Anatomical and biochemical characterisation of a barrier to radial O2 loss in adventitious roots of two contrasting Hordeum marinum accessions

A barrier to radial 02 loss (ROL) in roots is an adaptive trait of waterlogging-tolerant plants.11orcleummarinum Iluds. is a waterlogging-tolerant species that, in contrast to its waterlogging-sensitive cultivated relatives, forms a tight barrier to ROL in basal root zones. To evaluate the nature of the barrier to ROL in roots, we combined measurements of ROL with histochemical and biochemical studies of two contrasting H. marinum accessions. H21 formed greater aerenchyma (up to 38% of cross-sectional area) and a tight barrier to ROL when grown under deoxygenated stagnant conditions, whereas the barrier was only partially formed in roots of H90 and aerenchyma was up to 26%. A tight harrier to ROL in roots of 1-121 corresponded with strong suberisation of the exodemiis. In agreement with anatomical studies, almost all aliphatic suberin quantities were greater in roots of 1121 grown under stagnant conditions compared with roots from aerated controls, and also to those in 1190. By contrast to suberin, no differences in root lignification were observed between the two accessions raised in either aerated or stagnant conditions. These findings show that in adventitious roots of H. marinum, suberisation rather than lignification restricts ROL from the basal root zones.