Abstract
In flooded soils, an efficient internal aeration system is essential for root growth and plant survival. Roots of many wetland species form barriers to restrict radial O2 loss (ROL) to the rhizosphere. The formation of such barriers greatly enhances longitudinal O2 diffusion from basal parts towards the root tip, and the barrier also impedes the entry of phytotoxic compounds produced in flooded soils into the root. Nevertheless, ROL from roots is an important source of O2 for rhizosphere oxygenation and the oxidation of toxic compounds. In this paper, we review the methodological aspects for the most widely used techniques for the qualitative visualization and quantitative determination of ROL from roots. Detailed methodological approaches, practical set-ups and examples of ROL from roots with or without barriers to ROL are included. This paper provides practical knowledge relevant to several disciplines, including plant–soil interactions, biogeochemistry and eco-physiological aspects of roots and soil biota.
Highlights
During flooding, the gas-filled pore spaces in the soil that normally facilitate O2 diffusion are filled with water and the diffusion of gases (e.g., O2) is highly impeded
For rice roots grown in aerated conditions, higher basipetal radial O2 loss (ROL) rates decreasing towards the root tip are characteristic patterns indicating roots with ‘weak’ or Plants 2021, 10, 2322 rates decreasing towards the root tip are characteristic patterns indicating roots with ‘weak’ or absent barriers to ROL
The patterns of ROL obtained with O2 microsensors might be similar to those obtained using the root-sleeving electrode (e.g., Figure 1; [28]), but in many cases the measured fluxes are lower when obtained by a microsensor
Summary
The gas-filled pore spaces in the soil that normally facilitate O2 diffusion are filled with water and the diffusion of gases (e.g., O2) is highly impeded (diffusion coefficient of O2 in water is 10,000 lower than in air; [1]). Lower O2 radial flux rates were obtained in roots of plants grown in flooded conditions in comparison to aerated controls [10]. For rice roots grown in aerated conditions, higher basipetal ROL rates decreasing towards the root tip are characteristic patterns indicating roots with ‘weak’ or Plants 2021, 10, 2322 rates decreasing towards the root tip are characteristic patterns indicating roots with ‘weak’ or absent barriers to ROL. Within the past few decades, active research has been conducted on the ecological importance of ROL to the flooded soil as well as the anatomical location, chemical characterization, interspecific differences and external triggers for barriers to impede ROL from roots [7,11,12,15,16,17,33,34,35,36,37,38,39]. Understanding the strengths and weaknesses of the different methods for ROL quantification as well as the potential uses of each method will provide the needed information for an appropriate method selection and accurate quantification of ROL from roots of plants grown in diverse conditions
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