Post-illumination transient O₂ -uptake is driven by photorespiration in tobacco leaves

This study aims to elucidate the molecular mechanism for the transient increase in the O₂ -uptake rate in tobacco (Nicotiana tabacum cv Xanthi) leaves after turning off actinic lights (ALs). The photosynthetic O₂ evolution rate reaches a maximum shortly after the onset of illumination with ALs and then decreases to zero in atmospheric CO₂ /O₂ conditions. After turning off the ALs, tobacco leaves show a transient increase in the O₂ -uptake rate, the post-illumination transient O₂ -uptake, and thereafter, the O₂ -uptake rate decreases to the level of the dark-respiration rate. Photosynthetic linear electron flow, evaluated as the quantum yield of photosystem II [Y(II)], maintained a steady-state value distinct from the photosynthetic O₂ -evolution rate. In high-[CO₂ ] conditions, the photosynthetic O₂ -evolution rate and Y(II) showed a parallel behavior, and the post-illumination transient O₂ -uptake was suppressed. On the other hand, in maize leaves (a C4 plant), even in atmospheric CO₂ /O₂ conditions, Y(II) paralleled the photosynthetic O₂ -evolution rate and the post-illumination transient O₂ -uptake was suppressed. Hypothesizing that the post-illumination transient O₂ -uptake is driven by C3 plant photorespiration in tobacco leaves, we calculated both the ribulose 1,5-bisphosphate carboxylase- and oxygenase-rates (Vc and Vo) from photosynthetic O₂ -evolution and the post-illumination transient O₂ -uptake rates. These values corresponded to those estimated from simultaneous chlorophyll fluorescence/O₂ -exchange analysis. Furthermore, the H⁺ -consumption rate for ATP synthesis in both photosynthesis and photorespiration, calculated from both Vc and Vo that were estimated from chlorophyll fluorescence/CO₂ -exchange analysis, showed a positive linear relationship with the dissipation rate of the electrochromic shift signal. Thus, these findings support our hypothesis.