Photosynthesis Gas Exchange System with Internal CO₂ Directly Measured

Abstract

In this study, a leaf gas exchange system was constructed and tested over an extended range of external CO₂ concentrations (c_a) while the concentration inside the leaves (c_i) was directly and continuously determined. For c_a, an infrared analyzer was used to compare c_a with a reference gas at concentrations as high as 50,000 μmol•mol⁻¹. For c_i, a glass cup sealed to the abaxial leaf surface equilibrated with internal CO₂, and the gas was circulated to another infrared analyzer. When stomata were open in the light in a sunflower (Helanthus annuus L.) leaf, c_i was about 275 μmol•mol⁻¹ in c_a of 400 μmol•mol⁻¹. If stomata closed in the dark, c_i increased until it exceeded c_a but in the light the reverse occurred and c_i decreased to only 60 μmol•mol⁻¹. When c_a increased in light, stomata began to close but c_i could be increased to 50,000 μmol•mol⁻¹, overcoming the closure. But inward CO₂ diffusion was further inhibited by water vapor diffusing outward. The inhibition agreed with theoretical calculations of von Caemmerer and Farquhar (1981). The system expanded the CO₂ concentrations at which CO₂ fixation could be measured while avoiding complications from calculating c_i when stomata close.