Underwater photosynthesis and respiration in leaves of submerged wetland plants: gas films improve CO2 and O-2 exchange

Many wetland plants have gas films on submerged leaf surfaces. We tested the hypotheses that leaf gas films enhance CO2 uptake for net photosynthesis (P-N) during light periods, and enhance O-2 uptake for respiration during dark periods. Leaves of four wetland species that form gas films, and two species that do not, were used. Gas films were also experimentally removed by brushing with 0.05% (v/v) Triton X. Net O-2 production in light, or O-2 consumption in darkness, was measured at various CO2 and O-2 concentrations. When gas films were removed, O-2 uptake in darkness was already diffusion-limited at 20.6 kPa (critical O-2 pressure for respiration, COPR >= 284 mmol O-2 m(-3)), whereas for some leaves with gas films, O-2 uptake declined only at approx. 4 kPa (COPR 54 mmol O-2 m(-3)). Gas films also improved CO2 uptake so that, during light periods, underwater P-N was enhanced up to sixfold. Gas films on submerged leaves enable continued gas exchange via stomata and thus bypassing of cuticle resistance, enhancing exchange of O-2 and CO2 with the surrounding water, and therefore underwater P-N and respiration.