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

Abstract

The root barrier to radial O 2 loss (ROL) is a trait enabling waterlogging tolerance of plants. The ROL barrier restricts O 2 diffusion to the anoxic soil so that O 2 is retained inside root tissues.We hypothesised that the ROL barrier can also restrict radial diffusion of other gases (H 2 and water vapour) in rice roots with a barrier to ROL. We used O 2 and H 2 microsensors to measure ROL and permeability of rice roots, and gravimetric measurements to assess the influence of the ROL barrier on radial water loss (RWL).The ROL barrier greatly restricted radial diffusion of O 2 as well as H 2 . At 60 kPa pO 2 , we found no radial diffusion of O 2 across the barrier, and for H 2 the barrier reduced radial diffusion by 73%. Similarly, RWL was reduced by 93% in roots with a ROL barrier.Our study showed that the root barrier to ROL not only completely blocks radial O 2 diffusion under steep concentration gradients but is also a diffusive barrier to H 2 and to water vapour. The strong correlation between ROL and RWL presents a case in which simple measurements of RWL can be used to predict ROL in screening studies with a focus on waterlogging tolerance.

The quantitative importance of key root traits for radial water loss under low water potential

“…Radial water loss (RWL) was measured by using a gravimetric approach as described by (Peralta Ogorek et al 2021) with some modifications. Briefly, root segments were positioned on a metallic mesh in a balance chamber with a relative humidity (RH) of 18-28% (HOBO UX100-011 Temperature and RH data logger, Onset).…”

“…The fitting of data was conducted in order to identify the time for which 15% of total pool of water had been lost (Fig. S1) representing the volume of water present in cells exteriorly to the ROL barrier in rice (Peralta Ogorek et al 2021). The same procedure was applied to agar cylinders.…”

“…This is true for aerenchyma formation in particular, which formed substantially in plants growing in dry as well as in wet soils, while forming much less at intermediate soil water content (Yamauchi et al 2021b). In addition, the ROL barrier forming in roots of plants exposed to waterlogging also restricts radial water loss (Peralta Ogorek et al 2021). Since ROL and RWL in roots both seem related to the formation of a barrier in the exodermis (Taleisnik et al 1999;Toulotte et al 2022), the present study tested the hypothesis that a root barrier to ROL is formed as a response to drought using hydroponics, where the water potential of the nutrient solution was manipulated with PEG-6000.…”

Radial Oxygen Loss from Plant Roots—Methods

“…where D is the O 2 diffusion coefficient in the agar medium (approximates that in water at the experimental temperature), C1 is the O 2 concentration at the root surface (mol m −3 ) calculated from the partial pressure, C2 is the O 2 concentration at a given radial distance away from the root surface r2 (m) and r1 is the radius of the root (m). ROL measurements can be conducted along the root when the aboveground parts of the plant are above water (i.e., as explained before for ROL measurements using root-sleeving electrodes) or can be conducted on excised root segments sealed with lanoline in both cut ends [52]. Provided that the main source for O 2 is O 2 from root aerenchyma (deoxygenated solutions still have traceable amounts of O 2 ), the slope of the O 2 gradient from the deoxygenated medium into the root surface determine the presence of barriers to ROL.…”

Corrigendum

“…Since its publication, the authors of Peralta Ogorek et al (2021) have brought to our attention errors in their article, where the apparent permeances of O 2 and H 2 reported were incorrect. The corrected Results subsection 'Apparent permeance (P a ) to O 2 , H 2 and water vapour' and Table 1 are shown below.…”