Complete submergence of plants prevents direct O₂ and CO₂ exchange with air. Underwater photosynthesis can result in marked diurnal changes in O₂ supply to submerged plants. Dynamics in pO₂ had not been measured directly for submerged rice (Oryza sativa), but in an earlier study, radial O₂ loss from roots showed an initial peak following shoot illumination. 

O₂ dynamics in shoots and roots of submerged rice were monitored during light and dark periods, using O₂ microelectrodes. Tissue sugar concentrations were also measured. 

On illumination of shoots of submerged rice, pO₂ 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 pO₂ was one order of magnitude higher than in darkness, enhancing also pO₂ in roots.The initial peak in pO₂ 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 pO₂ after longer periods of submergence.