Fig. 1.

Adaptive significance of an ROL barrier for roots in waterlogged soil. (A) Oxygen molecules diffusing longitudinally through aerenchyma toward the root apex may either be consumed by respiration of root cells or diffuse radially to the rhizosphere (called radial oxygen loss, ROL). Although ROL aerates the rhizosphere, it also reduces the supply of oxygen to the root apex. (B) Many wetland species form an ROL barrier in the basal parts of roots. ROL barrier promotes longitudinal oxygen diffusion by preventing losses to anaerobic soils. (i) In anaerobic soils, enhanced movement of oxygen toward the apex enables active respiration of the cells at the root tips. Because the root tip does not have an ROL barrier, (ii) the ROL around the root tip detoxifies toxic reduced substances in the waterlogged soil. (iii) The barrier also blocks the entry of potentially toxic compounds in highly reduced soils.