driven sediment oxygenation (Racchetti, Longhi, Ribaudo, Soana, & Bartoli, ; Racchetti et al., ; Ribaudo et al., ; Soana et al., ). Whereas the above‐mentioned studies provide indirect evidence of O 2 leakage from roots, direct measurement of ROL in V. spiralis was only recently shown via planar optode application (Han, Ren, Tang, Xu, & Xie, ; Han et al., ). These studies demonstrated O 2 leakage of the roots, analysed the effect of irradiance on ROL, and showed a link between ROL and the immobilisation of porewater P.…”