Stagnant deoxygenated growth enhances root suberization and lignifications, but differentially affects water and NaCl permeabilities in rice (Oryza sativa L.) roots

It has been shown that rice roots grown in a stagnant medium develop a tight barrier to radial oxygen loss (ROL), whereas aerated roots do not. This study investigated whether the induction of a barrier to ROL affects water and solute permeabilities. Growth in stagnant medium markedly reduced the root growth rate relative to aerated conditions. Histochemical studies revealed an early deposition of Casparian bands (CBs) and suberin lamellae (SL) in both the endodermis (EN) and exodermis, and accelerated lignification of stagnant roots. The absolute amounts of suberin, lignin and esterified aromatics (coumaric and ferulic acid) in these barriers were significantly higher in stagnant roots. However, correlative permeability studies revealed that early deposition of barriers in stagnant roots failed to reduce hydraulic conductivity (Lp(r)) below those of aerated roots. In contrast to Lp(r), the NaCl permeability (P-sr) of stagnant roots was markedly lower than that of aerated roots, as indicated by an increased reflection coefficient (sigma(sr)). In stagnant roots, Psr decreased by 60%, while sigma(sr) increased by 55%. The stagnant medium differentially affected the Lp(r) and P-sr of roots, which can be explained in terms of the physical properties of the molecules used and the size of the pores in the apoplast.