Photosynthetic O2 Exchange Kinetics in Isolated Soybean Cells

Abstract

Light-dependent O2 exchange was measured in intact, isolated soybean (G. max. cv. Williams) cells using isotopically labeled O2 and a mass spectrometer. The dependence of O2 exchange on O2 and CO2 was investigated at high light in coupled and uncoupled cells. With coupled cells at high O2, O2 evolution followed similar kinetics at high and low CO2. Steady-state rates of O2 uptake were insignificant at high CO2, but progressively increased with decreasing CO2. At low CO2, steady-state rates of O2 uptake were 50-70% of the maximum CO2-supported rates of O2 evolution. These high rates of O2 uptake exceeded the maximum rate of O2 reduction determined in uncoupled cells, suggesting the occurrence of another light-induced O2-uptake process (i.e. photorespiration). Rates of O2 exchange in uncoupled cells were half-saturated at 7-8% O2. Initial rates (during induction) of O2 exchange in uninhibited cells were also half-saturated at 7-8% O2. Steady-state rates of O2 evolution and O2 uptake (at low CO2) were half-saturated at 18-20% O2. O2 uptake was significantly suppressed in the presence of nitrate, suggesting that nitrate and/or nitrite can compete with O2 for photoreductant. Apparently, 2 mechanisms (O2 reduction and photorespiration) are responsible for the light-dependent O2 uptake observed in uninhibited cells under CO2-limiting conditions. The relative contribution of each process to the rate of O2 uptake appears to be dependent on the O2 level. At high O2 concentrations (.gtoreq. 40%), photorespiration is the major O2-consuming process. At lower (ambient) O2 concentrations (.ltoreq. 20%), O2 reduction accounts for a significant portion of the total light-dependent O2 uptake.