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

The root barrier to radial O2 loss (ROL) is a trait enabling waterlogging tolerance of plants. The ROL barrier restricts O2 diffusion to the anoxic soil so that O2 is retained inside root tissues. We hypothesised that the ROL barrier can also restrict radial diffusion of other gases (H2 and water vapour) in rice roots with a barrier to ROL. We used O2 and H2 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 O2 as well as H2. At 60 kPa pO2, we found no radial diffusion of O2 across the barrier, and for H2 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 O2 diffusion under steep concentration gradients but is also a diffusive barrier to H2 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.