Fig. 3

Anatomical characteristics of adventitious roots of rice grown in a well-drained soil (left) or in a flooded soil (right). Rice roots develop constitutive aerenchyma and its amount increases significantly when growing in a flooded soil. Suberization of exodermal and lignification of sclerenchyma cell walls are incipient in adventitious roots of rice growing in well-drained conditions, but the deposition of these polymers significantly increases when plants grow in flooded soils, forming complete suberized and lignified lamellae. These two polymers act as an apoplastic barrier impeding radial O₂ loss. In roots growing in well-drained conditions, barriers to radial O₂ loss are not constitutively formed, therefore, radial diffusion of O₂ occurs along the root. In contrast, roots growing in flooded soils develop tight barriers to radial O₂ loss in their basal parts, thus O₂ effectively diffuses towards the root tip where O₂ leaks into the rhizosphere since a barrier is not formed in this part of the root. Asterisk keys indicate aerenchyma spaces; arrowheads and arrows point to lignified and suberized cells, respectively. Created with BioRender.com