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.