THE OXYGEN AND CARBON-DIOXIDE COMPENSATION POINTS OF C-3 PLANTS - POSSIBLE ROLE IN REGULATING ATMOSPHERIC OXYGEN

The O-2 and CO2 compensation points (O-2 inverted right perpendicular and CO2 inverted right perpendicular) of plants in a closed system depend on the ratio of CO2 and O-2 concentrations in air and in the chloroplast and the specificities of ribulose bisphosphate carboxylase/oxygenase (Rubisco), The photosynthetic O-2 inverted right perpendicular is defined as the atmospheric O-2 level, with a given CO2 level and temperature, at which net O-2 exchange is zero, In experiments with C-3 plants, the O-2 inverted right perpendicular with 220 ppm CO2 is 23% O-2; O-2 inverted right perpendicular increases to 27% with 350 ppm CO2 and to 35% O-2 with 700 ppm CO2, At O-2 levels below the O-2 inverted right perpendicular , net O-2 uptake and reduction are accompanied by net O-2 evolution, At O-2 levels above the O-2 inverted right perpendicular, net O-2 uptake occurs with a reduced rate of CO2 fixation, more carbohydrates are oxidized by photorespiration to products of the C-2 oxidative photosynthetic carbon cycle, and plants senesce prematurely, The CO2 inverted right perpendicular increases from 50 ppm CO2 with 21% O-2 to 220 ppm with 100% O-2, At a low CO2/high O-2 ratio that inhibits the carboxylase activity of Rubisco, much malate accumulates, which suggests that the oxygen-insensitive phosphoenolpyruvate carboxylase becomes a significant component of the lower CO2 fixation rate, Because of low global levels of CO2 and a Rubisco specificity that favors the carboxylase activity, relatively rapid changes in the atmospheric CO2 level should control the permissive O-2 inverted right perpendicular that could lead to slow changes in the immense O-2 pool.