Photosynthesis gas exchange system with internal CO 2 directly measured

Abstract

In this study, a leaf gas exchange system was constructed and tested over an extended range of external CO 2 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 with a reference gas at concentrations as high as 50,000 μmolmol -1. For c i, a glass cup sealed to the abaxial leaf sir- face equilibrated with internal C0 2, 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 CO 2 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 CO 2 concentrations at which CO 2 fixation could be measured while avoiding complications from calculating c i when stomata close.