Root O₂ consumption, CO₂ production and tissue concentration profiles in chickpea, as influenced by environmental hypoxia

Abstract

Summary Roots in flooded soils experience hypoxia, with the least O2 in the vascular cylinder. Gradients in CO2 across roots had not previously been measured. The respiratory quotient (RQ; CO2 produced : O2 consumed) is expected to increase as O2 availability declines. A new CO2 microsensor and an O2 microsensor were used to measure profiles across roots of chickpea seedlings in aerated or hypoxic conditions. Simultaneous, nondestructive flux measurements of O2 consumption, CO2 production, and thus RQ, were taken for roots with declining O2. Radial profiling revealed severe hypoxia and c. 0.8 kPa CO2 within the root vascular cylinder. The distance penetrated by O2 into the roots was shorter at lower O2. The gradient in CO2 was in the opposite direction to that of O2, across the roots and diffusive boundary layer. RQ increased as external O2 was lowered. For chickpea roots in solution at air equilibrium, O2 was very low and CO2 was elevated within the vascular cylinder; the extent of the severely hypoxic core increased as external O2 was reduced. The increased RQ in roots in response to declining external O2 highlighted the shift from respiration to ethanolic fermentation as the severely hypoxic/anoxic core became a progressively greater proportion of the root tissues.

Novel functions of the root barrier to radial oxygen loss – radial diffusion resistance to H2 and water vapour

“…The radial profiles followed the approach described in Colmer et al . (2020). The root segments and gas pressures used for profiling were prepared similarly to those used for O 2 or H 2 intrusion.…”

“…We hypothesised a strong positive correlation between RWL and ROL and therefore we measured rates of ROL using a microoptode on intact plants without, a weak or with a tight barrier. ROL was measured using the slope of O 2 concentrations within the DBL (Henriksen et al ., 1990; Colmer et al ., 2020). The ROL measurements were taken 40–60 mm behind the root tip on the exact same plant from which the root segments for RWL measurements were taken.…”

“…Radial profiles of pO 2 or pH 2 were taken to verify that the measured gas pressures were independent of the exact microsensor position in the cortex. The radial profiles followed the approach described in Colmer et al (2020). The root segments and gas pressures used for profiling were prepared similarly to those used for O 2 or H 2 intrusion.…”

Lateral roots, in addition to adventitious roots, form a barrier to radial oxygen loss in Zea nicaraguensis and a chromosome segment introgression line in maize

“…The measurements of radial O 2 fluxes in the diffusive boundary layer and associated internal tissue profiles followed the approach of Colmer et al (2020). An intact plant was placed in a glass chamber (250 9 60 9 62 mm), hereafter referred to as a 'flume', into which deoxygenated nutrient solution flowed with a mean flow velocity of 0.63 mm s À1 (see Supporting information Fig.…”

Measuring ROS and redox markers in plant cells

“…These electrodes have been used to measure O 2 concentrations in a variety of plant cell and tissue suspensions, and in recent decades, micro-electrodes have been used to measure O 2 concentrations in plant tissues including chickpea roots, growing potato tubers, and castor plant stems. [186][187][188] The best spatially resolved signal to date has been achieved with a miniaturised electrode whose 3 mm tip has been used to define 191,192 However optic fibres, while they don't consume O 2 , encounter similar challenges as Clarke-type electrodes, in that they can be invasive and physically damaging. The signal can also be impacted by chlorophyll fluorescence and incident light.…”