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 x 10(-7) cm(2) s(-1), 96% less than that at the apex, which was >= 1.59 x 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.