Steady-state room temperature fluorescence and CO2 assimilation rates in intact leaves

Steady-state room temperature variable fluorescence from leaves was measured as a function of CO₂ pressure in Xanthium strumarium L. and Phaseolus vulgaris L. Measurements were made in a range of light intensities, at normal and low O₂ parital pressure and over a range of temperatures. At low CO₂ pressure fluorescence increased with increasing CO₂. At higher CO₂ pressure fluorescence usually decreased with increasing CO₂ but occasionally increased slightly. The transition CO₂ pressure between the responses could be changed by changing light, O₂ pressure, or temperature. This breakpoint in the fluorescence-CO₂ curve was a reliable indicator of the transition between ribulose 1,5-bisphosphate (RuBP) saturated assimilation and RuBP regeneration limited assimilation. The fluorescence signal was not a reliable indicator of O₂-insensitive assimilation in these C₃ species.