Oxygen dynamics in submerged rice (Oryza sativa)

Complete submergence of plants prevents direct O-2 and CO2 exchange with air. Underwater photosynthesis can result in marked diurnal changes in O-2 supply to submerged plants. Dynamics in pO(2) had not been measured directly for submerged rice (Oryza sativa), but in an earlier study, radial O-2 loss from roots showed an initial peak following shoot illumination. O-2 dynamics in shoots and roots of submerged rice were monitored during light and dark periods, using O-2 microelectrodes. Tissue sugar concentrations were also measured. On illumination of shoots of submerged rice, pO(2) increased rapidly and then declined slightly to a new quasi-steady state. An initial peak was evident first in the shoots and then in the roots, and was still observed when 20 mol m(-3) glucose was added to the medium to ensure substrate supply in roots. At the new quasi-steady state following illumination, sheath pO(2) was one order of magnitude higher than in darkness, enhancing also pO(2) in roots. The initial peak in pO(2) following illumination of submerged rice was likely to result from high initial rates of net photosynthesis, fuelled by CO2 accumulated during the dark period. Nevertheless, since sugars decline with time in submerged rice, substrate limitation of respiration could also contribute to morning peaks in pO(2) after longer periods of submergence.