Assessment of O 2 diffusivity across the barrier to radial O 2 loss in adventitious roots of Hordeum marinum

Summary•  The subapical regions of wetland plant roots can develop a barrier to radial O 2 loss (ROL), but barrier resistance has rarely been quantified in terms of its O 2 diffusivity. •  Barrier resistance in adventitious roots of the waterlogging‐tolerant Hordeum marinum was assessed from measurements of ROL using cylindrical platinum electrodes while either varying shoot O 2 partial pressures or cooling the rooting medium. Anatomical features were examined using fluorescence microscopy. •  When grown in stagnant agar nutrient solution, a barrier to ROL was induced over the basal half of 100–120‐mm‐long roots. Autofluorescence in hypodermal cell walls indicated that putative suberin deposition was coincident with barrier expression. Root cooling revealed a significant respiratory component in barrier resistance. Eliminating the respiration effect by manipulating shoot O 2 partial pressures revealed an O 2 diffusivity for the barrier of 5.96 × 10 −7  cm 2  s −1 , 96% less than that at the apex, which was ≥ 1.59 × 10 −5  cm 2 s −1 . •  It is concluded that the ROL barrier is a manifestation of two components acting synergistically: a physical resistance caused principally by secondary cell‐wall deposits in the outer hypodermal layer; and respiratory activity in the hypodermal/epidermal layers; with physical resistance being the dominant component.