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

The subapical regions of wetland plant roots can develop a barrier to radial O sub(2) loss (ROL), but barrier resistance has rarely been quantified in terms of its O sub(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 sub(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 sub(2) partial pressures revealed an O sub(2) diffusivity for the barrier of 5.96 10 super(-7) cm super(2)&nb sp; s super(-1), 96% less than that at the apex, which was greater than or equal to 1.59 10 super(-5) cm super( 2) s super(-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.