Functional and chemical comparison of apoplastic barriers to radial oxygen loss in roots of rice (Oryza sativa L.) grown in aerated or deoxygenated solution

Radial oxygen loss (ROL) and root porosity of rice (Oryza sativa L.) plants grown in either aerated or deoxygenated (stagnant) conditions were combined for the first time with extensive histochemical and biochemical studies of the apoplastic barriers in the roots' peripheral cell layers. Growth in stagnant solution significantly affected structural and, consequently, the physiological features of rice roots. It increased adventitious root porosity by about 20% and decreased the ROL towards the base to zero at a distance of 40 mm from the apex. By contrast, roots of plants grown in aerated solutions revealed the highest rates of ROL at 30 mm from the apex. Differences in the ROL pattern along the root were related to histochemical studies, which showed an early development of Casparian bands and suberin lamellae in the exodermis, and lignified sclerenchyma cells in roots of plants grown in deoxygenated solution. In agreement with anatomical studies, absolute contents of suberin and lignin in the outer part of the roots (OPR) were higher in plants grown in deoxygenated solution. Regardless of growth conditions, the levels of suberin and lignin increased along the roots towards the base. It is concluded that radial oxygen loss can be effectively restricted by the formation of a suberized exodermis and/or lignified sclerenchyma in the OPR. However, the relative contribution of suberin and lignin in the formation of a tight barrier is unclear. Knowing the permeability coefficient across OPR for roots of plants grown in both conditions will allow a more precise understanding of the mechanisms controlling ROL.