Apoplastic barriers effectively block oxygen permeability across outer cell layers of rice roots under deoxygenated conditions: roles of apoplastic pores and of respiration

P>Despite the importance of the barrier to oxygen losses of the roots of hygrophytes growing in wet environments devoid of oxygen, there are few data available on permeability coefficients for O-2 across outer root cell layers (P-OPR) and how they may change in response to low O-2. A gas perfusion technique was used to measure the P-OPR of rice (Oryza sativa) plants grown in either aerated or deoxygenated solution. The contributions of the apoplast and of living cells to the overall P-OPR were characterized either by blocking apoplastic pores with precipitates of brown Cu-2[Fe(CN)(6)] or by killing cells with 0.1 N HCl. Compared with that of plants from aerated hydroponics, the P-OPR of plants grown in deoxygenated medium was smaller by an order of magnitude. Precipitates resulting from CuSO4/K-4[Fe(CN)(6)] treatment only formed in plants grown in aerated solution, where they reduced the P-OPR by 5-20%. Killing of root segments with HCl increased P-OPR in plants grown in both conditions by 20-55%. The results indicated that apoplastic barriers effectively restricted radial O-2 loss. The relative role of the respiratory O-2 consumption of root peripheral layers increased as P-OPR decreased.